Tropical Indian Ocean Clouds 9780824885434

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INTERNATIONAL INDIAN OCEAN EXPEDITION METEOROLOGICAL MONOGRAPHS Number

h

The International Indian Ocean Expedition Meteorological Monographs, published by the East-West Center Press, contain detailed discussions and supporting data on the various components of the general atmospheric circulation over the Indian Ocean, as well as the results from measurements of atmosphere-ocean interaction made as part of the expedition's observational program. Manuscripts are solicited, and should be sent to C. S. Ramage, Department of Geosciences, University of Hawaii, Honolulu, Hawaii, U.S.A. 96822.

Editorial committee C. S. RAMAGE

M. A. ESTOQUE

MICHAEL GARSTANG

Tropical Indian Ocean Clouds

TROPICAL INDIAN OCEAN CLOUDS by Andrew F. Bunker and Margaret Chaffee

East-West Center Press

Honolulu

The publication of this volume has been aided by Grant No. GA-386 from the National Science Foundation. Contribution No. 2089 from the Woods Hole Oceanographic

Copyright © 1969 by East-West Center Press University of Hawaii All Rights Reserved Standard Book Number: 8248-0083-4 Library of Congress Catalog Card Number: 69-17882 Printed in the United States of America First Edition

Institution.

Acknowledgment Many individuals and organizations planned the International Indian Ocean Expedition (IIOE), assembling men and equipment, making cloud observations, processing data, and publishing this manuscript. The authors are grateful to all for their help. A C-54Q aircraft was loaned to the Woods Hole Oceanographic Institution ( W H O I ) by the U.S. Navy through the Office of Naval Research. The National Science Foundation and the Office of Naval Research jointly funded the modification of the aircraft for meteorological and oceanographical research. The three visits to the Indian Ocean and the data processing were supported by the National Science Foundation under Grant 22389. Claude Ronne contributed greatly to the success of the cloud study. He designed the camera system on the C-54Q and operated it during the first visit to the Indian Ocean. He also measured cloud heights and estimated cloud amounts and types from the films obtained on all three visits. Particular thanks are due Captain William Ewing and the crew of the C-54Q. We are grateful to the meteorologists at the International Meteorological Center (IMC), Colaba, and at Santa Cruz Airport, Bombay, for their help while we were operating out of Bombay and for flying with us as observers.

Time-lapse movies made on Indian Ocean flights by the U.S. Weather Bureau's Research Flight Facility (RFF) and on flights between Aden, Gan, and Singapore by the Royal Air Force (RAF) significantly enhanced the material incorporated in this study. The streamline charts presented in this monograph were drawn by meteorologists at IMC, and were revised on the basis of new data by the staff and graduate students of the Department of Geosciences, University of Hawaii. We are grateful to C. S. Ramage for his help in all aspects of our work in the Indian Ocean.

Andrew F. Bunker (1 ) Margaret Chaffee (2) Woods Hole Oceanographic Institution

Woods Hole, March 7,1968

Contents Abstract

1

Temperature and Moisture 12 Clouds and Weather 13

I. Introduction and Meteorological Background

2

IV. Northeast Monsoon

15

A . Cloud and Streamline Charts 15 B. Statistics of Cloud Heights and Amounts 15

II. Cloud Data Collection, Processing, and Presentation

4

C. Seasonal Charts 15

A . Cloud Data Sources U

Radiation 15

B. Method of Measuring Clouds from A i r c r a f t

Temperature and Moisture 16 Clouds and Weather 16

Equipped with Cloud Time-Lapse Motion Pictures 5

V. Comments on Certain Features of Tropical Indian Ocean Clouds 17

C. Supplementary Observations 7 D. Format of Presentation of Cloud and

A . Published Papers 17

Supplementary Data 8

Cloud Climatology 17

E. Format of Presentation of Seasonal Charts

Relation of Clouds to the Monsoon

f o r the Southwest and

Circulation and Subtropical Cyclones 17

Northeast Monsoons 9

Orographic Clouds 17

Radiation 9

Haze 17

Temperature and Moisture 9

B. Heights and Extent of Middle and High

Clouds and Weather 9

Clouds over the Arabian Sea 18

F. Method of Compiling Cloud Statistics 10

C. Orientation of Cumulus 19

III. Southwest Monsoon

11

D. Rain Bands 19 E. Turbulence within Clouds 20

A . Cloud and Streamline Charts 11 B. Statistics of Cloud Heights and Amounts 11

Notes and References

22

Figures 1-346

24

C. Seasonal Charts 11 Radiation 11

Abstract Photographs of clouds were taken with time-lapse cameras mounted on research and military aircraft flying over the Indian Ocean. The positions of the cloud images on the films were measured and the cloud types and amounts determined. From the measurements, cloud heights were computed. Cloud cross sections are presented to show the height, type, and amount of clouds existing over the Indian Ocean during the southwest and northeast monsoons. Satellite and still photographs of clouds, streamline charts, mean temperature and humidity charts, and solar radiation charts are included to supplement the cloud cross sections. The cloud data are presented with a minimum of comment and interpretation since the purpose of the IIOE was to collect data in a form that would make them available for study by others. Averages and extremes of cloud heights and amounts have been compiled for several regions of the Arabian Sea and the equatorial Indian Ocean during the two monsoons. The statistics for the southwest monsoon show quantitatively how the cloud heights and amounts increase from west to east across the Arabian Sea. The increase in cumulus during the northeast monsoon from 22N to the equator along the 70E meridian is defined by the averages and extremes of height and amount. Several prominent features of Indian Ocean clouds are analyzed and discussed briefly. Photographs of middle and high clouds were measured intensively over a short section of a flight made through multilayered clouds during the peak of the southwest monsoon. It is concluded that mois-

ture brought up to the different levels by cumulus activity over India and transported westward by the easterlies produced the many layers of clouds. Organization of cumulus into lines was frequently noted over the Indian Ocean in limited areas. The spacing of the rows varied widely and did not confirm predictions based on wind speed and latitude. A case of rainfall organized into bands was observed and studied off the west coast of India between 19N and UN. The bands were aligned roughly parallel to the southwest wind. The distance between the bands averaged 23 kilometers. Turbulence within cumulus was measured and found to be moderate, as is usual for oceanic clouds. The greatest root-mean-square vertical velocity observed was 174 cm second-1. The maximum updraft observed was 553 cm sec-1, and the maximum downdraft 510 cm second"1.

I.

Introduction

The International Indian Ocean Expedition was organized to make as complete a study as possible of the physical, chemical, biological, and geological oceanography and of the meteorology of the Indian Ocean. About a dozen national committees planned research programs and worked with the Special Committee on Oceanic Research to coordinate their plans. The United States meteorology program (3) was planned to obtain data that would shed light on all phases of monsoons. Since clouds influence atmosphere and ocean, and since cloud forms reveal many processes operating in the atmosphere, cloud photography was made an integral part of the observational program. As a result a large number of time-lapse movies and still photographs of clouds were obtained from aircraft and satellites over the Indian Ocean. This collection of photographic data is the basis for the present monograph. The southwest and northeast monsoons of eastern Africa, India, and southwestern Asia have been recognized from prehistoric times as a predominant annual feature of the weather pattern, and their description appears in many textbooks (4). These descriptions have been based on several centuries of surface observations over land and oceans and a few years of upper-air observations over India and adjacent countries.

and Meteorological Background Thus we do know something of the onset time of monsoon rains in different regions; of rainfall occurrence and amounts; and of pressure patterns, cyclone tracks, and oscillations in the strength of the monsoon. While this information does allow statistical studies of certain features of the monsoon, it is still too limited in geographical and vertical extent to reveal the structure and dynamics of monsoon systems. Most of the statistics of cloud cover and type over the Indian Ocean have been compiled from ground stations or from ship observations and are available in several atlases (5). Statistics of the height, type, and amount of clouds from aircraft observations are lacking over most regions of the Indian Ocean. A study has been made by Rao (6) of the middle and high clouds and of thunderstorms occurring over India and the Bay of Bengal based on commercial jet-aircraft pilot reports. The statistics presented constitute an important contribution to our knowledge of clouds over the Bay, which will not be superseded by observations made during the HOE, since relatively few flights were made over the Bay of Bengal by IIOE aircraft. An investigation (7) of the intertropical front over northwest India has been based on RAF pilot observations of clouds. To fill the gaps in our knowledge of clouds over the Indian Ocean, the Expedition research aircraft were equipped with time-lapse movie cameras which were operated on all flights. The Transport Command of the RAF cooperated enthusiastically by mounting and using time-lapse cameras on many of their aircraft flying on the Aden-Gan-Singapore route. During 1963 and 1964 many thousands of feet of color and black and white film were used in photographing clouds over the Arabian Sea and the Indian Ocean.

A small portion of the cloud data has already been used in studies of Indian Ocean meteorology (8, 9, 10). It is the purpose of this monograph to present useful analyses of data so that those interested in further studies of monsoons can obtain the information that they need. In general, although movie pictures were taken at 2- to 10second intervals, measurements were made only on about one frame in a hundred. Hence, more information is contained on the films than is presented here. The time-lapse films and 35 mm stereo-pairs taken by the RAF between Aden and Singapore are on file either in the Department of Geosciences, University of Hawaii, or in the Woods Hole Oceanographic Institution and are available for detailed studies of clouds in specific regions on particular days.

IL

Cloud Data

A. CLOUD DATA SOURCES

The cloud sections and maps presented in this monograph were compiled from three sources of time-lapse films of clouds photographed from aircraft. The Woods Hole Oceanographic Institution's C-54Q aircraft was equipped with two Bolex 16 mm ciné cameras, which were used alternately to photograph clouds from the port side of the aircraft at intervals of 2 seconds. Two K-100 movie cameras were used alternately to photograph clouds from the starboard side of the aircraft at intervals of about 10 seconds. The cameras were battery operated, with the motor rheostatically controlled so that the correct time-lapse interval could be maintained (11). The Bolex had a 57-degree Angénieux lens, which was interchanged between the two cameras. Written dates and times were photographed at the beginning and end of each roll, and usually at 15- to 20-minute intervals during the exposure of the film. Navigational information, i.e., time, true airspeed, ground speed, heading, pressure altitude, and drift, were presented on a cabin instrument panel, which was photographed every 20 seconds on black and white film by a BeattieColeman 35 mm camera. The cameras were operated throughout the daylight hours on every Indian Ocean research flight, for a total of 34 days. The U.S. Weather Bureau Research Flight Facility's two DC-6 aircraft made flights over the Indian Ocean in May, June, and July of 1963 and in February and March of 1964. Observational missions were flown on 41 days during these 5 months. The aircraft were equipped with 16 mm

Collection, Processing, and Presentation color time-lapse cameras, which were pointed either perpendicular to the fuselage or out the nose of the aircraft, and which took a picture every 2 seconds. Specially built 35 mm time-lapse cameras were run at one frame every 5 seconds with black and white film during the second period. These black and white films were of limited use because of the difficulty of viewing them and because of the poor contrast between clouds. Royal Air Force Transport Command aircraft which flew between Aden, Gan, and Singapore were provided with two battery-powered K-100 16 mm time-lapse cameras. Figure 1 shows an installation at the window of a Britannia aircraft. Usually only a single 100-foot roll was exposed on each flight, so there was no complete coverage from take-off to landing. The films were generally of good to excellent quality. A i r crews provided wind data, navigational checks, and altitude and air speed information as well as marking time intervals on the film. Usable film was available from twenty-eight of these flights during February, March, July, August, and September of 1964. A chart of the Indian Ocean is presented as Figure 2 to show those routes flown by the research and military aircraft during the IIOE which yielded usable cloud photographs.

B. METHOD OF MEASURING CLOUD HEIGHTS FROM AIRCRAFT EQUIPPED W I T H TIMELAPSE MOTION PICTURE CAMERAS Ronne (12) developed a technique of measuring the heights of clouds from time-lapse motion pictures taken by a camera, with a known

field of view and a fixed time interval between exposures, mounted in an aircraft flying with a known speed at a known altitude. Bunker (13) modified the technique, and wrote a computer program to determine cloud heights. Figure 3 shows the geometry involved in determining the distance between a cloud and the aircraft by counting the number of frames required f o r the cloud to progress from the center of the field of view to the edge. It is assumed that measurements are made using a projector that has a frame counter and that the field of view of the projector gate is known. The equation f o r the cloud distance is: Distance =

( A i r speed) (A Frame Count) ( A T ) /tan (V2 field of v i e w )

The aircraft speed to be used in the equation depends upon the wind field in the region of the clouds and of the aircraft. If clouds near the aircraft and at about the same altitude are being measured, the true air speed should be used. I f the clouds are f a r away and at an altitude where the winds are unknown, better accuracy is obtained by using the ground speed of the aircraft. If the winds are known both in the region of the aircraft and in that of distant or high clouds, then it is best to use the speed of the aircraft relative to the air in the vicinity of the clouds. This refinement can rarely be applied. All measurements in the present study were made using the ground speed of the aircraft. Figure 4 shows that the height of a cloud can be obtained by summing the vertical distance of the cloud above (or below) the level line, the altitude of the aircraft, and the curvature of the

6

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INDIAN

OCEAN

CLOUDS

T A B L E 1: Cloud Height Computations before and after the Level-line Correction 2 6 June 1963, Flight Altitude 4 5 2 0 Meters Before Correction

A f t e r Correction

GMT

Latitude, deg. min.

Longitude, deg. min.

Type, code*

Amount, octas

Distance, t meters

Base, meters

Tops, meters

Base, meters

Tops, meters

0826 0831 0838 0846 0852 0858

11 11 11 11 11 12

61 61 62 62 62 63

2 2 2 2 2 2

2 2 2 2 2 2

28,304 29,834 27,156 16,664 21,419 21,419

-2204 -2530 -1937 -273 -579 -766

-1072 -1337 -579 370 278 275

550 550 550 550 550 550

1682 1743 1908 1193 1407 1621

27N 33N 39 N 47 N 54N 00N

28 43 02 24 43 00

E E E E E E

' I n t e r n a t i o n a l synoptic code f o r low clouds, t Distance of the cloud f r o m the aircraft.

earth away from the tangent line. The altitude angle of the cloud is found by projecting the picture upon a screen on which a scale is drawn giving the proper angles for the particular camera lens used. The height of the tangent line above the earth's surface at a distance, d, from the point of tangency is given by the approximate equation, height=dV2R,

where R is the radius of the earth. This equation gives the curvature correction with an accuracy of a few per cent in the distance ranges encountered. The measurement of the altitude angle is the crux of the system and warrants consideration. First a screen must be modified for each cameraprojector combination. The outline of the projector gate should be drafted on the screen and the screen distance adjusted, so that when measurements are to be made the frame always fits the outline exactly. On the vertical center line of the screen, a drafting of the tangent of the elevation angle is made with zero at the center of the screen. In cases where the aircraft is flying at low levels with unlimited visibility, the horizon line in the picture should be adjusted to coincide with the zero angle line. At this time it is well to note the spot at which the horizon line intercepts the trailing edge of the aircraft wing (if it shows in the photograph). Knowing this spot is a great help in locating the level line when the horizon is obscured by haze or clouds. For this spot to be at

all useful, the aircraft must, of course, be flying in a straight line, and without any pitching or rolling. In practice we have found that at low flying altitudes the horizon or level line is frequently obscured by clouds or haze, while at high flying altitudes the horizon is below the level line. To overcome this difficulty, use is made of a meteorological phenomenon. It is well known that on any given day in an area with no rain or fronts, the bases of cumulus vary in height by only a small fraction of the mean height. Hence, if the mean height of the base is known from radiosoundings or from observations, then the level line can be computed from measurements of the altitude angle of the cumulus bases. This technique greatly improves the accuracy of height measurements when hazy conditions prevail. Table 1 shows the effectiveness of the correction. It is obvious from computations of bases and tops where the level line correction was not used that very poor guesses were made as to the position of the level line. The last column presents the heights of the cloud tops, using the correction part of the program and a previously measured cloudbase height of 550 meters. The over-all average accuracy of the determinations of cloud height is estimated to be between ± 10 to 15 per cent of the height. This value was found by estimating the errors of the quantities used in the determination, and by propagating these errors to a final precision index. The ground speed, aircraft altitude, film frame

CLOUD

count, exposure interval, and camera constants can all be determined with an accuracy of a few per cent. The measurement of the altitude angle of a cloud is more difficult and has a probable error of about ± 10 per cent even after applying the technique of the level line correction. This error varies with the diffuseness of the cloud being measured; cumulus clouds, which characteristically have sharp outlines, can be measured more accurately, whereas layers of stratus, with few or no sharply defined features, have errors considerably greater than the average. On missions flown by the r f f in the summer of 1963, the 16 mm time-lapse color movies were taken from the side of the aircraft at 30 frames per minute, and were electronically timed by coded lights on the left-hand side of each frame. Unfortunately, these lights were not always visible in the frame, and hence the time of exposure of the film was questionable. On the winter expedition to the Indian Ocean the 16 mm camera was mounted in the nose of the aircraft. Since the cloud heights cannot be measured by the same photogrammetric technique from film taken by a forward-looking camera, the heights were estimated visually. Only three of the upper-level flights (4, 5, and 20 February 1964) were reduced in this manner (Figs. 234, 238, and 254). The heights indicated on these sections and maps for the cumulonimbus and the high-cloud forms are probably within 20 per cent of the true value. The 35 mm black and white films were virtually useless for this type of reduction. On the

DATA

COLLECTION,

PROCESSING,

AND

PRESENTATION

7

low-level flights, the time-lapse interval of 5 seconds was too long, so that the record is discontinuous. In heavy cloud cover or layered clouds, the various shades of gray on black and white film are difficult to differentiate. Because of these difficulties only the 35 mm films obtained on the low-level flight of 20 February 1964 were used. Only estimates of the cloud cover were made. Generally, enough information was provided with the RAF films to locate with reasonable accuracy the section of the flight during which the film was exposed. Time checks on the film, along with navigators' positions, allowed computation of the ground speed of the Britannia. Between Aden and Gan the aircraft usually flew at 5.5 km to 6 km, and between Gan and Singapore, at 6.7 km to 7.3 kilometers. The maps and sections for these flights were prepared mostly by estimates because of the deficiency of necessary data for photogrammetric measurements. Between Gan and Singapore, cumulonimbus were frequently encountered; measurements could not be made because the cloud bases seldom were visible or their heights known. C. SUPPLEMENTARY OBSERVATIONS

Stereo-pairs of color photographs of interesting cloud formations were taken by a 35 mm still camera from RAF aircraft flying between Aden and Singapore. The meteorological satellites tiros vi, vii, and viii made passes over the Indian Ocean during the Expedition period. Cloud photographs from eighteen of the passes have been selected for presentation since on each pass one or more of the observing aircraft were in the area of photographic coverage. More complete, systematic studies of Indian Ocean clouds from satellite data have been made (1U, 15). Solar and sky short-wavelength radiation observations were made from two Eppley pyreheliometers mounted on the whoi C-54Q aircraft. One instrument was mounted on the top of the aircraft to receive direct, scattered, and reflected solar radiation. The other instrument was mounted on the bottom of the aircraft to receive the hemispherically integrated, reflected, and scattered light from the sea, clouds, and atmosphere below the aircraft.

8

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INDIAN OCEAN

CLOUDS

D. FORMAT OF PRESENTATION OF CLOUD AND SUPPLEMENTARY DATA Cloud cross sections and cloud maps were prepared f r o m all usable film f r o m the three airc r a f t sources. The cloud sections are presented in a standardized schematic manner. Flights f r o m north to south and west to east are plotted f r o m left to right on the section and conversely plotted f o r flights in the opposite directions. All of the cloud data pertaining to a given day are presented together. A small map showing the flight path is inserted in the corner of each cross section. Whenever available or necessary, a satellite photograph and a cloud m a p are given. Photographs of clouds taken f r o m the a i r c r a f t are presented whenever they contribute to the understanding of the cloud forms. Since the charts and photographs are not analyzed or interpreted as a whole in this monograph, the legends to figures are used to call attention to characteristic or significant cloud formations. Clouds or cloud formations t h a t have been the subject of study elsewhere are commented on and the reference to the study cited. Measurements of the base and tops of average cumulus-type clouds were plotted on cross sections with time as the abscissa, at 15-minute intervals. The geographical position of particular clouds can be found by r e f e r r i n g to the map inserted in the cross section. The WHOI C-54Q airc r a f t usually traveled 3.0 degrees of latitude per h o u r ; the RFF DC-6, about 3.9 degrees per h o u r ; and the RAF Britannias, about 5.0 degrees per hour. Cloud amounts were determined visually f r o m the projected image of the clouds on the screen. The amount of sky coverage by clouds within the field of view of the camera was considered characteristic of the whole sky. The number of octas of the field of view filled by a particular type of cloud is the cloud amount used in this study. The amount of cumulus cloudiness in octas is indicated continuously below the base line of the section. The middle- and high-cloud cover is drawn at the measured height; the amount in octas shown by a digit, and the type of cloud, by the standard cloud symbol (16). On occasion, the tops of distant cumulus congestus or cumulonimbus were visible and measurable. These are indicated by a dotted line which does not extend to the base of the section. When organized cumulus were encountered, the strength of the organization and the orientation of the lines relative to the flight path are indicated by dotted lines f o r weak organization, dashed lines f o r moderate, and solid lines f o r

strong. Shearing of cumulus is indicated by a small arrow. The a r r o w gives the direction of shear relative to the flight line. Representation of clouds on cloud maps presented some problems, especially in areas of multilayered clouds or heavy cirrus over cumulus. The following procedure was finally devised and followed. All clouds were plotted on the side of the flight path which the camera was viewing as long as it was possible to maintain clarity in the map presentation. But if cloud layers became too congested, only the lower-level clouds were plotted on that side; the higher-level clouds were plotted on the opposite side. Cumulus humulis clouds are represented by dots, cumulus congestus clouds by a "cauliflower type" form, and cumulonimbus anvils are drawn on these "cauliflowers" with thin lines in the direction of anvil shear. Stratus clouds are indicated by cross-hatched wavy lines, altocumulus clouds by single wavy lines, thin cirrus clouds by longer "filament-like" lines, and heavy cirrostratus or cirrocumulus by cross-hatched lines. Stippling along the flight line indicates t h a t the a i r c r a f t was flying in clouds. In cases where two or three a i r c r a f t were working in the same area at the same time, the cloud maps were combined. The map f o r 20 Febr u a r y 1964 (Fig. 252) shows clouds plotted f r o m f o u r flights; two RFF flights at two levels, a WHOI flight, and an RAF flight. Flight lines of a i r c r a f t crossing areas photographed by TIROS are marked directly on the TIROS pictures. Capital letters indicate geographical positions on the cloud sections or maps and their counterparts on the picture. Positions A, B, C on an RAF flight f r o m Aden to Gan and D, E, F on an RFF flight f r o m Mauritius to Gan, which appear on Figure 235 (cloud m a p ) , are the same as shown on Figure 236 (TIROS picture). Heavy haze was a phenomenon encountered during the summer monsoon. Amounts of haze are indicated on the sections by the usual haze symbol oo, and the denseness by filling in the symbol, i.e., a totally filled-in symbol indicates t h a t haze obscured clouds more than 3 km or 4 km away. Kinematic analyses of the surface and 700 mb winds are presented to aid in interpreting the cloud data. The operational charts d r a w n at IMC, and now on file at the Institute of Tropical Meteorology, Poona, India, were used as the basis f o r the series. Staff and graduate students of the Department of Geosciences, University of Hawaii, added data received later, checked continuity, and made the final streamline and isotach analyses.

CLOUD

Daily 1200 GMT charts have been prepared for the periods 15 June to 10 July 1963 (Figs. 91 to 142), 5 August to 10 September 1964 (Figs. 149 through 222), and 3 February to 7 March 1964 (Figs. 266 through 333). Charts for eight other days separate from the three series are presented without continuity (Figs. 81 through 90, 143 through 148). They were drafted from the IMC operational charts. Isotachs in knots are indicated by dashed lines, where the wind-speed field can be delineated with some confidence. Cyclonic and anticyclonic centers are indicated by C and A, respectively. Equatorial vortices are indicated by E. In using these charts it should be kept in mind that the Indian Ocean is vast, and even with the radiowind stations which were added during the Expedition, the number of wind observations is too small to guarantee that all vortices have been detected and accurately positioned. Continuity procedures lessen the probability of vortices escaping detection. The data presented have been separated into two sections, corresponding to the southwest monsoon and the northeast monsoon. E. FORMAT OF PRESENTATION OF SEASONAL CHARTS FOR THE SOUTHWEST AND NORTHEAST MONSOONS Radiation

The radiation recorded every 5 minutes by the upward-looking Eppley pyreheliometer was processed by the computer to give daily totals of solar and sky short-wavelength radiation falling on a level surface. This daily total was derived by assuming the instantaneous reading to be typical for that latitude, longitude, date, and time of day, and to be integrated over all of the solar altitudes and hour angles that the sun passed through on that day at that place. Values, in Langleys per day, were plotted on a chart, averaged by 1-degree squares, and smoothed. Radiation isopleths were drawn for levels below 1000 m and above 3000 meters. The precision of the daily insolation values is lower than the daily totals measured by groundbased equipment. No automatic device was used to keep the radiometer level. Data recorded while the aircraft was banking or climbing were not used, but the effects of small variations in the pitch and roll of the aircraft could not be eliminated. The method of computing the daily insolation from a single reading is not exact. No

DATA

COLLECTION,

PROCESSING,

AND

PRESENTATION

9

attempt was made in the computation process to correct for the absorption of solar radiation by the atmosphere at low solar altitudes. A comparison of the average values with climatological averages at Bombay indicates that errors are probably less than 5 per cent. Maximum values observed at 600 mb under clear skies also indicate an inaccuracy of about 5 per cent. Distribution of the high-level albedo is depicted by isopleths of the fraction of incoming radiation. Albedo values observed below the cloud bases are not presented since reflected light at these levels amounts to only 1 or 2 per cent of the incoming radiation, and varies very little with geographical area. Temperature and Moisture

Vertical cross sections of potential temperature and mixing ratio are presented to aid in understanding the mechanisms producing the clouds. The sections were constructed from measurements made along mean southwest and mean northeast monsoon streamlines over the Arabian Sea. During the southivest monsoon, 80 soundings were made along a mean low-level streamline which curves from the equator off East Africa northeastward past the coast of Somalia and across the Arabian Sea to the Bombay region. The sections, which incorporate observations from research aircraft and from the research vessels, Atlantis II and Argo, extend from the surface to 300 millibars. Geographical coordinates of the mean streamline are given at the bottom of each section. Thirty-three dropsondes were available for the northeast monsoon sections which extend from the surface to 600 millibars. The abscissa represents distance in kilometers. The sections lie along a line from offshore of Bombay to the equator at about 68E. Thus they deviate from a mean streamline south of about 6N where air crosses the sections from east to west. Clouds a n d Weather

To aid in determining the limits of various types and layers of clouds, cloud occurrence charts were constructed. Areas in which many observations were made are enclosed by dotted lines. In regions with few observations, the individual tracks of the aircraft are indicated by solid lines. The six charts for each season show where at least one observation was made of: (a) 3 octas

10

TROPICAL

INDIAN

OCEAN

CLOUDS

or more of cumulus, LI, clouds (16) ; (b) any amount of cumulus congestus, L2, or cumulonimbus, L3; (c) more than 4 octas of any type of middle cloud; (d) more than 4 octas of any type of high cloud; (e) any observation of rain; and (/) any observation of moderate or thick haze. There are some deficiencies in these data arising from obscuration of clouds at one level by clouds at another level. In general, this problem is not serious, since the whoi aircraft usually was flown back to its base at a different level from that outbound, and one RFF DC-6 flew a low-level track while the other aircraft was flying at a high level. F. METHOD OF COMPILING CLOUD STATISTICS

The cloud information contained in the cloud cross sections has been grouped by areas and by monsoon season over the Indian Ocean and averages and extreme ranges of heights and amounts have been found for each group. For the southwest monsoon, eight areas have been selected for averaging. Six, which lie along the mean lowlevel streamline from East Africa to near Bombay, coincide with the temperature and moisture cross sections. The remaining two areas lie along the equator between Gan and Sumatra. For the northeast monsoon, four areas were selected over the Arabian Sea between the Coast of India and the equator. Three areas were chosen along the flight path of the RAF aircraft from Aden to Gan to Singapore. For each area average cloud height and amount were determined visually from the cloud cross sections. Biases of various types undoubtedly enter when this method is used, but no other method is practical when the number of individual clouds photographed exceeds 1 billion. Vast numbers of small cumulus were omitted from the cross sections. Thus the average height tabulated must be higher than a true average based on an actual cloud-by-cloud determination. Since the ratio of tall clouds to small varies from area to area and from time to time, no meaningful correction can be made. Extreme cloud heights and amounts in the selected areas have been tabulated to clarify the statistical picture of the clouds. In determining the maximum and minimum sky coverage the rule was followed that the sky coverage remain constant for about 50 kilometers. Without such a rule, most maxima and minima would degenerate to 8 octas and to 0 octas, respectively. Low stratus was observed too infrequently for statistics of this cloud type to be meaningful.

m. Southwest Monsoon A. CLOUD AND STREAMLINE CHARTS The cloud cross sections, cloud maps, and and individual cloud photographs for the southwest monsoon are presented in this section as Figures 5 through 80. The periods covered are from 11 May 1963 to 10 July 1963, and from 26 July 1964 to 9 September 1964. The streamline charts follow the cloud charts in chronological order as Figures 81 through 222. The first series of aircraft observations was started during the pre-monsoon period of 1963 and extended through the fully developed monsoon phase in mid-July. From 11 May 1963 until 12 June 1963, when the monsoon rains arrived over Gujarat (17), the synoptic situation was dominated by cyclones and anticyclones over the Bay of Bengal and the Arabian Sea. The surface wind pattern slowly changed in direction from varying from northwest to southwest winds to a nearly steady southwest wind. The monsoon strength increased after 12 June 1963 until early July, when heavy rain predominated. The 1963 monsoon rainfall was almost normal (17) with deviations from the normal in the various districts of India ranging between + 4 6 per cent and —32 per cent. The second observation period encompassed the fully developed monsoon and the weakening phase. Many short "breaks" in monsoon rains occurred in the Bombay region in early September 1964. The "retreat" of the monsoon commenced during the last week of September (18). In 1964 monsoon rainfall was nearly normal except in Punjab where it was 60 per cent in excess, and in Gangetic West Bengal, where it was 20 per cent deficient (18).

TIROS

B. STATISTICS OF CLOUD HEIGHTS AND AMOUNTS For each area (see page 12) averages of cloud heights and amounts have been entered in Table 2; extreme values of the heights and amounts have been entered in Table 3. The most conspicuous feature revealed by the tables is the marked increase in the height and amount of cumulus over the region off Somalia to India. Along the same path the amount of middle and high clouds also increases rapidly, but the average height shows no systematic change. Along the equator no trend can be determined. The extreme heights given in Table 3 are less than the extremes found by Rao (6) over India and the Bay of Bengal.

C. SEASONAL CHARTS Radiation

The short-wavelength solar and sky radiation values averaged over the summer monsoon are presented in Figures 223 and 224. These charts were constructed from 1566 individual observations. In interpreting the values it should be noted that approximately 930 cal c m - 2 d a y - 1 is received from the sun by a horizontal surface outside the earth's atmosphere at 20N during the months of June, July, August, and September. Apart from providing information useful for heat-budget studies, the charts give a quantitative measure of the extent of the clouds and the absorption and scattering of radiation by clouds and haze above the aircraft.

12

TROPICAL

T A B L E 2:

INDIAN

OCEAN

CLOUDS

Average Cloud Heights and Amounts Observed during the Southwest Monsoon

Middle C l o u d

Area

Height of Cumulus and Cumulus Congestus, meters

A m o u n t of A l l Cumulus, octas

High C l o u d

Height of Cumulonimbus, meters

Height, meters

8200

4000

9300

Amount, octas

Height, meters

70E t o India, 12N t o 20N

2400

65E t o 70E, 11N t o 19N

1700

3500

9100

60E t o 65E, 10N t o 18N

1600

4100

8700

55E t o 60E, 07N t o 15N

1400

4200

8800

50E t o 55E, 05N t o 13N

1500

4000

9300

45E t o 50E, 02S t o 06N

3300

3700

9000

70E t o 85E, 04S t o 04N

3200

7900

5400

8000

85E t o 95E, 02S t o 06N

3900

6000

5400

7400

Centers of low insolation appear on both charts of low- and high-level clouds off the west coast of India, showing the extent and density of the clouds. A secondary minimum occurs at about U N , 60E. Figure 225 presents the high-level observations of albedo. Most of the reflected light comes from the tops of cumulus and of middle clouds below the aircraft. A smaller amount comes from scattering by the haze and from reflection by the sea surface. Thus, albedo maxima coincide with areas of greater low cloudiness. Temperature and Moisture

Changes in potential temperature and mixing ratio of the air along a line that approximates the main streamline below 700 mb are presented in Figures 226 and 227. The cross sections show an important feature of the stability structure of the monsoon air. The increase of potential temperature with height is not as great as the increase of the potential temperature of an adiabatically ascending saturated parcel, hence the air is conditionally unstable. Further, with large water-vapor content in the

Amount, octas

lower levels the air is latently unstable. Under these conditions, cumulus convection would be suppressed unless the lower air is lifted. Lowlevel convergence frequently occurs over the eastern Arabian Sea (8), thereby releasing the latent instability of the air and producing active convection and rain. As the monsoon air rises over the Western Ghats, the convective process intensifies, and the rainfall becomes heavy. The cool air near the surface between 7N and 9N is the result of heat transfer to the cold, upwelling water off the Somali coast. This feature has been discussed in detail (20). The general warming of the air as it crosses the Arabian Sea is caused by turbulent transfer of sensible heat from the sea to the air, and by the release of the latent heat of condensation of water vapor. The mixing-ratio cross section shows that the air mass accumulates water vapor as it crosses the Arabian Sea. The increase above 500 mb reflects primarily vertical transport of water vapor by tall cumulonimbus over the land, and westward transport of the vapor by the upper tropospheric easterlies. Discussions of the fluxes of sensible

SOUTHWEST

T A B L E 3:

MONSOON

13

Extremes of Cloud Heights and Amounts Observed during the Southwest Monsoon

High Cloud

Middle Cloud

Area

Height of Cumulus Tops, meters

A m o u n t of A l l Cumulus, octas

70E t o India, 12N t o 20N

Max. 10,200 Min. 6 0 0

8,800 1,900

13,400 6,000

65E t o 70E, 11N t o 19N

Max. 10,000 Min. 700

8.000 2.200

13,400 8,500

60E t o 65E, 10N t o 18N

Max. 6,300 Min. 700

6,200 2,200

11,800 7,500

55E t o 60E, 07N t o 15N

Max. 3,800 Min. 700

5,800 2,500

10,800 7,000

50E t o 55E, 05N t o 13N

Max. 3,000 Min. 8 0 0

5,800 2,200

11,800 8,400

45E t o 55E, 02S t o 06N

Max. 6 , 6 0 0 Min. 9 0 0

6,600 2,400

10,200 8,500

70E t o 85E, 04S t o 04 N

Max. 10,400 Min. 700

6,400 2,800

10,400 5,800

85E t o 95E, 02S t o 06 N

Max. 7,800 Min. 1,000

6,000 2.800

8,000 6,400

Height, meters

and latent heat through the monsoon atmosphere are given by several authors (21). Clouds and Weather

Figures 228 through 231 show the distributions of cumulus, cumulus congestus or cumulonimbus, middle clouds, and cirrus, and thus delineate the extent of the monsoon cloud system. Since the numerous tracks flown still left many areas unsurveyed, no definitive outline of the cloud system can be drawn. Cumulus humilis are widely spread over the Arabian Sea except for the region northeast of Somalia. East of 60E, cumulus congestus and cumulonimbus predominate (Fig. 229). Sky covered by cumulus humilis drops below 3 octas in many areas, since the small cumulus are incorporated into the more actively growing clouds. Cumulus congestus and cumulonimbus occur over a wide belt stretching from 24N along the coast of India to the equator. Another more irregular belt extends westward from India to about 60E. These clouds are also widespread along the equator east of 70E.

Amount, octas

Height, meters

Amount, octas

Figure 230 shows that the distribution of middle clouds is widespread, but that the spacing is irregular. The atmosphere over the eastern Arabian Sea is cloudier than over the rest of the Arabian Sea. The distribution of cirrus presented in Figure 231 follows that of the pattern of middle clouds. Its apparently greater irregularity may result from obscuration by middle cloud formations. The monsoon rain (Fig. 232) was concentrated to the west of the Bombay area, with a narrow branch extending to the equator. The small area of rainfall in the Gulf of Aden was associated with a cyclone on 1 September 1964. Much rain probably fell from the cumulonimbus observed along the Gan-to-Singapore route, but it could not be confirmed by the photographs taken from the flight level of jet aircraft. Figure 233 shows that haze is the most widespread monsoon weather phenomenon over the Arabian Sea. Along the equator, haze is infrequent and is less dense (see page 141).

14

TROPICAL

T A B L E 4:

INDIAN

OCEAN

CLOUDS

Average Cloud Heights and Amounts Observed during the Northeast Monsoon

Height of Cumulus and Cumulus Congestus Tops, meters

Area

High Cloud

Middle Cloud A m o u n t of A l l Cumulus, octas

Height of C u m u l o n i m b u s Tops, meters

Height, meters

Amount, octas

Height, meters

Amount, octas

Trace

-

-

0

11,800

Trace

68E t o India, 15N t o 22N

800

70E t o India, 15N t o 08N

1,300

2

-

-

0

10,500

Trace

70E t o 78E, 08N t o 03N

2,300

3

9,000

5,000

2

10,500

1

70E t o 75E, 02S t o 03N

3,400

4

8,400

4,400

4

9,100

2

07N,60E t o 2N, 70E

2,800

4

-

7,000

2

7,500

4

0 0 N , 75E t o 03N, 85E

3,700

4

9,400

5,300

4

9,600

4

03N,85E t o 0 6 N , 95E

3,900

4

9,600

5,700

3

9,800

5

T A B L E 5:

Extremes of Cloud Heights and Amounts Observed during the Northeast Monsoon

Middle C l o u d

Area

Height of Cumulus Tops, meters

A m o u n t of A l l Cumulus, octas

Height, meters

68E t o India, 15N t o 22N

Max. 8 0 0 Min. 8 0 0

Trace 0

—

70E t o India, 15N t o 08N

Max. 2,000 Min. 800

6 0

70E t o 78E, 08N t o 03N

Max. 9,000 Min. 6 0 0

70E t o 75E, 02S t o 03N

High Cloud Amount, octas

Height, meters

Amount, octas

0 0

13,000 10,000

2 0

-

0 0

11,000 10,000

Trace 0

4 0

7,000 1,800

6 0

11,000 10,000

3 0

Max. 11,000 Min. 500

7 1

5,800 2,800

8 0

12,000 6,500

8 0

0 7 N , 60E t o 0 2 N , 70E

Max. 5,200 M i n . 1,000

8 0

7,000 4,000

7 0

8,300 5,500

7 1

00N,75E t o 0 3 N , 85E

Max. 10,600 M i n . 1,000

7 2

6,500 3,000

7 0

12,000 6,400

8 0

03N,85E t o 0 6 N , 95 E

Max. 10,200 Min. 2,000

7 1

6,000 4,200

7 0

11,600 6,400

8 0

-

_

iv. Northeast

Monsoon

A. CLOUD AND STREAMLINE CHARTS The cloud cross sections, cloud maps, TIROS photographs (Figs. 234 through 265) and streamline charts (Figs. 266 through 333) are presented f o r the northeast monsoon in the same manner as the data f o r the southwest monsoon. Observations were made during the period 4 February 1964 through 7 March 1964. Several figures consist of only the flight paths flown out of Bombay, since no clouds were observed. Other charts show only a trace or a f e w octas of clouds of any type. The clear skies of the northeast monsoon result from dry air subsiding out of the Asiatic anticyclone. Only after the air has traveled a f e w hundred kilometers over the water does it accumulate enough water vapor to produce clouds. Southward in the equatorial convergence zone many cumulus and cumulonimbus were encountered. B. STATISTICS OF CLOUD HEIGHTS AND AMOUNTS F o r each area (see page 14) averages of cloud heights and amounts are presented in Table 4; extreme values of the heights and amounts are presented in Table 5. The most conspicuous feature of the northeast monsoon is near absence of clouds of any type over the northeastern Arabian Sea. The maximum cloudiness observed on any day was 2 octas of cirrus. N o middle cloud of any kind and only a trace of cumulus were observed. The monsoons have little influence on clouds in the equatorial belt. Heights and amounts during the northeast monsoon only slightly exceed those of the southwest monsoon.

C. SEASONAL CHARTS Radiation

The solar and sky short-wavelength radiation averages f o r the winter monsoon are presented in Figures 334, 335, and 336. These averages are based on 1323 individual observations. During February and early March the solar radiation falling on a horizontal surface at 20N outside the earth's atmosphere increases from about 700 Langleys to 800 Langleys per day. A t the equator the insolation is nearly constant at about 900 Langleys per day. In the northern Arabian Sea, insolation is high as a result of generally clear skies. South of India and along the equator, clouds associated with the equatorial trough are present and the insolation decreases and becomes more variable. A word of explanation is necessary to understand the radiation values presented along the equator. According to the chart, more radiation is received near the surface than above 3000 meters. This impossible situation came about since only one flight was made out of Gan to the west along the equator. On the low-level westbound leg the clouds consisted mainly of cumulus and thin high clouds (see Fig. 255), and hence a large amount of radiation was received. On the high-level eastbound leg large cumulonimbus and thick altostratus which had developed (see Fig. 256) greatly diminished the insolation. Thus the apparently peculiar radiation pattern results from too f e w observations and a rapidly changing cloud distribution. In other regions, many observations were made on many days, thereby giving a more realistic distribution.

16

TROPICAL

INDIAN OCEAN

CLOUDS

Temperature and Moisture Cross sections have been constructed to show the variations in potential temperature ( F i g . 337) and water vapor ( F i g . 338) along a line f r o m the region offshore of Bombay to the equator at 68E. The abscissa is distance expressed in kilometers f r o m the Bombay region. In the northern part the section parallels the mean streamline. South of about 1400 km out of Bombay, the section no longer represents changes along the streamline. From there to the equator the section describes the structure of the easterlies (see page 1 8 9 ) . The air flowing off the northwest coast of India is stable and dry. Usually there is a strong subsidence inversion in the lowest 500 meters. Since the inversion occurs at different heights on different days, it is not pronounced on the mean cross section. The inversion prevents rapid upward diffusion of water vapor which thus accumulates below the inversion. In the southward flowing air the subsidence inversion weakens and cumulus are formed which have sufficient buoyancy to break through the inversion and carry water vapor into the air aloft. South of 6N, towards the equatorial convergence zone, stability is less and there is more water vapor at all levels. This distribution is produced by low-level convergence and the continuing action of cumulus congestus and cumulonimbus. Clouds and Weather A s mentioned on page 15, the main characteristic of the northeast monsoon air over the northern Arabian Sea is the absence of clouds ( F i g . 339 through 343). Significant amounts of middle and high clouds occurred only south of 12N. These cloud types were associated primarily with vortices in the near-equatorial trough and with active cumulonimbus development. N o haze comparable to southwest monsoon haze was observed ( F i g . 344). A f e w local patches drifted out over the sea f r o m the fires of coastal towns and cities.

v. Comments on Certain Features of Tropical Indian Ocean Clouds A. PUBLISHED PAPERS Six papers have been published recently which describe the weather, clouds, storms, and circulation of the monsoons during the IIOE. The contents and findings of these studies will be reviewed briefly, since they either add to the description of clouds over the Indian Ocean or aid in the interpretation of the cloud observations by describing many of the systems producing the clouds. Cloud Climatology

The present description of clouds is made more nearly complete and useful by two studies of Indian Ocean clouds utilizing satellite photographs. Sadler (14) has constructed maps of mean monthly cloudiness from the U.S. Weather Bureau nephanalyses of TIROS data. Raman (15), also using satellite data, has made a study of the annual variations of the near-equatorial trough and its associated clouds. Relation of Clouds to the Monsoon Circulation and Subtropical Cyclones

Ramage (22) has used IIOE data to establish the circulation of the atmosphere over the Arabian Sea during the summer monsoon. This investigation is particularly pertinent to the present cloud study in that it describes the inflow of moist air into the rain area of India and the effect of the release of latent heat of water vapor upon the circulation of the atmosphere. The intensification of the Indo-Pakistan heat-low by subsiding warm air originating from the rain area is described. Miller and Keshavamurthy (8) have analyzed the monsoon circulation and the structure of a subtropical cyclone over the northeastern Arabian Sea from 26 June 1963 to 10 July 1963. The study should be referred to for an understanding of the

clouds observed during this period (see Figs. 21 through 42, and Figs. 113 through 142). Orographic Clouds

The TIROS vil photograph presented in Figure 251 shows a clearly defined line of cumulus, oriented perpendicular to the coast. Bunker (10) investigated the wind field and cloud formations in the area on this and other days, and found that cloud lines frequently formed off the southwestern coast of India during the northeast monsoon. The locations of the clouds were downwind and to the north of deep valleys in the Western Ghats. It was concluded that the clouds formed along lines of convergence between air blowing through the valleys and air moving southward over the Arabian Sea. Haze

Srivastava and Ronne (5) made a study of the haze in which the cumulus clouds of the eastern Arabian Sea were imbedded. Near the sea surface, where the relative humidity was about 70 per cent, the visibility was good. Near the cloud-base level, the relative humidity was 90 per cent, and the air was very hazy. During vertical ascents of the C-54Q aircraft a sharp transition from no haze to a hazy condition took place as the relative humidity increased beyond 80 per cent. This condition suggested that the haze was due to condensation of water vapor on sea-salt particles. To test this idea, visibilities were measured at different relative humidities and compared with visibilities computed from the visibility relation given by Aufm Kampe and Weickmann (23). The sea-salt distribution, as measured by Woodcock (24), and the size distributions of drops formed by condensation of water vapor on salt particles, as given by Keith and Arons (25), were used in the computations. The range of visibility

18

TROPICAL

INDIAN

OCEAN

CLOUDS

found by both means agreed well. Hence it was concluded that the haze was caused by the condensation of w a t e r vapor on sea-salt particles. Another type of haze, which was not reported in (9), is the " d r y " haze encountered over the western Arabian Sea, Somalia, A r a b i a , and Iran. In these^ regions the relative humidity is usually less than 50 per cent; and, hence, haze cannot be produced by the condensation of w a t e r vapor on hygroscopic nuclei. Dust and sand f r o m the desert areas are the chief sources of this haze, which extends up to about the 4-km to 5-km level (see Figs. 22 and 3 0 ) . T h e wind direction over the western A r a b i a n Sea veers slowly f r o m southwest at the surface to west at 3 kilometers. Between 3 km and 6 km it veers rapidly to northeast. Hence deserts lie upwind of the air below about 4.5 km, and dust f r o m them is the most likely source of the haze. T h e haze over the eastern A r a b i a n Sea between 2 km and 4 km may be produced, at least in part, by the same dust blown f r o m Somalia and transported across the A r a b i a n Sea.

B. HEIGHTS A N D E X T E N T OF MIDDLE A N D H I G H CLOUDS OVER T H E A R A B I A N SEA One outstanding characteristic of the southwest monsoon atmosphere is the existence of many cloud layers above the tops of the cumulus. The cloud cross sections reveal that discontinuous patches and layers of middle and high clouds usually exist hundreds of kilometers west of the Indian coast. W i t h the exception of areas with intense monsoon rain and regions close to the shoreline, the middle and upper clouds are found f r o m 1 km to 10 km above the tops of the cumulus. T o measure all of the individual cloud layers f r o m the time-lapse films would have been an impossibly large task, and the presentation of the measurements impossibly bulky. Hence a 350 km section of the WHOI 26 June 1963 flight was selected f o r detailed measurement. E v e n in this short section, not all of the thousands of individual small clouds could be measured. Heights of the more extensive clouds presented in the ensuing cross section ( F i g . 345) w e r e measured; heights of smaller clouds w e r e estimated. T h e widths of cumulus are drawn schematically and are greater than true scale. The thicknesses of the middle and high clouds are estimated to be 200 meters. I t is impossible to i d e n t i f y accurately points on the tops and bases of layered clouds. The a i r c r a f t flew at 4250 m on a 65-degree

heading, which was nearly downwind relative to the lower tropospheric wind and nearly upwind relative to the upper tropospheric wind. The timelapse camera was pointed at 335 degrees. Since there was a great variation in the distances that cloud layers extended normal to the flight path, these distances are entered on the section; and since the maximum visual range f r o m the a i r c r a f t was about 60 km, this is the greatest value entered on the section, although some cloud layers may have extended f a r t h e r . I f no values are given, the clouds w e r e only a f e w kilometers in extent. N o t e the large gap between the cumulus tops and the lowest middle clouds. I t is obvious that middle and high clouds were not produced locally f r o m w a t e r vapor transported by the cumulus. Cumulonimbus tops observed f r o m j e t a i r c r a f t over India averaged 13 km in height (6, 26) and occasionally reached 18 kilometers. Since easterlies prevailed above 4 km, w e conclude that the middle and high clouds w e r e produced f r o m moisture brought up in regions east of the A r a b i a n Sea. N o monsoon storm was present on 26 June 1963 in the Bombay region (see Figs. 113 and 114, and 8), hence it is likely that the cirrus present over the A r a b i a n Sea originated f r o m cumulonimbus over the eastern coast of India where a cyclone had developed. V i e w i n g the films gives the observer the impression of a chaotic sky w i t h clouds that do not easily f a l l into the categories of the international cloud codes devised f o r ground observers (16). The altostratus and altocumulus just above flight level fit the M9 category best, but resemble f r a c tostratus of the low cloud group L7. This layer of clouds may have spread out or have been sheared f r o m the tops of cumulus that reached 4500 m over the W e s t e r n Ghats. The great number of different heights at which the altocumulus were observed probably resulted f r o m variations in both the stability of the environmental air and the buoyancy of the rising air. Individual cumulus towers rise and finally spread out at the particular level where hydrostatic equilibrium is attained. T h e bases of altocumulus at 6 km, shown at 300 km on F i g u r e 345, m e r g e into a featureless sheet of altostratus. They thus definitely belong in the M 5 category. These cloud bands and others extending 60 km or more perpendicular to the wind probably result f r o m medium scale vertical motions in the atmosphere. W a v e s of larger scale may have been present in the atmosphere which produced the cirrus observed between 9 km and

COMMENTS

ON

CERTAIN

11 kilometers. Alternatively, they may have been anvil-shaped cirrus, H3, distorted by wind shears into H2 clouds during their passage f r o m the east coast of India to the Arabian Sea. C. ORIENTATION OF CUMULUS The organization of cumulus into rows has been observed (27) over nearly all p a r t s of the earth. The Indian Ocean is no exception (see Figs. 46 and 47). On 22 of the observing days, orientation was noted and indicated on the cloud cross sections. Undoubtedly, many more cases occurred but were not observed because of obscuration by middle clouds or because the organization could not be recognized since the a i r c r a f t was flying too close to the bases of the cumulus. The organization of clouds into rows usually occurs over limited a r e a s ; the regime may break down within a few tens of kilometers a f t e r having extended f o r a few hundreds of kilometers. One such rapid transition occurred on 30 August 1964 at 05N, 55E (see Fig. 69). There, about 40 fairly regular rows of cumulus, spaced 4.4 km apart, were oriented nearly parallel to the wind which was blowing f r o m 260 degrees. On the southern side of the organized area, the narrow rows of clouds lost their organization, but a larger-scale organization was evident. The cumulus appeared to be randomly distributed within wide bands of clouds. On the northern edge, the distance between cloud rows rapidly became greater and finally ended in a wide layer of stratocumulus, which dissipated f a r t h e r north. It is of interest to note t h a t the 4.4 km spacing between the rows of cumulus does not agree with the value predicted (28). The equation f o r the spacing is given a s : L = (V/sin

(200)

where L is the band spacing in meters, V is the wind speed above the boundary layer in meters per second, and (/> is the latitude. Using the observed wind speed at 500 m of 11 m s e c - 1 at 05N, the distance between the rows should have been 24 kilometers. Thus some factor other than wind speed and latitude must have controlled the spacing. D. RAIN BANDS Rainfall during the southwest monsoon is characterized by its intermittent, showery nature. Variations occur over a wide range in the time scale. At the low-frequency end of the spectrum,

FEATURES

OF

TROPICAL

INDIAN

OCEAN

CLOUDS

19

breaks in monsoon rains last f o r a number of days or even weeks. Shorter breaks or variations occur with all frequencies right down to large hour-byhour variations during periods of heavy falls. These variations result f r o m the convective nature of the rain which is influenced by stability of the air, buoyancy of convective parcels, convergence or divergence of the air a t various levels, and orography. While flying the research missions over the Arabian Sea, it was noticed t h a t on several occasions rain was organized into long bands nearly parallel to the wind direction. The most clear-cut case occurred on 28 June 1963 when the WHOI C-54Q a i r c r a f t was flying a 165 degree heading at the 300 m level about 50 km off the west coast of India (Fig. 26). The surface wind was blowing f r o m 270 degrees at 9 m s e c - 1 . At the 700 mb level around 18N, the wind was f r o m about 260 degrees at 5 m s e c - 1 . At U N the 700 mb wind was f r o m 280 degrees at 10 m s e c - 1 . The camera, being mounted normal to the centerline of the a i r c r a f t , was pointed about 15 degrees to the left of the downwind direction and hence roughly parallel to the rain bands. The time-lapse camera films revealed t h a t the rain occurring between 19N and U N was concentrated into long bands parallel to the wind direction. The bands extended downwind in welldefined lines until the haze obscured them f r o m view. Visibility varied considerably in the monsoon haze, but was generally between 20 km to 30 km in the rain-free areas. The bands varied greatly in width, rain intensity, and absorption of solar radiation. Under one band, only 0.18 Langleys m i n - 1 were received by the short-wavelength pyranometer, while f r o m a non-rain zone immediately adjacent, 1.68 Langleys m i n - 1 were received. This large variation in the insolation occurred because in the rain bands the clouds extended f r o m a few hundred meters up to the cirrus level, while between the bands the skies were clear except f o r a few traces of cirrus. Zones bordering small rain bands (2 km to 10 km wide) were not clear, but had varying amounts of cumulus and middle and high clouds. A f r a m e count of the time-lapse films was made to determine the number, width, and arrangement of rain bands and intermediate zones between 19N and U N . A plot of the occurrence or nonoccurrence of rain against the f r a m e number is presented in Figure 346A. Latitudes are d r a f t e d above the f r a m e numbers. Twenty-two distinct rain bands were counted between 19N

20

TROPICAL

T A B L E 6:

INDIAN

OCEAN

CLOUDS

Cloud-Turbulence and Cloud-Fluctuation Values*

Lat.

Long,

Height,

(J w.

4/8 Middle

MONSOON Cloud

FIGURE 230. Middle-cloud occurrence during the southwest monsoon. The shaded areas indicate that at least one observation was made of more than 4/8 middle clouds.

SOUTHWEST >4/8

so" r FIGURE 231. High-cloud occurrence during the southwest monsoon. The shaded areas indicate that at least one observation was made of more than 4/8 high clouds.

MONSOON

High Cloud

FIGURES

SOUTHWEST Rain

MONSOON Areas

FIGURE 232. Rain occurrence during the southwest monsoon. The shaded areas Indicate that at least one observation was made of rain.

SOUTHWEST MONSOON Haze

Wg|||f

FIGURE 233. Haze occurrence during the southwest monsoon. The shaded areas indicate that at least one observation was made of medium to heavy haze.

230-233

141

142

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 234. Cloud cross section of 4 February 1964 RFF flight from Mauritius to Gan. Cumulonimbus rising to 9 kilometers are associated with a low-level vortex at 10S, 72E.

F I G U R E 235. Cloud m a p of 4 February 1964 RFF and R A F flights from Aden to Gan, and from Mauritius to Gan, respectively. See Figures 268 and 269.

F I G U R E 236. Photograph from T I R O S VII, orbit 3400, 1022 G M T 4 February 1964. R A F and RFF aircraft paths and positions are indicated.

FIGURES

234-240

143

F I G U R E 237. C l o u d c r o s s section of 5 F e b r u a r y 1 9 6 4 R A F flight f r o m G a n to S i n g a p o r e . C u m u l o n i m b u s or c u m u l u s c o n g e s t u s were o b s e r v e d o n all G a n to S i n g a p o r e f l i g h t s d u r i n g this period.

F I G U R E 238. C l o u d c r o s s s e c t i o n of 5 F e b r u a r y 1 9 6 4 R F F flight f r o m G a n to C o c o s I s l a n d . C u m u l o n i m b u s are a s s o c i a t e d with a vortex at 3 S , 8 7 E .

F I G U R E 239. C l o u d m a p of 5 F e b r u a r y 1 9 6 4 R A F a n d R F F f l i g h t s f r o m G a n to S i n g a p o r e a n d f r o m G a n to C o c o s , respectively. T h e s e f i g u r e s a n d s u b s e q u e n t o n e s s h o w that the equatorial region e x p e r i e n c e s g r e a t c u m u l u s activity t h r o u g h o u t F e b r u a r y a n d M a r c h . S e e F i g u r e s 2 7 0 a n d 271.

F I G U R E 240. P h o t o g r a p h f r o m T I R O S V I I , orbit 3 4 1 4 , 0 9 0 4 G M T , 5 F e b r u a r y 1964. R A F a n d R F F aircraft p a t h s a n d p o s i t i o n s are indicated.

144

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 241. Cloud cross section of 7-8 February 1964 R A F flight, from Singapore to Gan. C u m u l o n i m b u s activity w a s noted on the southern horizon. See Figures 274 and 275. F I G U R E 242. Cloud cross section of 8 February 1964 R A F flight from Gan to Aden. See Figures 276 and 277.

F I G U R E 243. Photograph from T I R O S VII, orbit 3458, 0830 GMT, 8 February 1964. R A F aircraft path and positions are indicated. The letter A marks the position of cumulonimbus.

FIGURES

F I G U R E 244. Cloud map of 12 February 1964 W H O I flight from Bombay to 20N, 66E and return. No low or middle clouds were observed on this flight. Only a trace of cirrus w a s noted north of the track. See Figures 284 and 285.

F I G U R E 245. Photograph from T I R O S VII, orbit 3516, 0642 G M T 12 February 1964. W H O I aircraft path is indicated. The absence of clouds along the northwest coast of India is very apparent in this photograph.

F I G U R E 246. Chart of 15 February 1964 W H O I flight over the Bombay coastal region. N o clouds of any description were observed during this flight. See Figures 289 and 290.

2 4 1- 2 4 6

145

146

TROPICAL

S M Level — Cu Coverage

INDIAN

OCEAN

CLOUDS

—I— 0200

I 0500

F I G U R E 247. C l o u d c r o s s section of 16 February 1964 R A F flight f r o m S i n g a p o r e to Gan. The c u m u l o n i m b u s rise to 1 1 kilometers. S e e Figures 2 9 2 a n d 293.

F I G U R E 248. C l o u d m a p of 17 February 1964 W H O I flight over the region west of B o m b a y . The only c u m u l u s observed were near the southwest corner of the box pattern at 0 6 2 1 G M T . N o n e were observed at this point at 1851 G M T . T r a c e s of cirrus were observed over the southern half of the box pattern. S e e Figures 2 9 4 and 295.

F I G U R E 249. C l o u d c r o s s section of 18-19 February 1964 R A F flight f r o m G a n to Singapore. C u m u l o n i m b u s are widespread a n d not associated with any vortex. S e e Figures 2 9 6 a n d 297.

February

18-19,

1964

FIGURES

No

clouds

—i—•—T—•—,—•—,—•—,—.——i0h 0500

0600

Distant

0700

Cb s to

spreading

SE with

0600

anvils

NW

j ~ S e a Level s i Cu Coverage

FIGURE 250. Cloud cross section of 19 February 1964 WHOI flight f r o m Bombay to Gan. Skies were clear to U N , then varying amounts of cloud were observed at many levels. See Figures 298 and 299.

FIGURE 251. Photograph f r o m TIROS VII, orbit 3816, 0415 GMT 19 February 1964. WHOI aircraft path is indicated. A most significant feature is the line of cumulus perpendicular to the coast of India at the position marked A. This case has been studied in detail (10).

247-251

147

148

TROPICAL

INDIAN

OCEAN

CLOUDS

FIGURES

252—255

149

F I G U R E 254. Cloud cross section of 20 February 1964 RFF flight from Bombay to Gan. Icing was encountered at 5 kilometers in stratus.

F I G U R E 255. Cloud cross section of 20 February 1964 W H O I flight along the equator. See Figure 256 for appearance of clouds.

150

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 256. C l o u d p h o t o g r a p h t a k e n at 1245 G M T d u r i n g 2 0 F e b r u a r y 1 9 6 4 W H O I flight. Aircraft h a d been in s t a t u s a n d c u m u l o n i m b u s for 150 k i l o m e t e r s a n d h a d just e m e r g e d f r o m c u m u l o n i m b u s in background. F I G U R E 257. P h o t o g r a p h taken by T I R O S V I I , orbit 3 6 6 2 , 0 3 4 5 2 2 F e b r u a r y 1964. W H O I aircraft p a t h i s Indicated.

F I G U R E 258. C l o u d c r o s s s e c t i o n of 2 2 F e b r u a r y 1964 W H O I flight f r o m G a n to B o m b a y . A vortex w h i c h h a d d e v e l o p e d in the nearequatorial t r o u g h p r o d u c e d c l o u d north of G a n . S e e F i g u r e s 3 0 4 and 305.

GMT

FIGURES

F I G U R E 259. Cloud map of 26 February 1964 W H O I flight from Bombay to 13N, 70E and return. N o clouds were observed north of 15N except for a trace of cirrus over land at 18N. C u m u l u s increased from % at 15N to % at 13N. See Figures 312 and 313.

F I G U R E 261. Cloud cross section of 28 February 1964 R A F flight from Gan to Singapore. Cumulonimbus reached 10 kilometers.

256-261

151

F I G U R E 260. Track chart of 28 February 1964 W H O I flight from Bombay to area southwest of Veravai. N o clouds were observed at any time during the flight. S e e Figures 316 a n d 317.

152

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 262. Cloud cross section of 1 March 1964 R A F flight from Aden to Gan. The camera was operated only from 0500 to 0630 GMT. See Figures 320 and 321.

F I G U R E 263. Cloud craft from Bombay turned on at 0200 of c u m u l u s on the and 325.

map of 2-3 March 1964 night flight of W H O I airto area southwest of Veraval. Cloud camera was GMT. The only clouds observed were small puffs western leg of the box pattern. See Figures 324

FIGURES

F I G U R E 264. Cloud cross section of 6 March 1964 R A F flight from Gan to Singapore. C u m u l u s towers rose to 6 kilometers. See Figures 330 and 331.

68® E

F I G U R E 265. Cloud m a p of 7 March 1964 W H O ! flight from Bombay to Diu to ION, 6 8 E to Bombay. N o clouds were observed north of 15N. See Figures 332 and 333.

70°

72"

262-265

153

F I G U R E 266. Streamline chart, 1200 G M T 3 February 1964, surface.

F I G U R E 267. Streamline chart, 1200 G M T 3 February 1964, 700 millibars.

FIGURES

266-269

F I G U R E 268. Streamline chart, 1200 G M T 4 February 1964, surface.

F I G U R E 269. Streamline chart, 1200 G M T 4 February 1964, 700 millibars.

156

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 270. Streamline chart, 1200 G M T 5 February 1964, surface.

F I G U R E 271. Streamline chart, 1200 G M T 5 February 1964, 700 millibars.

FIGURES

270-273

157

F I G U R E 274. Streamline chart, 1200 G M T 7 February 1964, surface.

F I G U R E 275. Streamline chart, 1200 G M T 7 February 1964, 700 millibars.

FIGURES

274-277

F I G U R E 276. Streamline chart, 1200 G M T 8 February 1964, surface.

F I G U R E 277. Streamline chart, 1200 G U T 8 February 1964, 700 millibars.

159

F I G U R E 278. S t r e a m l i n e chart, 1 2 0 0 G M T 9 F e b r u a r y 1964, surface.

F I G U R E 279. S t r e a m l i n e chart, 1 2 0 0 G M T 9 F e b r u a r y 1964, 7 0 0 millibars.

FIGURES

278-281

F I G U R E 280. Streamline chart, 1200 G M T 10 February 1964, surface.

F I G U R E 281. Streamline chart, 1200 G M T 10 February 1964, 700 millibars.

161

F I G U R E 282. S t r e a m l i n e chart, 1 2 0 0 G M T 1 1 F e b r u a r y 1964, surface.

F I G U R E 283. S t r e a m l i n e chart, 1 2 0 0 G M T 1 1 F e b r u a r y 1964, 7 0 0 millibars.

FIGURES

282-285

F I G U R E 284. S t r e a m l i n e chart, 1 2 0 0 G M T 12 F e b r u a r y 1964, surface.

F I G U R E 285. S t r e a m l i n e chart, 1 2 0 0 G U T 12 F e b r u a r y 1964, 7 0 0 millibars.

163

F I G U R E 286. Streamline chart, 1200 G M T 13 February 1964, surface.

F I G U R E 287. Streamline chart, 1200 G M T 13 February 1964, 700 millibar*.

FIGURES

286-289

F I G U R E 288. S t r e a m l i n e chart, 1 2 0 0 G M T 14 F e b r u a r y 1964, surface.

F I G U R E 289. S t r e a m l i n e chart, 1 2 0 0 G M T 14 F e b r u a r y 1964, 7 0 0 millibars.

40*

50°

60°

70°

80°

90°

I00°E

165

166

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 290. Streamline chart, 1200 G M T 15 February 1964, surface.

40°

50°

60°

70*

80*

90"

100®

F I G U R E 291. Streamline chart, 1200 G M T 15 February 1964, 700 millibars.

FIGURES

290-293

F I G U R E 292. S t r e a m l i n e chart, 1 2 0 0 G M T 16 F e b r u a r y 1964, surface.

F I G U R E 293. S t r e a m l i n e chart, 1 2 0 0 G M T 16 F e b r u a r y 1964, 7 0 0 millibars.

167

168

TROPICAL

INDIAN

OCEAN

CLOUDS

FIGURE 294. Streamline chart, 1200 GMT 17 February 1964, surface.

FIGURE 295. Streamline chart, 1200 GMT 17 February 1964, 700 millibars.

FIGURES

294-297

F I G U R E 296. Streamline chart, 1200 G M T 18 February 1964, surface.

F I G U R E 297. Streamline chart, 1200 G M T 18 February 1964, 700 millibars.

169

170

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 298. Streamline chart, 1200 G M T 19 February 1964, surface.

F I G U R E 299. Streamline chart, 1200 G M T 19 February 1964, 700 millibars.

FIGURES

298-301

171

F I G U R E 302. Streamline chart, 1200 G M T 2 1 February 1964, surface.

F I G U R E 303. Streamline chart, 1200 G M T 2 1 February 1964, 700 millibars.

FIGURES

302-305

F I G U R E 304. S t r e a m l i n e chart, 1 2 0 0 G M T 2 2 F e b r u a r y 1964, surface.

40'

50"

60"

70"

80"

90"

I00"E

F I G U R E 305. S t r e a m l i n e chart, 1 2 0 0 G M T 2 2 F e b r u a r y 1964, 7 0 0 millibars.

40"

50"

60"

70"

80"

90*

I00"E

173

174

TROPICAL

INDIAN

OCEAN

CLOUDS

FIGURES

40°

50°

60°

70°

80°

90°

306-309

I00°E

F I G U R E 309. Streamline chart, 1200 G M T 24 February 1964, 700 millibars.

40°

50°

60°

70°

80°

90°

I00°E

175

176

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 310. Streamline chart, 1200 G M T 25 February 1964, surface.

F I G U R E 311. Streamline chart, 1200 G M T 25 February 1964, 700 millibars.

FIGURES

310-313

F I G U R E 312. Streamline chart, 1200 G M T 26 February 1964, surface.

F I G U R E 313. Streamline chart, 1200 G M T 26 February 1964, 700 millibars.

178

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 314. Streamline chart, 1200 G M T 27 February 1964, surface.

F I G U R E 315. Streamline chart, 1200 G M T 27 February 1964, 700 millibars.

FIGURES

314-317

179

F I G U R E 316. Streamline chart, 1200 G M T 28 February 1964, surface.

F I G U R E 317. Streamline chart, 1200 G M T 28 February 1964, 700 millibars.

70*

80"

180

TROPICAL

INDIAN

OCEAN

CLOUDS

FIGURES

318-321

181

182

TROPICAL

INDIAN

OCEAN

CLOUDS

F I G U R E 322. Streamline chart, 1200 G M T 2 March 1964, surface.

F I G U R E 323. Streamline chart, 1200 G M T 2 March 1964, 700 millibars.

FIGURES

322-325

F I G U R E 324. Streamline chart, 1200 G M T 3 March 1964, surface.

F I G U R E 325. Streamline chart, 1200 G M T 3 March 1964, 700 millibars.

183

F I G U R E 326. S t r e a m l i n e chart, 1 2 0 0 G M T 4 M a r c h 1964, surface.

F I G U R E 327. S t r e a m l i n e chart, 1 2 0 0 G M T 4 M a r c h 1964, 7 0 0 millibars.

FIGURES

326-329

F I G U R E 328. Streamline chart, 1200 G M T 5 March 1964, surface.

F I G U R E 329. Streamline chart, 1200 G M T 5 March 1964, 700 millibars.

185

F I G U R E 330. Streamline chart, 1200 G M T 6 March 1964, surface.

F I G U R E 331. Streamline chart, 1200 G M T 6 March 1964, 700 millibars.

FIGURES

330-333

F I G U R E 332. Streamline chart, 1200 G M T 7 March 1964, surface.

F I G U R E 333. Streamline chart, 1200 G M T 7 March 1964, 700 millibars.

187

F I G U R E 334. Short-wavelength solar and sky radiation below 1000 meters. Average daily radiation in Langleys per day observed during the period 12 February 1964 to 7 March 1964.

F I G U R E 335. A s in Figure 334, but for short-wavelength radiation above 3000 meters.

F I G U R E 336. A s in Figure 334, but for albedo above 3000 meters.

FIGURES

334-338

322

,320

189

600

—

— 600

700

T,

—

800

—

— 800

900

—

—

1000

300 —

700

o

XI --

O)

to

900

10 Q)

— 1000 l 1400

i

1200

l 1000

Distance

I

i 800

600

o

(kilometers)

FIGURE 337. Average potential temperature (K) cross section for the northeast monsoon. The section extends from 19N, 72E to the equator at about 68E.

—600

O XI

—700

800

-800

—

O) 3

to 10 d)

- 9 0 0

-1000

I 2000

Distance

FIGURE 338. As in Figure 337, but for mixing ratio (grams per kilogram).

(kilometers)

190

TROPICAL

INDIAN

OCEAN

CLOUDS

NORTHEAST MONSOON 3/8 or more cumulus (LI)

50" E

60°

70°

FIGURE 339. Cumulus (LI) occurrence during the northeast monsoon. The shaded areas Indicate that at least one observation was made of more than % small cumulus.

NORTHEAST

MONSOON

Growing cu 8 cb's

50" E

FIGURE 340. Cumulus congestus or cumulonimbus occurrence during the northeast monsoon. The shaded areas indicate that at least one observation was made of these clouds.

FIGURES

F I G U R E 341. Middle-cloud occurrence during the northeast monsoon. The shaded areas indicate that at least one observation w a s made of more than 4/8 middle cloud.

F I G U R E 342. High-cloud occurrence during the northeast monsoon. The shaded areas indicate that at least one observation w a s made of more than 4/8 high cloud.

339-342

191

192

TROPICAL

INDIAN

OCEAN

CLOUDS

NORTHEAST Rain

FIGURE 343. Rain occurrence during the northeast monsoon. The shaded areas indicate that at least one observation was made of rain.

FIGURE 344. Haze occurrence during the northeast monsoon. Shaded areas indicate that at least one observation was made of medium to heavy haze.

MONSOON Areas

FIGURES

12

-

10

"

-

—

60

i r - ^ S "

oJ 6 E

Ì

Flight

4

343-346

_

193

12

-'0

*TcT

^

6o

Path

Aircra't

60

¡nctouds

-

-C •2" Q) ? £

„ -8 B

Haze -

I

g

O

s

I 250 OC N 69.40'E

_ , Distance

B s I— 275

s

s

I

2

S

m i » , 'kilometers)

FIGURE 345. Detailed cloud cross section of 26 June 1963 WHOI f l i g h t over t h e Arabian Sea. The distances t h a t c l o u d layers extend f r o m the a i r c r a f t t o w a r d 335 degrees is indicated in kilometers near the clouds. See Figures 21, 22, 23; 113 and 114.

Legend

•

Rain

Band Latitude

19° N 1

18°

17° 1

I

16° 1

15°

14°

I

I

13°

12° 1

11° N I

Start 165° heading

i

r"*

0

Turn to 2 7 0 ° heading

• i

T T

~ 1000

2000

3000

T 5000

4 000

Frame

Number

FIGURE 346. (A) Occurrence of rain d u r i n g t h e 28 June 1963 WHOI f l i g h t southward, 50 kilometers off the Indian west coast. Frame numbers of t h e 16 m m f i l m are labeled along t h e abscissa. Solid bars indicate where rain was observed.

Ki l o m e t e r s 0 1 1 0

10 1 •

20 I J I 100

30 1 1 •

I 200

L

40 1

50 1

Ii I 300

•

S p a c i n g in (B) Frequency of spacing between centers of rain bands observed 2 8 June 1963. The abscissa gives t h e distance between centers f r a m e c o u n t s and kilometers. The o r d i n a t e gives t h e n u m b e r of currences of spacings w i t h t h e indicated distances. See Figures 26, 117, a n d 118.

on in oc25,

60 1 « i 7 400

a

Frames

70 I

•

r 500

80 1

90 1 •

1 600

^ 6000

m

k"