Cloud Structure and Distributions Over the Tropical Pacific Ocean [Reprint 2020 ed.] 9780520328983

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CLOUD STRUCTURE AND

DISTRIBUTIONS

OVER THE TROPICAL PACIFIC OCEAN

CLOUD STRUCTURE AND DISTRIBUTIONS OVER THE TROPICAL PACIFIC OCEAN By Joanne S. Malkus and Herbert Riehl

U N I V E R S I T Y

OF

C A L I F O R N I A

P R E S S / B E R K E L E Y 19 6 4 /

AND

LOS

ANGELES

University of California Press, Berkeley and Los Angeles, California Cambridge University Press, London, England Copyright © 1964 by The Regents of the University of California Library of Congress Catalog Card N u m b e r : 64-22418 Designed by Jane Hart Printed in the United States of America

Acknowledgments

In a large-scale u n d e r t a k i n g such as this, involving aerial cloud p h o t o g r a p h y and synoptic d a t a collection and analysis over a large o c e a n , the efforts of m a n y persons and t h e c o o p e r a t i o n of m a n y institutions are required. T h e key to the succcss of this project lay in the o u t c o m e of the precision time-lapse p h o t o g r a p h y and its reduction to cloud m a p s and m e a s u r e m e n t s . O u r chief p h o t o g r a p h e r , C l a u d e R o n n e of the W o o d s Hole O c e a n o g r a p h i c Institution, a d a p t e d the m e t h o d s , m a d e all the flights, and contributed b o t h the f u n d a m e n t a l s and m u c h w o r k t o the reduction a n d analysis of t h e m o t i o n pictures. H e was ably assisted b y Miss M a r g a r e t C h a f f e e , w h o also d r a f t e d all the d i a g r a m s . T h e flights themselves were m a d e possible by the spirited c o o p e r a t i o n of the Pacific Division of the Military T r a n s p o r t Services ( M A T S ) ; their individual aircraft c o m m a n d e r s and n a v i g a t o r s went m a n y times b e y o n d the call of d u t y t o p r o v i d e us with the needed positions and in-flight d a t a .

In assembling the synoptic d a t a , P r o f e s s o r Riehl w a s h e l p e d by t h e U.S. Navy's Joint T a s k F o r c e Seven H e a d q u a r t e r s in H a w a i i and b y P r o f e s s o r Colin R a m a g e of the University of H a w a i i . M r . William G r a y joined in the m a p analysis a n d c o n tributed m a n y valuable ideas t o the discussions. M r s . M a r y C. T h a y e r ably p r e p a r e d m a n y stages of the m a n u s c r i p t . T h e w o r k was mainly s u p p o r t e d by the Office of N a v a l R e search, with i m p o r t a n t c o n t r i b u t i o n s f r o m the N a t i o n a l Science F o u n d a t i o n and the National H u r r i c a n e P r o j e c t of t h e U.S. W e a t h e r Bureau. T h e authors are also grateful to the University of C h i c a g o and the W o o d s H o l e O c e a n o g r a p h i c Institution f o r the use of their facilities during the long period required t o collate and analyze this material.

University of California, Los Angeles Colorado Stale University

J. S. M . H. R.

Contents

I. Introduction, B a c k g r o u n d , and M o t i v a t i o n o f the S t u d y . . . REFERENCES

II. M e t h o d s o f D a t a C o l l e c t i o n and A n a l y s i s . 1. M e t h o d s a n d

p r o c e d u r e s of

observation

Survey

and

"whole

3. Q u a n t i t a t i v e c l o u d Distances,

sizes,

4. C l o u d m a p p i n g : orientation

and

.sky"

TABLE 2.1 S u m m a r y of Pacific P h o t o g r a p h i c

code

Summer,

measurement heights

t h e d e t e r m i n a t i o n of

lì

REFERENCES

2. O v e r - a l l r e s u l t s a n d m e t h o d s of a n a l y s i s

row

II

1957

Flights;

6

viii

Contents

III. Results of Flight I

12

1. Synoptic s t u d y a n d over-all c l o u d survey Leg

1: Honolulu-Kwajalein,

Leg

2: Kwajalein

Leg

3: Guam-Manila,

July

Guam,

July

July

10-11,

11-12,

12-13,

1957

1957

195 7

2. C l o u d s t u d y : C l o u d cross sections

12

REFERENCES

12

TABLE 3.1-Cloudiness,

21

TABLE 3.2-Heights

28

TABLE 3.3-Cloudiness,

35

TABLE 3.4-Heights

55

Flight of Clouds, Flight of Clouds,

I, Leg

1

Flight

I, Leg

54 1

I, Leg 2 Flight

I, Leg 2

Leg

1, Flight

1

36

TABLE 3 . 5 - C l o u d i n e s s , Flight I, Leg 3

Leg

2, Flight

1

41

TABLE 3.6 C l o u d C o d e , O r g a n i z a t i o n , a n d P r e c i p i t a t i o n ;

Leg

3, Flight

1

44

3. Organization Results

study

from

Flight

Orientation Spacing

44

of

1, Leg 1 cloud

of cloud

Cross-wind Relation

45

lines

45 48

organization

lo air structure

from

Flight

I, Leg 2

50

Results

from

Flight

I, Leg 3

50

of

Summary

Flight

of Flight

I

60

Flight I, L e g 2

61

TABLE 3 . 9 - C l o u d C o d e , O r g a n i z a t i o n , a n d Precipitation; Flight I, L e g 3

62

53

1 organization

study

54

IV. Results of Flight II

63

Synoptic study a n d over-all c l o u d s u r v e y Leg

1: Honolulu-Kwajalein,

Leg

2: Kwajalein-Guam,

Leg 3: Guam-Wake,

59

48

Results 4. Summary

57 58

TABLE 3 . 8 - C l o u d C o d e , O r g a n i z a t i o n , and Precipitation;

45

rows

organization of cloud

Flight I, L e g 1

56

July July

July

24-25,

25-26,

26-27,

1957 1957

1957

64

REFERENCES

65 70

TABLE

4 . 1 - C l o u d i n e s s , Flight II, Leg 1

121 122

TABLE

4.2 H e i g h t s of C l o u d s , Flight II, Leg 1

123

78

TABLE

4 . 3 - C l o u d i n e s s , Flight II, Leg 2

124

87

TABLE

4 . 4 - H e i g h t s of C l o u d s , Flight II, Leg 2

125

C l o u d cross sections

96

TABLE

4 . 5 - C l o u d i n e s s , Flight II, Leg 3

126

Leg 1, Flight

11

96

TABLE

4 . 6 - H e i g h t s of C l o u d s , Flight II, L e g 3

127

Leg 2, Flight

11

98

TABLE

4 . 7 - C l o u d i n e s s , Flight II, Leg 4

128

Leg 3, Flight

11

100

TABLE

4 . 8 - H e i g h t s of C l o u d s , Flight II, Leg 4

129

Leg 4, Flight

11

102

TABLE

4.9-Cloud Code, Organization, and Precipitation;

Organization

study

102 TABLE

4 . 1 0 - C l o u d C o d e , O r g a n i z a t i o n , a n d Precipitation;

TABLE

4.11-Cloud Code, Organization, and Precipitation;

TABLE

4 . 1 2 - C l o u d C o d e , O r g a n i z a t i o n , and Precipitation;

Leg

4: Wake-Honolulu,

July

27-28,

1957

Leg I

organization

106

Leg 2

organization

106

Leg 3

organization

110

Leg 4 Results

organization of cloud

Organization

modes of Flight

Flight II, L e g 2 Flight II, L e g 3

110 mapping and

110 the flow

field

S u m m a r y of Flight II Summary

Flight I L Leg 1

II organization

study

114 119 120

Flight II. Leg 4

130 131 132 133

Contents

134

V. Results of Flight III 1. Synoptic study a n d over-all cloud survey Leg 1: Honolulu-Wake, August 16-17, 1957 Leg 2: Wake-Guam, August 17-18, 1957 Leg 3: Kwajalein-Honolutu, August 21-22, 1957 Leg 4: Honolulu-San Francisco, August 26-27, 1957

135 135 147 159 167

2. Cloud Leg 1, Leg 2, Leg 3, Leg 4,

176 176 186 186 186

cross Flight Flight Flight Flight

sections III 111 III III

3. Organization study Organization charts and tables Cloud-mapping results of Flight

188 188 193

III

REFERENCES

207

TABLE 5.1-Cloudiness, Flight III, Leg 1

208

TABLE 5 . 2 - H e i g h t s of Clouds, Flight III, Leg I

209

TABLE 5.3 Cloudiness, Flight III, Leg 2

210

TABLE 5 . 4 - H e i g h t s of Clouds, Flight III, Leg 2

211

TABLE 5.5-Cloudiness, Flight III, Leg 3

212

TABLE 5 . 6 - H e i g h t s of Clouds, Flight III, Leg 3

213

TABLE 5.7 Cloudiness, Flight III. Leg 4 TABLE 5.8 Heights of Clouds, Flight III, Leg 4 TABLE 5.9 Cloud Code, O r g a n i z a t i o n , a n d Precipitation;

214 215

Flight III, Leg 1 TABLE 5 . 1 0 - C l o u d Code, Organization, a n d Precipitation;

216

193

Flight III, Leg 2 TABLE 5.1 l - C l o u d Code, O r g a n i z a t i o n , a n d Precipitation;

217

1

Leg 2 Leg 3

196 198

Flight III, Leg 3 TABLE 5.12 Cloud Code, Organization, a n d Precipitation;

218

Leg

Leg 4 4. S u m m a r y of Flight III Summary of Flight III organization

study

flights

2. S u m m a r y and conclusions Relationship of cloud structure patterns Cloud organization

220

REFERENCES

222 to planetary

Appendix: Tropical Whole Sky C o d e

219

.220

VI. Comparisons, Summary, and Conclusions 1. C o m p a r i s o n of the three Pacific

Flight III, Leg 4

202 206 206

flow 222 222

225

224

Introduction, Background, and Motivation of the Study

This is a study of cloud formations and distributions over the tropical Pacific Ocean in relation to the large-scale flow patterns. The primary tools are quantitative photography and synoptic analysis. The investigation was designed to learn more of the two-way interaction between the structure of the lowlatitude atmosphere and its dynamics and thermodynamics, and also to determine the factors governing tropical convection, cloud formation, and precipitation. Recent developments in tropical meteorology show the important role played by cumulus convection in the conversion of heat energy from latent into sensible form and in its transport to great heights. Towering cumulonimbus clouds have been shown to be essential in maintaining the heat and mass budgets

and exports f r o m the equatorial trough zone and hurricanes. Furthermore, it has been established that differential release of latent heat by convective systems creates the large-scale temperature differences in the horizontal which provide pressure forces that can drive the air motions against friction. Quantitative models of the maintenance of the lower trade flow and of the hurricane system have been developed on this basis. It thus appears that convective clouds are not merely a decorative feature of the tropical atmosphere, but are in fact the working cylinders in this part of the heat engine. Clouds probably also play a large, but still essentially unknown role in the radiation budget of this energy source region. Last but not least, tropical clouds are the main water supply over 30 per cent of

2

Introduction, Background, and Motivation

the earth's s u r f a c e — a n area in which h u m a n development has been limited in considerable part because of the unreliability and inadequacy of the rainfall. We therefore wish to determine how tropical clouds of various types are distributed over these oceans in space a n d time, a n d how their presence, development, and f o r m s are related to, affect, and are affected by larger-scale systems. T h e classical picture of convection in the tropics might suggest a statistically r a n d o m and, o n the average, invariant distribution of clouds over low-latitude oceans. In the vertical, the tropical a t m o s p h e r e is usually conditionally unstable f r o m cloud base u p w a r d , frequently to the tropopause; buoyant energy is thus nearly always available f o r a noninteracting convective parcel. Horizontally, s h a r p air-mass contrasts are virtually nonexistent; the low-level atmosphere is well loaded with moisture and closely coupled by vertical stirring with an almost infinite extent of h o m o g e n e o u s sea surface. O n e might expect, a priori, u n i f o r m or r a n d o m bunching of tropical c u m u lus clouds, with c u m u l o n i m b u s build-ups and precipitation distributed here, there, and everywhere, and cloud patterns that statistically show little diurnal, weekly, or even seasonal variations. T h e organization, if any, might be expected to be the hexagonal or roll convection typical of laboratory experiments over a heated surface. This classical picture of tropical convection has gradually faded. T h e first blows c a m e with the W y m a n n - W o o d c o c k [ 1 ] trade-wind expedition and the entrainmcnt theory of Stomme! [ 2 ] . Clouds were f o u n d t o exchange air with their surroundings, and consequently the degree of instability of the t e m p e r a ture lapse rate lost its unique significance as the factor determining cloud growth. T h e moisture distribution of the ambient air proved critical in controlling in-cloud parameters, and most tropical cumuli were observed to lose buoyancy by drying out well below the level where a stable layer was reached. It is true that c u m u l o n i m b u s towers reaching the high troposphere were observed occasionally, but conditions permitting their growth proved rare. T h e evolution of the W o o d s Hole studies of individual clouds (during the period 1 9 4 7 - 1 9 6 0 ; for example [ 3 - 6 ] ) f u r t h e r emphasized the controlling effects of the environment and the inhibitory factors against penetrative

development. R u n a w a y cloud growth to the cumulonimbus stage was f o u n d to require special circumstances, among which were a d e e p moist layer, a large horizontal dimension, and probably synoptic-scale convergence in the low levels. T h e correlation between vertical development and degree of conditional instability proved disappointingly low; a more unstable lapse rate was usually found with a sky of widely separated, stunted trade cumulus than when towering cumulonimbus were rampant, as in the hurricane [ 7 ] . T h e background for the classical convection picture collapsed entirely with the w o r k of Riehl and collaborators which carried tropical meteorology both upward and equatorward [ 8 - 1 1 ] . In contrast to the lower trades, the upper tropical atmosphere is characterized by extreme restlessness. Even in the inversiondominated regions where steady conditions prevail in the belowinversion moist layer, this constancy vanishes upward, and large space and time variations in wind, humidity, and stability set in aloft. Tropical rainfall studies revealed an enormously skewed distribution, with the m a j o r fraction of the monthly precipitation falling on two or three days; even the annual average is a poor indication of the expectation for a given year. E q u a t o r w a r d f r o m the trades the inversion weakens and disappears, and inconstancy at all levels becomes pronounced; the steady trade flow becomes perturbed into deepening waves and vortices characteristic of the equatorial trough zone, which itself operates in a fluctuating manner. In the downstream and equatorial regions of the low-latitude oceans the tropical disturbance becomes a significant feature which dominates the circulation patterns and their transports; there the so-called "meridional tropical cell" becomes only a rather misleading average over intermittency; the crucial u p w a r d and poleward exports of energy are confined to disturbances, and within these, f u r t h e r confined to the region of organized penetrative cloud towers! T o m a k e f u r t h e r progress in the large-scale meteorology of the tropics and its connection as energy source to middle latitudes, as well as to advance our knowledge of the natural convective process, we are therefore now required t o study these widely different scales of motion in their context to each other. This study represents a first attempt, largely descriptive, to relate synoptic and cloud scale p h e n o m e n a . W e wish to de-

Introduction, Background, and Motivation termine how cloud f o r m s are distributed in relation t o the threedimensional structure of the a t m o s p h e r e over a n entire ocean, and to what degree the cloud variations in space a n d time are related to changes in atmospheric structure and flow. This is a necessary requirement preceding a n understanding of the mechanisms of interaction, hopefully leading to an eventual model of how cloud growth is regulated by and in t u r n regulates t h e large scale dynamics, thermodynamics, and transfer processes of the tropical atmosphere and its c o m p o n e n t parts. Specifically, we are interested in the following questions: 1) H o w variable are the cloud patterns between t h e different regions of the oceans, and in the same region as a function of time and synoptic situation? 2) H o w c o m m o n are c u m u l o n i m b u s build-ups and precipitation, and how are they distributed relative to synoptic disturbances? 3) H o w organized are tropical cloud f o r m a t i o n s a n d does the organization depend to any degree o n the large-scale flow? A r e "cloud lines" confined to island streets a n d t h e spiral arms of hurricanes, or are geometrical patterns the rule? H o w do clouds distribute themselves in an incipient and deepening storm? H o w are they arranged in an u n disturbed situation? 4) Is middle and upper cloudiness in the tropics p r o d u c e d independently of c u m u l o n i m b u s towers, a n d if so, h o w commonly and under what conditions? 5) What role do high tropospheric flow patterns play in tropical convection? M u s t the three-dimensional structure of the atmosphere to great heights be considered in analyzing clouds, and if so, what p a r a m e t e r s are critical? 6) Would a quantitative three-dimensional large-scale picture of the atmosphere containing a limited n u m b e r of key p a r a m e t e r s and calculations permit an accurate prediction of cloud structure, or are there so m a n y c o m plexities or r a n d o m f a c t o r s at w o r k that apparently identical synoptic structure permits hopelessly wide variation in cloud forms?

3

A s a corollary to question 6, c a n the state of t h e tropical sky be coded in a meaningful fashion, with relatively few numbers, so that t h e code n u m b e r chosen provides significant information about the synoptic p a t t e r n and the physical processes at w o r k therein? With this background and set of questions in mind, a c o m b i n a tion photographic-synoptic study of the tropical N o r t h Pacific O c e a n w a s undertaken in the s u m m e r of 1957. Its m e t h o d s and p r o c e d u r e s are described in the next chapter. 7)

REFERENCES 1. Wyman, J., et al., 1946: Vertical motion and exchange of heat and water between the air and sea in the region of the trades. Unpublished report on file at Woods Hole Oceanographic Institution. 2. Stommel, H., 1947: Entrainment of air into a cumulus cloud. J. Meteor., 4, 91-94. 3. Malkus, J. S., 1954: Some results of a trade cumulus cloud investigation. J. Meteor., 11, 220-237. 4. Malkus, J. S., and C. Ronne, 1954: On the structure of some cumulonimbus clouds which penetrated the high tropical troposphere. Tellus, 6, 351-366. 5. Malkus, J. S., 1958: On the structure of the trade-wind moist layer. Pap. in Phys. Oceanog. and Meteor., Mass. Inst, of Tech. and Woods Hole Ocean. Inst., 13, No. 2, 47 pp. 6. Levine, J., 1959: Spherical vortex theory of bubble-like motion in cumulus clouds. J. Meteor., 16, 653-662. 7. Malkus, J. S., 1960: Recent developments in the studies of penetrative convection and an application to hurricane cumulonimbus towers. Cumulus Dynamics, Pergamon Press, London, 65-84. 8. Riehl, H., 1951: On the role of the tropics in the general circulation. Tellus, 2, 1-17. 9. Riehl, H., 1954: Tropical Meteorology. McGraw-Hill, New York. See chaps. 3 and 12. 10. Riehl, H., 1962: General atmospheric circulation of the tropics. Science, 135, 13-22. 11. Riehl, H„ and J. S. Malkus, 1958: On the heat balance in the equatorial trough zone. Geophysica 6:3-4, 503-538.

Methods of Data Collection and Analysis

1.

METHODS

A N D PROCEDURES OF

OBSERVATION

To record and analyze cloud forms over wide stretches of ocean, an aircraft photography program was clearly required. Through the kind cooperation of MATS, it was possible to utilize the normal Pacific military transport aircraft as platforms for the photography. Owing to night scheduling of most passenger planes, however, it was found necessary to use the cargo Globemasters (C-124A), which were spacious, comfortable to work in, and had good visibility. Their most serious limitation was the low operating altitude, which never exceeded 10,000 ft. Under undisturbed or suppressed conditions, most cumulus tops were

at or below flight level and no problem resulted, although the maximum distance from the aircraft to the cloud recorded exceeded 100 miles* on only a few occasions. Under the highly disturbed equatorial trough or typhoon conditions often encountered on the western and equatorial flight legs, the altitude limitations were serious, and the camera observed long stretches of completely "socked-in," black, rainy, and obscured scenery. Were this program to be repeated, it would be preferable to await replacement of the Globemasters by the turbopropellered • A l l miles q u o t e d in t h i s t e x t a r e nautical otherwise.

miles u n l e s s

noted

Methods of Data Ci

C-130 to achieve a much higher flight level; 30,000 ft would be desirable if possible. The chosen method of photography, namely time-lapsed Kodachrome (16 mm) motion pictures, was dictated by the dual purpose of the inquiry: to provide an over-all statistical survey of cloud forms over great distances, and simultaneously to permit relatively accurate cloud mapping in selected regions. The camera equipment, method of modification, and operation thereof have been described in detail by Ronne [1, 2], The important points to keep in mind with regard to the results are (1) the necessity for accurately known and controllable speed of film exposure, and (2) the necessity for frequent time marks recorded directly on the film in order to locate the photographs, to tie them in with the aircraft navigator's reports, and to check the rate of film exposure. Frames were exposed at all times at very nearly one per second; frequent (5-15 minute) time marks permitted checking this speed during reduction, and corrections were made when a departure exceeded 5 per cent. The one-second interval was chosen from experience in flight testing at anticipated airspeeds. Since the Globemaster flew at close to 100 meters/sec, frames could be selected for stereo-pairs with a base line of 100 meters and any increased multiple desired. The one-second interval further provided a visually pleasing continuous record when the films were projected at conventional speeds; an 8-hour flight leg could be watched in about half an hour and encompassed mentally by the analysts. Shorter time intervals of exposure would have been uneconomical in film use and would have made the results too unwieldly and boring to watch in projection. Any much larger interval between frames would have resulted in "jumping" and bewilderment. Since the motion picture camera was aimed exactly normal to the fuselage (from one of the rear windows of the cargo compartment), only a strip on one side of the aircraft path could be recorded. It was planned to record cloud forms on the opposite side by means of 35-mm stills taken every 10 or 15 minutes; unhappily the still camera failed to operate properly and only about one-third of the total flight distance was so covered. In

ion and Analysis

5

general, the loss was not severe, as the observer usually noted similar cloud distribution on both sides, although occasionally evidence of a distant disturbance was confined to one direction. The side chosen for photography was determined by anticipation of the position of the most interesting cloud forms, as suggested in the crew's preflight briefing at the take-off weather station. Owing to the generally east-west flight paths, this was northerly or southerly, the latter being more common (see Table 2.1). Navigational information—including hourly position, air speed, ground speed, heading, and drift winds—was very satisfactorily provided by the regular flight crew, who generously supplied the photographer with a completed table at the conclusion of each flight leg. Cooperation between navigator and photographer enabled recording on the film of the exact time (hourly; first position one hour after take-off) of each position reading, which was supplemented by additional time marks rarely separated by more than 15 minutes. Altogether, three complete flights across the Pacific were accomplished in July and August, 1957. More than 8,000 ft of film was exposed, all of excellent photographic quality. The three flights are summarized in Table 2.1; the routes and times are illustrated in Figures 2.1-2.3. Each of these flights was naturally subdivided into three or four individual legs, of 6-12 hours (1,400-2,800 miles) duration, separated by refueling and rest stops. The hours listed in Table 2.1 define the period when the camera was actually running; on the three starred legs the photography had to stop because of darkness (see Figs. 2.2 and 2.3) before the aircraft's destination was reached. On flights I and II in July, the legs succeeded each other in rather rapid succession, with only 15-17 hours elapsing between the end of one and the start of the next. Flight II was particularly fortunate in that a complete circuit of the area was achieved in 75 hours. In August the flights scheduled were much sparser, and Flight III, which was intended to be an equally rapid circuit extending all the way back to San Francisco, was seriously interrupted; only its first two legs form a continuous flight (the Guam-Kwajalein leg was, unfortunately, made in darkness).

6

Methods of Dala Collection and Analysis

TABLE

2.1

SUMMARY OF PACIFIC PHOTOGRAPHIC FLIGHTS; S U M M E R ,

Origin

1957

End Date & Time (GCT)

Flight Altitude (ft)

Camera aimed

Place

Date & Time (GCT)

Honolulu

7/10 2107Z

Kwajalein

7/11 0620Z

8,000

S

Normal trade —> easterly wave

Kwajalein

7/11 2205Z

Guam

7/12 0408Z

8,000

S

Very weak wave —» suppressed —> build-ups near G u a m

Guam

7/12 2125Z

Manila

7/13 0408Z

8,000

S

Typhoon

II, Leg 1

Honolulu

7/24 2322Z

Kwajalein*

7/25 0635Z

8,000

S

Normal trade —> undisturbed

II, Leg 2

Kwajalein

7/25 2126Z

Guam

7/26 0330Z

8,000

S

Build-ups —• suppressed —> moderately disturbed

II, Leg 3

Guam

7/26 2122Z

Wake

7/27 0348Z

9,000

s

Distant disturbance —» suppressed trade

II, Leg 4

Wake

7/27 1819Z

Honolulu

7/28 0225Z

9,000

s

Initial moderate build-ups— very suppressed most of flight

III, Leg 1

Honolulu

8/16 2134Z

Wake

8/17 0735Z

8,000

N

Normal trade —> trough, cb build-ups, disturbed

III, Leg 2

Wake

8/18 0115Z

Guam

8/18 0635Z

8,000

N

C b build-ups —> typhoon outskirts

III, Leg 3

Kwajalein

8/21 2327Z

Honolulu*

8/22 0555Z

9,000

S

Normal trade —» weak disturbance —» normal trade

III, Leg 4

Honolulu

8/26 2015Z

San Francisco*

8/27 0308Z

7,000

N

Inversion-dominated trade

Flight

I,

Leg 1

I, Leg

2

I, Leg 3

Place

•Photography ended before destination, owing to darkness; see maps.

General character of flight

Methods of Data Collection and Analysis

2.

OVER-ALL RESULTS AND METHODS OF ANALYSIS

Projection of the motion pictures a n d a simultaneous preliminary study of synoptic charts, radiosondes, etc., immediately gave rise to some provocative results, which guided the f u r t h e r directions of the analysis. A m o n g these were the following: 1)

L a r g e time variations in cloud distribution, f o r m , type a n d structure in the same locality, intimately coupled with changes in t h e large-scale flow. T h e s e variations were m o s t m a r k e d in d o w n s t r e a m and e q u a t o r w a r d p o r tions, and were less p r o n o u n c e d in the u p s t r e a m a n d inversion-dominated trade.

2)

Typical " w h o l e - s k y " conditions which p r o m i s e d an effective coding scheme. Regional preferences for types of cloud patterns, again

3)

4)

5)

7

coupled with the large-scale air structure characteristic of the a r e a : that is, u n d i s t u r b e d t r a d e regimes gave rise to a consistent sky appearance, while easterly wave, inversion-domination, t y p h o o n , etc., showed both recognizable cloud patterns a n d preference for occurrence in certain regions of the ocean. Rarity of penetrative c u m u l o n i m b u s build-ups and significant precipitation; these were confined entirely to either recognizable disturbances o r at least to regions where synoptic-scale low-level convergence w a s inferrable qualitatively f r o m the charts. P r o n o u n c e d and prevalent organization of convective clouds into well-marked lines, roughly parallel to t h e low-level flow, particularly u n d e r disturbed and convergent conditions and only noticeably absent at t h o s e

8

Methods of Data Collection and Analysis

Fig. 2.2. Routes, dates, and times for Flight II, July 24-28, 1957. places and times where the trade inversion was present or dominant. Consequent upon these preliminary findings, the cloud study was divided into two closely interlocking procedures: the first, an over-all survey by means of a "whole sky" code; and the second, a quantitative analysis to be applied to construct cloud maps and cross sections. We next briefly describe these procedures. Survey and "whole sky" code To encompass each flight leg, prints were made from the motion picture film at about 15-minute intervals and arranged in a long strip in the direction of the flight. Frames that best characterized the interval in question were printed. The results of this procedure were striking, in that a coherent progression of cloud forms

and sky types succeeded each other logically and bore meaningful relation to the synoptic maps. A sky code was composed and is reproduced and illustrated pictorially in the Appendix. It consists of seventeen basic sky types—ten for trade-wind skies and seven for disturbed skies. Five qualifying postscripts were added, primarily applying to undisturbed skies. Five qualifying postscripts were added, primarily applying to undisturbed or trade-wind skies. The code was first constructed with only Flight I data available but was found equally suitable for the remaining two. Three independent observers agreed on the precise code number more than 75 per cent of the time and differed only slightly on the remainder. A single observer was able to reproduce his own results later even more satisfactorily. The code numbers were entered at the correct positions on the base map of each leg independently of

Methods of Data Collection and Analysis

9

Fig. 2.3.-Routes, dates, and times for Flight III, August 16-27, 1957. the synoptic charts and have been used as underlays f o r these in the presentation of results to follow. A final check o n t h e coding was made by projecting the films slowly with all o b servers present, but few modifications were required. T h e r e a d e r will be able to get a good visual image of the cloud f o r m s d u r i n g any portion of any flight by looking at the c o d e n u m b e r a n d referring t o the sample print of each contained in the A p p e n d i x . This is, in itself, a result of some significance.

Distances,

T h e m o r e general usefulness of this type of sky coding in tropical meteorology has been advocated in a review of this coding endeavor by Alaka [ 3 ] . Its simplicity in digesting a n d presenting information over large regions m a y b e contrasted with the expense and labor generally a t t e n d a n t u p o n securing meteorological data over wide expanses of ocean. Actually, t h e photography stage could even be by-passed; code reports n o t e d

T h e o t h e r t w o uses of t h e cloud photographs involve quantitative m e a s u r e m e n t s u p o n individual clouds and cloud groups. First, it w a s desired to construct a cloud cross section f o r each flight leg, t o show the a m o u n t of cloudiness (low, middle, a n d h i g h ) a n d t h e heights of layer clouds, a n d to represent cumulus structure, all as a function of time and distance along the leg. F o r the cumulus, heights of bases, m e a n tops, and m a x i m u m

d o w n at fixed time intervals by observers flying on any tropical oceanic routes would still be very useful, both in routine forecasting operations and, in particular, in the interpretation of satellite a n d rocket p h o t o g r a p h s . 3.

QUANTITATIVE

sizes,

and

CLOUD

MEASUREMENT

heights

10

Methods of Data Collection and Analysis

tops are t o be shown, as well as directions of cloud shears and precipitation w h e r e present. F o r cloud measurements, aerial p h o t o g r a m m e t r i c techniques were adapted as described in detail in a m a n u a l by R o n n e [ 2 ] . Tables were constructed giving the n o r m a l distance f r o m the c a m e r a t o a selected cloud or a part thereof as a function of the time required f o r the image to pass across any fraction of the view field. F r o m this, a distance scale (feet per photograph inches and t e n t h s ) follows which is used t o determine cloud dimensions. E r r o r s in distance determination rarely exceeded 5 p e r cent, since t h e constants in the calculation were altered whenever t h e aircraft speed or rate of f r a m e exposure altered that much. T h e rate of f r a m e exposure was accurately timed a n d regulated by a rheostat. T h e aircraft airspeed a n d groundspeed were provided by the crew; the excellence of their navigation is attested t o by the fact that their estimated times of arrival were invariably met within 1-2 minutes. T h e vertical a n d horizontal extent of a cloud were obtainable with nearly equal accuracy if its variation in distance f r o m the c a m e r a ( d u e to shear, etc.) was negligible in comparison to the distance itself. F o r very large cumulonimbus, or highly slanting towers, several points were located independently a n d the entire cloud m a p p e d out thereby. T o determine actual heights of cloud bases and tops above the sea surface, flight altitude must be k n o w n and either the c a m e r a must be accurately level or the horizon visible. Flight altitude rarely varied by 5 0 ft f r o m that prescribed. H o w e v e r , the horizon presented problems. Despite careful leveling of the c a m e r a when the aircraft was in t r i m m e d flight, this condition could not be counted o n to prevail at all times, particularly when flying on automatic pilot. Fortunately a horizon was visible o f t e n enough to use it as reference. F r o m it, a "true h o r i z o n " was calculated and d r a w n on the projection screen. T h e "true h o r i z o n " is a line tangential to the earth's surface if t h e observer is at sea level. If the observer is in an aircraft, it is a line parallel to but above this tangent line, the exact height above being determined by the altitude of the observer. T h e r e f o r e the height of the true horizon above the earth's surface is a combined function of the curvature of the earth and the height of the observer. T h e angular separation of the

true and visual horizon is known as the " d i p angle"; it m a y be determined in minutes f r o m the simple f o r m u l a 9 = 0.98V©,

(2.1)

where D is the altitude of the aircraft in feet; or the angle m a y be read f r o m tables ( f o r example, McNeil [ 4 ] ) . Graphically, it appears on a p h o t o g r a p h as a line parallel to a n d a b o v e the visual horizon. T h e distance between the lines is determined once the dip angle and focal length of the c a m e r a are known. M o s t of the legs of the Pacific flights were m a d e at altitudes of 8,000 and 9 , 0 0 0 ft, and for these heights the dip angle of 1 ° 2 8 ' was used. Consequently, o n the projected image with an effective focal length of one foot, as used here, the separation between the two horizon lines is 1 X tangent 1° 2 8 ' ; that is, 0 . 0 2 5 6 ft. T h e actual procedure for obtaining cloud heights w a s therefore as follows: T h e extent of a given cloud above a n d below the true horizon was measured and converted t o height using the predetermined length scale for that cloud. T h e n a correction for earth's curvature and atmospheric refraction is required which depends on distance; namely C = 0.574M2,

(2.2)

where the correction C is in feet and M is the distance in statute miles t o the object. This correction (always additive) a m o u n t s to 5 , 7 4 0 ft at 100 miles, 1,453 ft at 5 0 miles, and a b o u t 2 3 0 ft at 2 0 miles. In the precise cloud mapping to b e described later, the correction was applied individually to all clouds m o r e than 2 0 miles distant. In constructing the cross sections it was applied only to distant build-ups and ignored f o r nearer and smaller clouds. Cloud base heights may therefore be as m u c h as 2 0 per cent in error, although an attempt was m a d e to confine base measurements to clouds 15-20 miles f r o m the c a m e r a . T h e heights of tops are correct within 15 per cent or better, and the absolute error rarely, if ever, exceeds 1,000 ft. T h e problem of representativeness is thus more difficult t h a n that of accuracy. T h e most severe limitation upon this investigation is placed by the sparsity and poor resolution of the synoptic d a t a used to analyze the flow field in which the clouds were i m b e d d e d and is plainly not set by the limitations of the cloud m e a s u r e m e n t s deduced f r o m the photography. T o be of the historic value of

Methods of Dala Collection and Analysis which it is capable, cloud photography, whether aerial or satellite, must be accompanied by aircraft dropping soundings and taking wind profiles at frequent intervals along the flight routes of the cameras. 4.

CLOUD MAPPING: THE DETERMINATION OF ROW ORIENTATION

The most striking aspect of the cumulus populations was their frequent line-up into rows or long streets. Preliminary inspection of the films suggested that cloud organization, on several scales, was the most fascinating feature of the data and that our method offered a unique opportunity for its quantitative study. Thus one of our main objectives became the detailed mapping of selected regions where cloud organization was seen or suspected. Consequently, some method of determining the geographical orientation of individual cloud rows was desired, to tabulate and include on the maps. A simple procedure was devised to measure the angle that the rows made with the flight path, which has been converted to the angle from true north in the tables. This determination could be made quantitatively from the motion picture with an accuracy of about ± 1 0 ° when the organization was moderately pronounced and within ± 5 ° when it was very strong. The procedure was to select a cloud line and to map the positions of two or more clouds therein. Since all that is required to determine the angle of the cloud lines is the ratio of the lateral (along the flight path) distance between members to their normal (to flight path) separation, a rapid graphical method was readily devised which required only three measurements per cloud pair and was independent of slight variations in camera or aircraft speed. The film was projected on a screen (size unimportant) and the frame counter of the projector was zeroed when one cloud of the pair was on the extreme right edge of the picture. The counter reading was recorded when the second cloud in the row appeared at the right and left edges, respectively. The lateral distance A.r between clouds is propor-

11

tional to the difference in readings when each cloud is at the right edge; and the normal difference Ay between them is obtained from the number of frames required for each cloud to cross the picture. The unit size of Ay relative to that of Ax depends on the camera angle only. In many cases three or four clouds per line were plotted, but it was almost always found that any two determined the orientation of the line within about 5°.

The greatest difficulty in applying this procedure lay in ensuring that the selected clouds were actually members of the same line, and were not from different but nearly adjacent ones. Where the organization was pronounced and/or the individual lines were widely separated, the selection was straightforward, but it became more of a problem when the lines were crowded and the organization was weak or confused. At least one determination of angle was made every 15 minutes (corresponding to the prints) and often several intermediate checks were also performed. The orientation has been represented by arrows at these intervals superimposed on the streamline charts; they will be discussed with the material for each flight leg in chapters iii-v. In addition, an organization code was devised, indicating four degrees of organization of cumulus rows, from absent to intense: X, 1, 11, and 111, respectively. M was coded when intervening clouds obscured the view and 0 when cumuli were sparse or absent. REFERENCES 1. Ronne, C., 1958: Modification of a Bolex camera for quantitative time-lapse photography. Unpublished manuscript on file at Woods Hole Oceanographic Institution. 2. Ronne, C., 1959: On a method of cloud measurement from aircraft motion picture films. Unpublished manuscript. Woods Hole Ocean. Inst., Ref. No. 59-29. 3. Alaka, M., 1960: The coding of clouds—A new approach. W.M.O. Bulletin, 9, 105-109. 4. McNeil, G. T., 1954: Photographic Measurements. Pitman Publishing Corp. New York.

dBk Results of Flight I

The tropical Pacific Ocean exhibited a typical, glamorous cloud display for our first crossing. Trade-wind cumuli first gave way to a classical easterly wave regime, with a cloud pattern we later learned to recognize as typical for such a disturbance, and later to a typhoon with well-defined rainbands which could be mapped from our film. The cumulus row organization was the outstanding feature of this circuit; its prevalence was not equaled in the two succeeding journeys. 1.

SYNOPTIC STUDY AND O V E R - A L L

CLOUD

SURVEY

The three legs of this flight (see Table 2.1 and Fig. 2.1) were centered in time close to OOZ on July 11, 12, and 13, 1957; thus the corresponding synoptic charts and soundings were selected for reproduction, although earlier, later, and intermedi-

ate analyses were carried out for understanding and continuity. Since the flights are in an area that is about 180° longitude across the globe from England, a rough estimate of local time is just 12 hours away from Greenwich time (this is exact for the Marshall Islands area). Leg 1 (Flight I, 1): Honolulu - Kwajalein, July 10-11, 1957 On July 10-11 (in Z time) the flight route from Hickam AFB (Honolulu, Hawaii) to Kwajalein (Marshall Islands, see Fig. 2.1) was covered by cloud formations coded as underlay in Figures 3.1-3.5; inversion clouds were observed near the Hawaiian Islands, going over to trade-wind clouds, then clouds indicating a moderate disturbance, and finally trade-wind clouds again. From the surface charts in Figures 3.1-3.3 it is apparent that the Hawaiian end of the flight route was located in north-

Results of Flight I

13

Fig. 3.1.-Surface chart, July 11, 1957, 00Z. Labels at right are pressures in mb above 1,000 mb; at left lines are labeled in feet as contours of the 1,000-mb surface. Underlay is sky code. Heavy solid lines mark positions of wave troughs in the easterlies.

14

Results of Flight I

Fig. 3.2.-Mean winds (knots) of lower layer with approximately uniform wind, July 11, OOZ; also top of layer (above station, denoted by T; in 1,000's of feet) and height of strongest wind in layer (below station, denoted by M; height in 1,000's of feet and maximum wind in knots).

Results of Flight I

15

Fig. 3.3.-Total precipitable moisture (cm precipitation or g m / c m - ) , July 11, 00Z. Successive 12-hourly values are entered for each station, progressing downward in time, with current values circled.

16

Results of Flight I

Fig. 3.4.-Shear winds from top of trade-wind layer to upper troposphere, July 11, 00Z (Flight I, Leg 1). Base of layer in 1,000's of feet written to right of station, and amount of shear (knots) at tail of arrow. Top of layer 40,000 ft.

Results of Flight I

A plot of the total precipitable water vapor content (Fig. 3.3) shows a course paralleling that of cloudiness (underlay), but the data are too scarce to determine the total moisture pattern uniquely from radiosonde humidity observations alone. Above the trade-wind layer, westerly shear existed everywhere (Fig. 3 . 4 ) . It was very strong and set in at about 8,000 ft in the early part of the flight and became less noticeable beyond Johnston Island as the trade-wind layer deepened. The high tropospheric circulation was characterized by an oscillating westerly current, with a large cyclone and anticyclone located along its borders (Fig. 3 . 5 ) . The major part of this leg passed on the northern side of the upper anticyclone. That this was a

easterlies curving clockwise as part of a subtropical Pacific High. The atmosphere over the ocean south of the islands had the appearance of an easterly trade current oscillating in a wavelike manner. The depth of the layer taking part in this oscillation increased from 9,000 ft near Hawaii to 20,000 ft in the Marshall Islands. The surface winds were from an easterly direction over the whole of the flight leg, with a velocity of 1518 knots around Hawaii and 12-15 knots in the western part of the flight. The winds were from the east-northeast at the start, becoming more southerly along the flight path until the trough of the easterly wave was passed, when the easterly winds again returned to a more northerly direction.

^Illl

0»

-

,

/ /

\\

N

/

¿»ISO

•fc 093

/

lI)iL^P*>70 onBp« a ^ i

17

M. *

1 07*

•

Ku»«i*

>

1

1 " 1

1

\

7

J

V

Fig. 3.5.-Mean winds and altimeter corrections with respect to mean tropical atmosphere for layer 250-150-mb, July 11, 00Z (Flight I, Leg 1). Isopleths are of the latter in 10's of feet. The thickness of the layer, in 10's of feet with the first digit omitted, is written to the right of the stations.

\

/

\

I1

(\Jo

\

\\

18

Results of Flight 1

strong tropospheric thermal high can be seen by the m e a n " D " (altimeter correction) values of the 2 5 0 - 1 5 0 - m b m a p (Fig. 3 . 5 ) . T h e m e a n thickness of t h e 2 5 0 - 1 5 0 - m b m a p agreed very well with the " D " values, indicating that the m e a n temperatures of the 2 5 0 - 1 5 0 - m b layer c o r r e s p o n d e d with the mean temperat u r e of the whole layer f r o m the surface to 150 m b ; this holds also o n most of the remaining flights.

of Q throughout the troposphere. T h e Kwajalein curve reflects the subsidence n o t e d there, but the very low value of Q at 8 5 0 m b is difficult t o explain. W i t h subsidence, the mid-tropospheric m i n i m u m of Q will tend t o b e lowered o r extended o v e r a T

'C

N e a r t h e H a w a i i a n Islands the u p p e r westerly current attained jetstream strength, a n d "jet s t r e a m " cirrus were observed there o n the films; these will b e tied in with u p p e r moisture patterns and synoptic picture in the cloud cross sections t o follow later (see section 2, this c h a p t e r ) . F a r t h e r downstream, the wave disturbance in the easterlies was located n e a r the southeasterly inflection point of the high-tropospheric flow, where the current lost cyclonic relative vorticity and presumably also absolute vorticity. Qualitatively, f r o m the viewpoint of the theorem of conservation of potential vorticity, the m a j o r cloud system was encountered in a region in which, dynamically, the existence of lower inflow and u p p e r outflow can be explained. Figures 3.6-3.10 show t h e variation of t h e r m o d y n a m i c properties and vertical wind structure along the flight path. As usually observed in the trades, little correlation exists between stable layers and vertical wind shear. At Hawaii (Fig. 3.6) a dry trade-wind inversion prevailed, while at J o h n s t o n (Fig. 3 . 7 ) only an isothermal layer was in evidence. F o r t h e region near the wave trough, the sounding n e a r M a j u r o (Fig. 3 . 8 ) may be taken as representative, with a nearly moist-adiabatic lapse rate and high moisture content t h r o u g h a deep layer. Kwajalein (Fig. 3 . 9 ) exemplifies the drying and stabilization that occurs in air moving through the region west of a wave disturbance that travels slower t h a n the basic current. T h e low-level winds of 12-15 knots were indeed s o m e w h a t higher than the progression speed of the wave, which was f o u n d by extrapolation between charts. T h e concentration of the convection area to its eastward is illustrated nicely by the coded sky conditions in Figures 3.13.4. 5

Figure 3.10, a plot of total energy ( Q = C,T + gz + Lq, in c a l / g m ) for the f o u r soundings, strikingly brings out the transition f r o m H a w a i i to M a j u r o as the tropical atmosphere acquires heat which gradually establishes a nearly uniform value

V

10 (Knots!

15

360

020 0 4 0 0€0 OBO

100 120 dd

140

160 160

200 220 240

260 280

(degrees)

Fig. 3.6.-Tephigram (above) and vertical wind plot (below) at Hilo, July 11, 00Z. The numbers next to the dew-point curve on the sounding are mixing ratios ( g m / k g m ) entered for significant levels.

Results of Flight I

19

deeper than average layer, but its absolute intensity should not increase from this cause alone; it is possible that the radiation sink is intensified in regions of reduced moisture, but this point cannot be settled at present. It may be noted that this was the only ascent in the succession of Kwajalein soundings which contained this feature.

10 15 20 25 V (knots) aDIa I StLayer

5 10 19 20 2 3

320 340 3CO 020 040 060 000 O I O 120 140

Fig. 3.7.-Tephigram and vertical wind plot at Johnston I., July 11, OOZ.

060 090 100 120 140 160 ISO dd (degrees)

Fig. 3.8.-Tephigram and vertical wind plot at Majuro, July 11, OOZ. The Q (energy content) and "D" (altimeter correction; not reproduced for individual stations) profiles showed some interesting correlations. Comparison of Majuro and Kwajalein 11 July OOZ soundings in Figures 3.8 and 3.9 (280 miles apart) showed Majuro with 4.9 gm/cm- total water vapor content and Kwajalein with only 2.9 gm/cm-. The difference in position relative to the surface trough, plus the difference in upper

20

Results of Flight I

Fig. 3.10.-Graphs of total energy content Q = CrT -f gz + Lq (cal/gm) for the soundings of Figs. 3.6-3.9. CP is the specific heat of air at constant temperature, T the temperature in degrees Absolute, g the acceleration of gravity and z the elevation (term converted into calories), L the latent heat of vaporization in calories per gm, and q the mixing ratio in gm per gm. 5

10

15

V (knots)

20

25

0 6 0 CeO 100 120 140

160

IBO 2 0 0

Od (degrees)

Fig. 3.9.-Tephigram and vertical wind plot at Kwajalein, July 11, OOZ. tropospheric flow seems to explain this moisture contrast. The Q profile at Kwajalein (Fig. 3.10) shows a sharp minimum near 850 mb. This would indicate little vertical mixing of air from the surface and consequently little vertical moisture transport. The M a j u r o profile shows a much less pronounced minimum in

Q, indicating that more vertical motion had taken place there than at Kwajalein; the air properties were more mixed at M a j u r o and the vertical energy profile showed smaller changes. The subsequent movement of the easterly wave into the vicinity of Kwajalein completely wiped out the intense minimum of Q within 12 hours. The II July 1 2 0 0 Z (approximate time of trough passage; data not shown here) profile was quite different from the 11 July O O Z profile of Figure 3.10. The total water vapor content at Kwajalein consequently jumped from 2.9

Results of 2

2

gm/cm to 5.1 gm/cm (see 12-hourly succession of values of Fig. 3.3). The "D" profiles (not reproduced) were similar at Kwajalein and Majuro up to nearly 600 mb, then varied, with the Majuro profile sloping to higher "D" values, suggesting a possible effect of convection of moisture to upper levels and condensation warming. The Kwajalein "D" profile showed that the " D " values diminished with height. This is perhaps partially a result of deficient vertical moisture transport and condensation warming, although Figure 3.5 also permits the hypothesis that upper pressure patterns were superposed at least partially independent of the lower flow: causal coupling between lower and upper tropical systems is not clearly understood at the present time and should be investigated. At Johnston Island the "D" and Q profiles showed only small variations in the three soundings of 10 July 1200Z to 11 July 1200Z. The total water vapor contents remained relatively constant. The Johnston Q profile showed intense minima at 11 July 00Z and 1200Z, illustrating suppression of convection as the moisture values (see Fig. 3.3) of 3.2 and 2.9 gm/cm- would indicate. The films showed relatively few, suppressed clouds ("minus" postscripts on sky code numbers) from about 100 miles east of Johnston to 200 miles beyond. The Lihue (western Hawaiian islands; not reproduced) Q profile showed only a weak minimum, indicating above average convection for that area. This was verified by the great amount of inversion clouds and cumulus along the first 150 miles of the flight. The 21.6° N, 163.2° W radiosonde ship Q profile (not shown) indicated a strong minimum and very little moisture. The cloudiness had nearly dissipated by this point (250 miles along the flight path). Hilo's Q profile (see Fig. 3.10) showed a stronger minimum than Lihue's, consequent from the lower moisture content (2.9 to 3.4 gm/cm 2 ). In this flight leg, cloud forms and synoptic and thermodynamic structure were thus excellently correlated. This will be brought out even more fully in section 2 of this chapter. Leg 2 (Flight I, 2): Kwajalein - Guam, July 11-12, 1957 On July 11-12 (in Z time) the flight route from Kwajalein (Marshalls) to Guam (Marianas; see Fig. 2.1) experienced

light I

21

cloud formations as coded on Figures 3.11-3.15. On this leg, suppressed cloudiness was encountered along most of the flight route, indicated by the repeated appearance of code 7 and 7 —. Such types of sky are considered rare for the location and season of the flight, which occurred near the equatorial boundary of the trades. While the observer spent the night at Kwajalein, the wave trough in the easterlies flown through on the previous day moved west. By the maptime of July 12 00Z (Fig. 3.11; time corresponds to aircraft's location about one-third of the way along the flight leg) it had advanced to a position about 200 miles to the west of Kwajalein. Thus Flight I, Leg 2 offered a second opportunity to observe this disturbance, but the cloud patterns encountered were quite different from those of Leg 1. There was hardly any trace of disturbance left along the path of the aircraft, except five or six very distant cumulonimbus on the southern horizon at the approximate position of the trough line. While the disturbance in the surface pressure field maintained itself from the preceding day, the surface winds and the mean winds for the easterly layer (Fig. 3.12) reveal clearly that a wavelike oscillation was no longer present in the wind field south of latitudes 12-15° N. A flight farther north should have produced a pattern of marked disturbance, but in the region south of about 12° N definite evidence of damping of the disturbance in the windfield supports the cloud evidence. After the wave, the surface chart shows that the flight passed through an easterly ridge; convection was most suppressed from Eniwetok through this ridge. The moisture values (Fig. 3.13) remained near 5 gm/cm 2 along the flight path—rather surprising in view of the suppression of cloudiness. This indicates that dynamic factors producing even slight damping of convective activity through subsidence are of much greater importance in determining the cloud types and their distribution than is the total water content of the atmosphere. It was only over a distance of 150 miles just east of Guam that convective activity resumed when the plane passed through the southeasterly winds at the outskirts of a deepening trough located west of the Marianas. On the total moisture chart a dry area is shown there, but the total moisture increased rapidly at Guam from 3.9 to 5.6 gm/cm 2 in the 12

22

Results of Flight I

Fig. 3.11.-Surface chart, July 12, 1957, 00Z. Isopleths are labeled as surface isobars on the right and as contours of the 1,000-mb surface in 10's of feet on the left. T denotes the position of Typhoon Wendy Underlay is sky code.

Results of Flight I

23

Fig. 3.12.-Mean winds (knots) of lower layer with approximately uniform wind, July 12, 00Z; also top of layer (circled, in 1,000's of feet) and height of strongest wind in layer (maximum height in 1,000's of feet; direction and velocity of maximum in code). Code notation at tail of arrow denotes mean wind.

24

Results of Flight I

Fig. 3.13.-Total precipitable moisture (cm precipitation or gm/cm 2 ), July 12, OOZ (Flight I, Leg 2). Successive 12-hourly values are entered for each station, progressing downward in time, with current values circled. hours from 12 July OOZ to 1200Z. The typhoon Wendy was located approximately 300 miles northwest of Yap and was moving east-northeastward on the 12th. On the 13th, it was a well-developed typhoon. The build-ups observed near Guam and the accompanying high moisture content are thought to be associated with the 12 July 1200Z sounding rather than with that of 12 July OOZ. We must look to the three-dimensional and high-level structure of the atmosphere, however, to gain a clearer understanding of the cloud structure. The wind pattern showed a deep easterly current ranging in depth from 16,000 ft to 30,000 ft at various places, with a strong shear to the west in mid-levels when the top of the easterly current was near 20,000 ft as at Kwajalein

and Eniwetok (Figs 3.16 and 3.17). The upper shear wind (Fig. 3.14), however, was nearly parallel and opposite to the lower wind over the whole route. Considering the latitude, the shear was very strong; it attained 55 knots or more over a large distance. This shear wind led over into the high-tropospheric flow pattern of Figure 3.15, which provides a hint concerning the damping of the wave disturbance in the easterlies and the convective activity. The center of the flight route was overlaid by a large high-level trough south of the upper low already noted in Figure 3.5. Air rounding the southern bend of this trough was likely to gain cyclonic relative and absolute vorticity. The upper trough is superposed on the lower forward (divergent) portion of the dissipating easterly wave, and, westward

R o u t e of Flight I

25

Fig. 3.14.-Shear winds from top of trade-wind layer to upper troposphere, July 12, 00Z. Base and top of layer denoted in 1,000's of feet. Direction (10's of degrees) and amount (knots) of shear at tails of arrows.

26

Results of Flight I

of Ponape, upon the lower ridge. These factors acting together would account for upper convergence placed above lower divergence, and consequent strong subsidence and suppression of convection. Because of uncertainties in the contour analysis, a few (dashed) streamlines have also been entered in Figure 3.15. Since the wind field changed only slowly with time, these streamlines may be taken as trajectories in the first approximation. The streamlines over the center of the route, which was nearly cloudless, execute the cyclonic bend, but near the western end they keep pointing toward lower latitudes, finally executing an anticyclonic bend. In this region upper convergence should have given way to divergence, and this is the area where at first

upper clouds and later convective build-ups were encountered by the aircraft. The soundings for Kwajalein and Eniwetok, July 12 00Z (Figs. 3.16 and 3.17) show conditionally unstable lapse rates with high moisture—additional clear evidence that vertical stratification of the atmosphere and its moisture content are not the prime determinants of cloudiness. From parcel reasoning there should have been large thunderstorms all over the area. At Guam (Fig. 3.18) a typical subsidence sounding was observed at 00Z on the 12th, as on Leg 1 at Kwajalein; the total Q was lower in this subsidence area than the lowest value on any of the other soundings (Fig. 3.19). Unless such soundings are held to be in error, the occurrence of abnormally low

Fig. 3.15. Mean winds and altimeter corrections with respect to mean tropical atmosphere for layer 250-150-mb, July 12, 00Z (Flight I, Leg 2 ) . Isopleths are of the latter in 100's of feet. T h e thickness of the layer, in 10's of feet with the first digit omitted, is written below the stations.

Results of Flight I T

27

-C T

/

-50 1

-40

-50 1

/

-20 1

-10 T

0 1

/

10 1

S

20 1

3C

S

-50

-40

"30

1

1

1 x

-20 1

*c -10 1

/

0 1

to

1 /

20 !

/

s /

02

— —

1»

x /

\

/

/

I

V

/ I

(knots)

/

A

X

dd

y\

/

S ? " , / \ x

'

(degrees)

Fig. 3.16.-Tephigram (above) and vertical wind plot (below) at Kwajalein, July 12, 00Z. The numbers next to the dew-point curve are mixing ratios (gm/'kgm) entered for significant levels.

values of Q in subsidence zones deserves detailed study. As remarked previously and illustrated in Figure 3.19, by 1200Z on the 12th, the Guam sounding had lost its subsidence character and pronounced Q minimum owing to the onset of lower convergence and convection. The ragged wind sounding (Fig.

X

/

X , X

V

Ihrtots)

X X y X" y

dd

y,

(degrees)

Fig. 3.17,-Tephigram and vertical wind plot at Eniwetok, July 12, 00Z.

3.18) in the lowest levels at Guam is thought to be orographically produced and not typical for the adjacent oceanic area. Comparison of synoptic situation and cloud patterns for this flight leg brings out the crucial role of dynamic factors in governing the latter, and demonstrates the important or perhaps

28 T

Results of Flight I

"C

Fig. 3.19.-Graphs of total energy content Q = C,T -f gz + Lq (cal/gm) for the soundings of Figs. 3.16-3.18, with the curve for Guam July 12, I2Z added (dashed). moisture largely confined to a shallow cloud layer closely coupled t o a uniform sea surface by vertical stirring; that is, where leaking u p w a r d of moisture is restricted, t h e cloud f o r m a tions may be expected t o be m o r e closely governed by the input of the system, or by air-sea exchange which is in t u r n held relatively steady by the steady conditions on each side n e a r the atmosphere-ocean b o u n d a r y . V (knols)

dd

(degrees)

Fig. 3.18.-Tephigram and vertical wind plot at Guam, July 12, 00Z. even controlling role played at least on occasions by upper tropospheric flows o n moist or low-layer cloud patterns. C o m parison of Legs 1 and 2 suggest that the effect of the upper layers u p o n convective cloud patterns m a y be stronger in downstream and e q u a t o r w a r d portions of t h e tropics t h a n in inversion-dominated trade portions where the existence of the inversion itself acts t o " d e c o u p l e " u p p e r and lower flows by ( a ) d a m p i n g synoptic disturbances d o w n w a r d and ( b ) keeping

Leg 3 (Flight I, 3): Guam - Manila, July 12-13,

1957

This flight leg, characterized by t h e synoptic observations of 0 0 Z on July 13, was completely d o m i n a t e d by t h e very large T y p h o o n Wendy, whose center w a s situated approximately 150 miles south of the flight r o u t e ( 1 2 ° N , 1 2 9 ° W ) . Although the central pressure of this t y p h o o n w a s only 9 9 0 m b ( m a x i m u m wind was 100 knots, as reported b y N a v y r e c o n n a i s s a n c e ) , it covered a huge area; the cyclonic circulation h a d an east-west extent of 1,900 miles at t h e surface. T h e sky code (underlay in Figs. 3 . 2 0 - 3 . 2 4 ) shows that the entire oceanic portion of the flight took place b e n e a t h a broken

Remits of Flight I

29

Fig. 3.20.-Surface chart, July 13, 1957, OOZ (Flight I, Leg 3). Isopleths are labeled as surface isobars in mb and as contours of the 1,000-mb surface in feet. Underlay is sky code.

30

Result« of Flight I

Fig. 3.21.-Mean winds (knots) of lower layer with approximately uniform wind, July 13, 00Z. The depth of the layer in 1,000's of feet is denoted to the side or the top of each station. The speed of the mean wind is either written at the tail of the arrow followed by a "k," or speed and direction appear there in code. The maximum wind is denoted in knots and the level of the maximum in 1,000's of feet (the M after the number denotes "1,000's of feet").

Results of Flight I

31

Fig. 3.22.-Total precipitable moisture (cm precipitation or g m / c m 2 ) , July 13, 00Z. Current values circled; earlier and later times above and below, respectively.

32

Results of Flight I

Fig. 3.23.-Shear winds from top of trade-wind layer to upper troposphere, July 13, 00Z (Flight I, Leg 3). Base and top of layer denoted in 1,000's of feet ( M ) , and direction and magnitude of shear in code near tails of arrows.

Résulta of Flight I

33

Fig. 3.24.-Mean winds and altimeter corrections with respect to mean tropical atmosphere for layer 250-150-mb, July 13, 00Z (Flight I, Leg 3). Isopeths are of the latter in feet. The thickness of the layer is given in 10's of feet with the first digit omitted. and overcast middle and upper cloud deck associated with the typhoon. At take-off from Guam, there were broken middle clouds and broken cirrus. Within 30 miles, suppressed low clouds began appearing under the high and middle overcast. Twenty miles farther ahead, cumulonimbus with a chaotic sky became evident. The low clouds had built up and were very ragged. From this point onward, cumulonimbus and chaotic sky persisted through the remainder of the flight leg until the Phillipine Islands; codes 14, 15 and 16 were used to describe all of the prints until landfall, where breaks and gradual clearing occurred with a reduction in convective activity from cumulonimbus to cumulus congestus. The surface chart (Fig. 3 . 2 0 ) , the low level winds (Fig.

3 . 2 1 ) , and total moisture chart (Fig. 3.22) require little elaboration. For the most part the upper shear (Fig. 3.23) revealed the well-established pattern for tropical storms: cyclonic circulation decreasing upward and going over into anticyclonic circulation at high levels. The upper anticyclone (Fig. 3.24) is located about 300 miles to the east of the surface typhoon center, both in the mean " D " and on the thickness chart. Throughout most of the region the major wind shear occurs in the upper troposphere, and the vertical shear seen in the clouds is not appreciable below 20,000 ft; this seems logical, as the vertical profile in typhoons does not show appreciable change in speed in the lower half of the troposphere but weakens or goes over to anticyclonic above 300 mb. Only Manila differed from the

34

Results of Flight I

000 020 040 060 C60 100 120 dd

(degrees)

Fig. 3.25.-Tephigram (above) and vertical wind plot (below) at Guam, July 13, OOZ. The numbers next to the dew-point curve are mixing ratios (gm/kgm) entered for significant levels. general pattern, in that no turning of the wind to the south in the upper troposphere was observed there. If the few data leading to the pattern of Figure 3 . 2 4 can be wholly trusted, Manila was in the circulation of an upper cyclone over the southern Phillipines rather than in the typhoon circulation. Soundings at Guam (Fig. 3 . 2 5 ) and Manila (Fig. 3 . 2 8 ) show

13 20 25 50 V (knots)

140

160 dd

ISO 200 220 240

(degrees)

Fig. 3.26.-Tephigram and vertical wind plot at Yap, July 13, OOZ.

some evidence of subsidence in the middle troposphere. Those for Yap (Fig. 3 . 2 6 ) and Palau (Fig. 3 . 2 7 ) have been added as samples of the probable thermodynamic structure of the atmosphere in the middle of the flight route, which, however, was closer to the typhoon core than were these stations. It is interesting, however, that a mid-tropospheric minimum of Q

Results of T

-50

-40 1

-30

-20 1

/

I

35

-C

T

-10 \

0 /

\

10 1^

20 1

-50 .1

^

-40

-30 l

-C

- 20 / 1

-10 i

/i

0

10

20 i /

13

at

/ / 22

T°'

/

/

/

/

1

/1

a

y

A

/

0*

, v

V [knots)

V ^

/

y

i

/i

A

y y*

A

y

y\

dd (degrees)

Fig. 3.27. -Tephigram and vertical wind plot at Palau, July 13, 00Z.

remains in evidence on the soundings at all four of these stations (Fig. 3 . 2 9 ) , despite their location in the outer rain area of a typhoon. T h e d o m i n a t i o n of the cloud structure by the synoptic situation is so p r o n o u n c e d on this leg that f u r t h e r emphasis is u n necessary. T h e cloud patterns are described in m o r e detail in the two sections to follow.

Fig. 3.28.-Tephigram and vertical wind plot at Clark Field, Manila, July 13, 00Z. 2.

C L O U D STUDY:

C L O U D CROSS

SECTIONS

In addition t o the over-all sky code, it was desired to obtain m o r e quantitative and detailed cloud information. In this section we shall discuss the amounts of lower, middle, and high cloud along the route of Flight I and the heights and sizes of cumuli-

.

36

Results of Flight I no question of either the reality or the suddenness of this jump, which occurred in a 5-minute (about 20 miles) interval and was checked by five or more independent measurements on either side. N o change in reel, film calibration, or aircraft speed occurred. Another interesting feature of cloud base was its pronounced consistency from cloud to cloud during the first half of the flight (to 0155Z or longitude 175° W ) and its marked variability thereafter. There is also no question that this feature is real, and it cannot be accounted for in any simple way by variation in cloud size or thickness. An attempt to correlate base height with diameter and penetration (which were clearly related) failed. Quite possibly, the subcloud or so-called "homogeneous" layer became less homogeneous on the downstream portion of the flight, perhaps owing in part to the decrease in low-level wind speed.

78 80 82 84 86

Fig. 3.29. -Graphs of total energy content Q = CPT + gz + (cal/gm) for the soundings of Figs. 3.25-3.28.

Lq

form clouds. As before, this is done for each leg separately and a summarizing cross section constructed for each (Figs. 3.30, 3.33, and 3 . 3 4 ) . Cloud amounts were estimated, to the nearest one-quarter sky cover, for cumuliform, stratiform (lower and middle), and upper (height 20,000 ft and u p ) . All heights were calculated by the method described in chapter ii, except where noted as estimated. Heights of stratiform decks were generally more difficult to determine than individual cumulus tops, but numerous identifiable points on each deck were chosen, and the results are apparently both self-consistent and in good agreement with synoptic information. Leg

1, Flight

I

Tables 3.1 and 3.2 give cloudiness and cumulus properties for this leg, respectively. An interesting feature of Table 3.2 is the behavior of cloud base, which jumped rather dramatically from 1,500 ft to 2,200 ft just upstream of Johnston Island. There is

T h e information in Tables 3.1 and 3.2 is summarized schematically in the cross section of Figure 3.30. The sky coverage by cumulus-type clouds ( 1 / 4 , 1/2, 3 / 4 , etc.) is shown by the continuous curve (hatched) at the bottom. A schematic cloud showing base height, average tops (solid), and maximum tops ( d o t t e d ) is shown for every 15 minutes of flight. The size of the schematic cloud is proportional to coverage and not to actual cloud diameter, the thinnest corresponding to 1/4 coverage and the widest is four times as big. Actual cumulus diameters were also measured on the film at frequent intervals; in general tower diameters were proportional and about equal to the heights penetrated. Cloud bodies, from which the towers emerged, were generally much wider but could not be reported meaningfully; frequently they were obscured and equally frequently so many ran together that the meaning of individual "cloud" was lost. Inversion stratus or stratocumulus is reported in the " R e m a r k s " column of Table 3.1. Its amount has not been denoted on the cross section, although Table 3.1 shows it went from complete coverage at the start of I.eg 1 to traces by Johnston Island. True middle cloudiness (exclusive of inversion cloud) was present only within the 350 miles eastward of the wave trough; its percentage sky coverage is indicated there by the black vertical columns plotted at the 400-mb level, the tallest one indicating 3 / 4 . The upper cloudiness was entirely cirriform at about 30,000-40,000 ft in three distinct and separate regions. The first

Height

Pressjre mb

feet

-

-

Flight

|

xT'

Fig. 3.30.-Schematic cloud cross section for Leg 1, Flight I (July 10-11, 1957), constructed from measurements on film. Lower cloudiness (cumulus) amount indicated, to nearest one-quarter coverage, by bottom hatched-under curve, middle cloudiness by black columns at mid-levels, and upper cloudiness by black columns at top; inversion stratus not included. Width of schematic cumuli indicate coverage (narrowest, one-quarter), not cloud size.

2 0 0

700

Line

^

Arrows are direction of measured cloud shear (north u p w a r d ) . Mean heights of cumuli are solid clouds; maximum heights in the 15-minute interval are dashed. Precipitation is noted coming f r o m cloud base in every 15-minute interval that is was observed. The tall build-up near the wave trough (close to M a j u r o ) was measured as reaching 42,000 ft. Several others exceeded 30,000 ft far to the south of the aircraft path at the trough line.

Height i n kilometers

38

Results of Flight I

was associated with the jet stream over and southwestward of the Hawaiian Islands. The second batch was in mid-leg (where no radiosonde ascent existed) and its origin is not therefore clear, although Figure 3.5 suggests that the high-level meandering westerly current may have been beginning there to lose anticyclonic vorticity and gain cyclonic at a northwest inflection point. The small region of relatively suppressed cumulus shown directly below on Figure 3.30 indicates that upper convergence was here superposed on the lower divergence of the subtropical ridge just eastward of the location where the troughing of the easterly wave sets in. The final patch of cirrus in the convective region of the wave could easily have been sheared off persisting anvils (build-ups to 30,000 ft are possible at or just west of here, although they would have been obscured to view on the film); however, it is not possible to determine their origin precisely. Upper cloudiness (to the nearest one-quarter) is denoted by the black columns along the top of Figure 3.30, the largest one representing 3/4. On the whole, the cloud study supplemented and agreed very well with the picture of the flight leg derived from the synoptic analysis and sky coding. The flight started out as a strong inversion-dominated one. There was an extensive stratus, intermittently stratocumulus, deck just below aircraft level (8,000 f t ) . These cloud forms weakened and died out approximately 300 miles out of Honolulu. The Lihue sounding (not reproduced) showed this strong inversion, agreeing perfectly with the measured height of cloud tops, as did that for Hilo (Fig. 3.6). By Johnston Island, the inversion had weakened to an isothermal layer and risen slightly. Multiple inversions and highly stratified moisture, with intervening dry wafers, were seen on the Hawaiian soundings and at the radiosonde ship, nearly disappearing by Johnston as the moisture became better mixed in the vertical. The inversion clouds had diminished to scattered "eyebrows" by Johnston; a few of these recurred in the suppressed region in mid-flight ( ~ 0 2 5 5 Z ) and again in the subsidence region ahead (westward) of the wave but were not pronounced enough to be entered on the cross section. For the first half of the flight the measured uniform height of cloud base agreed very closely with the lifting condensation level (LCL) of air having the average properties of the subcloud layer; dur-

ing the last part, the variable cloud base corresponded with variations in LCL between air at the very surface and that about half-way up through the subcloud layer (Majuro, Fig. 3.8), substantiating the previous suggestion that this layer was less well mixed in the downstream portion. Disturbed weather on the horizon to the south began to appear about 300 miles west of Johnston Island, but was not included as a significant feature on the cross section; the cloud codes in Figures 3.1-3.5, however, indicate it by the postscript "d." For the next 350 miles it gradually rose on the horizon toward the water vapor maximum associated with the easterly wave. It then became interspersed with the wave weather zone, which predominates on the cross section from about 0400Z (178° E longitude) to the trough line, crossed at 0546Z (-~172° E longitude). The trough was extremely obvious on the cloud films, and the severe weather broke suddenly once it was passed. Shear of the clouds from the southwest was pronounced in the vicinity of the trough to its east, as shown schematically on the cross section. The direction of the shear was determined from the films; it suggests an increase of wave amplitude with height (as is common) and perhaps even a closed cyclonic center at 15,000-20,000 ft, although the synoptic data were too sparse to confirm this. The one fault of the sky coding is revealed by the cross section right at the wave trough, where a build-up was actually measured to 42,000 ft. At least two others reaching heights of 30,000 ft and more were recorded on the horizon. Owing to their great distance and to the fact that the towers happened to be still growing and rounded-looking and had not yet glaciated outward into typical anvil appearance, their cumulonimbus character was not detected on the prints. At this stage, all observers coded the sky type as 9 and estimated these clouds as congestus reaching 23,000-30,000 ft. Actually, the code should have been 12 at the wave trough, and the coding system has underestimated the intensity of the disturbance, which it may well be expected to do on other occasions unless supplemented by actual determinations of cloud tops. However, for the sake of maintaining an objective and simple code usable by inspection only, the code numbers have not been altered. We do not

Results of Flight I

39 Preisure mtj - 200

Fig. 3.31.-Cross section of relative humidity along Leg 1, Flight 1 (July 10-11, 1957), with the cloud cross section as underlay. The wave disturbance in the easterlies lay between the longitudes of Majuro and Kwajalein.

Height in kilometers

Fig. 3 . 3 2 . - C r o s s section of total energy content Q ( c a l / g m ) along Leg 1, Flight 1 (July 10-11, 1957), with t h e cloud cross section as underlay. Note low values at mid-levels in the subsidence regions and the near disappearance of this mid-tropospheric minimum in the convection area of the wave.

Results of Flight I know yet whether a typical moderate easterly wave in the Marshall area commonly possesses cumulonimbus towers to 40,000 ft; we suspect rather that the concentration and number of such towers is both a measure of and a necessity for the intensity of the disturbance. If so, the code remains by and large adequate because a high concentration of cumulonimbi to these heights would surely lead to some with recognizable anvils and thereby correct coding, or, perhaps more likely to a "socked-in" sky condition such as that coded for the typhoon-dominated Leg 3 of this flight. Finally it was attempted to make cross-sectional analyses from the radiosondes of vertical distributions of significant properties along the flight leg and relate them with the cloud cross sections as underlays. Although the mid-portion of the flight was unhappily without a radiosonde, this attempt (with reservations due to data sparsity and time variations) proved sufficiently successful to justify the effort for Leg 1. Figures 3.31 and 3.32 show the results for relative humidity and Q, total energy. Sections for temperature, potential temperature, lapse rate, and mixing ratio were also made but did not add anything to our knowledge. The relative humidity section of Figure 3.31 forms a beautiful link between cloud and synoptic patterns. The extreme moisture stratification at the outset of the flight is brought out clearly, with the main inversion cloud deck appearing at inversion base, or just at the region of rapid upward drying. A relative humidity maximum associated with the jet-stream cirrus is also evident aloft. In the region of the wave, a close fit again occurs; one can see the effects of the lifting in the convection zone and subsidence to the westward. The middle-cloudiness region is one of high humidity at what must have been the level (15,000-20,000 ft) of the maximum wave amplitude. Figure 3.32, a cross section of total energy Q, supplements the discussion in section 1. We see the gradual downstream warming of the air, with the violent effects of the wave superposed. The lowest air increases its total energy to about longitude 180° W, as expected, but then the surface values go down in the wave's convection zone, presumably owing to the upward export by convection described previously. The minima occurring in mid-troposphere in subsidence zones are well

41

brought out. Whether the minimum west of the wave is due to increased radiation sink relative to upward transport, whether the normal sink unbalanced by input is adequate to explain it, or whether a lateral advection (from north) of lower heat content air occurs here we cannot say. Since the change in Q at mid-levels through the wave trough is - 2 cal/gm, or about 8 ° C in equivalent potential temperature, this seems to exceed a reasonable rate of radiation loss and suggests at least some contribution of advection of cooler a n d / o r drier air, probably the latter. Leg 2, Flight

I

Tables 3.3 and 3.4 give the estimated cloudiness by categories and the results of the height calculation for Leg 2. These are illustrated schematically in the cross section Figure 3.33. This leg was as interesting as Leg 1, in a rather negative way. First, the repeat passage through the easterly wave trough, now about 200 miles west of Kwajalein as discussed in section 1, showed almost complete suppression of convection near the aircraft path, even at the troughline. On the southern horizon, however, five or six cumulonimbus build-ups were visible in this vicinity. They were measured as 75-150 miles away from the aircraft and their tops reached 32,000 ft. One of these has been entered in the cross section in dotted lines to indicate its removal from the flight path. It is not certain whether these build-ups were remains of the old disturbance or a portion of the equatorial trough. They receded and disappeared by 2240Z (longitude 165° E ) , and very suppressed cumulus with traces of upper cirrus then prevailed. Many of these suppressed cumuli, particularly in the vicinity of Eniwetok, showed pronounced shear from the northeast in the levels near their tops at 5,000-6,000 ft. Although this surprised us at the time, such shear was later verified on the Eniwetok sounding from 6,000-8,000 ft (Fig. 3.17). The excellent agreement and representativeness of cloud shears found throughout this study suggest their use for synoptic analyses in regions where standard upper air data are not available. No middle clouds were present until a patch at 0055Z (near midleg, or about longitude 158° E ) . These were definitely not produced by cumulus; they occurred at the line of the upper-

42

Results of Flight 1 Height 200

Fig. 3.33. -Schematic cloud cross section for Leg 2, Flight 1 (July 11-12, 1957), constructed from measurements on film. Notation generally the same as Fig. 3.30, except that amount of upper cloudiness is denoted by top hatched-under curve. The dotted tall cloud at about 2240Z denotes the cumulonimbus build-ups visible 150-250 miles south of the aircraft.

in

kilometers

Heigh'

Pressure mb

in

feet

Height in kilometers 400 Possibility cirrus

of

4/4

coverage

20,000 500-

H e a v y , layered 600

stratus

Stratus layers range from flight level to 18,000 feet. 3/4 + coverage

700 —

10,000 — Socked 8,000 -

Fhght

Line

•••m

ftmwtimM

mfflm/MM i

m mm*

\*kmmmt)tmmt$x Camera off

6,000-

850 4,000 -

Cumulus over land only

2,000-

Land Manila

0400

0300

1000

3S-

0200

0100 130°

2300

0000

2200

Yao

F i g . 3 . 3 4 . - S c h e m a t i c c l o u d cross section f o r L e g 3, Flight I ( J u l y 13-14, 1 9 5 7 ) , constructed f r o m measurements on film. M i d d l e overcast at —• 16,000-18,000 f t prevailed, so that cloud f o r m s a b o v e that could not be observed. Hatcheu-under c u r v e at 500 m b height is amount o f m i d d l e cloud. T h e portions that are mostly blank with precipitation falling w e r e " s o c k e d i n . " O n l y those cumuli whose tops and bases c o u l d be seen w e r e entered.

Guam

44

Results of Flight I

level trough (see Fig. 3.15). Clouds on the horizon did not appear again until 400 miles beyond Eniwetok, when suddenly they came into view 100-200 miles away on the horizon. These were probably very intense cumulonimbus building to 30,00035,000 ft, but they were not measured precisely, owing to the poorly defined horizon. Cirrus was radiating outward from them. This cloud pattern was maintained until about 250 miles east of Guam when a middle cloud deck again appeared suddenly. This time the films definitely showed that it was formed by intense shearing of cumulus build-ups which rapidly increased as Guam was approached. These cumuli, some of whose tops measured 26,000 ft, showed well-defined shear from 240° at this level and just below, in excellent agreement with Figure 3.14. Soon after the appearance of the middle cloud deck, the area becomes very unstable and an intense convection zone over the last 150 miles is observed. There are many cumulus congestus, although no measured tops exceeded 26,000 ft, so that the code number 9o was actually the best choice. This strong convection zone is on the upstream side of a deep easterly surface trough located west of Guam which was developing at the time into Typhoon Wendy. That the patterns were changing rapidly at the end of this leg was noted earlier in section 1, where the rapid increase in moisture at Guam by 1200Z on the 12th was remarked. It proved impossible to make coherent analyzed cross sections of this flight leg, particularly in respect to Q. It had been intended to composite one, using Ponape and Truk, which were located off the flight route to the south. Their values of Q, however, proved excessively high, and compositing gave an absurd pattern, due primarily to excessive moisture values in mid-levels. We concluded that these soundings were probably more representative of the equatorial trough zone, which the pictures suggested curved off to several hundred miles to the south of the mid-portions of the flight. It would thus appear that although mid-level temperatures are fairly homogeneous latitudinally in the tropics, humidity and Q values may in fact show strong gradients, particularly at the boundary of the equatorial trough itself. If this proves to be the case, relatively very low mid-level values of Q in subsidence zones of waves (and high values in convection zones) may have a partially advective explanation as suggested.

Leg 3, Flight I Cloudiness for this typhoon-dominated leg is given in Table 3.5. The altostratus shield was overhead at Guam at take-off and persisted throughout the entire flight over the ocean, only breaking after landfall over the Phillipines. Within 30 miles of take-off, small suppressed cumuli began to appear in lines under the overcast, gradually building up into cumulonimbus whose tops were lost from view in the overcast. The important cumuliform clouds on this leg, which were undoubtedly giant towers penetrating to great heights, could not be studied or mapped here owing to altitude limitations and the black, "socked in" conditions which prevailed over a major segment of the flight path. The cloud heights table is therefore not reproduced and the small amount of information that could be gained about cumuli, mostly smaller ones, is entered in the cross section in Figure 3.34. The "socked in" portions are indicated schematically. Lower cloudiness is recorded on the lower hatched curve and middle cloudiness by the hatching near 500 mb. The upper regions are blank because nothing could be inferred about them; in all probability, several more sheets of clouds and intermittent penetrative towers occurred, most likely where the precipitation was heaviest. Even in the most disturbed portions of the flight, breaks generally occurred between bands of intense precipitation, which were definitely both concentrated and intermittent. It was found possible to measure the duration of precipitation and socked-in regions quite accurately on the film. These "rainbands" are mapped and discussed in the following section on organization of clouds.

3.

ORGANIZATION

STUDY

One of the most striking aspects of the cumulus populations observed in all three flights across the tropical Pacific was their organization into rows or lines. Cumulus clouds were observed to line up in the majority of the 15-minute prints made from the motion pictures, and the impression prevailed among the observers that the rows were approximately parallel to the low-level wind flow. During some sections of the flights, however, organization was far more pronounced than on others,

Results of Flight I

and apparently random cumulus distribution was encountered in some places. Furthermore, several scales of organization were frequently encountered, and in some disturbed regions a double organization was suspected, with some portions of the parallel cloud lines greatly amplified at periodic intervals along the rows, giving the appearance of a superposed crosswind organization. The following questions therefore arose: 1) Under what conditions are tropical cumulus clouds organized into rows and under what conditions do they appear in random bunches? 2) What determines the direction of the rows? Are they usually or always parallel to the low-level flow? Or are they oriented with or affected by vertical shear in the flow? 3) Where and how does the apparent cross-wind organization become superposed upon the rows? In order to work toward a physical understanding of cumulus organization, a quantitative descriptive study of the films was first required with the above questions in mind. This discussion presents results from Flight I, using the techniques outlined in chapter ii. Results

from

Flight / , Leg I

This leg proceeded through a relatively undisturbed trade regime at first, then into and across an easterly wave trough just east of Kwajalein. For the first half-hour out of Hawaii, signs of a marked trade inversion were visible and no noticeable organization of the cumuli into rows was detected. Organization into rows then began to be observed and had become moderate by the time Johnston Island was passed. It remained fairly pronounced from there until after passage through the wave trough. The results of the organization study for this first leg of the flight are presented in Table 3.6 and in Figure 3.35. Orientation of cloud rows. During the portion of the flight from one hour out to the wave trough there is little question that the cloud rows were closely parallel to the low-level flow. The cloud lines were at first nearly along the flight path (from 070°), gradually becoming more southerly as the trough was

45

approached, and reaching 110° just east of the trough line. Superposition of the cloud orientation arrows upon the streamline chart (Fig. 3.35) shows coincidence within the accuracy of the drawing of the streamlines for this portion. The lines took a pronounced bend at the trough line itself, and shortly after crossing it (within 50 km) had become oriented from 010°. There is no way of knowing whether the flow in this region was actually this far backed to the north or not, because the wind at Kwajalein (another 400 km westward of the trough line) was only easterly or slightly south of easterly at this time. Nor does the time section for Kwajalein show any pronounced backing to northerly prior to passage of the wave there at about 1200Z on July 11 (during the overnight stop of the flight). The orientation of the cloud lines in the vicinity of the wave trough is shown here in Figure 3.36, the construction of which is discussed below. Spacing of cloud lines. Beginning at the time when the flow and cloud lines became obviously veered from south (about halfway between Honolulu and Kwajalein), an attempt was made to determine their spacing (normal distance between rows). At first, this averaged about 4 km, a distance comparable to the depth of the moist layer. The clouds were average or small-sized trade cumuli here, and none was congestus or penetrative. As the flow became further veered and the wave trough was approached, the row spacing became wider and wider until it reached about 25 km just east of the wave trough and 30 km just west of it. It appeared that the spacing was being widened by superposition of a larger scale organization upon the smaller (4 km) scale and that some of the cloud rows were being enhanced while those between were being suppressed. For example, at the halfway point evenly sized rows at 4-km normal intervals were found. One hour (-— 370 km) later, it appeared that every fourth row was being built up at the expense of the intervening three. Rows of cumuli 4 km apart were still measured, but every 16 km a much more developed row was found, in which the larger clouds were close to congestus. By the time the aircraft had reached a location about 300 km east of the trough, the major lines had become 24-26 km apart and the minor ones intervening had been suppressed almost to the vanishing point. After passage through the trough, three (and

46

Results of Flight I

Fig. 3.35.-Organization of cumulus, Leg I, Flight I, on lower layer winds. Arrows give measured direction of cloud lines. Code symbols are organization intensity: X absent, 1 weak, 11 moderate, 111 strong, or M no measurement.

Results of Flight I

47

Fig. 3.36.-Schematic distribution of cumuliform clouds near wave trough in easterlies, Leg I, Flight I. Orientation of cloud rows and spacing between them was measured exactly from the film, as was the distance between the amplified cloud groups, denoted schematically in the figure as a single large cloud. Individual clouds have not been entered nor drawn to scale.

48

Results of Flight I

only t h r e e ) lines of congestus clouds were observed oriented f r o m 0 1 0 ° and exactly 3 0 km apart, with n o intervening cumuli and with a p r o n o u n c e d suppressed or almost clear area f r o m the last one westward extending t o the n e a r vicinity of Kwajalein. T h e few suppressed cumuli to the west of the last lines of congestus were quite randomly distributed and no orientation was detectable; signs of a trade-wind inversion were again visible, as might be expected prior t o the advent of an easterly wave. Cross-wind organization. F r o m a b o u t 300 km east of the wave t o the trough itself a marked cross-stream organization became a p p a r e n t , superposed on the parallel lines. T h a t is, groups of extra large clouds appeared at intervals along the lines (in this case at 120-km intervals; see Fig. 3 . 3 6 ) while intervening portions of the lines were suppressed to the point of being clear or nearly clear spaces, giving the impression of large cloud groups lined u p normal to the flow; these groups are shown schematically in Figure 3 . 3 6 as a single large cloud. Within each group, individual clouds retained their parallel ( t o flow) orientation. It is possible that this superposed crosswind organization is related to vertical shear, which the cloud slope ( a n d c o m m o n knowledge of the model easterly wave) showed previously was from the southwest on the cast side of the wave. It is also possible that the pronounced backing of the lines to 0 1 0 ° o n the west side of the trough was related to northerly shear there, and that we are seeing only the crosswind organization with the parallel totally suppressed. It was

TABLE

not possible, however, to resolve this point with the data of Flight I. Relation of cloud organization to air structure. T h e important first question raised in the introduction to this section—namely, under what conditions are tropical cumuli organized into rows— can face a preliminary examination f r o m the results of Leg 1. The clouds were well organized in rows in all portions of the flight except the first and last hours. D u r i n g the first hour, an inversion-dominated trade regime prevailed, and during the last, an inversion-dominated pre-wave situation. It seems that organization occurred over all portions of this leg where we may reasonably have expected synoptic-scale convergence and where inversion domination did not occur. Fortunately, radiosonde and radiowind observations were available f o r Lihue (very early part of flight), J o h n s t o n , and Kwajalein n e a r the flight time. Table 3.7 shows some pertinent features of the soundings presented earlier f o r this leg (Figs. 3.7 and 3.9 for Johnston and Kwajalein; see Hilo, Fig. 3.6, for early p o r t i o n ) . T h e information in T a b l e 3.7 is indicative but by no means adequate to establish the criteria for cloud rows. It is clear that the mere absence of vertical wind shear is not enough t o give rise to cloud rows (there were none in t h e vicinity of Kwajalein despite weak s h e a r ) , although a possible inhibitory effect of strong shear is not precluded, since no organization occurred near Lihue and there was an 11-knot shear through the cloud layer with a 3 0 ° wind turning. T h e cloud layer near Lihue was actually quite deep, although its lapse rate was

3.7

F E A T U R E S O F V E R T I C A L A I R S T R U C T U R E AT T I M E O F F L I G H T I , L E G 1

Place 00Z/llth

LCL (ft)

Inversion base (ft)

Inversion strength (°C)

Depth of cloud layer (ft)

Lapse rate of cloud layer ( ° C / 1 0 0 m)

Lihue Johnston Kwajalein

2,000 3,200 2,000

7,750 7,400 4,050

3 0 0

5,750 4,200 2,050

0.46 0.56 0.57

Vertical shear of cloud layer (knots) 1 1 from 215° Negligible Negligible

Results of Flight I

49

winds. Notation same as Fig. 3.35.

50

Results of Flight I

only marginally unstable (12 per cent in excess of wet adiabatic). In the vicinity of Johnston Island, where organization began to be clearly detected on films, the cloud layer was fairly deep (4,200 f t ) , and its lapse rate exceeded wet adiabatic by 36 per cent. Off Kwajalein, the cloud layer lapse rate remained this steep, but the cloud layer was only a little more than 2,000 ft deep; cumulus clouds and organization were suppressed.

of clouds. Thus although the cloud rows, such as they were, could be said to be closely parallel to the low-level air flow, the measured angle wobbles considerably in the mid-portion of the flight. It is interesting to note that this part of the leg manifested the poorest development of clouds in all of Flight I, with a clear space 168 km across occuring near the beginning of the leg. It might appear that upper lows and lower ridges somehow suppress organization as well as suppressing cumuli.

Results

Results

from Flight I, Leg

2

The organization study for this leg is summarized in Table 3.8 and Figure 3.37. During the 17-hour stopover at Kwajalein, the wave trough passed the station (at about midnight local time, or 1200Z), causing showers, and moved on westward. It thus moved underneath an upper-level cold low and was damped out to near the vanishing point. Leg 2 of the flight passed the remains of this trough shortly out of Kwajalein. Only a few build-ups far to the south were visible and the cloud lines remained from about 106° throughout the early portion of the flight, indicating the total demise of the disturbance. The flight then passed through a low-level ridge and gradually into moister, converging flow as Guam was approached. Cloud organization was far weaker and less pronounced on this leg than on the previous one and was coded as weak or absent for 76 per cent of it. It was only listed as moderate during the last hour before Guam, when cross-wind organization was also noted, with large-scale cloud groups spaced about 100 km apart along the parallel lines. In connection with the possible effects of vertical wind shear on organization, it is noteworthy that the Guam winds showed a 14-knot shear from the south through the cloud layer (2,000-9,000 f t ) with a wind direction turning through 060°! It thus does not appear that moderate shear inhibits cloud rows, although it may quite likely be responsible for the superposed cross-wind organization. Unfortunately no soundings were available for the earlier portions of the flight, because the nearest Marshall Island stations failed to report. With organization weak, the orientation of cloud lines was hard to determine over most of the earlier part of the flight because it was difficult to decide just what constituted a "line"

from Flight

I, Leg 3

After a 17-hour stopover at Guam, Flight I continued to Manila. Leg 3 was entirely dominated by Typhoon Wendy, a minimum-strength typhoon with central pressure about 990 mb. Its center was located at about 129° E, 12° N. On the nearest approach, the flight passed about 200 km north of storm center. The entire flight was beneath an overcast of middle and upper clouds. However, the cumulus organization was the most intense of the three legs of Flight I; it is recorded in Table 3.9 and shown superposed on the low-level flow in Figure 3.38. Figure 3.39 shows a schematic map of the rainbands, constructed from the film. The numbers along the flight path (other than Greenwich times) correspond to prints made from the motion-picture films, and may be ignored. Up to 2245Z (see Fig. 3.39 for location), the orientation of the cumulus lines could be determined accurately by measurements. The largest convective lines consisted of swelling cumuli or cumulonimbi penetrating the overcast; they are drawn more or less to scale on the figure. Lines of smaller cumuli were interspersed between these; they had the same orientation but could not be drawn on the figure. For the next hour (from 2240Z to 2340Z), the aircraft passed through an extremely black, rough, precipitating region, during which the camera was turned off. This is indicated as a single cloud band on the figure, although it was more likely a group of several closely packed bands. It was unlikely to have been a typhoon band, since it was 1,300 km from the storm center and not in the region of flow converging into the storm. From this portion of the flight onward, the orientation of cloud bands could not be determined by measurement as previously because of the blackness and confusion of typhoon

Results of Flight I

51

Fig. 3.38. Organization of cumulus, Leg 3, Flight I, on lower layer winds. Notation generally same as Fig. 3.35. Orientation of cloud rows, however, could not be measured exactly after the first "socked-in" region and was estimated thereafter. Double arrows indicate limits of estimation. Results suggest flow around typhoon may have been more elongated north-south than these streamlines suggest.

52

Results of Flight I

Fig. 3 . 3 9 . - M a p of cumulus and rainbands (visual not radar) along Leg 3, Flight I, on surface streamlines. The first f o u r bands after G u a m are lines of cumulus, some building up into the middle overcast. A f t e r that the orientation, spacing, and size of the black, socked-in, raining regions have been mapped. The difference between these and their surroundings was sufficiently marked to make this mapping fairly accurate. (The numbers along the flight path [other than Greenwich times] correspond to prints made f r o m the motion-picture films, and may be ignored.)

Results of Flight I conditions. Nevertheless it was clear f r o m the film that a succession of intense convective bands approximately normal to the flight p a t h was being encountered. T h e film would become extremely black and rainy at periodic intervals, with partial clearing between b a n d s during which the sea, lower scud, a n d upper overcast were visible. O n the east side of the t y p h o o n center, the b a n d s were spaced at 5 0 - k m intervals, each b a n d being at first about 2 0 km across with 3 0 km between bands. A s the center was approached, the 5 0 - k m spacing was retained, but some bands were 30 km or m o r e across with correspondingly smaller intervening spaces. T h e "socked in" region about 170 km f r o m G u a m just north of the storm center was the blackest encountered on all three of the Pacific flights. T h e aircraft probably was passing lengthwise through an active convection band winding around to the north of the storm center (see Fig. 3 . 3 9 ) . A f t e r leaving this, the 5 0 - k m spacing of b a n d s resumed, with an extremely wide one ( a b o u t 5 0 - k m across) just before land was sighted at 0 3 0 0 Z . A f t e r crossing the islands and coastline, the bands gradually weakened and disappeared toward landing, but retained the same spacing and orientation. A comparative study of the radar and visual hurricane rainbands has been made for several Atlantic storms, particularly f o r Daisy, 1958, by Malkus, R o n n e , a n d Chaffee [ 1 ] , using a photographic technique identical with that used here. T h e coincidence between the black, socked-in areas o n the film and the hard-core radar echoes was f o u n d to be nearly perfect. W e may therefore expect that a radar m a p of the flight p a t h north of W e n d y would have looked very similar to Figure 3.39. 4.

SUMMARY

OF

FLIGHT

53

consistent with what might h a v e been expected before this p r o g r a m was u n d e r t a k e n : namely, trade-wind skies prevailing in t h e easterly current a r o u n d the subtropical high cell n e a r and just down stream of Hawaii, going over into incipient or m o d e r a t e trade disturbances in the Marshalls area a n d to m o r e severe tropical storm or t y p h o o n in the western low-latitude N o r t h Pacific. In fact, one might expect this to be the typical or c o m m o n picture encountered on all such flights, with t h e reservations that Leg 2 ( e q u a t o r w a r d b o u n d a r y of the t r a d e s ) might often show portions of the equatorial trough and its deepening disturbances, and Leg 3 might not always contain a t y p h o o n . This expectation is tested with the results of Flights II and III and their comparison with the present flight; in this way we examine the time variations over these portions of the o c e a n and their relation to changes in the over-all flow.

TABLE

3.10

DISTRIBUTION OF SKY T Y P E S , F L I G H T I

(Percentages)

Trade-wind Strong Trade-wind skies disturbance disturbInver- Nor- SupNor- Supance to sion mal pressed Total mal pressed Total typhoon 1 2 3 Over-all

20 0 0 9

57 12 0 29

11 4 0 6

77 16 0 39

23 36 0 21

0 48 0 15

23 84 0 36

0 0 100 25

I

Table 3 . 1 0 summarizes the distribution of sky types along the route of Flight 1. T h e r e are some overlapping categories in the table: for example, cloud code 3i was counted as b o t h n o r m a l trade-wind sky and inversion. Also, the degree of suppression of Leg 2 as shown by the films and previous discussion is slightly understated in this presentation. Nevertheless, the results are quite

O n the other hand, a not necessarily expected result w a s the extreme rarity and concentration of precipitating c u m u l u s and cumulonimbus clouds. Their distribution is summarized in Table 3.11. It is seen that all such large cloud build-ups were confined to disturbed areas or to places w h e r e low-level convergence ( a n d upper divergence) could be inferred qualitatively f r o m the

54

Results of Flight I

synoptic charts. N o precipitation reaching the sea surface was seen at any other localities in the entire flight. TABLE

3.11

DISTRIBUTION OF PRECIPITATION ON F L I G H T I

Leg

Percentage of intervals with precipitation

1 2 3

14 8 90

Remarks All in wave convection zone All within 150 miles of Guam All in typhoon vicinity

A n over-all result beginning to appear f r o m this work is the significant if not controlling role played by large-scale flow patterns u p o n cloud structure. Cloudiness and showers appear to be far less randomly distributed than the classical picture suggested, which gives hope that they may be forecast accurately enough f o r m a n y purposes when m o r e d a t a and understanding concerning the large-scale dynamics of the tropical atmosphere become available. It is clear, f u r t h e r , that u p p e r level conditions are of crucial importance in governing what we observe in the tropics at low levels, f r o m the turbulence and cumulus scale up; what we see is the result of interaction, not yet fully understood, between upper and lower synopticscale systems. T h e futility of attempting to understand or predict tropical cloud or precipitation physics p a r a m e t e r s out of context is becoming apparent, and this context a p p e a r s to include u p p e r tropospheric ( t o 2 0 0 m b anyway) flow patterns. Summary

of Flight

I organization

study

Flight I showed highly organized cumulus formations. Over those portions of the flight where organizations could be coded

satisfactorily ( a b o u t 6 , 3 0 0 k m ) , no organization or a random distribution was coded 9 p e r cent of the time; weak organization, 30 per cent; moderate organization, 34 per cent; and strong organization, 28 per cent. Most of the absent or weak organization reports occurred on Leg 2, and most of the intense organization was confined to Leg 3. It may not be coincidental that the degree of disturbance present h a d a similar distribution. In all but one portion of the flight just to the west of a wave trough, the cloud lines were almost certainly closely parellel to the low-level air flow; in the portion just west of the wave trough they m a y have been backed m o r e to the north than the lower wind was, but since no wind data were available there, this is not certain by any means. T h e spacing of the cloud lines appeared to increase as convergence ( p r e s u m a b l y ) increased and was surely smaller in regions of undisturbed trade cumulus and larger as cumulonimbus development was a p p r o a c h e d . T h e reason for this is not clear, but it may be simply that larger scale modes (longer wave lengths) become possible as the convectivc layer deepens. Cross-wind organization (exemplified here as periodic amplification of clouds at intervals along the lines and suppression in between) was evident in two areas of convergence, namely just to the east of the easterly wave trough on Leg 1 and at the end of Leg 2 as the convergent region near G u a m was crossed. There is a suggestion here that this cross-wind mode of organization is related to the shear of the wind in the vertical; in the next chapter, Flight II provides some fine evidence to test this point.

REFERENCE I. Malkus. J. S., C. Ronne, and M. Chaffee, 1961: Cloud patterns in Hurricane Daisy, 1958. Tellus, 13, 8-30.

TABLE

3.1

CLOUDINESS, FLIGHT I, LEG 1; HONOLULU K W A J A L E I N , J U L Y 1 0 - 1 1 ,

Time (GOT)

Cumuliform Type; amount ?

2145 2200 2210 2230 2245 2304 2308 2326 2341 2355 0010 0025

High, > 20,000 ft

,

?

À

1/4 +

£3 £3 £3

1/4 1/41/4

£3 £3

A £3 A c A o A

1/4 + 1/41/41/41/4 1/4 +

— u

1/41/4 -

Remarks Heavy inversion — 7,600 ft Inv. almost ended

Socked in

£3

0040 0055 0110 0130 0145

£3

A Û c A «o A A

1/4 1/4 + 1/4 1/4 1/4 +

0155 0210 0225 0242 0255 0310 0325 0340 0355 0414 0430 0443

^ c

1/2 1/2 1/4 1/4 1/2 1/2 1 '2 1 2 1/4 + 1/2 1/2 + 3/4

0500 0515 0531 0546 0601 0620

Middle, 10,000-20,000 ft

1957

£3 Ci ¿3 £3

A A A

A

A ¿3 A ¿3 A o A

A

¿3

Sea clear 1/2 3/4 + 3/4 + £3 Û 1/41/4 + ¿3

A A

— »

— "

1/4 + 1/4 trace

Very thin inv. layer 8,000 ft

Abrupt change in cu base

Ci begins ~ 0150Z in dist. 1/4 1/4 + 1/4 + 1/4 + J — ^ trace -i— 1/4 1/4 ^ — trace trace 3/4 3/4 l> ~ 1/4

s ^ ^

3/4 3/4 1/2 +

J —

trace ? ? 1/4 -

Upper sky obscured; ci estimated

56

Results of Flight I

TABLE

3.2

H E I G H T S OF CLOUDS, F L I G H T I , LEG 1; F L I G H T A L T I T U D E 8 , 0 0 0 F T , JULY 1 0 - 1 1 , 1 9 5 7

Cumulus Time (GCT)

Base (ft)

Mean tops (ft)

2145 2200 2210 2230 2245 2304 2308 2326 2341 2355 0010 0025 0040 0055 0110 0130 0145 0155 0210 0225 0242 0255 0310 0325 0340 0355 0414 0430 0443 0500 0515 0531 0546 0601 0620

? ?

7

?

? Socked in 5500 4000 2800 3800 4000 7500 6000 6500 5000 3200 5500 6500 5000 5000 6500 6500 5000 3800 5000 3500 5500 6000 8000 7000 8000 ?

1500? 1000? 1800 1800 2000

Sea clear 7000 12,000 13,000 4000 5000

1650 1700 1500 1500 2200 2200 2200 2400 1700 1700 1700 1600 1600 1500 .— 1400 13-1800 12-1900 — 1500 12-1800 15-1800 14-1800 14-1700 14-1700 10-1200 10-1200

Max. tops (ft) —8,000 — 11,000 8,000 8,500 8,000 9,000 8,500 8,500 8,200 8,500 8,300 7,500 6,000 9,300 7,500 9,800 9,500 7,000 7,400 3,800 7,000 9,000 15,000 10,500 9,500 17,000 21,000 30,000? 16,000 25,000 + 42,000 4,000 5,000

Middle cloud (ft)

—

— — —

High cloud (ft) ? 7 32,000 32,000 32,000

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

— — — — — — — — —

21,000 21,000 —

16,000 16-18,000 16-19,000 —

39,000 38,000 38,000 40,000 40,000 24,000 40,000 40,000 trace 35,000 ? 38,000

1

Remarks Inv. undercast Inv. patches Socked at 2203Z Out 2230Z; cu small Thin inv. Cu base sudden rise Cu tops uniform Cu in clumps Abrupt base change Larger cu are dist. Cu start to lean W N W Bimodal height distrib. Ci start 0150Z Base beginning to vary Base variable Base variable Nearby cu sparse, stunted Cu stunted; ci disappearing New small ci lower Max cu 60 miles; ci 92 miles Ci far background Altocu 0406Z; extreme shear to E N E Ci background Cu shearing at 8,000 ft Thin altocu

?

42,000

—

—

—

—

Trough line; break through Many sm. unorg. cu Sm. unorg. cu

Results of Flight I

57

T A B L E 3.3 CLOUDINESS, F L I G H T I , L E G 2 ; K W A J A L E I N - G U A M , J U L Y 1 1 - 1 2 ,

Time (GCT)

Cumuliform Type; amount

Middle, 10,000-20,000 f t

High, >20,000 ft

2225

Q

1/4 -

trace

2240

O

1/4

4

trace

2255

^

1/4

-

2308

f'\

trace

trace trace trace trace

2310

1/4 •

2320 2335

1/4

Û

2350

1/4 1/4

1/4

1/4

trace

0035

1/4-

0055

1/4

0110 0122

1/2 1/2

0140

1/2

-

3/4

0155

1/2

-

3/4

0210 Ci ^

0242 0255

Ci Ci

0313

trace

1/2

^

Ô

3/4

^

3/4

Ci 3 7 - 4 2 , 0 0 0 ft

3/4 Y

-

1/2 trace 1/4

Stratus very thin

1/2 1/2

1/2 1/4 1/4 1/4

-

+

-

trace

1/2 +

0335 0355

1/4

Leftover cb's on horizon; no trade cu Cb at 77 miles; ci background Cb at 77 miles; ci background Ci on horizon Ci on horizon Ci on horizon Patches thin ci High, thin ci Ci patches over sky

1/4 -

1/4

0227

-

1/4

0006 0020

0406

Remarks

trace

2210

1957

1/4

1/2

1/2

—4/4

Dist. shearing cu into ci; as-sc start 0342Z, from cu Large shearing cu Middle cl'd from cu

58

Results of Flight I

TABLE

3.4

HEIGHTS OF CLOUDS, FLIGHT 1, LEG 2 ; FLIGHT ALTITUDE 8 , 0 0 0 FT, JULY 11-12, 1 9 5 7

Time (GCT)

Base (ft)

2210

M e a n tops (ft)

Max. tops (ft)

2000 18-2100 18-2200 1800 1800

14-2000

4500

5,600

0006 0020 0035

1700 1500 14-1700

5500 5000 4000

8,200 6,000

0055

13-1600

4500

6,300 9,300

1700

5000

10,500

0140 0155 0210 0227 0242

3300 5000 3000 2400 5500

6,000

38,000

4,200 3,200

trace trace

6,100

Sea clear Sea clear

15-1700 1300 1400 1400?

5000 7000 4000 5000?

—

4000?

0355

1300?

20,000

26,000

20,000

26,000

7

trace 37,000 37-42,000 33,000 33-42,000 33-42,000? 33-42,000? 38,000? 38,000? ~ 39,000 —39,000

6,000?

0335

3500

19,000

8,500 8,300

2200 3500 5500

1500? 14-1700

41,000 41,000 41,000 —36,000 25,000 23,000

8,000 8,000

14-1700? 1700?

0406

trace trace

32,000

7,000 3,000 4,600 19,000

0255 0313

High cloud (ft)

N o cu nearby

2225 2240 2255 2308 2310 2320 2335 2350

0110 0122

Middle cloud (ft)

14,000 19,000 14,000 19,000

Remarks Distant cb's a n d ci trace 3 2 , 0 0 0 cb at 77 miles M o r e cu nearby now Base variable Clear areas, tiny c u Cu tops shear f r o m N E Enter cl'dless area Cu base variable Ci lower Ci lower again V e r y thin st. at — 0 0 4 5 Z with large holes Ci thicker on horizon Ci changing overhead Several layers ci Ci overhead; horizon bad Few sm. cu foreground Base variable? N o horizon; hazy N o horizon; hazy 3 6 , 0 0 0 f t cu at 100 miles; shear t o N E

38-40,000

T w o layers middle cl'd f r o m cu

38-40,000

T w o layers middle cl'd f r o m cu

Results of Flight I

TABLE

59

3.5

CLOUDINESS, FLIGHT I, L E G 3 ; G U A M - M A N I L A , J U L Y 1 2 - 1 3 , 1 9 5 7

Time (GCT) 2130 2148 2205 2220 2240 2342 2357 0010 0015 0030 0040 0100 0119 0133 0149 0204 0219 0235 0256 0311 0328 0343 0400 0425

Cumuliform Type; amount

O

A A a

A A

A A A A A A A A A A A A A A

Middle, 10,000-20,000 ft

3/4 -4/4 3/4

a a a a a a a a a a a a a a a a a a

3/4 3/4 1/2 3/4 3/4 3/4 3/4 3/4 3/4 3/4 1/2 3/4 3/4 3/4 3/4 3/4 3/4 1/2 1/2

éè + Socked in + +

+ + + +

+ + + + + + +

A A A

éS éó

tìi A

A A

A A A A

A A A

High, > 2 0 , 0 0 0 ft 3/4 7 7 7 7

3/4 3/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4 3 / 4 -!3/4 +

Camera off 7 7 7 ? 7 7 7 7 7 7 7 7 7 7 7 7 7 --

1/2 + 1/2

Remarks N o cu; piane climbing Two layers st. Cu begin 2155Z Some cu in st. deck Very black rainband Tremendous shear on cu Shear cb from SW

Socked-in off and on Little clearer now

Socked 0135-0149Z Clearer area Mostly socked in Mostly socked in Mammatus above Socked in 0320Z Breaks in st. Landfall; clearing Land; sun through breaks

TABLE

3.6

C L O U D C O D E , O R G A N I Z A T I O N , AND P R E C I P I T A T I O N ; F L I G H T I , L E G 1, J U L Y 1 0 - 1 1 ,

1957

(Camera aimed south)

Time (GCT)

Cloud code

Organization code

2145 2200 2210 2230 2245 2304 2308 2326 2341 2355 0010 0025 0040 0055 0110 0130 0145 0155 0210 0225 0242 0255 0310 0325 0340 0355 0414 0430 0443 0500 0515 0531 0546 0601 0620

2c lc 1c 0 3i 3i 3i 3 4 4 4 4 4 4s 4s 4 4 4d 4d 4d 4d 4d 4ds 4ds 4ds 8o 8so 9 9 9o 9o 8o 6 3d 3i

M M M 1 1 1 1 1 11 1 1 11 11 111 111 111 11 1 1 1 1 1 1 1 1 11 11 11 11 11 1 M 11 11 M 111 X X

Directions of rows ( ° from N ) — — — —

070 070 070 070 070 070 070 075 075 080 082 082 082 085 095 091 090 095 100 100 100 095 105 105 105 110 ? Bend 010 — —

Precip. (seen on film) No? No No No No No No No No No No No No No No No No No No No No No No No No No Yes Yes Yes Yes Yes Yes Yes No No

Remarks Inversion cl'd; obscures cu Inversion cl'd; obscures cu Inversion cl'd; obscures cu Too sparse to measure

Cross-wind org. begins Cross-wind org. Cross-wind org. Cross-wind org. Trough line; rows bend Large-scale org. imposed No org. No org.

Results of Flight I

61

TABLE 3.8 C L O U D CODE, ORGANIZATION, AND PRECIPITATION; F L I G H T I , LEG 2 , JULY 1 1 - 1 2 ,

1957

(Camera aimed south)

Time (GCT)

Cloud code

Organization code

Direction of rows from N )

Precip. (seen on

2210

6

1

106

No

2223

6

1

106

No

2240 2255 2308 2310 2320 2335

3d 3d Od 3 77-

1 1 1 1 1 M

106 106 106 106 106

No No No No No No

2350 0006 0020 0035 0055 0110 0122 0140 0155 0210 0227 0242 0255 0313 0335 0355

7 5 5 5o

8o 9o

M 1 1 11 1 M X or 1 X 1 11 1 X 1 11 11 11

0406

9o

11

lo

5 lo

5 5o 5o lo lo1 1

090 108 -—108 — 108 108 108 108 108

No No No No No No No No No No No No No No No Yes

108

Yes

090 125 120 095 070

film)

Remarks

Larger-scale org; no sm. cu seen Larger-scale org; no sm. cu seen

Clear space 168 km across; no cu

Org. weak and confused Org. weak and confused N o org. Very weak org. Better org. Poor org.; hard to measure Nearly random Better org. Better org. Cross-wind org. Blobs 100 km apart Cross-wind org. Blobs 100 km apart

62

Results of Flight I

TABLE

3.9

C L O U D C O D E , O R G A N I Z A T I O N , AND P R E C I P I T A T I O N ; F L I G H T I , L E G 3 , J U L Y 1 2 - 1 3 ,

1957

(Camera aimed south)

Direction of rows ( ° from N )

Time (GCT)

Cloud code

Organization code

Precip. (seen on film)

2130 2148 2205

16 15 15

M 111 111

M 125 130

No No No

2220

15

111

125

Yes

2240

16

2342 2357 0010

15 15 14

0015

14

1 1 1

196

Yes

0030

14

11 1

196

Yes

0045

15

111

196

Yes

0100

15

111

196

Yes

0119

15

11 1

—190

Yes

0133

15

1 11

—190

Yes

0149 0204 0219 0235 0256 0311

15 15 15 15 15 15

11 1 111 111 111 111 111

Indet. ~010 ~-010 ~010 ~010 ~0I0

Yes Yes Yes Yes No No

M Yes M Camera off, owing to black, socked-in conditions M Yes M Yes M M Yes 111 196

Remarks N o cu Lines of big cu interspersed with small Lines of big cu interspersed with small Socked in Socked in Socked in Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart Lines normal to flight path about 50 miles apart T o o dark

Land; lines still normal to flight path

A Results of Flight II

The second Pacific photographic flight was in many ways the most successful and interesting of the series. Its track and timing are shown in Table 2.1 and Figure 2.2. The entire circuit of the area, from Hawaii to the Marshalls to Guam and return via Wake Island was accomplished in just 75 hours, with the four legs succeeding one another during the daylight hours of four successive days (July 24-28, 1957). Some 2,400 feet of time-lapse motion pictures resulted from this circuit. A fascinating sequence of situations was encountered, which gains further informative value in comparison with those of the other two flights of the series. The first leg, from Hawaii to the Marshall Islands (Kwajalein), passed through a nearly undisturbed trade regime, with a minimal wave disturbance, to be contrasted with the

moderate wave on the same leg of Flight I. The second leg (Kwajalein to G u a m ) skirted the northern edge of the equatorial trough and passed through two mildly disturbed zones which offered intriguing cloud patterns for quantitative mapping. The two return legs, made farther north (from Guam to Wake and Hawaii) showed remarkable suppression for the tropical oceans. Particularly outstanding was a clear region about 400 miles in extent near the end of Leg 4; during much of it not a cloud was visible in the sky from horizon to horizon. This situation forms a particularly spectacular contrast with the same legs of Flight III (three weeks later), during which rampant cumulonimbus and storm breeding were at work in the same location. On the whole, Flight II offered excellent opportunity for

64

Results of Flight II

pursuit of the important questions raised b y the data of Flight I. I n particular, the d a t a emphasize the concentration of large towers and precipitation into convergent zones, the sudden spatial changes in cloud regime, and, especially, the organization of individual clouds into lines and patterns. It becomes possible f r o m these observations to suggest criteria for both the existence and the orientation of these formations.

1.

SYNOPTIC STUDY AND O V E R - A L L

CLOUD

SURVEY

T h e four legs of this flight were centered in time close to O O Z on July 25, 26, 27, and 28, 1957; thus t h e corresponding synoptic charts and soundings were selected f o r reproduction although earlier, later, and intermediate analyses were carried out for understanding and continuity.

Fig. 4.1.-Surface chart, July 25, 1957, OOZ. Labels on isobars at right are pressures in mb above 1,000 mb; at left lines are labeled in feet as contours of the 1,000-mb surface. Where two wind arrows appear at a station, the dashed arrow is the 2,000-ft wind. The heavy line is the weak easterly wave trough. Underlay is sky code.

Results of Flight II

Leg 1: Honolulu - Kwajalein, July 24-25, 1957 The second round trip opened again on the Honolulu-Kwajalein route, but on this occasion the cloud forms encountered were characteristic of the nearly undisturbed trades. Except for small segments of cumulus activity somewhat above average, only trade-wind skies were coded. By and large, the cloud thickness kept increasing as the aircraft passed toward the equatorial margin of the trades. The cloud distributions in Figures 4.1-4.5 and 4.38 (cross section discussed in section 2)

65

may thus serve as a model of what happens in the undisturbed trades. Moreover, since the low-level streamlines (Fig. 4.2) roughly paralleled the course of the aircraft almost to the end of the area where photography was possible, these figures also describe the cloud distribution along a streamline. However, we cannot infer the cloudiness along an air trajectory, since in view of the prevailing windspeed of 15 knots in the trades, the streamline field would have had to remain steady for a period of 6 days in order to serve as a trajectory indicator.

Fig. 4.2.-Mean winds (knots) of lower layer with approximately uniform wind, July 25, 1957, 00Z. Circled figure is height of top of layer (1,000's of feet). Numbers are coded DDFF for layer at each station, with direction (DD) in 10's of degrees and speeds (FF) in knots. Isopleths are streamlines. Underlay is sky code.

66

Res

The surface map (Fig. 4.1) gave weak indication of cyclonic curvature of isobars between Honolulu and Johnston. A corresponding bend could have been entered also in Figure 4.2, but it was omitted owing to lack of direct evidence. The wave trough indicated by the heavy line in Figure 4.1 was located primarily on the basis of cloud evidence and is left off the other charts because of its poor development. In the process of cloud coding, we found a sharp break in sky type between the two prints separated by the heavy line in Figure 4.1. The later, more detailed cloud analysis confirmed the existence of a sharp transition there between deeper and more shallow convective layers, and the organization study (see Fig. 4.42)

of Flight n

suggested a veering of the low-level flow in the weak convective zone just east of the trough line, with organization vanishing directly the trough is crossed. Comparison of Figures 4.1 and 4.2 brings out the pronounced crossing of the streamlines toward lower pressure over almost the entire route. The vertically integrated moisture pattern (Fig. 4.3) correlated well with the upper tropospheric flow pattern. There was low precipitable moisture content and upper cyclonic flow over the first part of the flight leg, and upper tropospheric ridging and corresponding high precipitable moisture over the second half of the flight leg. This moisture pattern was almost identical

Fig. 4.3.-Total precipitable moisture (gm or cm per cm 2 ) for July 25 1957, 00Z. Underlay is sky code.

Results of Flight II

with the moisture pattern of the day before and the day after, indicating quasi-conservatism of this feature. Actually, there was little change in the total precipitable water content for the five days from July 24 through the 28th for the whole tropical Pacific from Hawaii to the Phillipines. The equatorward increase of total precipitable water shown in Figure 4.3 is to be expected under undisturbed trade conditions. None of the stations along the route reported the trade inversion, but deep stable layers with rapid drying were ob-

67

served at Lihue (Fig. 4.6) and were more marked at Johnston (Fig. 4.7), indicating subsidence southwest of the weak disturbance drawn on Figure 4.1. The total water content doubled from Johnston to Kwajalein, and at the end of the daylight hours, cirrus began to be observed as the aircraft passed under an area of anticyclonic flow in the high troposphere (compare Figs. 4.5 and 4.38). Presumably increasing build-ups would have been recorded on the last 400 miles of the flight had photography still been possible; in fact, a very sudden transition

Fig. 4.4.-Shear winds from top of trade-wind layer to upper troposphere, July 25, 1957, 00Z; base and top of layer shown (in 1,000's of feet) by figures followed by M. Coded figures are direction (10's of degrees) and magnitude (knots) of shear vector. Isopleths are streamlines of shear vector. Underlay is sky code.

68

Results of Flight II

from code number 4 — to code number 8 occurred on the last print of the leg (Fig. 4 . 5 ) . With the increasing moisture, the vertical gradients of Q decreased markedly (Fig. 4 . 9 ) . The depths of the trade-wind layer increased along with the moist layer to attain a thickness of at least 16,000 ft at Kwajalein, compared with about 8 , 0 0 0 ft in the northeast trades (cf. Figs. 4 . 6 - 4 . 8 ) . The upper shear (Fig. 4 . 4 ) and high tropospheric flow charts (Fig. 4 . 5 ) provide the most interesting information. North of Hawaii and northeast of the Marianas, pronounced shears of the cyclonic turning led to the cyclonic circulations

noted in Figure 4.5. These were associated again with dry conditions in the lower troposphere (Fig. 4 . 3 ) . Hilo and Johnston had more complicated shear patterns; the succession of high-level data at Johnston, entered in Figure 4.5, suggested southward travel of a small, high tropospheric cyclone across that station. If the solution drawn is correct, the disturbance seen in Figure 4.1 northeast of Johnston would be a weak low-level system associated with the upper cyclone. Its position well to the east of the upper low would be typical of what is frequently observed. The weakness of the cloud system, can, of course, be ascribed to the stable and dry conditions prevailing

Fig. 4.5.-Mean winds and altimeter corrections ( D ) with respect to mean tropical atmosphere for layer 250-150-mb, July 25, 1957, 00Z. D value (ft) symbol above station; thickness of layer (10's of feet, first digit omitted) below stations. Underlay is sky code.

Results of Flight II in the area. N o precipitation could be discerned f r o m the films of this leg. Intense surface convergence lasting for at least one day would be needed before the m e a n structure of the lower troposphere could be changed appreciably a n d useful w o r k gained f r o m the release of latent heat. With an initial deep T

V

(knots)

°C

69

moist layer, the situation near Hawaii might well have led t o intense weather bands. T h e most obvious contrast between this leg a n d the corresponding one of Flight I was the absence here of the moderatestrength easterly wave and its attendant huge towers, middle and high cloud, and precipitating convective zone. T h e weak disturbance on this leg can be plainly seen not to be a true Riehl-type easterly wave merely from its absence of high towers a n d precipitation; it bears a quite different relationship

da (degrees!

Fig. 4.6. Tephigram and vertical wind plot at Lihue (Hawaii), July 25, 1957, 00Z. Tephigram (above) gives dew-point curve on left with mixing ratios entered in gm/kgm.

Fig. 4.7.-Tephigram and vertical wind plot at Johnston Island, July 25, 1957, OOZ. Tephigram (above) gives dew-point curve on left with mixing ratios entered in gm/kgm.

70

Results of Flight II

to a quite different upper flow configuration than did the classical wave of Flight I. In fact, a major conclusion of this whole study is the vital role played by the interaction of upper and lower flow patterns in controlling tropical clouds and precipitation. Leg 2: Kwajalein

- Guam,

July 25-26,

1957

The cloud pattern on this leg was among the most spectacular

Majuro

of the series, and provided opportunity for most of the mapping and organization study described later in section 3. This was the only leg of Flight II in which code numbers beyond 8 and 9 (weak or trade-wind type disturbance) were recorded; it also contained 77 per cent of Flight II's print-intervals with precipitation. As shown in Figures 4.10-4.14 and 4.39, the towering clouds were grouped into two disturbed zones, separated by a relatively suppressed interval of about 240 miles.

Sounding

Fig. 4.8.-Tephigram (Majuro) and vertical wind plot (Kwajalein), July 25, 1957, 00Z. Tephigram (above) gives dew-point curve on left with mixing ratios entered in gm/kgm.

80

82

84

86

88

90

Fig. 4.9.-Graphs of total heat content Q = CrT + Lq + gz for the soundings of Figs. 4.6-4.8.

Results of Flight II T h e first convective z o n e e n c o u n t e r e d contained all the cumulonimbus coded o n the circuit, and w a s apparently associated with an easterly wave d i s t u r b a n c e moving at 8 - 1 0 knots. Some evidence f o r the t r o u g h ' s existence was f o u n d on the previous day's synoptic charts, but n o n e o n the following day's; it probably dissipated by passage b e n e a t h the upper tropospheric, nearly stationary cyclone between Eniwetok and G u a m . Owing to weak evidence of its development in the flow field it has not been entered o n all t h e synoptic charts, although its

71

effect u p o n the cloud configuration was most noteworthy, as will be seen in sections 2 and 3. T h e second convective zone contained beautifully organized and sheared rows of towers which marginally m a d e the c u m u lonimbus category. Its termination has been entered as a solid line on Figure 4.10 only. T h e evidence suggests that it is a reflection of the upper trough directly above it, and is not in any sense a progressive easterly wave disturbance. T h e moisture pattern was fairly representative of the cloud

Fig. 4.10.-Surface chart, July 26, 1957, OOZ. Labels on isobars at right are pressures in mb above 1,000 mb; at left lines are labeled in feet as contours of the 1,000-mb surface. The heavy lines relate to the disturbances discussed in the text. Underlay is sky code.

Fig. 4.11.-Mean winds (knots) of lower layer with approximately uniform wind, July 26, 1957, 00Z. Circled figure is height of top of layer (1,000's of f e e t ) . Numbers are coded D D F F for layer at each station, with direction ( D D ) in 10's of degrees and speeds ( F F ) in knots. Isopleths are streamlines. Underlay is sky code.

Results of Flight 11

patterns that were observed. At Kwajalein (Fig. 4.15) there is almost total saturation up to 750 mb and then rapid drying above. The total moisture content is high (5.2 gm/cm 2 ). This would favor isolated cumulonimbi with little upper cloudiness, as observed. Eniwetok (Fig. 4.16) had less lower tropospheric moisture than Kwajalein, but more upper level moisture. This would agree with the greater amounts of upper clouds that were observed near the middle of the flight. Guam's sounding (Fig. 4.19) showed less lower and more upper moisture. This fits in with the independent cirrus layers and relatively suppressed cumuli on the latter part of the flight leg

73

(code numbers 5 and 7 ) , also emphasized later on the cross section (Fig. 4.39). The total moisture chart (Fig. 4.12), drawn with the aid of continuity, is rather unsatisfactory in that gradients between stations over most of the Pacific were no larger than the 12hour fluctuations at the radiosonde stations. Nevertheless, the moisture pattern of Figure 4.12 corresponded well with the upper tropospheric flow. The highest values of moisture occurred along the first part of the flight leg, where upper level anticyclonic flow prevailed. The moisture values then gradually decreased and an area of lower moisture was present along

1957, 00Z. Underlay is sky code.

74

Results of Flight II

the second half of the flight, under upper cyclonic flow. A m i n i m u m could easily have been centered u n d e r the upper level low, although the synoptic data are m u c h t o o sparse here t o determine details. T h e lowest values of moisture were all above 4 g m / c m 2 , however, and the cloud patterns showed few large areas with very suppressed conditions. It is important to point out that the gradients of moisture were much less than those in the cloudiness, and the sharp transitions in the

cloud patterns must be studied in their relationship to the dynamic effects of the synoptic systems. The upper tropospheric flow pattern was a stable f e a t u r e ; it showed little change between the 24th and 28th of July. T h e high-level cyclone between Eniwetok and G u a m showed very slow westward movement between the 25th and t h e 27th. T h e moisture patterns, associating themselves with the upper tropospheric flow, also showed only small changes during this

Fig. 4.13. Shear winds from top of trade-wind layer to upper troposphere, July 26, 1957, 00Z; base and top of layer shown (in 1,000's of feet) by figures followed by M. Coded figures are direction (10's of degrees) and magnitude (knots) of shear vector. Isopleths are streamlines of shear vector. Underlay is sky code.

Results of Flight II period. Nevertheless, there was a trend. It is noteworthy, relative to Figure 4.12, that total moisture gradients h a v e weakened from the previous day. This m e a n s that the u p p e r cold low dominating the area of the route (Fig. 4 . 1 4 ) was becoming more moist. As an interesting corollary, cyclonic u p p e r shears (Fig. 4 . 1 3 ) were noticeably w e a k e r t h a n on the preceding day ( a n d stronger than on the next d a y ) , suggesting that release of latent heat of condensation was altering the structure of the upper disturbance.

75

A deep easterly current extended f r o m the surface t o 14,0002 0 , 0 0 0 ft along the entire flight leg. This current practically paralleled the p a t h of the aircraft. Over the first half of t h e flight leg, this deep easterly flow averaged 16-18 knots; over the second half it averaged 10-12 knots. There was a distinct trough in the shearing pattern of the wind f r o m the t o p of t h e easterly current to the upper troposphere. T h e wind shear w a s f r o m the southwest during the first half of the leg, veering to become northwest and north on the last third.

Fig. 4.14.-Mean winds and altimeter corrections ( D ) with respect to mean tropical atmosphere for layer 250-150-mb, July 26, 1957, 00Z. D value ( f t ) symbol above station, thickness of layer (10's of feet, first digit omitted) below station. Underlay is sky code.

76

Results of Flight II

F r o m comparison of the sky code with the analysis of Figure 4.14, it is evident that the true cumulonimbi died away well to the east of the upper anticyclone under southwesterly flow aloft, that the clear area was also situated east of the upper center, and that the spectacular leaning clouds (coded 6 and 9 o ) filled the center and the area just to its west, as analyzed. The cloud shear shown later on the cross section of Figure 4.39, however, suggests that the low center may have been

somewhat farther to the west. For a full understanding of this case, detailed calculations following the upper low would be needed, but in view of the widely spread data there is little hope that this can be accomplished. There is also no obvious explanation for the cloudiness seen on the southern horizon for long stretches, indicated by the frequent presence of the sky code postscript " d " . Possibly the whole area, to the north and south, had occasional deep T °C

•'

_40

130

O

-JO

to

¿Q

24 22 20 -

ie --

o

5

10 15 20 2 5 30 V (knots)

'•"•'< 060 080 dd

ol

I

l

*•'

J

(aegreesj

Fig. 4 . 1 5 . - T e p h i g r a m a n d vertical w i n d plot at K w a j a l e i n , J u l y 26, 1957, 0 0 Z . T e p h i g r a m ( a b o v e ) gives d e w - p o i n t c u r v e on left with mixing ratios e n t e r e d in g m / k g m .

Fig. 4 . 1 6 . - T e p h i g r a m a n d vertical wind plot at E n i w e t o k , J u l y 26, 1957, OOZ. T e p h i g r a m ( a b o v e ) gives dew-point curve o n left with mixing ratios e n t e r e d in g m / k g m .

Results of Flight II

convective bands lying parallel to the wind. More likely, an equatorial trough was attempting to form south of the route and the attendant convergence was reflected in the sky picture. Larger-scale cloud organizations, which cannot be encompassed in 100-mile wide strips such as ours, may be sought in the satellite photographs; it is hoped that enough meteorological data will be simultaneously available to relate these to the flow patterns. T -50

-40

-30

-20

-

/

-10

0

10

/ \ /

II

T -50

-40

-30

-20

>C -10

0

10

20

20

30

\

j

>

V Unots)

Figures 4.15-4.19 contain a full set of soundings along the route. At Kwajalein there was distinct evidence of subsidence coupled with the wind sounding, which looks very unreliable. At Eniwetok, Ponape, and Guam evidence of current or previous subsidence exists for a shallow layer centered variably from 700 to 500 mb, with higher moisture above and below.

°C

•/

/

77

1 1 1 1 1 / 1

1

dd (iiflreiil

Fig. 4 . 1 7 , - T e p h i g r a m a n d vertical wind plot at P o n a p e , J u l y 2 6 , 1957, OOZ. T e p h i g r a m ( a b o v e ) gives dew-point c u r v e o n l e f t w i t h mixing ratios e n t e r e d in g m / k g m .

5

ie 20 2t V I Knols I

040 0€C 080 100 120 I4C 160 .00 dd [degrees!

Fig. 4 . 1 8 . - T e p h i g r a m a n d vertical wind plot at T r u k , J u l y 26, 1957, OOZ. T e p h i g r a m ( a b o v e ) gives d e w - p o i n t curve on left w i t h m i x ing ratios entered in g m / k g m .

78

Results of Flight II

T h e moisture pattern varies in the opposite sense to the cloud difference, with higher moisture on Flight I everywhere along the leg except at G u a m , which was much drier. N o evidence of build-ups to the south was seen on Flight I, despite greater moisture at the south-lying stations of P o n a p e and T r u k and far less pronounced Q minima. This comparison again serves to emphasize the importance of dynamic factors in tropical cloud development, and it points to the existence of critical interactions between upper-level synoptic scale patterns and low-level meso-scale patterns, which, although not resolvable with the present data network, are reémphasized by the cross sections to come in section 2. Leg

5 D i i5 20 j (ki ots ,

020 040 060 060 100 120 140 160 d d 1degrees )

Fig. 4.19.-Tephigram and vertical wind plot at Guam, July 26, 1957, 00Z. Tephigram (above) gives dew-point curve on left with mixing ratios entered in gm/kgm. Very low values of Q were observed (Fig. 4 . 2 0 ) , considering the area, season, and cloud developments. C o m p a r e d with the same leg for Flight I, two weeks previous, the over-all upper flow and synoptic structure are not too different, except that in the earlier leg the easterly wave at the start of the flight is almost entirely defunct and the upper low is farther to the n o r t h — f a c t o r s perhaps related to the much more vigorous cloud development on the present flight.

3: Guam

- Wake,

July

26-27,

1957

T h e mildly disturbed area flown through on the day before did not extend to the present route, but was visible for a long distance o n the southern horizon (codc postscripts " d " ) . Marked subsidence and trade-wind skies alternated along the flight path, which passed through clockwise curving flow in the low levels (Figs. 4 . 2 1 - 4 . 2 2 ) just south of the center of a weak subtropical anticyclone. Only one zone of markedly above-average convective cloudiness was encountered en route (code n u m b e r s 8 at about longitude 155° E ) where occasional strongly sheared towers up to 3 0 , 0 0 0 ft were found, together with a trace of high cirrus which may have been leftover anvils. This frail activity could have been the northerly r e m n a n t of the second (western) convective zone encountered the previous day, provided that it was nearly stationary and slanted eastward with increasing latitude. T h e lower trade-wind layer was deep ( 2 0 , 0 0 0 ft or m o r e ) and the mean direction of this layer shifted f r o m southeast through cast to northeast on the flight. As we shall demonstrate quantitatively in section 3, the orientation of cloud lines underwent a similar turning. T h e total moisture chart (Fig. 4 . 2 3 ) reveals an area of minimum moisture along the path, even though the moisture depression compared to the surroundings was small. This low moisture was consistent with the clouds, which were sparse a n d suppressed for the region and season. At G u a m , the total precipitable moisture was only 63 per cent confined below 7 0 0 mb,

Results of Flight II

79

Figs. 4.15-4.19 (July 26, 1957, 00Z).

80

Results of night II

Fig. 4.21.-Surface chart, July 27, 1957, 00Z. Labels on isobars at right are pressures in mb above 1,000 mb; at left lines are labeled in feet as contours of the 1,000-mb surface. Where two wind arrows appear at a station, the dashed arrow is the 2,000-ft wind. Underlay is sky code.

Results of Flight II

81

Fig. 4.22. Mean winds (knots) of lower layer with approximately uniform wind, July 27, 1957, 00Z. Circled figure is height of top of layer (1,000's of feet). Numbers are coded D D F F for layer at each station, with direction ( D D ) in 10's of degrees and speeds ( F F ) in knots. Isopleths are streamlines. Underlay is sky code.

82

Results of Flight II

Fig. 4 . 2 3 . Total precipitable moisture (gm or cm per c m - ) for July 27, 1957, 00Z. Underlay is sky code.

Remits of Flight II

83

Fig. 4.24. Shear winds from top of trade-wind layer to upper troposphere, July 27, 1957, 00Z; base and top of layer shown (in 1,000's of feet) by figures followed by M. Coded figures are direction (10's of degrees) and magnitude (knots) of shear vector. Isopleths are streamlines of shear vector. Underlay is sky code.

84

Results of Flight II

Fig. 4 . 2 5 . - M e a n winds a n d altimeter corrections ( D ) with respect to m e a n tropical atmosphere f o r layer 250-150-mb, July 27, 1957, 00Z. D value ( f t ) symbol above station; thickness of layer (10's of feet, first digit omitted) below station. Underlay is sky code.

Results of Flight II

^

s

85

1

i

*

i

i

^

i

i

v

i

1 y

1

S

„

(:

/ ;

\

•

i

A

\ '

i

\

.

:

: \ -

l

\


J-

5 10 15

20

/ £ knots)

25

\ •

10

i5

I knots]

20

050 032 034 036 020 040 060 060 ICO 120 140 160 80 200 ad (degrees I

080 100 120 140 do

(degrees)

Fig. 4 . 2 6 . - T e p h i g r a m and vertical wind plot at G u a m , J u l y 2 7 , 1957, 00Z. Tephigram ( a b o v e ) gives dew-point curve on left with mixing ratios entered in gm/kgm.

Fig. 4 . 2 7 . - T e p h i g r a m and vertical wind plot at W a k e Island, J u l y 2 7 , 1957, 00Z. Tephigram ( a b o v e ) gives dew-point curve on left with mixing ratios entered in gm/kgm.

/

86

Results of Flight II

Fig. 4.28. Graphs of total heat content Q = C,T + Lq + gz for the soundings of Figs. 4.26-4.27.

Results of Flight II and the u p p e r troposphere was h u m i d , in agreement with the very p o o r cumulus development with good u p p e r clouds ( c o d e 7; see also cross section, Fig. 4 . 4 0 ) . A t Wake, t h e total moisture was 87 p e r cent concentrated below 7 0 0 mb, with a b o v e - n o r m a l cumulus and n o u p p e r clouds at all. As previously f o u n d , the total moisture configuration a n d its height distribution bore a clear relation to the u p p e r t r o p o spheric flow pattern, which h a d undergone a r e m a r k a b l e change since the day before. A s already noted on Leg 2, the u p p e r cyclone was weakening and drifting south and west. This development is clearly brought out by Figures 4 . 2 4 and 4.25, which show that the big upper trough was narrowing to a shear line, with cutting-off of the u p p e r low center in the southern portion of the trough. This sequence is quite similar to w h a t has been n o t e d in the middle latitude westerlies w h e n a t r o u g h is eliminated f r o m the circumpolar flow. T h e u p p e r shear line was crossed by the aircraft about 2 0 0 miles out of G u a m , with a change in cloud type f r o m code n u m b e r 7 to n u m b e r s 3 and 4. It is not possible f r o m Figure 4 . 2 5 to resolve the question of whether the high cumuli coded as 8 were indeed a n o r t h ward extension or remnant of the disturbance crossed the day before, although there is no other obvious reason for their existence to be found on any of the charts. T h a t their tops sheared f r o m southwest, however, suggests that the cyclonic center analyzed on Figure 4.24 should have been d r a w n as a much n a r r o w e r ellipse with its axis f a r t h e r west. T h e t h e r m o d y n a m i c and wind soundings (Figs. 4 . 2 6 - 4 . 2 8 ) have the structure that could be expected.

Leg 4: Wake - Honolulu, July 27-28,

1957

O n this last leg of the second round trip the aircraft passed at first along the southern border of a weak cyclone (Figs. 4 . 2 9 - 4 . 3 0 ) that had been forming slowly over the preceding two days in a nearly stationary position. Some high congestus and considerable organization of the cumuli were in evidence just to the south and east of the center. T h e remaining 25 per cent of Flight l i s precipitation occurred here, in the first 150 miles out of W a k e . This section has been coded as 8; although Figure 4 . 4 1 gives the highest cloud top as 4 2 , 0 0 0 ft, one giant c u m u -

87

lonimbus alone was seen and most cumuli did not exceed 16,000 ft elevation, so that 8 is the appropriate code number. A f t e r this, cloudiness decreased markedly and for the remainder of the flight leg was phenomenally suppressed, culminating in the spectacular, nearly 400-mile wide, clear stretch extending eastward f r o m longitude 170° W , until stunted trade cumuli set in again about 3 0 0 miles out of Hawaii. F r o m the end of the precipitation zone o n w a r d , the flight was in a b r a n c h of the trades that was strongly curving clockwise (Figs. 4 . 2 9 - 4 . 3 0 ) . Evidently, the heat exchange between sea and air in such a case does not lead to the type of downstream variation of cloudiness f o u n d in earlier studies of the Pacific northeast trade [ 1 ] . Subsidence plus turning of the wind to a southeasterly direction, that is, in the direction along the ocean isotherms or t o w a r d colder ocean temperatures, prevented development of convective cloudiness with u p w a r d heat transport. T h e detailed structure of such a situation in the western Atlantic trade has been described by M a l k u s [ 2 ] , T h e total moisture pattern (Fig. 4 . 3 1 ) c o n f o r m s to the deductions a b o u t ascent along the extreme western part of the route and descent along the central and eastern parts. Values of moisture above 5 g m / c m - occurred n e a r Wake, dropping off to 2.5 g m / c m - bracketing the clear zone, and then rising again to 3.5 g m / c m - n e a r Hawaii, in excellent agreement with the cloud p a t t e r n . Owing to the presence of the surface low, there is no trade layer at W a k e , b u t instead a layer of fairly uniform northwesterlies u p to 9 , 0 0 0 ft (Fig. 4 . 3 4 ) . In Figures 4 . 3 5 and 4 . 3 6 the depth of t r a d e layer is shown to be about 9 , 0 0 0 ft at Johnston and Lihue; however, there is evidence ( f r o m the aircraft crew's r e p o r t ) that it w a s considerably shallower t h a n 9 , 0 0 0 ft in and just beyond the clear zone. T h e importance of this to the orientation of c u m u l u s lines will be raised in section 3. T h e u p p e r shear was mostly westerly (Fig. 4 . 3 2 ) , indicating that the low pressure center was of the subtropical type, associated with the meridional temperature gradient and opening n o r t h w a r d into a trough at high levels but not at low levels. A short region of u p p e r shear from south to southeast is f o u n d on the east of t h e clear zone, which bears an interesting relation to the c u m u l u s organization to be discussed later.

88

Results of Flight II

Fig. 4.29.-Surface chart, July 28, 1957, 00Z. Labels on isobars at right are pressures in mb above 1,000 mb; at left lines are labeled in feet as contours of the 1,000-mb surface. Where two wind arrows appear at a station, the dashed arrow is the 2,000-ft wind. Underlay is sky code.

Results of Flight II

89

Fig. 4.30.-Mean winds (knots) of lower layer with approximately uniform wind, July 28, 1957, 00Z. Circled figure is height of top of layer (1,000's of feet). Numbers are coded D D F F for layer at each station, with direction ( D D ) in 10's of degrees and speeds ( F F ) in knots. Isopleths are streamlines. Underlay is sky code.

90

Results of Flight II

Fig. 4.31. Total precipitable moisture (gm or cm per cm-) 28, 1957, 00Z. Underlay is sky code.

for July

Results of Flight II

91

Fig. 4.32.-Shear winds from top of trade-wind layer to upper troposphere, July 28, 1957, 00Z; base and top of layer shown (in 1,000's of feet) by figures followed by M. Coded figures are direction (10's of degrees) and magnitude (knots) of shear vector. Isopleths are streamlines of shear vector. Underlay is sky code.

»2

Result« of night II

Fig. 4 . 3 3 . - M e a n winds and altimeter corrections ( D ) with respect to mean tropical atmosphere for layer 250-150-mb, July 28, 1957, 00Z. D value ( f t ) symbol above station; thickness of layer (10's of feet, first digit omitted) below station. Underlay is sky code.

Results of Flight II

/

I

/

I

I

.

93

/

0 5 10 15 20 Fig. 4.34.—Tephigram and vertical wind plot at Wake Island, July 28, 1957, 00Z. Tephigram (above) gives dew-point curve on left with mixing ratios entered in g m / k g m .

•O 120 '40 0 0

(degrees)

Fig. 4.35. -Tephigram and vertical wind plot at Johnston Island, July 28, 1957, OOZ. Tephigram (above) gives dew-point curve on left with mixing ratios entered in g m / k g m .

94

Res

of Flight II T

The high-altitude contours (Fig. 4.33) can readily be drawn with change from cyclonic to anticyclonic curvature over the western part of the route. In the east, analysis was helped considerably by a moving weather ship. The upper cyclone previously found near Johnston apparently was still in the area. T h e region entirely free of clouds and filled with haze (code 0 ) was situated under and just to the east of this system. There are many ways to draw the contour curvature in the upper layer. If the curvature is drawn as in Figure 4.33, upper convergence is indicated along the flight path extending to the area just west of Hawaii.

-50

-40

T

-30

-20

1 — n

°C

-10

0

iO

I - ->" I

I -

20^

s

The available soundings (some reproduced in Figs. 4.344 . 3 6 ) showed basic correlation with the cloud picture. The Wake sounding possessed considerable moisture to very high altitudes on all soundings from 27 July 12Z to 28 July 12Z. The values were 5.9, 5.3, and 5.2 g m / c m - respectively. The 28 July 00Z sounding was conditionally and convectively unstable to the upper troposphere. These high moisture values correlate with the surface low just east of Wake and the upper tropospheric anticyclonic curvature in the vicinity. Johnston's three soundings of 27 July 12Z to 28 July 12Z showed very small moisture amounts of 2.8, 2.7, and 4.0 g m / c m - respectively. The 27 July 12Z sounding had a very strong inversion at 800 mb. By 28 July 00Z, this inversion had broken up into three smaller inversions at 670, 780, and 850 mb, with moisture amounts concentrating just below these. Motorboating moisture values occurred above 700 m b on all these soundings. Midway's moisture values dropped off in the three time intervals from 5.4 to 4.4 to 3.4 g m / c m - with the push into that area of the strong surface ridge from the east. The northeast-southwest extent of the ridge line gave clearing conditions sooner to the south of Midway at the flight times than at Midway itself. Lihue's soundings (see Fig. 4.36) were mostly of the typical trade-wind type, with unstable conditions in the lower atmosphere and a stronger-than-normal capping inversion layer above. Moisture values were 3.3 and 3.5 g m / c m - on the 28 July 00Z and 28 July 12Z observations. The clouds in the vicinity at flight time fit these categories of inversion domination and slightly more-suppressed-than-average trade regime.

Fig. 4 . 3 6 . - T e p h i g r a m a n d vertical wind plot at L i h u e ( H a w a i i ) , July 28, 1957, 0 0 Z . T e p h i g r a m ( a b o v e ) gives dew-point c u r v e on left with mixing ratios entered in g m / k g m .

Two radiosonde ships gave reports just north of the flight path at 28 July 00Z. The one at 2 4 ° N, 171° W ( 1 8 0 miles north of the flight leg) gave a value of 2.4 gm/cm 2 , with all moisture in the lowest 200 mb. The other ship 120 miles to the northeast of this one gave the much higher value of 3.8 gm/cm-, with its moisture extending to above 700 mb. This apparent concentration of low moisture amounts between Johnston Island and the ship at 2 4 ° N, 171° W fits quite well with

Results of Flight II

95

Fig. 4.37. - G r a p h s of total heat content Q = CrT + Lq + gz for the soundings of Figs. 4.34-4.36.

the upper tropospheric cyclone and with the northeast-southwest extent of the surface trough. Signs of inversion cloud were observed at or below flight level intermittently along the flight path, beginning at the end of the convection zone just beyond Wake. Such cloud was particularly thick and pronounced in the places coded 2 and 2c from 2010Z to just beyond 2110Z. Signs of inversion domination persisted through and beyond the clear zone but there was not enough moisture for thick inversion cloud. In the dry layers, values of Q sank almost to 76 cal/gm (Fig. 4.37). We suspect that the over-all cloud suppression along the last two-thirds of the flight leg was rather unusually pronounced for the summer trades. Four basic factors are believed to be responsible: 1) The surface streamlines diverge along most of the last two-thirds of the leg. 2) The intense surface ridge encountered halfway along the

flight caused suppression by dynamic principles. The trade cumulus weakened abruptly once the ridge was passed; note the rapid transition from code numbers 6 to 3— and 0 as the ridge is crossed in Figure 4.30. 3) The upper tropospheric flow pattern showed a trough in both the 250-150-mb thickness and in the mean "D" values about 600 miles east of Wake, and exhibited the upper low north of Johnston. These patterns cause upper-level convergence with consequent low-level divergence and descent throughout most of the troposphere. 4) Deficient moisture due to the above dynamic processes. There were no corresponding legs of Flight I with which to compare these last two of Flight II. However, identical legs of Flight III (flown in reverse direction three weeks later) will provide as extreme a contrast as the tropics can muster, with spectacular cumulonimbi towering into the same sky that was so devoid of convective display on this run.

96 2.

C L O U D CROSS

Results of Flight II

SECTIONS

T h e cloud structure along each of the four legs of Flight II is shown in m o r e detail in the cross sections (Figs. 4 . 3 8 - 4 . 4 1 ) . These sections have been constructed f r o m the data presented in Tables 4.1-4.8, two f o r each leg. T h e first table gives the type of cloud observed and the cloudiness, to the nearest oneq u a r t e r sky cover, f o r t h e cumulus category and also for high clouds. Averages for each interval centered at the time given ( s a m e times as the prints and the cloud code entries of the previous figures) were estimated f r o m the film. T h e r e was n o true middle cloudiness ( 1 0 , 0 0 0 - 2 0 , 0 0 0 f t ) seen on Flight II; inversion stratus and stratus-like shelves originating f r o m cumuli were c o m m o n l y below 10,000 ft and are entered in the " R e m a r k s " column where evident. T h e second table of the pair gives cloud heights: the cumulus bases, average cumulus tops, m a x i m u m cumulus tops, and the heights of any upper sheets present. Again, the level of inversion stratus has been entered in " R e m a r k s . " Cloud base could probably be determined to 10 per cent, except in those intervals where the aircraft was rocking badly or the horizon was indistinct. Several base heights were usually determined in each 15-minute interval, and if agreement was poorer than 10 per cent, the base was listed as "variable" with the range given. This occurred in only 4 out of the 106 periods composing this entire flight, and we may thus conclude that cumulus base was usually rather homogeneous over intervals of about 5 0 miles. This uniformity in cloud base is in good agreement with the rather stronger low-level wind speeds prevailing over the route of Flight II c o m p a r e d to Flight I, where cloud base was often m o r e variable at a given spot, suggesting a less well-mixed subcloud layer. In fact, the most variable cloud base on Flight II was observed just out of W a k e on Leg 4 (see table 4 . 8 and Fig. 4 . 4 1 ) , where the surface wind-speed was under 5 knots. W e have no wind evidence for the variable base period (roughly 0 2 1 2 - 0 3 1 7 Z ) of Leg 1 except f o r the rather unruffled appearance of the sea. T h e m a x i m u m cloud t o p was defined by the highest tower measurable in the interval, which frequently w a s rather far f r o m the camera and occasionally was achieved by only one indi-

vidual. F o r tops above 2 0 , 0 0 0 ft, accurate height determination could usually be m a d e to better t h a n 1,000 ft. In fact, the p r o b lem in constructing these cross sections lay m o r e in achieving representativeness than in accuracy, particularly with respect to the determination of the average cumulus top, which often had to be m o r e or less subjective. Sometimes the average size of cumuli varied f r o m the f o r e g r o u n d to the background of the picture, or withinin the interval, and occasionally no meaningful average existed. W e have attempted to represent the typical conditions, during each period, that prevailed within a b o u t 30-50 miles of the aircraft. O n c e a cloud had been chosen as "average," its t o p height could be f o u n d to about 10 per cent. T h a t this attempt had some usefulness is suggested by the logical sequence of heights in relation to the synoptic patterns and the generally good agreement of the height of average tops with the upper limit of the "moist l a y e r " when soundings were available. Clearly a large b o d y of very small cloudlets was ignored in this procedure. One of the most striking features of the cloud structure which shows u p on the cross section, but is even m o r e spectacular on the films themselves, is the occasional almost unbelievably abrupt change in regime. M a n y times all cloud p a r a m e t e r s ( m e a n and m a x i m u m tops, base, and cloudiness) altered entirely in a distance of travel of 5 miles or less. Available evidence suggests that these transitions are both too a b r u p t and of too large amplitude to be explained merely as results of changes in the thermal and moisture structure of the air; dynamic changes on this scale, particularly in the ambient convergence and vertical motion fields, are suggested and should be investigated in connection with cumulus growth. Leg

1, Flight

II

Tables 4.1 and 4.2 give cloudiness a n d cloud heights f o r this leg, respectively. T h e corresponding cross section (Fig. 4 . 3 8 ) is probably typical of the nearly undisturbed trade flow along a streamline in this region. The flight begins in a dry inversion-dominated regime, with streamers but no solid overcast of inversion stratus at 8 , 0 0 0 ft and almost no cumuli below. A f t e r a b o u t 2 4 0 miles we enter the convection zone of the weak trough disturbance denoted

Results of Flight I I

97 Pressure mb —200

Height in feet

Height in km

12 2

—

40,000

- II 6

-110 - 10 4 300 ~

9 8

- 92

30,000

- 85 79 - 400 - 73 -6.7 20.000

- 500

-

6.1

-55 49 600

43 37

10,000 8,000 6.000 4,000 2,000

oiaea Johntton

* j

ohxi

Fig. 4 . 3 8 . - S c h e m a t i c cloud cross section for Leg 1, Flight II, constructed from measurements on film (see also Tables 4.1 and 4 . 2 ) . Horizontal coordinate is time along flight leg, increasing toward left. Vertical coordinate is feet on left side, pressure in mb and height in km on right. L o w e r cloudiness (cumulus) amount indicated by bottom hatched-under curve; maximum amplitude here is 1/2. Upper cloudiness is indicated by black columns at top (trace, 1/4, 1/2 trace shown h e r e ) . Width of schematic cumuli is proportional to coverage not cloud size. Mean heights of cumuli shown by solid clouds; dashed cloud on top denotes m a x i m u m top in the (approximately) 15-minbase, ute interval. At dashed cloud indicates variable base. Inversion stratus is entered schematically where observed. Arrows on a cloud denote sheared tops (north upward ) . Two-digit figures entered above Honolulu, Johnston, and Majuro are relative humidities. The position of the weak wave trough of Fig. 4.1 is denoted by " T . "

98

Results of Flight II

in Figure 4 . 1 . Its cloud cross section is in m a r k e d contrast with that of the easterly wave encountered on Flight I, Leg 1. Not a trace of middle or u p p e r cloud m a r s the unusually deep blue of the sky aloft, and n o cumulus top exceeds 13,000 ft n o r produces any discernible shower. However, the enhancement and organization of convection relative to the previous and subsequent flight portions is obvious, as is the gradual rise of the inversion stratus to 9,400 ft. T h e cumuli are clumped into large, iceberg-like blobs, with rather wide clear spaces in between, and the blobs are elongated and lined u p parallel to the low-level flow. O n e of the sudden, spectacular changes in regime occurred between the prints at 0 1 0 6 Z and 0 1 2 3 Z (see Table 4 . 2 ) . This time average cumulus activity a n d 5 0 per cent cloudiness changed to highly suppressed and stunted cumuli filling less than 25 p e r cent of t h e sky. This change was, in fact, the primary basis f o r locating the heavy line in Figure 4.1. A n other sudden change f r o m above-average cumulus activity to none whatsoever ( c o d e n u m b e r 8o gives way t o 0 ) occurs just after 0 3 3 8 Z ; n o information whatsoever o n this is available f r o m the sparse synoptic data. A final, even more sudden change in the opposite sense, f r o m scantiness and suppression to plentiful and vigorous cumuli (code 8 ) , occurs on the last print of the flight, just before the camera h a d t o be shut off owing to darkness. H e r e we are already beneath the center of the u p p e r anticyclone and about 4 0 0 miles east-northeast of Kwajalein. It seems probable that this marks the beginning of the convection zone of a disturbance then a b o u t 100 miles west of Kwajalein which the flight encountered on Leg 2 the next day. T h e cirrus commencing at 0 4 2 4 Z , with stronger patches near the end of the photography, is in good agreement with the moister u p p e r levels as the downstream portion of the leg is reached. T h u s while it is true, as said previously, that by and large the convective cloud thickness increased on this leg from the polar t o the equatorial margin of the trade, we can see that actually this took place not gradually or uniformly but in a series of three jerks, with suppressed, almost clear regions intervening between regions of normal and above-normal cumuli. Unfortunately, the sounding and wind data are too sparse to

determine whether any variations o n this scale occurred in the moisture or any other p a r a m e t e r s . T h e cloud slants and shears were generally weak o n this leg, except in the two regions shown in Figure 4.38, where higherthan-average cumuli tilted fairly sharply b a c k w a r d , approximately along the flight path, near their tops and some had shelves protruding upstream. O n the whole, the cumuli showed normal forward slant at low levels, turning to slight back slant above. Beyond the clear spot at a b o u t 0 3 3 8 Z , the f o r w a r d slant extended higher, indicating a higher and m o r e pronounced tradewind maximum (contrast Figs. 4.7 and 4 . 8 ) . Leg

2, Flight

II

T h e cloud structure of Leg 2 was distinctive o n this circuit as being the most disturbed, the best organized, and containing the most upper cloud. O u r study so f a r suggests that these factors are generally correlated in the tropics. Tables 4.3 and 4 . 4 and the cross section (Fig. 4 . 3 9 ) show that the high towers ( > 3 0 , 0 0 0 f t ) were entirely confined t o the two disturbed zones described earlier. Most of the cirrus, of which this flight was never wholly free, were produced as anvils and streaming, sheared-ofT shelves by the cumuli, except in the last 300 miles where several apparently independent cirrus layers appeared and gradually filled the sky. T h e first disturbed zone, which the aircraft entered on takeoff from Kwajalein, contained the highest cumulonimbi. M a n y of these attained 4 0 , 0 0 0 ft and created a 5 0 per cent sky cover of anvil cirrus at this level. At first these high towers so d o m inated the scene that we have called t h e m the average clouds as well as the highest. Between them, the sky w a s clear, except for some precipitating stratus shelves protruding f r o m the c u m u lonimbus bodies. These terminated abruptly at 8 , 0 0 0 ft, where the relative humidity took a sharp d r o p f r o m 100 per cent, becoming 29 per cent at 10,000 ft (see also Fig. 4 . 1 5 ) . T h e aircraft was in and out of precipitation until 2 2 4 4 Z . At this time the orientation of the small cumuli backed to north of east (section 3 and Fig. 4 . 4 6 ) , so it has been denoted as the wave trough by a " T " on the section and by the heavy line on Figures 4.10-4.11. T h e transition to suppressed cloud conditions appeared as a relatively gradual one, however, with

Height f ee t

Pressure m b

in

-

Height ¡n km

200

40,000

- ¡2.2 11.6

-n.o 10.4 9.8

300

9.2 8.5 7.9 400

7.3 6.7

6.1

500

5.5 4.9 -

600

4.3 3.7

700

tOO

Flight

3. I g

Line

-

4

1.8

'00 - 850 1.2

0.6

2I22E Kwoj.

Fig. 4.39. - S c h e m a t i c cloud cross section for Leg. 2, Flight II, constructed from measurements on film (see also Tables 4.3 and 4.4). Notation same as Fig. 4.38 except now high cloudiness is shown by upper hatched-under curve; maximum amplitude is 3 / 4 + . Maximum amplitude of lower hatched-under curve (cumulus coverage) is 3/4. Precipitation is indicated as falling from the cloud base for each (approximately) 15-minute interval during which it was seen. " T " denotes the position of the easterly wave trough (first disturbance encountered out of Kwajalein). Relative humidities are shown above Kwajalein (slightly to right), Eniwetok, and Guam.

100

Results of Flight II

the congestus and anvils retreating s o u t h w a r d ; the latter were observable o n the horizon for nearly 3 0 0 miles farther. T h e succeeding clear space was not entirely cloud free, showing traces of stunted cumuli t h r o u g h o u t . It was interrupted by an interval of scattered congestus, and the 3 0 , 0 0 0 - f t cirrus, connected with the next disturbance, began to be observed aloft almost at its beginning.

have been drawn more elongated or f a r t h e r westward. N o significant cloud shears were seen in the last p o r t i o n except t h e c o m m o n forward lean of the trade cumuli u p to the wind m a x i m u m at about 6,000 ft and backslant above. This has n o t usually been noted on the sections, except where they were either very pronounced or unusual.

T h e second disturbance, extending f r o m 0 0 5 0 Z to 0 I 4 0 Z , was the photogenic zone of highly sheared and organized towers, m a p p e d and described f u r t h e r in the next section. Its tallest cumulus did not exceed 3 8 , 0 0 0 ft, and m o s t of the high towers terminated by 3 5 , 0 0 0 ft. T h e section shows several levels at which stratus shelves p r o t r u d e d (generally t o w a r d the east) f r o m the towers. T h e most spectacular, however, was at 3 0 , 0 0 0 ft, where several single "anvils" measured 5 0 miles in length. T h e end of this regime could be called infinitely abrupt, because, as we shall see, the convection zone consisted of four distinct cloud lines and no more. A f t e r this, the remainder of the flight was dull, with intervals of sparse congestus succeeding the suppressed region which directly followed t h e disturbance. T h e sky was gray, as m a n y interrupted cirrus layers gradually thickened to a near overcast. Again we see that although the over-all trend in cumulus structure w a s in agreement with the moisture trend, in this case a diminution f r o m Kwajalein to G u a m , the diminution took place in a series of overshooting oscillations along the path, which apparently were dynamically controlled.

Leg

T h e m a r k e d cloud shears on this leg were confined t o the regions of high and active cumuli, which penetrated into the shearing layer aloft. In the first convection zone, the cloud shear was measured to begin at about 2 4 , 0 0 0 ft. It was f r o m southwest, in good agreement with the upper shears and upper winds shown on Figures 4 . 1 3 - 4 . 1 4 . Directly after the wave trough, the congesti showed a shear f r o m east-southeast beginning at 8 , 0 0 0 ft. T h e wind at Eniwetok (Fig. 4 . 1 6 ) began to veer in this m a n n e r , but not until 14,000 ft. In the second disturbance, the cloud shears were indicated by the 30,000-ft anvils. T h e s e were mostly f r o m west-southwest becoming more nearly west at the last, in general agreement with Figures 4.13 and 4.14, but indicating p e r h a p s that the cyclonic center should

3, Flight

II

Tables 4.5 and 4.6 report the cloud structure for this leg. As illustrated f u r t h e r by the cross section (Fig. 4 . 4 0 ) , it w a s undistinguished, being unusually sparse and suppressed f o r t h e region and season. T h e only significant upper sky cover was left behind a b o u t 200 miles out of W a k e . Beneath this, the cumuli were so sparse and suppressed as to be almost nonexistent. At first, their bases were the highest in the entire circuit ( — 3 , 0 0 0 f t ) — a not u n common feature of suppressed zones. T h e transition to clear skies aloft was achieved by a slow southward retreat of the cirrus layers. Equally gradual w a s the strengthening of cumulus activity t o w a r d mid-flight, which the section has somewhat exaggerated. Only three or f o u r congestus towers exceeded 2 0 , 0 0 0 ft. H o w e v e r , their strongly sheared tops and general a p p e a r a n c e suggested a connection with the second disturbance of Leg 2, as did the scattered, u n connected anvil cirrus at 4 2 , 0 0 0 ft, which could either h a v e been left over in time or advected in aloft by the generally southerly flow at those levels (see Fig. 4 . 2 5 ) . T h e last half of the flight leg was cloudless aloft and suppressed below. Strong inversion domination with scattered pieces of inversion stratus below flight level succeeded the congestus, gradually rising to flight level ( 9 , 0 0 0 f t ) and disappearing after about 200 miles. F r o m here on, below-normal t r a d e c u m u l u s skies alternated with almost entirely clear zones. A slight revival of cumulus activity, in iceberg-like blobs, set in n e a r W a k e as we approached the outskirts of the surface low t o the northeast. T h e only clouds that showed marked shear o n this leg were the few high congesti, which put out long streamers t o w a r d northeast, beginning at about 2 1 , 0 0 0 ft. T h a t the lower cumuli remained nearly vertical is consistent with the rather d e e p and uniform trade layer prevailing over most of the route.

Fig. 4 . 4 0 . - S c h e m a t i c cloud cross section for Leg 3, Flight II, constructed from measurements on film (see also Tables 4.5 and 4.6). Notation same as Fig. 4.38 except that time increases to right. M a x i m u m cumulus coverage 1/2; maximum upper cloudiness 1/2 + .

102 Leg

4, Flight

Results of Right II

II

T h e cloud structure o n this leg is shown in Tables 4.7 and 4.8, f r o m which the cross section (Fig. 4 . 4 1 ) was constructed. T h e flight began in an area of precipitating congestus associated with the low-pressure center near W a k e . T h e 42,000-ft m a x i m u m cumulus noted on the section w a s due to one lone c u m u l o n i m bus at the outset; n o n e of the o t h e r precipitating towers exceeded .25,000 f t elevation. A b o u t 3 0 0 miles out of W a k e , precipitation ceased a n d cumulus activity began diminishing. T h e remainder of the flight achieved the record suppression for the entire study. D u r ing the mid-portions of the flight, cumuli occasionally p o k e d their tops above flight level ( 9 , 0 0 0 f t ) but the recurring inversion stratus shown o n the section m a d e it evident that subsidence and stabilization must h a v e shallowed the moist layer so that it m o r e resembled that at Johnston (Fig. 4 . 3 5 ) t h a n that left behind at W a k e (Fig. 4 . 3 4 ) . F o r about 2 0 0 miles, beginning just before 2100Z, a nearly solid, thick stratus u n d e r cast m a d e it almost impossible to detect the cumuli a n d sea surface below. Such conditions are more to be expected of the trade regime upstream of H a w a i i t h a n of this portion of the tropics. Farther along on the flight, in the region d o m i n a t e d by the upper low and surface ridge, cumuli became t o o suppressed t o spread o u t any longer into inversion stratus, which gradually disappeared by 2 3 1 2 Z . T h e u p p e r low is made evident o n the cross section by the fragments of cirrus apparently p r o d u c e d by the high-level convergence n e a r its center. T h e suppression culminated in the 400-mile wide clear area, seen on the section between 2 3 2 6 Z and 0 1 1 0 Z ( f r o m longitude 171.5° W to about longitude 164.5° W ) . During m o r e t h a n 8 0 per cent of the time the aircraft spent in this area, the sky was totally cloud-free as far as the eye could see. A b o u t 4 0 0 miles out of Hawaii a suppressed, inversion-ridden t r a d e c u m u lus regime again set in, which showed some very interesting organization patterns ( t o be studied in the next s e c t i o n ) . T h e only cloud shears on this leg were observed in t h e convection zone near its beginning. In the most active region, the tall towers sheared f r o m due west, beginning at a b o u t 2 4 , 0 0 0 ft, in good agreement with the analysis of Figure 4 . 3 3 , which

shows a rather flat trough line in the u p p e r c o n t o u r s r u n n i n g through that area. Just at the end of the convection z o n e , before the first clear spot, the congesti showed p r o n o u n c e d s h e a r f r o m t h e east, with elongated shelves p r o t r u d i n g f r o m t h e m t o w a r d the west at 9 , 0 0 0 ft. It is interesting t o note ( o n Fig. 4 . 3 4 ) that this is the direction of the shear vector between the trade-wind layer and the wind at 10,000 ft on t h e W a k e wind sounding. T h e previous higher towers did not seem to show this shear, despite their p r e s u m e d exposure to t h e same wind field. Indeed, it seems to be a general result f r o m this study that cumuli will only exhibit a really m a r k e d shear or shelving if the ambient wind change is located at a level where s o m e or most of their towers are running out of buoyancy; large, vigorous chimneys will penetrate, with no appreciable slant, shear regions where their smaller neighbors are d r a w n out into shelving streamers. Frequently in the cross sections (see, for example, early p a r t of Leg 2; Fig. 4 . 3 9 ) we h a v e entered shearing a r r o w s f r o m cloud bodies that penetrate m u c h higher. Actually, this should be interpreted to m e a n that towers that terminate n e a r the level of the arrow shear in this m a n n e r ; such fine distinctions could not readily be incorporated on a schematized cross section.

3.

ORGANIZATION

STUDY

Probably the m o s t interesting aspect of tropical cloud distributions is their high degree of organization on m a n y different scales. T o w e r i n g cumulonimbi and precipitation are c o n c e n trated into the convergent zones of disturbances, and within these zones they are generally organized into restricted locations and intriguing patterns, with wide inactive spaces between. Within a given synoptic system, several meso-scale transitions in cloud regime are c o m m o n l y e n c o u n t e r e d , o f t e n discontinuously a b r u p t . In each cloud regime, moreover, the individual cumuli frequently line themselves u p into rows, which m a y extend f o r 5 0 or 100 miles or more. It is with this last aspect of organization that we shall primarily concern ourselves h e r e ; probably the larger scales are better e x a m i n e d using higherflying aircraft, or space vehicles, whose c a m e r a s can e n c o m p a s s a greater area.

Pressure mb

001?. 2

Oil 2 Z

021 ? l

Fig. 4.41. -Schematic cloud cross section for Leg 4, Flight II, constructed from measurements on film (see also Tables 4.7 and 4.8). Notation same as Fig. 4.38 except that time increases to right. Maximum cumulus coverage 3/4; maximum upper cloudiness 1/4 + . Precipitation entered below cloud base for every interval where it was seen.

Height ¡ n km

104

Results of FUghl II

On Flight I, row organization was recorded more than 90 per cent of the time cumuli were visible. The orientation of the rows was measured on the photographs. Mostly it was parallel, within 5° or 10°, to the low-level streamlines, or within the accuracy that these could be drawn from the sparse wind data. Occasionally, however, another mode of organization was detected which produced lines at a very high angle to the wind. One such case was carefully mapped (see Fig. 3.36). Here a "cross-wind" mode was found to arise by the enhancement, at fixed intervals along the parallel rows, of individual clouds relative to their neighbors, so that lines almost at right angles to the flow were superposed on the parellel ones, giving the sky a checkerboard appearance. On another occasion, it was suspected that the "cross-wind" organization alone was seen and measured, and that the intervening portions of the parallel rows had been completely suppressed. Unfortunately, no wind data were available at this location to determine the angle and thereby to test the suspicion, which is an important one for the interpretation of satellite photographs, particularly if one hopes to make deductions about the wind field from them. Clearly, we wish to establish criteria for the development of the common parallel mode and the spacing of its rows, and also for the spacing and scale of the rarer "cross-wind" mode when it occurs. For these purposes, our treatment of Flight I gave only preliminary suggestions. The parallel mode appeared to be best developed under convergent and disturbed conditions, and most often absent (replaced by random cumuli) under inversion domination in the upstream and poleward portions of the trades. The spacing of the lines seemed to bear a relation to the depth of the convective layer, becoming wider where the cumulus towers grew taller. In the case of the "cross-wind" mode, there was a hint that wind shear had something to do with its appearance. However, since only one case was studied, this point could not be pursued. Fortunately the data of Flight II were well suited for the continuation of this organization study and, in particular, to evolve a criterion for the presence and orientation of the "cross-wind" mode. Here the study has been divided into two parts. The first consists in coding the degree of row organization and measuring its orientation for each of the 15-minute intervals of the

prints and sky coding. This was done previously for Flight I, and the comparison proves very illuminating. Second, detailed cloud maps were made for several regions of the flight where cross-wind organization was suspected. These maps have been related to the wind field and soundings. The methods of analysis were those described in chapter ii. Code symbols X, 1, 11, and 111 were used to denote degrees of row organization: from absent, through weak, moderate, and strong. The symbol M was used when the cumuli were obscured from view and the symbol 0 was added to denote those cases where cumuli were so sparse that a discussion of their organization would be pointless. Each print interval on the flight was given one of these code symbols, determined from projection of the film as well as examination of the strip of prints made from it. At each interval where organization was coded 1 or higher, the orientation of the rows was carefully measured, as described in chapter ii. Tables 4.9-4.12 give these results for each leg of Flight II, together with sky code and precipitation. Figures 4.42-4.45 show the same information as underlays on the lowlevel wind map for each leg. Here the direction of line-up has been indicated by an arrow to facilitate its comparison with the wind field. Table 4.13 summarizes the over-all organization study for Flight II. TABLE CUMULUS R o w

Leg

Intervals

1 2 3 4 Over-all

27 24 24 31 106

4.13

ORGANIZATION ON F L I G H T

No. of intervals organization reported X 1 11 111 M 10 5 9 3 27

7 8 10 9 34

8 9 1 5 23

0 0 0 0 0

0

0 0 1 5

2 2 3 9

6

16

II

% with ore present

% with ore absent

56 71 46 45 54

37 21 38 10 25

Results of Flight II

Ignoring the M and 0 cases, we have 84 print intervals where cumulus were visible and their organization could be judged. Of these, organization was absent on 32 per cent, weak on 40 per cent, moderate on 27 per cent, and strong on 0 per cent. For Flight I, the corresponding figures were: absent 9 per cent, weak 30 per cent, moderate 34 per cent, and strong 28 per cent.

105

Flight II thus showed outstandingly less cloud organization than Flight I, even allowing for some subjectivity in coding, which was minimized. Flight II was likewise a much less disturbed circuit than Flight I, except for the Kwajalein-Guam leg, which was more disturbed and comparably well organized on Flight II. Table 4.13 shows that this leg was much the best-

Fig. 4.42.-Organization of cumulus, Leg 1, Flight II, on lower layer winds (Fig. 4.2). See also Table 4.9. Arrows give measured direction of cloud lines; head arbitrarily chosen to conform to wind field. Code symbols are organization intensity: 0, almost no cumulus; X, organization absent; 1, weak; I I , moderate; 111 strong; M missing because cumulus obscured.

106

Results of Flight II

organized one of Flight II, with organization present 71 per cent of the time. It was also the only leg of this circuit which possessed cumulonimbi and any significant flight time in disturbances. The last leg of Flight II showed record suppression, while the last leg of Flight I contained a typhoon. The synoptic analyses make it evident that whereas much of the ocean experienced low-level convergence at the time of Flight I, it had largely been replaced by divergence on Flight II. On the whole of Flight II, only one-third of the X's (organization absent) occurred with sky code numbers higher than 4, and some of these had the postscript "minus"; furthermore 65 per cent of the 11's (moderate organization) occurred with codes 6 or higher. Thus our suggested relationship between convergence and row organization is strengthened by the analysis of Flight II. However, no single definite quantitative criterion for its development has been offered, and the solution is not likely to prove simple. As we shall see, inversion-domination alone is not adequate to prevent rows, nor is above-average cumulus activity absolutely certain to ensure them. We shall now examine the row orientation and development on each leg. Leg

1

organization

Leg 1 was the second-best-organized of Flight II, with organization present 56 per cent of the time, although it fell very far short of the 80 per cent exhibited by its counterpart on Flight I. Table 4.9 gives the organization parameters and Figure 4.42 relates them to the flow as underlays on the lower layer wind chart. By and large, the cloud rows are seen to be lined up with the trade winds, although medium-scale wobbles are found in them relative to the smoothly drawn streamlines. Our wind data are much too sparse to determine whether the flow field also meandered slightly. Near the end of the leg, the cloud row orientation suggests that the streamlines could have been analyzed with a sharper ridge, while in the convection zone at the early part, the veering and then shift to parallel (to flight path) of the cloud rows confirmed the location of the trough line as did the sudden cessation of organization at that point. As mentioned previously, this trough was entered almost entirely on the basis of cloud evidence; other evidence for it was nearly nonexistent.

No clear appearance of a cross-wind mode was found on this leg, although a faint suspicion of one was met in the convection zone of the trough. It did not seem obvious enough to attempt mapping. Leg 2

organization

As mentioned, Leg 2 was the best-organized leg of Flight II, with organization present 71 per cent of the time. Nevertheless, it still fell slightly short of the poorest-organized leg of Flight I (same track, also Leg 2), which showed organization 76 per cent of the time. The organization parameters are given in Table 4.10 and illustrated graphically as underlays on the lower layer wind chart in Figure 4.43. We see that cloud rows were almost entirely confined to the two disturbed zones of high convection; the last third of the flight approaching Guam showed random cumulus, such as there were. A "cross-wind" mode of organization was obvious in both disturbed zones of this leg, but is not indicated in Figure 4.43; it was left for the mapping and detailed discussion to follow. Only a parallel mode is shown here. The reason for this is that where both modes are present, the parallel rows are generally closer spaced and composed of smaller clouds, so that they are readily identified and their orientation is quickly determined by our rapid method. The wider-spaced cross-wind mode usually requires a detailed and laborious map-making effort to define. Thus we have adopted the convention that when the parallel mode is identifiable and measurable it is the one that will be treated in our tables and figures, and the presence of the crosswind one will be indicated in the "Remarks" column of the tables where possible. In the first disturbance, the cloud rows turn as does the wind in a classical easterly wave, from veered behind to backed ahead of the trough line. There was some corrobative evidence in the wind field, and so the streamlines have been drawn to conform to the organization arrows. The second disturbance showed little systematic turning of the rows. Its end, however, is clearly seen on Figure 4.43 as the organization code changes from moderate to absent. As we shall see presently, mapping these two portions gives particularly interesting results.

Results of Flight II

107

o

Cumulus Organization on Lower Layer July

Winds

26, 0 0 Z

Fig. 4.43.-Organization of cumulus, Leg 2, Flight II, on lower layer winds (Fig. 4.11). See also Table 4.10. Notation same as Fig. 4.42. Arrows give direction of parallel mode only, although "cross-wind" mode may enter coded intensity. See text.

Kusai*

\

108

Results of Flight II

See also Table 4.11. Notation same as Fig. 4.42.

Results of Flight II

109

Fig. 4.45.-Organization of cumulus, Leg 4, Flight II, on lower layer winds (Fig. 4.30). See also Table 4.12. Notation same as Fig. 4.42, except that near the eastern end of the leg the three arrows at a large angle to the streamlines refer to a dominant "crosswind" mode of organization; choice of arrowhead discussed in text.

110 Leg 3

Results of Flight II

organization

Leg 3 showed organization only 46 per cent of the time, although the 38 per cent where rows were absent was primarily confined to the first third of the flight, which was also highly suppressed. Beyond that, organization was usually in existence, particularly where normal or above normal cumuli were active. Table 4.11 gives the organization parameters, which are illustrated relative to the lower flow in Figure 4.44. Where recorded, the cloud rows arc seen to closely parallel the streamline analysis, except for the first two arrows which are slightly off and occur in a region far from wind observations. After this, the slow turning of the cloud rows from southeast to east to northeast is apparent and in excellent agreement with the rotation of the trade layer. No evidence at all of a cross-wind mode was detectable on this leg. Leg

4

"cross-wind" mode—one that dominates to the extent that for nearly 200 miles the parallel mode is entirely suppressed. The latter begins to come back about 250 miles out of Hawaii, making the checkerboard pattern, and by the final observation is sufficiently strong again to govern the direction of our last arrow, which we see conforms exactly to the streamline field. This case is a very fortunate one for mapping and investigating the cross-wind mode, as we shall see, but it has less fortunate implications regarding the deduction of the wind field from satellite photographs. Essentially it means that such deductions cannot be made infallibly, at least in any simple or obvious fashion. Even when only one mode of trade cumulus organization is present, there is no 100 per cent assurance that this is a parallel mode; it may be a dominant cross-wind mode, as this case shows. As we see next, the orientation of the cross-wind mode bears no simple relation to the wind at any level, although it tells us something about its height derivative.

organization

On Leg 4, organization is present only 45 per cent of the time; on the other hand, it is coded as absent only 10 per cent of the time. The apparent discrepancy is due to the wide clear areas, in which there were insufficient cumuli to be organized or otherwise. Table 4.12 presents the organization parameters, which are illustrated in Figure 4.45. The latter shows some startling and intriguing results. For the only time on this circuit we see cloud lines perpendicular to the flight path, which surely are rarely to be expected in the tropical easterlies. North-south organization is measured both near the beginning and near the end of the flight. When we examine that near the beginning, however, we find that it is just the parallel mode, with the somewhat unusual orientation due to the presence of the trough and succeeding high cell. In fact, up to the clear area, the cloud lines rotate in beautiful conformity to the low-layer winds. Beyond this, we find a clearly documented case of trade cumulus rows strongly lined up at a sharp angle to the flow field. (The choice of which way to aim the arrowhead here is explained later, when a map of this region is related to wind structure.) There is no doubt of the winds here being directed as analyzed. The cumulus rows are definitely showing a true

Results

of cloud

mapping

Cloud maps have been prepared of the two disturbed portions of Leg 2, where the presence of a double mode of row organization had been detected from watching the films. The first (Fig. 4.46) runs from 2130Z to 2332Z on July 25. That is, it includes the convection zone of the easterly wave trough, running from take-off at Kwajalein through the trough line itself (denoted by " T " at 2244Z) and a little way into the subsidence zone to its west. This map has been schematized only very slightly, in that it was impossible to locate exactly and measure every cloud in the field of view. However, almost every cloud that topped 35,000 ft within a 100-mile strip to the south of the flight path is included, except a few that were too obscured by neighbors in the foreground. Some that could be detected on the film as being present but were not viewed long enough to measure precisely are entered in dashed outline. For cumuli under 20,000 ft, no aim at complete coverage was made, particularly beyond 40 miles from the aircraft. A fair proportion of the nearby rows of small cumuli has been located. All anvils denoted by solid elongated figures have been measured carefully and are entered closely to scale; this includes all visible coherent anvils

Results of Flight II

111

Fig. 4.46.-Cloud map along Leg 2, Flight II, between 2130Z and 2332Z, July 25, 1957. Map runs from near take-off at Kwajalein (right) through easterly wave trough of Fig. 4.11 (denoted by " T " ) to subsidence zone to its west. Compare also Figs. 4.39 and 4.49. Large clouds located precisely are drawn in solid lines, roughly to scale. Estimated large clouds (generally too obscured by neighbors for measurement) are sketched in with dashed lines. Several more towers (mainly where noted) were obscured by rain and other clouds. Heights of towers and some anvils (elongated figures) are given in 1,000's of feet above sea level. Plus sign indicates rapid growth at time of measurement. A few of the numerous lines of small trade cumuli are shown, although many more were present except in regions marked "clear." Shower symbols denote rain showers passed through by aircraft, with double symbol signifying moderate to heavy showers. Note change in orientation of small cumuli as trough line is passed. Note suggestion of "cross-wind" orientation of larger clouds ( f r o m northeast to southwest) and that anvils extend toward approximately east-northeast. T h e clouds began shearing between 23,000 ft and 24,000 ft in the same direction as the cross wind rows, as determined f r o m careful measurement on the film.

112

Results of Flight II

that bore some relation to a living cloud body below and a few that did not, marked "dead." We see that here the parallel mode is well developed, probably clearer and more pronounced than the "cross-wind." The parallel rows change direction very nicely across the wave

trough; the change is sharp and discontinuous, often being observed as a 45° bend in a single cloud line. There is, however, a strong suggestion of another mode, quite likely resulting from the regularly-spaced enhancement to the cumulonimbus stage of selected members of the parallel rows. This organiza-

Fig. 4.47.-Cloud m a p along Leg 2, Flight II, between 0027Z and 0140Z July 26, 1957. Compare with Figs. 4.39, 4.48, and 4.49. Notation similar to that of Fig. 4.46, except a larger fraction of the nearby small trade cumulus rows are shown (they were fewer here). Note the pronounced banded structure of the large clouds, oriented from northnortheast to south-southwest, with anvils streaming toward east-northeast, becoming more nearly toward east at the end.

Results of Flight II

113

tion had an angle of 40°-50° to the parallel rows (wind). These "cross-wind" lines could be readily seen on the film by sighting at the proper angle along the large clouds, at those places where the aircraft passed through precipitation. The most pronounced (the first four) were evenly spaced, with a little more than 40 miles between them. It is significant that the rows are oriented exactly parallel to the shear in the congestus towers, which set in at 24,000 ft. We can see this on the map in the way the bulbous towers at 25,000-30,000 ft are displaced from their bodies. Still another direction of orientation was provided by the high anvils, which are drawn out to the right (eastward) of the "cross-wind'* rows. It is also significant that the western two rows, the last of which is barely discernible, are composed almost wholly of clouds that are still rising rapidly. It appears that these rows were forming as the aircraft passed by; possibly the wave disturbance moves along by creating new rows in its forward portion. The map of the second disturbance on the flight leg appears in Figure 4.47, and three frames of the film are shown in Figure 4.48. Actually this spectacular situation can only be appreciated in projection, as its scale is too large to be encompassed in one photograph. Most striking was the contrast between west and east sides of the normal rows: the former gave the appearance of an abrupt towering wall of icebcrgs (Fig. 4.48, a) while the latter (Fig. 4.48, b) showed only the sheared-off anvils streaming above, over a tunnel-like, nearly vacant sea below. One of these large clouds is seen as a whole in Figure 4.48, c.

Fig. 4 . 4 8 . - T h r e e f r a m e s f r o m the movie film illustrating t h e c l o u d c o n f i g u r a t i o n m a p p e d in Fig. 4 . 4 7 . U p p e r f r a m e ( 4 . 4 8 , a ) s h o w s the f o r w a r d ( w e s t e r n , left side in Fig. 4 . 4 7 ) side of t h e s e c o n d ( f r o m r i g h t ) c l o u d b a n d ; c e n t e r f r a m e ( 4 . 4 8 , b) s h o w s t h e s t r e t c h e d - o u t anvils on the eastern side of the c l o u d line; a n d lower f r a m e (4.48, c ) s h o w s in entirety the nearest c l o u d in t h e w e s t e r n m o s t ( f a r t h e s t left) b a n d .

The map (Fig. 4.47), which includes the entire disturbance, is only slightly schematized. Some messy stratus shelves have been omitted, but most of the nearby rows of small cumuli are there. Here the "cross-wind" mode is plainly dominant and the parallel mode, although measurable, is suppressed to nearextinction. This time the "cross-wind" mode makes an angle of about 15° to the parallel. The anvils (providing a third direction of cloud orientation) stretch out to east-northeast at first, becoming more nearly eastward at the end. Again we find that all the members of the westernmost row are growing vigorously; rates of rise of towers were measured

114

Results of Flight II

at 6-12 m/sec. To test whether this was indeed related to motion of the disturbance, we sought remnants of old rows to the eastward of the map region. Two such were readily found; they consisted of leftover anvils and cirrus remnants (at about 30,000 ft elevation) at approximately the same spacing as the rows. The farthest one was much more dissipated than the nearest. Furthermore, the right-hand row shown on the map already consisted mostly of stretched-out anvils, with a few dead cloud bodies below. Thus we suggest that a tropical disturbance may propagate by creating new rows ahead of itself, as those behind die off. This could explain how, in coordinates relative to the disturbance, the cloud patterns may remain nearly invariant, despite miles of travel; this question was raised in our study [3] of cloud distributions in hurricanes. Organization modes and Using these two maps and from Flight I, we may now ence and orientation of the

the flow field adding that of the case referred to advance a hypothesis for the occurcross-wind mode of organization.

The background for this suggestion lies in some beautiful experimental work on convection by Avsec [4], coupled with a recent theoretical development by W. Malkus [5]. Avsec made quantitative laboratory studies of the convective regimes in fluids heated from below, either initially at rest or in laminar motion in a channel. In the former case, the polygonal regimes of Benard were investigated; in the latter the cells took the form of long rolls, elongated infinitely in the direction of the shear between the channel bottom and the fluid velocity, with updraft spacing comparable to the depth of the convective layer. In the old days, meteorologists frequently used these experimental results as a framework to explain some types of atmospheric convection. Theoretical models analogous to the laminar treatments were made, using "eddy exchange coefficients" in place of those for molecular processes. The fact that these were pushed too far led to a discrediting of entire approach; workers began to doubt that laminar results had any meaning for the fully-turbulent atmosphere. The convection work by W. Malkus puts the problem back in proper perspective and shows that the apparent analogy between atmosphere and experiment is actually not coincidental, if sought under the proper condi-

tions. Without arbitrary assumptions or coefficients, his work demonstrates that when fully turbulent convection maintains (many modes or scales of motion are all active together), the dominant mode remains that which first breaks out in the laminar convective case. Thus Avsec's work can be quite helpful to our study, though the conditions he treated were very much simpler than those with which we are faced. In dealing with convection over the tropical oceans, we believe we are often treating motions in a moving fluid fairly uniformly heated from below and with fairly uniform boundaries. We therefore hypothesize, extending Avsec, that the orientation of the rows or cells will be governed by the shear between the converting fluid layer and its boundaries. If the convective layer is exposed to shear only at its bottom boundary, then the cloud rows will line up with this shear, or with the low-level trade wind. This is the common case, particularly in the more poleward regions of the tropics, since the uniform east wind layer is commonly deeper than the cloud layer. If, however, the convective layer is exposed to shear from its top boundary as well, it may show two modes of row organization—the second lining up with the shear vector between the motion of the convective layer and that of its top boundary. Thus we may expect the "cross-wind" mode in tropical cumuli to be rarer than the parallel, and to come in when either (a) the convective cloud layer is sufficiently deep to reach the strong shear region above the uniform easterlies, or (b) the shear layer is abnormally low and invades the normal cumulus regime. We may expect the relative development of the two modes to depend, first, on the relative magnitudes of the two shear vectors, and second, in those cases of deep cloud layer, possibly also on the vertical distribution of static stability within the layer. We now return to our three mapped cases of "cross-wind" mode in order to connect them with the motion field, using the hypotheses evolved above, and to determine whether the latter will work out. The three cases are shown schematized in Figure 4.49, with the important features of the wind field summarized on the left. The latter were deduced from a combination of the nearest wind sounding, the aircraft crew's log, and the synoptic analyses. Though we cannot vouch for their details, the main outlines are probably sound.

Realtà o( Flight n

115

Winds [-—2 0 kt

Winds

Flight 0 0 2 7 I July

r—20 kt - -H

1 w

E

I

S

II

Leg 2

-

0 1 4 0

26,

19 5 7

Flight 0 4 3 0

Z

July

II,

I -

Z

Leg I

0 5 3 0 1957

Z

Fig. 4 . 4 9 . - S c h e m a t i c s u m mary of cloud organization in the maps shown in Figs. 4.46 and 4.47 and from Fig. 3.36 from Leg 1, Flight I. The reconstructed winds and their relation to cloud orientation are shown to the left of each diagram. The mean low-level wind is denoted by L and is hypothesized to govern the small cumulus line-up (parallel mode). The shear between the lower and upper parts of the cloud layer is labeled "rows" and hypothesized to govern the line-up of large clouds or "cross-wind" mode. The mean wind in the upper convective layer is generally that reached at 22,000-25,000 ft, as indicated by the numbers at the end of the arrows from a southeasterly direction (see Fig. 4.16). The final arrow is the westerly wind at 30,00040,000 ft. The shear between the trade-wind layer and this uppermost layer is believed to govern the orientation of the anvils. Note the dominance of the parallel mode in case a, of the cross-wind" mode in case b, and the nearequal development of the two modes in case c. Note the nearly equal spacing of the "cross-wind" rows ( A

!

20 1

/

/

'

1.0 y 13

1

/

1

/I

y\

/ 2.0 U / /i

1.1

/

Fig. 5.36. -Tephigram and vertical wind plot at Weather Ship N (Nectar: 3 0 ° N , 140° W ) , August 27, 1957, OOZ. Tephigram ( a b o v e ) gives dew-point curve on left with mixing ratios entered in gm/kgm.

V (knots)

DD (tens

degrees)

Fig. 5.35.-Tephigram and vertical wind plot at Lihue ( H a w a i i ) , August 27, 1957, 00Z. Tephigram ( a b o v e ) gives dew-point curve on left with mixing ratios entered in gm/kgm.

s

m i

/

1

174 -20 /

-10

\

i

y

The Pacific subtropical high in summer formed the subject of a classic study by Nieburger, Johnson, and Chien [ 2 ] . On this occasion, we can find nothing in its pressure, wind, temperature, or stability distributions which perceptibly differs from their findings. In addition, the flight route along the trade trajectory from Ship N to Hawaii provided the material for a series of intensive studies of tropical meteorology by Riehl and collaborators [ 3 - 5 ] . These works described and modeled quantitatively the mean flow, structure, and energy transactions along this route, which vary little in summer. The synoptic conditions along Leg 4, Flight III, resemble the mean section they constructed in every testable feature, but even the sky coding suggests that the accompanying cloud structure has surprises to offer.

20

10

\

Results of Flight III

2.6

\

^

/

\

/ /

\

S

9.4

/

^

^ 10.6

I

1

Fig. 5.37a.-Tephigram at Weather Ship at 2 9 . 2 ° N, 136.8° W, August 27, 1957, 00Z. Tephigram gives dew-point curve on left with mixing ratios entered in gm/kgm. T °C

À

1

y

i

/

x

'

/

/

\0.9 \

/

\

Td

/

msg.

i

/

i

/

l

/

i

vF

i

Fig. 5.37b.-Tephigram at Weather Ship at 2 7 . 5 ° N, 129.4° August 27, 1957, 0500Z. Dew-point curve largely missing.

W,

It is indeed observed that inversion stratus predominates near Ship N, and that downstream at Hawaii the inversion has risen, the moist layer deepened, and trade cumuli prevail. However, the film shows no gradual transition in cloud forms, but a backand-forth oscillation with amazingly abrupt transitions between regimes. As the basic synoptic analysis exhibits uniform progression in air structure, these variations in cloudiness have to be investigated from a scale of measurement below that of the synoptic scale. We regarded this alternation of cloud features as sufficiently interesting to warrant a detailed study in sections 2 and 3. Fig. 5.38. -Schematic cloud cross section for Leg 1, Flight III, constructed from measurements on film (see also Tables 5.1 and 5 . 2 ) . Horizontal coordinate is time along the flight leg, increasing toward the left. Vertical coordinate is feet on left side, pressure in mb and height in km on right. Lower cloudiness (cumulus) amount given by lower hatched-under curve; maximum amplitude here is 3 / 4 + at 0538Z. Middle cloudiness (10,000-20,000 ft) given by black marks in middle: maximum amplitude here is 4/4 (example at 0 5 0 0 Z ) . Upper cloudiness is indicated by upper hatched curve; maximum amplitude here 4/4 (example at 0 4 0 0 Z ) . Width of schematic cumuli is proportional to coverage, not to cloud size. Mean heights of cumuli shown by solid clouds; dashed cloud on top denotes maximum top in the (approximately) 15-minute interval. Inversion stratus and precipitation entered schematically where observed. Question marks at cloud base denote uncertainty in its determination. Arrows on a cloud represent sheared tops at level indicated in direction shown (north upward). The positions of the trough lines in Fig. 5.2 are denoted by " T . "

Results of Flight III

175

Height

in

ft.

? ? ?

40,000

200

'IMMU^^W^

l> 20,000

« 1 rrrrn>T> ÌTTTTTTT7J77T[\. Woke

07002

| 0500Z

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0400Z

JTRRY.

^ T-m-rrrrr>—-RJT 0300 Z

0200Z

in kn

176 2.

Results of Flight III

CLOUD CROSS SECTIONS

The cloud structure along each of the four legs of Flight III is shown in more detail in the cross sections (Figs. 5.38, 5.41, 5.44, 5.45 [lower] ). The sections have been constructed from the data in Tables 5.1-5.8, two for each leg. The first table gives the type of cloud observed and the cloudiness, to the nearest one-quarter sky cover, for cumulus, middle (10,000-20,000 ft), and high (above 20,000 ft) clouds. This was estimated from the film to approximate an average for each interval centered at the time given (same time as the prints and cloud code entries of the previous figures). The second table of the pair gives cloud heights: the cumulus bases, average cumulus tops, maximum cumulus tops, and the heights of middle and upper sheets, where present. The level of inversion stratus has been entered in the "Remarks" column, except for Leg 4 (Table 5.7), where it was the predominant sheet cloud. The same methods were used in constructing the cross sections here as on Flights I and II, and the same types of problems were encountered in attempting to characterize the cloud structure satisfactorily. On Flight III, Legs 1 and 2 are noteworthy for high towers, precipitation, and middle and high cloudiness, whereas Legs 3 and 4 are noteworthy for the absence of these features. Leg 1, Flight 111 Tables 5.1 and 5.2 give cloudiness and cloud heights, respectively, for this leg. The cross section constructed therefrom is shown in Figure 5.38. By and large its features both confirm and supplement our analysis. The weak wave trough (marked "T" between 2300Z and 0000Z) east of Johnston is seen indeed not to be a true easterly wave, as it contains no high towers or precipitation in its only slightly developed convection zone. The western half of the section is particularly valuable. The cirrus between 0101Z and 0215Z appears to be independent of cumulus and is quite high (37,000 ft). If we reexamine Figure 5.6, we see that it overlies the portion coded 5 and 7 where upper air trajectories are losing anticyclonic vorticity and gain-

ing cyclonic; the cirrus thus appears to be produced at the mean level of this chart by the convergence indicated in the flow. The cirrus that began at 0215Z, however, had quite a different character and was recorded by the observer in flight as anvil cirrus, coming from already visible cumulonimbus towers in the distance. This cirrus, at 30,000-33,000 ft, soon thickened to overcast at 0350Z and probably remained so, although obscured by alto-clouds, until the aircraft broke out at 0602Z into the region coded 11, with rows of cumulonimbus towers and rapidly diminishing upper cloud. The 150-mile stretch of raining altostratus overcast from 043 3Z to 0538Z is noteworthy for its lack of towers on the section, although some were occasionally faintly visible far to the north. Figures 5.2-5.3 show no reason for this suppression of convection in the large-scale dynamics or moisture pattern. Visually, the whole region resembled the outer rain area of a tropical storm, and we believe the suppressed rainy portion may have been one of compensatory subsidence between convection bands (cf. [6]). The spectacular emergence at 0602Z into rows of cumulonimbus towers against blue sky is understated on the section, in both its suddenness and its magnitude. It was not possible to schematize the fact that the high towers were much rarer and increasingly distant after that time, although subsidence is suggested on the section in several ways: diminishing upper cloud, lower mean cumulus tops, and a recurrence of inversion stratus below the aircraft. Selected still photographs of the cloud progressions through this disturbance are shown in Figures 5.39 and 5.40, with which it is also advisable to review Figure 5.8. The straight vertical development of these high towers is noteworthy; little shear of the cumulonimbi was noted in this disturbance, up to the anvil level. Following Riehl and Simpson [7], we suggest that this may have been a factor favoring the subsequent development. In contrast, the patterned, highly sheared towers of Leg 2, Flight II, were in systems that did not deepen. We suggest a study, perhaps using satellite photos, to relate cloud shears and patterning to the subsequent development, or lack thereof, of the synoptic disturbance containing them; it is possible that a valuable forecasting tool could be developed.

Fig. 5.39.-Still photographs taken in disturbed region east of Wake on Leg 1, Flight III. Fig. 5 . 3 9 a . - T a k e n at 0434Z in region between cumulonimbus rows denoted "no cu" on Fig. 5.8a. Rain beginning from stratus overcast. Camera aimed southwest.

Flg. 5.39b. Taken at 0545Z in main rainband on Fig. 5.8a. Anvils seen overhead art from line of cumulonimbus (now crossing flight path) also shovn in Fig. 5.8b. Air extremely rough. Camera aimed northwest.

Figs. 5.40_Still photographs taken in disturbed region just east of Wake on Leg 1, Flight III. This series taken in clearing region at and just west of the X in Fig. 5.8a. All taken between 0600Z and 0637Z.

Fig« 5.40a. Taken at 0600Z, aiming about due south, showing major line of cumulonimbi extending to southwest of flight path. Note calm seas and cat's paws.

Fig. 5.40b. - T a k e n at 0615Z, aiming just south of west, showing second line of cumulonimbi being approached by aircraft. Note clearing aloft.

Fig. 5.40c.-Taken at 0637Z, aiming about south, showing a line of high towers extending southwest or west-southwest. This is probably the same line as that shown ahead of the aircraft in Fig. 5.40b and west of the X on the map in Fig. 5.8a.

Height i

Pressur«

feet

mb

r

I

I

I

i

i

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0600

I

I

I

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'

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Fig. 5.41.-Schematic cloud cross section for Leg 2, Flight III, constructed f r o m measurements on film (see also Tables 5.3 and 5.4). Notation same as Fig. 5.38. T denotes trough line located about 180 miles west of W a k e at OOZ, August 18, 1957 (Fig. 5.13).

Height in km

Fig. 5.42.-Still photograph showing cloud structure just west of trough line. Taken at 0246Z with camera aiming west of north (same as motion-picture camera). Shows line of dying cumulonimbus lined up from northeast. Aircraft is flying in cloud-free lane between two such lines. Note glassy sea. See also Figs. 5.41 and 5.53.

184

Results of Flight III

R u u l t s of Flicht I D

1S5

Height

Height

feet —

/

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30,000

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F i g . 5.44.-Schematic cloud cross section f o r Leg 3, Flight I I I , constructed f r o m measurements on film (see also Tables 5.5 and 5.6). Notation same as Fig. 5.38. Dotted cloud up to 23,000 f t at 0019Z is 82 miles away. Maximum cumulus coverage is 1 / 2 + at 2342Z. M a x i m u m middle cloud coverage is 1/4— at 2342Z. Maximum upper cloud coverage is 1 / 2 + at 0309Z.

186 Leg

2, Flight

Results of night III

III

This leg recorded the highest cumulonimbus towers (45,000 f t ) of the three circuits and was bettered only by typhoon-dominated Leg 3, Flight I, for upper sheet clouds and precipitation. Figure 5.41 shows its cross section, for which the appropriate tables are 5.3 and 5.4. During the beginning portion, from take-off at Wake to the trough line marked " T " just after 0200Z, we are repeating the disturbed zone of the previous day, and indeed note some development. Both the average and the highest cumulus are considerably higher; a trace to one-quarter anvil cirrus has appeared (Table 5 . 3 ) ; and the inversion stratus is practically gone. All this suggests a weakening of any subsidence that was present. The cloud m a p of this trough zone, however (see section 3 ) , still holds some surprises. After passage through the trough line, we first find thickening anvil cirrus, with dead or dying cloud bodies below (Fig. 5 . 4 2 ) , then a brief near-clear space around 0303Z-0320Z, with only distant cumulonimbus and a little anvil cirrus aloft. This is apparently the break between the cyclone's circulation and that of the equatorial trough (see especially Fig. 5 . 1 4 c ) . At 0334Z, a quite different cloud regime sets in, beginning with the abrupt onset of a cirrus overcast. This had a clearly defined edge and we noted "Enter cirrus shield" (Table 5 . 3 ) . The overcast appeared to lower, thicken, and started to rain at 0447Z. However, occasional breaks in the overcast (Table 5.4) showed that there was still cirrus above. From the onset of the cirrus shield, local convection began to diminish, with occasional bands that attained the congestus stage at first, then only to scud and stratocumulus as Guam was approached. Altogether, however, the overcast gave the impression of emanating from distant convective towers (Fig 5.43) to the north, which would fit in well with the analysis of an equatorial trough there (Figs. 5.13, 5.14). Leg

3, Flight

III

The suppressed and sparse nature of Leg 3 convection is well illustrated by its cross section (Fig. 5.44) and Tables 5.5 and 5.6. No cumuli near the flight route achieved even the modest

height of 15,000 ft. The dotted 23,000-ft tower at 0019Z denotes two congesti at 82 miles away. Only one short patch of cirrus appears near the middle. Comparing this cross section and its synoptic analysis with those for the same legs of Flights I and II, we would indeed b e hard pressed to explain the poorer cloudiness on this occasion, were it not for the very unsteady conditions and rapid development of subsidence along the route which we have inferred in section 1. In this connection it is noteworthy that changes in cloud development appear to lead rather than follow those in moisture content. We saw in Figure 5.23 that although the moisture along this route had decreased in the last 12 hours, it was still appreciably higher than on other occasions (cf. Flight II, Leg 1, Hawaii-Johnston segment) where the cloudiness was much better. The interesting features of this leg, namely some regions with cloud patterning and abrupt transitions in regime, will be studied in section 3 where some typical photographs will also be displayed. Leg

4, Flight

III

Tables 5.7 and 5.8 give the figures for this leg. The cloud cross section constructed from them is shown in Figure 5.45, b, together with some mixing ratios and inversion base heights at Honolulu and Weather Ship N. Above it, in Figure 5.45, a, we have reproduced the mean summer cross section for this route by Riehl et al. [ 3 ] . This mean section shows the various layers, and in particular the thickening of the cloud layer and the rising and weakening of the inversion with downstream distance. Those authors had four stations along the route, one at each end and two intermediate. They analyzed the changes as gradual, including the growth of the cumuli, which by poking their tops into the dry air aloft progressively deepen the moist layer. The section from our photographs (Fig. 5.45, b) represents a fascinating comparison. The prevailing synoptic conditions resemble the mean even better than the steady trade flow might lead one to expect. The mixing ratios and their changes between the ship and Honolulu on the section are almost identical to the moisture accumulation found by Riehl et al. In our case the trade inversion rises from a strong single one at the ship to a

double weakened structure at Honolulu (see also Figs. 5.35, 5 . 3 6 ) . A n d it is true that the cumuli are taller at Hawaii than at the upstream end of the section and that the inversion stratus is vanishing near the downstream end. However, the change is by no means gradual, but takes place in a series of overshooting oscillations. On the cross section, we have tried to indicate cumulus below the stratus, whenever it was possible to detect them through holes. Thus those places where no cumuli are shown below stratus decks were by and large free of convective clouds. T h e outstanding feature of Figure 5.45, b, is the series of transitions between cumulus and stratus regimes, occurring on roughly the time scale of print intervals (approximately 15 minutes or 50 miles). W e found this alternation so interesting that a way to map this whole leg was devised, as discussed in the next section.

35,000

Fig. 5 . 4 5 . -Upper: Observed mean summertime vertical air structure along route of Leg 4, Flight III (after Malkus [ 5 ] ) . Isopleths without arrows are potential temperature labeled in degrees Absolute. Isopleths with arrows are streamlines and trajectories. Cumuli entered schematically to show thickening of cloud layer toward downstream end. Lower: Schematic cloud cross section for Leg 4, Flight III, constructed from measurements on film (see also Tables 5.7 and 5.8). Notation same as Fig. 5.38. Maximum cumulus coverage 1/2 at 2038Z (and other places). Maximum upper cloudiness 1/4 + at 0043Z. N o middle cloud. Some mixing ratios are entered at ship and Hickam (from Figs. 5.36 and 5.35, respectively). Inversion base denoted by solid line; double at Hickam to denote double structure. (Distance scale approximately the same on upper and lower figures.)


V> \-

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% Flight

40

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cu

of

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0340Z

Flight

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Fig. 5.51.-Cloud map for Leg I, Flight III, 0138Z-0400Z, August 17, 1957. For location see Fig. 5.1. Heights of some larger clouds indicated in 1,000's of feet to nearest thousand. No attempt made at complete mapping. Most large clouds included up to 4 0 miles from aircraft. Correct spacing and lengths of lines where they occur.

»I

Cle

t

0255 Z

a

196

Results of Flight III

above was very gradually thickening but showed no change accompanying the cumulus disappearance. These sudden transitions in cloud regimes on the meso-scale warrant further study, which probably will require a special program of observations. The final map for Leg 1 has already been shown as Figure 5.8a. Here we found two or three widely spaced rows of cumulonimbus towers, lined up from southwest across the flight path and spewing forth an anvil overcast toward the east. The cloud photographs showed vertical growth until about 30,000 ft; the anvils sheared off sharply eastward. It appears that the tower line-up was about parallel to the low-level flow and the wind shear was either weak or in the plane of the flow up to anvil level. No checkerboard array or other mode of cumulus orientation was visible. Leg 2. A map (Fig. 5.53) was made of the only organized portion of Leg. 2. This began at 0133Z, shortly after take-off from Wake, and continued to just east of longitude 160° E. By the map's end the aircraft had left the cyclone circulation, gone under the cirrus shield, and entered the equatorial trough regime. Comparison of Figure 5.53 with Figure 5.47 shows that the organization arrows near the trough line must be regarded with some skepticism. It is not clear at which ends of the arrows the heads should be located, nor even if any arrows at all should be drawn. A fairly clear line-up from northeast develops after the troughline, however. This was noted by the observers in flight. At about 0248Z they commented that the aircraft was flying in a clear lane between two rows of huge cumulonimbus towers with this orientation.

Fig. 5.52.-(After Avsec, 1939, Figs. 46 and 47.) Experimental transformation of polygonal cells into longitudinal bands, by the setting in motion of a layer of air heated from below. Depth of layer 20 mm. Compare with cloud configurations mapped in Fig. 5.51.

The wind conditions provide a clue to the confused situation near Wake. At take-off, the surface wind was nonexistent; the rather strong wind on Figure 5.18 is thought unrepresentative owing to local rainsqualls. Up until about 0237Z the sea was glassy, marred only by cat's-paws under the clouds. After that a slight wind from northeast appeared, but even Figure 5.42 (taken at 0246Z) still shows cloud reflections and cat's-paws, indicating weak surface winds. The map, however, contributes two valuable supports to the synoptic analyses. The first is the break between the cyclone and the trough circulation shown by the clear spot just before

0237 2

Fig. 5.53. - C l o u d map for Leg 2, Flight III, 0133Z0340Z, August 18, 1957. For location see Fig. 5.1. Map begins just after take-off from Wake, runs through wave trough ( T ) and into beginning of equatorial trough circulation at about 0330Z. Almost all large clouds mapped. Heights given in 1,000's of feet to nearest thousand. A plus sign by a height means still growing when measured. Anvil orientations measured carefully by means of numerous points; anvil heights given on anvils when accurately determined. Lineups doubtful, as can be seen. 0334 z

198

Results of Flight III

0 3 2 0 Z a n d then the onset of the cirrus shield with a veering of the small cumulus lines t o a m o r e easterly orientation. T h e second is the d r a m a t i c change in anvil direction after t h e point m a r k e d " T . " T h e latter was a striking feature of the film and was detectable in an utterly different angle t a k e n b y t w o anvils not 2 0 miles apart. T h i s evidence confirms the analysis of Figures 5.16 and 5.17 a n d shows that at this time the u p p e r a n d lower troughs were vertically superposed. T h u s all evidence points t o this cyclone's being in t h e predevelopment or breeding stage rather t h a n in the development stage, and implies that something h a d to h a p p e n t o force it to d e e p e n : perhaps the subsequent superposition of the u p p e r anticyclone. However, the m e c h a n i s m appears loaded a n d ready t o be triggered, in that high towers a b o u n d showing little midtropospheric shear or ventilation, a n d a good outflow channel exists toward the east at high levels. O n e wonders how f r e q u e n t ly a system is loaded like this without firing and, in particular, w h a t determines the ultimate effectiveness with which the firing takes place. Leg 3. Despite its undistinguished cumuli, Leg 3 was t h e best organized of the third circuit a n d showed the only region where the intensity of rowing was coded as "strong." T h e d a t a f r o m this leg provide some valuable insights into the m e c h a n i s m s forming and maintaining c u m u l u s streets. T h e two segments of this leg which showed m o d e r a t e or strong rowing were m a p p e d (location shown o n Fig. 5 . 4 8 ) . Because of its greater simplicity, t h e later (in time) m a p is discussed first. This m a p (Fig. 5 . 5 4 ) shows as intense, clearcut, a n d simple a case of c u m u l u s rowing as we have f o u n d . It plainly exemplifies the parallel m o d e existing by itself. T h e entire regime, a b o u t 2 8 0 miles in extent, has been m a p ped. At first the small cumuli are lined u p f r o m 0 8 0 ° , in excellent agreement with the navigator's wind at the time. T h e n t h e lineu p shifts to 115°, which it m a i n t a i n s for the remainder of the regime. O n this occasion t h e t r a d e s were two-dimensional u p to at least 2 5 , 0 0 0 f t (Fig. 5 . 2 7 ) . T h e surface wind was estimated ( f r o m waves and w h i t e c a p s ) as f r o m 1 1 0 ° - 1 2 0 ° . T h e cloud tilt was measured f r o m t h e film to lie in this same plane, confirming little or no wind t u r n i n g with height (Fig. 5 . 5 5 ) .

During the flight, we were able to watch the f o r m a t i o n of several new segments of cloud rows. This went as schematized in Figure 5.56, a. As shown m u c h earlier by M a l k u s [ 1 0 - 1 2 ] , small cumuli lean downshear, b e c o m i n g more slanting as their u p d r a f t is exhausted. These small cumuli were noted to lie d o w n flat along the shear but remain visible. T h e n several new little towers spring f r o m the prostrate body, as shown. Sometimes these separated into individual cumuli, or sometimes they amalg a m a t e d into a bigger cloud which could go through the cycle on a larger scale. Frequently another small cloud f o r m e d d o w n shear at Stage 3, and the same process repeated to add a segment to the row, which o f t e n appeared to elongate rapidly in the d o w n s h e a r direction. T h u s we could expect some tendency for parallel c u m u l u s rowing u n d e r these circumstances, even without the roll convection of Avsec. However, Figure 5 . 5 4 shows that the c u m u l u s streets m a y be 6 0 miles or more long, with few or no clouds between rows. F u r t h e r m o r e , the spacing widens with increasing cloud height. Finally, longish rows were sometimes observed to spring into being almost simultaneously along their length. We thus postulate Avsec roll convection in the convective layer, lined u p along the shear between the flow and the o c e a n b o u n d a r y , that is, along the low-level wind. This is shown schematically in Figure 5.56, b. In the region of convergence a n d ascent, the mixed subcloud layer is thickened u p to or a little above the condensation level ( L C L ) and clouds b r e a k out. In the intervening regions of divergence and descent, the mixed layer fails to reach the condensation level and cumuli are prevented f r o m forming. O u r previous work [13, 14] shows that the top of the trade-wind mixed layer is commonly within 100 m of the condensation level, so that a very small amplitude is all that is needed to confine the cumuli in a regular m a n ner. Fig. 5.56, c puts Figures 5.56, a and b together, showing h o w the process described in Figure 5.56, a is confined to the roll crests. A c r u d e attempt to test the roll concept was m a d e f r o m F i g ure 5.54 by measuring the ratio of row spacing to t h e depth of the convective layer, defined by the height of the cumulus t o p s . Encouragingly, this came out as a constant ratio f o r the regime, with a value between 4 and 5; the row spacing widened in linear

Resalte of Flight III

Flight August

III, 22,

Leg

Many

\

cu

ENDS

V

Clear Are a

®

tiny \ « ' '• % unorg.

x.. „

W

x

t 0512 2

small in

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8

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u n o r g.

3

1957

15 ©

199

Flight

V. V

Path

0450 Z

0420 Z

Fig. 5.54.-Cloud map for Leg 3, Flight III, 0406Z-0537Z, August 22, 1957. For location see Fig. 5.1. Heights denoted in 1,000's of feet to nearest thousand. When heights of individual clouds are given, these are generally the highest in their line. All cloud lines and nearly all clouds within 30 miles of aircraft are shown.

proportion to the vertical cloud development. Avsec found he could maintain a ratio of spacing to height in his experiments varying between 1 and 5, although 2 was the most common value. Again this regime ended rather suddenly, but here we may have a clue to its demise. At 0512Z, near the end, the 9,000-ft cumuli began showing a shear from north, so that some towers no longer sheared parallel to the rows; in any event, some alteration in the wind field is suggested. The map of the earlier organized portion of Leg 3 is shown in Figure 5.57. It is one of the most remarkable of all the maps. It begins at 2327Z, shortly after take-off from Kwajalein in a regime of unorganized 5,000-8,000-ft cumuli which show a shear from roughly south or southeast, in good agreement with the Kwajalein wind profile (Fig. 5.26) and our shear chart analysis in Figure 5.24. This regime suddenly ends in a clear stretch about 25 miles wide. Then the organized portion which

is the main subject of the map sets in. This regime lasts for 100 miles along the flight path and then terminates abruptly (Fig. 5.58). Throughout it, the cloud shear was measured to be east-west, which suggests that the bend in Figure 5.24 should be a little farther west. This is corroborated by the navigator's report, which showed westerlies at 0019Z at 9,000 ft (but with the trade layer up to this level by 0 1 1 9 Z ) . There is little doubt that the main direction of organization, shown by the solid lines in Figure 5.57, is the parallel mode. The surface wind at Kwajalein was northeast. The sea wave direction in flight was measured as from about 030°. The cloud rows are from 020° in the first third of the regime and from 0 4 0 ° in the last two-thirds. This is the first case of rowing studied in which the wind turns rather gradually within the lower cloud layer. We suggest, using Figure 5.59, that this is the reason for the cloud elongation at a high angle to the main rows. Actually the rows show up as just a greater concentra-

200

Results of Fllfht III

Fig. 5.55.-Still photograph showing cloud lines mapped in Fig. 5.54. Camera aimed just west of north (180° opposite to movie camera, from opposite side of aircraft). This still was one of a stereo pair confirming line-up from 115°. Note leaning of small clouds along the line, also from 115°, especially the right-hand one in foreground which is lying down along the line.

Results of Flight III

201

Fig. 5.56.-Schematic illustration of our hypothesis regarding cumulus row formation when low-level wind and shear in lower cloud layer lie in the same plane, a. Effect of shear. Three stages, left to right: (1) Little growing cloud tilts downshear. (2) Lies downshear as updraft dies. (3) New little clouds grow from prostrate body. b. Effect of Avsec rolls (wind and shear into paper). Wavy line denotes top of mixed layer, raised in zones where roll-motion is convergent and upward; depressed where roll-motion is divergent and downward. Cloudlets break out where mixed layer reaches to condensation level (LCL), that is, roll crests, c. Combination of shear effect in part a and Avsec rolls in part b produces cumulus rows parallel to flow which elongate downshear. WIND

ojgn ® Fig. 5.57.-Cloud map for Leg 3, Flight III, 2327Z, August 21 to 0006Z, August 22, 1957. Map starts shortly after take-off from Wake; for location see Fig. 5.1. Map has been schematized; not all visible clouds have been drawn. Lines, their length and spacing, are to scale. Height range for each line is indicated in 1,000's of feet.

WIND

AND

b.

•-o-

SHEAR

--QCg} 3

SHEAR

Cumulus

AND

o

Row

Formation

j>

202

Results of Flight III tion or areal density of clouds along the lines as compared to the interstices, which are not clear but merely contain fewer and smaller cumuli. This m a p supports our hypothesis that both elongation along the shear and Avsec rolls may contribute to trade cumulus rowing. In Figure 5.54 these effects combined to produce clear-cut parallel rows, whereas here in Figure 5.57 they seem to have worked at cross purposes to make the stippled pattern. Comparison of Figures 5.55 and 5.58 is striking. Thus we have apparently gained a beginning insight into row mechanisms. However, it remains a mystery why the cumuli near 2327Z were disorganized, why a little further on they lined up, and why the regime ended so suddenly where it did. Leg 4. The Leg 4 study provided the final and probably the biggest surprise of Flight III. The leg we had expected to be the dullest proved to be one of the most interesting of the series. After viewing the film and constructing the coding and cross section, we decided to investigate the meso-scale fluctuations in cloud regime by mapping this entire leg. A schematic technique was developed in order to cover so much ground ( 1 , 3 2 0 miles); the result is shown in Figure 5.60. No concentration upon individual clouds was made, but row lengths and spacings are roughly to scale. A further schematization has been made in Figure 5.61, which best brings out the character of the leg.

Fig. 5.58.-Selected frames from movie illustrating map in Fig. 5.57. a (upper): Frame taken at 2342Z, August 21, 1957. Note rows running from left foreground to right background (from 0 2 0 ° ) . Note individual clouds shearing and lying down from right to left (approximately east-west), b (lower): Frame taken at 0006Z, August 22, 1957. Note abrupt end of mapped regime.

We see five different identifiable regimes which replace each other abruptly and periodically along the flight, each lasting from 20 to 100 miles. Cumulus regimes were wider and more frequent near Hawaii. As the aircraft progressed upstream, stratus regimes began to appear more frequently and to widen. Referring back to Figure 5.60, we see that the transition between regimes rarely occupied as much as 10 miles and was often instantaneous. First, one must inquire whether this remarkable meso-scale organization is typical of this portion of the trades. We have a small bit of evidence that it is at least not unusual. On August 6, 1957, one of the authors made the outbound flight from San Francisco to Honolulu by commercial airplane and took still photographs every 15 minutes or oftener. Cloud conditions, described as typical by the crew, resembled those of Leg 4, Flight III, and suggested similar transitions in regime, insofar as could be established from the stills.

Results of Flight III

1

WIND

o a

fa

WIND

cb

WIND

t i O o

0

®

o

cxp>

WIND

2

b.

SHEAR

Cumulus when Wind

and

Row

Formation

Shear at right

angles

Fig. 5.59.-Schematic illustration of our hypothesis regarding cumulus row formation when low-level wind and shear in lower cloud layer are at roughly right angles to each other, a. Same as Fig. 5.56, a. Effect of shear on lower cloud layer shown elongating cumuli downshear. b. Avsec rolls lined parallel to wind, but at right angles to shear. Cloud rows develop along wind as suggested in Fig. 5.56, b, but clouds elongated normal to rows. c. Combination of a and b, showing clouds stretching downshear normal to Avsec rolls. Preference for clouds to break out along roll crests, but some spreading into interstices.

203

Second, it is important to seek the reason for these mesoscale transitions in cloud regime. Among other things, they must produce considerable change in albedo and radiation balance (cf. Riehl [ 1 5 ] ) . The first thing to discover is whether these transitions are associated with changes in the sea-surface temperature; this could be readily determined by an aircraft study that supplemented the photography with an airborne radiometer [ 1 6 ] . Dramatic changes in cloud regime are known to occur at the boundaries of the Gulf Stream and Cromwell Current. Also our own previous work on tropical clouds [13, 17] indicates that changes of only several hundredths of a degree Centigrade in sea temperature have noticeable effects upon the cumuli above. In any event, the evidence from this flight series suggests that rather small circulation changes may effect the sudden transition from a cumulus or clear regime to one of stratus, or vice versa. If the physical cause can be found, a clue to a new attack on modification might be provided. This is conceivable, since these small circulation changes may be highly amplified to large radiation changes via the cloud transitions. It is the ocean's excess radiation income over outgo in this region which is used to evaporate the sea water which provides the major part of the energy driving the atmosphere. The map (Fig. 5.60) of Leg 4 was also used to study cumulus rows. The orientations measured therefrom have been recorded in Table 5.2 and drawn on Figure 5.49. The cumulus line-up vacillates between 0 9 0 ° and 125°. This line-up (the three arrows in Fig. 5.49 at near right angles to the flight path) is hypothesized to be a cross-wind mode, oriented with the shear between the lower trade layer and a slightly stronger wind from south of east above. The Lihue rawin (Fig. 5.35) shows a 10-knot surface wind from 050°, strengthening and veering to 100° by 5,000 ft. The intermittent intrusion of the shearing layer down into the cumulus layer is suggested by the navigator's report. At 2140Z, his flight-level (7,000 f t ) wind is from 053°. At 2240Z, it is from 121°, and at 2340Z it is back to 083°. Thus the alternation from parallel to cross-wind mode and back can be explained by the sporadic superposition of southeasterly flow and shear at the level of the cloud tops.

Flight ill Leg 4 , Part II Ajg. Slialut

26-27, R«g>fli(

1957 Suppress«« Cumuljs Reg.*«

Cleor

R««in«

Cumulua

Cumulus

Sfro'ul

fteg.T

5"«tus

n form STIO'I

R«g.m«

| C uwulws

b^^'ganiKd

1 't*

—J. - 23392

-B

'OOO

' 00'2 2

"T

Fig. 5.60. -Schematic cloud map for entire photographed part of Leg 4, Flight III. No attempt made to map individual clouds; rows are roughly to scale in spacing and length. Stratus denoted by hatching, both alone and when over cumulus. Stratocumulus denoted by hatching with bulges.

Results of Flight III

205

N Clear Unorganized PVN^

Organized

cumulus

."A

cu - correct

direction

Stratus T-l.j-'H ; M vrl

Mixed

stratus

and

cumulus

PACIFIC

VW

Flight August Route-

ScaleHawaii 2I°N

I62°W

III

Leg

26-27,

Hawaii toward

4

1957 San

Francisco

120 n.mi. to one inch along flight

path.

Schematic

Map

of

Cloud

Fig. 5 . 6 1 . - F u r t h e r schematized m a p for Leg 4, Flight III. M a d e f r o m Fig. 5.60. W h e r e regime is organized cumuli, orientation of rows is s h o w n as m e a s u r e d .

Regimes

206 4.

SUMMARY OF FLIGHT

Results of Flight III The precipitation on Flight III was entirely concentrated into the disturbed legs, as brought out in Table 5.15.

III

Table 5.14 summarizes the distribution of sky types along the route of Flight III. We see at a glance a sharp break between the character of Legs 1-2 compared with that of Legs 3-4. Legs 1 and 2 were very disturbed and Legs 3 and 4 very undisturbed. Some of this difference is due to time changes and some to space changes. TABLE

III

(Percentages)

1 2 3 4 Over-all

5 0 0 78 19

18 0 38 15 19

5 0 27 15 11

27 0 65 100 47

5.15

Leg

Percentage of intervals with precipitation

1 2 3 4

25 82 0 0

III

Remarks

5.14

DISTRIBUTION OF SKY T Y P E S , F L I G H T

Trade-wind skies Inver- NorSupsion mal pressed Total

TABLE

D I S T R I B U T I O N O F P R E C I P I T A T I O N ON F L I G H T

Trade-wind disturbance SupNorma] pressed Total 41 5 31 0 22

5 0 4 0 3

45 0 35 0 25

Strong disturbance to typhoon 30 95 0 0 29

The character of Leg 4 is presumably typical for the region and season; it would be rare to observe disturbed skies in this part of the trades in summer. Leg 3 also shows up with a sky distribution quite similar to those of the same route on Flights I and II (Leg 1 on each). This Hawaii-Marshalls region appeared in our sample to be fairly steady in flow and over-all cloud character; it generally contained one or two easterly waves. These ranged in strength from barely detectable by swelling cumulus in slightly bent rows, to moderate with high towers, some sheet clouds, and rain. We suspect that the downstream portions of this leg could be even more disturbed on occasion. The routes from Hawaii to Wake (Leg 1) and Wake to Guam (Leg 2 ) showed violent time fluctuations within our sample, from suppression and wide clear spaces to typhoon skies. These variations were well related to changes in the over-all circulation patterns, particularly to those at upper levels.

Summary

of Flight

All in disturbance near Wake Entire leg disturbed None None

III organization

study

Although Flight III showed the poorest cumulus organization of the series, it nevertheless contributed several links in our attempt to understand the rowing process. Its evidence provided support for the criteria developed in chapter iv for the orientation of the parallel and cross-wind modes, but unhappily the wind information was too sparse to provide the rigorous test that we desired. However, some insight was gained into the necessary conditions for the appearance of cumulus rows: it seems that they are produced by a combination of roll-type convection in the subcloud layer, plus an elongation of the cumuli along the shear vector by a process described earlier by Malkus [13, 14]. The absence of definite rows in the storm-breeding section of this flight also suggested that a minimum wind strength is needed to effect a line-up. A new kind of meso-scale organization was discovered and mapped on Leg 4 of this flight. It consisted of rapid transitions (in a few miles) between regimes of cumulus, stratus, and clear, which may prove to have important implications about the airsea heat budget and thermal-dynamic relationships. Our study provides leading questions about cloud organization on these many scales which it will now be possible to resolve by specially designed observation programs. Some suggestions regarding these are made in the concluding chapter.

Results of Flight III REFERENCES 1. Riehl, H., 1948: On the formation of typhoons. J. Meteor., 5, 247-264. 2. Neiburger, M., D. S. Johnson, and C. W. Chien, 1961: Studies of the structure of the atmosphere over the eastern Pacific Ocean in summer. I. The inversion over the eastern North Pacific Ocean. Univ. of Calif. Press, Berkeley and Los Angeles. 3. Riehl, H., T. C. Yeh, J. S. Malkus, and N. E. LaSeur, 1951: The north-east trade of the Pacific Ocean. Quart. J. Roy. Met. Soc., 77, 598-626. 4. Riehl, H., and J. S. Malkus, 1957: On the heat balance and maintenance of circulation in the trades. Quart. J. Roy. Met. Soc., 83, 21-29. 5. Malkus, J. S., 1956: On the maintenance of the trade winds. Tellus. 8, 335-350. 6. Malkus, J. S., C. Ronne, and M. Chaffee, 1961 : Cloud patterns in Hurricane Daisy, 1958, Tellus, 13, 8-30. 7. Riehl, H., and R. H. Simpson, 1958: Mid-tropospheric ventilation as a constraint on hurricane development and maintenance. Proc. Tech. Conf. on Hurricanes, Miami Beach, Fla., Nov. 19-22, 1958, D4, 1-10. Am. Meteor. Soc. 8. Koteswaram, P., 1961: Cloud patterns in a tropical cyclone in the Arabian Sea viewed by Tiros I meteorological satellite.

9.

10. I I. 12. 13. 14.

15.

16. 17.

207 Sei. Rep. No. 2, Met. Div., Hawaiian Inst, of Geophys., Univ. of Hawaii. Contr. No. A F 1 9 (604)-6156. Avsec, D., 1939: Thermoconvective eddies in air. Application to Meteorology. Scientific and Tech. Pub. of Air Ministry Works of Inst, of Fluid Mech. of Fac. of Sei., Paris, N o . 155. Published (in French) at Ed. Blondel la Rougery. 7, Rue St. Lazare, Paris. Malkus, J. S., 1949: Effects of wind shear on some aspects of convection. Trans. A m . Geophys. Union, '30, 19-25. Malkus, J. S., 1952: T h e slopes of cumulus clouds in relation to external wind shear. Quart. J. Roy. Met. Soc., 78, 530-542. Malkus, J. S., 1954: Some results of a trade cumulus cloud investigation. J. Meteor., 11, 220-237. Malkus, J. S., 1955: T h e effects of a large island on the tradewind air stream. Quart. J. Roy Met. Soc., 81, 538-550. Malkus, J. S., 1958: On the structure of the trade-wind moist layer. Pap. in Phys. Oceanog and Meteor., Mass. Inst, of Tech. and Woods Hole Ocean. Inst., 13, No. 2, 47 pp. Riehl, H., 1962: Radiation measurements over the Caribbean during the autumn of 1960. Atmos. Sei. Res. Rep. No. 2, Dept. of Atmos. Sei., Colorado State Univ., Ft. Collins, Colo. Richardson, W., and C. W. Wilkins, 1958: An airborne radiation thermometer. Deep Sea Res., 5, 62-71. Malkus, J. S., 1957: Trade cumulus cloud groups: Some observations suggesting a mechanism of their origin. Tellus. 9. 33-44.

208 Results of Flight III M "8 z o 3a u

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"

+ -

? ? ? ?

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?

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4/4 4/4 4/4 4/4 4/4 4/4 4/4 4/4

-

1957

High, > 2 0 , 0 0 0 ft —7

+

17-18,

trace trace 1/41/4 + 1/4 — 1/4 + 1/2 1/2 1/4 3/4 + 3/4 + 4/4 4/4 4/4

Remarks

T r a c e inv. 8,000 f t T r a c e inv. 8,000 ft Inv. gone

F o r e g r o u n d clear F o r e g r o u n d clear; distant cb's F o r e g r o u n d clear; enter cirrus shield Broken cirrus, overcast Cirrus lowering Cirrus b e c o m i n g middle ovc. Ovc. thinned between time checks Darkening Breaks in ovc.; anvils visible Dark a n d disturbed D a r k a n d disturbed Scud is tiny cu Scud is tiny c u Scud is tiny c u Ceiling and visib. low; dark a n d disturbed

NOTE: NO significant cloud shears below anvil level. For anvil shear see cloud map (Fig. 5.53).

Results of Flight III

TABLE

211

5.4

HEIGHTS OF CLOUDS, F L I G H T I I I , L E G 2 ; FLIGHT A L T I T U D E 8 , 0 0 0 F T , AUGUST 1 7 - 1 8 ,

1957

Cumulus Time (GCT)

Base

Mean tops

M a x . tops

(ft)

(ft)

(ft)

0118 0133 0148 0207 0223 0238 0248 0303 0320 0334 0351 0405 0417 0432 0447 0500 0515 0530 0545 0600 0615 0630

2,400 2,400 2,400 2,700 2,500 2,000 1,700 1,700 1,700 2,300 2,000 2,000 2,000 1,700 1,100 1,100 7

20,000 15,000 15,000 12,000 12,000 8,000 5,000 2,500 30,000 7,000 7,000 7,000 7,000 9,000 9,000 9,000 10,000 4,000 2,500 2,000 2,000 3,000

44,000 44,000 38,000 30,000 30,000 42,000 40,000 40,000 45,000 45,000 10,000 9,000 10,000 17,000 17,000 15,000 17,000 4,500 2,500 2,000 2,000 3,500

1,200 1,300 1,300 1,300 1,500

Middle cloud (ft)

High cloud (ft)

Remarks Plane still climbing

— — — — —

.— — — — — — — —

—20,000 —19,000 ,-17,000 — 15,000 — 11,000 — 10,000 — 9,000 — 8,500

— —

38,000 38,000 38,000 ~39,000 —39,000 37-40,000 35-40,000 35-40,000 —32,000 —30,000 —26,000 7 7 7 7 ? 7 7 7

Cirrus anvils Cirrus anvils Cirrus anvils Cirrus anvils Cirrus anvils Cirrus anvils; no intermed. cl'ds Enter cirrus shield Cirrus ovc. Cirrus lowering Rain beg. — 0 4 2 0 Z Ovc. lowering Ovc. now middle cl'd Breaks in ovc.; anvils visible Cl'ds at flight level Cirrus above st.

Ovc. just above aircraft

212

Results of Flight III

TABLE

5.5

CLOUDINESS, F L I G H T I I I , L E G 3 ; K W A J A L E I N - H O N O L U L U , AUGUST 2 2 ,

Time

Cumuliform

Middle,

High,

(GCT)

Type; amount

1 0 , 0 0 0 - 2 0 , 0 0 0 ft

> 2 0 , 0 0 0 ft

2327 2342 2357 0006 0019 0032 0054 0109 0124 0139 0154 0209 0224 0239 0254 0309

1/2 1/2 + 1/4 + 1/4 trace 1/4trace —

A

—

trace 1/4-

o

trace — trace 1/4 1/2 1/2 1 / 4 --

o

1/4 1/4?

0325 0341 0402 0416 0431 0450 0502 0517 0534 0550

O O

A

1/4

-

-

Cu shear from ~ S E Small cu tilt from ~ W Small cu tilt from , ~ W Small cu regime ending Cu are distant Coming into cu Cu tiny; almost none Clear; trace cu in far distance Foreground nearly clear Small cu tilt from ~ E ; no shear visible in larger cu

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

trace 1/4 1/2 1/2 1/2

— — — — —

—

/

— —

— —

A

— —

—

+

1/2 1/2

—

1/4 1/4 trace 1/4 -

O

1/4

—

1/2 1/4 + 1/4

Remarks

—

^

^

1/4 trace trace trace trace trace trace —

1957

Cirrus appears

No obvious shear or tilt in cu No obvious shear; the cu above lit. level are distant Inv. cloud beginning No shear; all cu vertical; inv. undercast .— 1 / 2 coverage Ci retreating; cu vertical Slight cu tilt along rows from E N E Slight tilt sm. cu along rows from E S E ; large cu vertical Rows from E S E ; large cu vertical Rows from E S E ; ci are in distance All cu look vertical All cu look vertical; ci retreating: inv. tr. Inv. tr.; too dark to see well

Results of Flight III

TABLE

213

5.6

H E I G H T S OF CLOUDS, F L I G H T I I I , L E G 3 ; F L I G H T A L T I T U D E 9 , 0 0 0 F T , A U G U S T 2 2 ,

1957

(Camera aimed east of south)

Cumulus Time (GCT) 2327 2342 2357 0006 0019 0032 0054 0109 0124 0139 0154 0209 0224 0239 0254 0309 0325 0341 0402 0416 0431 0450 0502 0517 0534 0550

Base (ft) ?

—2000 1800 1800 1600 2000

1800 1800 1800 1400 1400 1400 1800 1800 1600 1600 1600 1600 1600 1600 1600 1600? 1600?

Mean tops (ft)

Max. tops (ft)

—6000 7,000 4000 5,000 6500 10,000 4000 14,000 1800 14,000 12,000 2500 trace of cu trace of cu 3000 8,000 8,000 4000 2300 - 4,000 2500 4,000 2500 3,000 3500 4,500 3500 7,000 10,000 3000 7000 13,000 missing 4000 1 3,000 2500 9,000 4000 7,000 4000 8,000 2500 9,000 2500 9,000 6000 11,000 6000 10,000

Middle

High,

10-20,000 ft (ft)

> 2 0 , 0 0 0 ft (ft)

Remarks Plane climbing Patch of alto-st. 13 mi. across

11,000

Two 23,000-ft towers at 82 mi.

40,000 40,000 40,000 40,000 40,000 40,000 40,000 40,000

Independent cirrus

Inv. near ovc. ~ 9 , 0 0 0 ft Cirrus ending Horizon bad Horizon bad Horizon bad Horizon bad Inv. starting; not too thick Inv. traces; getting dark

214

Results of Flight I I I

TABLE

5.7

CLOUDINESS, F L I G H T I I I , L E G 4 ; H O N O L U L U - S A N FRANCISCO, A U G U S T 2 6 - 2 7 ,

1957

Time

Cumul i f o r m

Inversion

High,

(GCT)

T y p e ; amount

stratus

> 2 0 , 0 0 0 ft

2023

trace

—

—

2038

1/2 1/4

—

—

Highly org.

trace

—

Shear f r o m ~ S W at 10,000 f t

3/4

—

N o visible shear

—

T r . 2d inv. st. at 14,000 f t

2053

Remarks L a r g e r cu in background

2100

1/4-

2114

1/4

2139

1/4

-

1/4

—

L e f t o v e r cb in dist. 2 1 2 1 Z

2152

1/2

-

—

—

I n v . c l o u d g o n e ; n o sign shear

—

1 /2 -

2206

none

2223

1/2-

2228

n o cu?

2247

trace

2306 2323 2339 2357

dist. trace trace £3 some cu underneath? O

0012 o

0043

o

0059

-

o

some b e l o w ? 1/4

¿3

+?

1/4

0200

1/2?

0215

1/4

-

1/4

+?

0230

Q

0245

?

0257

1/2

—

F o r e g r o u n d clear

trace

—

Shear f r o m S W ; clear patch ahead

3/4

—

St. o v e r clear patch

inv. st. ending

—

dist. trace

—

F o r e g r o u n d clear

trace

—

C u area just ended

4/4

-

4/4

_-_>

1/4 1/4 trace

1/2

0132 0147

1/4? 1/2

-

—

missing

0028

0116

1/2

at 5,000

4/4

-

. j

—

Flat inv. undercast

—

T r a d e cu r e g i m e

trace

Inv. st. overcast

1/4-

M i x e d r e g i m e ; cu and inv.

1/4 + trace

M i x e d r e g i m e ; cu vertical

—

4/4

—

C u tops p o k e through inv.

trace

—

C l e a r spot ahead

-

—

Inv. regime

trace

—

C u r e g i m e ; n o shear

3/4

—

4/4

4/4 1/4

-

—

C u tops p o k e u p

—

Cu r e g i m e

Results of Flight III

TABLE

215

5.8

H E I G H T S OF CLOUDS, F L I G H T I I I , L E G 4 ; F L I G H T A L T I T U D E 7 , 0 0 0 F T , A U G U S T 2 6 - 2 7 ,

1957

( C a m e r a aimed west of n o r t h )

Cumulus Time (GCT) 2023 2038 2053 2100 2114

Base

M e a n tops

(ft)

(ft)

1000 950 M 1400?

3000 2000 M M

2139

M

M

2152 2206 2223 2228 2247 2306 2323 2339 2357 0012 0028 0043 0059 0116 0132 0147 0200 0215 0230 0245 0257

1500

3000

—

? M 1500-2000 —

1600 —

1500 M 1700 1800 1900? 2000 —

—

—6000 M 3000 —

5500 —

4000 M 6200 5000 5500 probably no cu 5500 —

?

9

2000 1600 7

4700 4700

1700

5000

?

M a x . tops (ft)

7500 10,000 10,500 11,500

below 4800 5500 —

6000 M 6000 —

8500 —

6000 6600? 7000 7300 7000 6500 —

7000 7000 7000 7300 7000

Inversion stratus

High, > 2 0 , 0 0 0 ft

(ft)

(ft)

Plane climbing C u org. C u org. Plane above inv. st. O n e large cu in b'kg'd sending out midl. cl'd, 12-15,000 f t H'vy inv. 2 1 3 4 - 2 1 4 3 Z

—

4000 4800

4800

M a n y sm. cu N o clouds Clear coming u p Some largish cu in dist. Inv. 2 2 2 8 - 2 2 4 6 Z Clear 2246-2312Z

— —

5700 5700 —

6000 6-7000 —

6400 6400 6400 6400 6000 6000 —

6000 only tr. 5000 5000 5000

Remarks

29,000 35,000 35,000 33,000 31,000

Inv. st. u p then down Ci begins 0 0 0 4 Z Few cu tops poking

Only tr. inv.; ci deer. Ci gone Cu with inv. patches Clear, 0146-0159Z Inv. with cu tops poking Inv. with cu tops poking Inv. with cu tops poking Inv. breaks 0 2 5 6 Z

216

Results of Flight III

io E o

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