Structure of an Arabian Sea Summer Monsoon System 9780824885366

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INTERNATIONAL INDIAN OCEAN EXPEDITION METEOROLOGICAL MONOGRAPHS

Number 1

The International Indian Ocean Expedition Meteorological Monographs, published by the East-West Center Press, is a series for the publication of monographs containing detailed discussions and supporting data on the various components of the general atmospheric circulation over the Indian Ocean, as well as the results f r o m measurements of atmosphereocean interaction made as p a r t of the expedition's observational program. Manuscripts a r e 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

Structure of an Arabian Sea Summer Monsoon System

STRUCTURE OF AN ARABIAN SEA SUMMER MONSOON SYSTEM

by Forrest R. Miller and R. N. Keshavamurthy

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. 196 from the Hawaii Institute

of Geophysics, University of Hawaii.

Copyright © 1968 by East-West Center Press University of Hawaii All Rights Reserved Library of Congress Catalog Card Number: 67-29576 Printed in the United States of America First Edition

Acknowledgment The authors are especially indebted to C. S. Ramage, director of the IIOE Scientific Program for Meteorology, for his comments, valuable suggestions, and encouragement while completing the research. Sincere thanks are due J. C. Sadler, C. R. V. Raman, and J. R. Nicholson for their helpful comments on the many aspects of this paper. The indirect contributions made by other research associates, too numerous to mention by name, during daily discussions at the International Meteorological Center, are gratefully acknowledged. For aid in editing the manuscript, sincere thanks to Mrs. Ethel McAfee. The authors also express appreciation to L. K. Oda, L. V. Medeiros, and V. K. Venkiteswaran for their skillful assistance in preparing the illustrations. A grateful acknowledgment also goes to Miss Noella Simoes, Mrs. Michie Hamao, and Miss Betsy Murashige for their untiring assistance in typing the text. Finally, a special acknowledgment to aircrew members of the U. S. Weather Bureau Research Flight Facility and the Woods Hole Oceanographic Institution, whose outstanding efforts in data collection have made possible this paper and many future research papers in the tropical meteorology of South Asia. Forrest R. Miller, University of Hawaii R. N. Keshavamurthy, India Meteorological Department

Honolulu, July 18, 1967

Contents

ABSTRACT

PART 1: INTRODUCTION

I.

Climatology of the Summer Monsoon . . . .

3

A. Mean Circulation for July U B. Branches of the Monsoon Current 6 C. Divergence of the Mean Resultant Winds 6

II. III. IV.

Early Monsoon Descriptions

8

Recent Monsoon Investigations

9

Data and Analytical Procedures

9

A. Observations 9 B. Analyses 11

PART 2: THE SUMMER MONSOON REGIME

V.

The Preactive Period A. Mid-Troposphere Circulation IS B. Low-Level Circulation H C. Upper-Troposphere Circulation 18 D. Weather and Temperature Distribution 20

13

VI.

The Active Period

PART 3: DISCUSSION

21

A. Mid-Troposphere Circulation 21

IX.

B. Low-Level Circulation 23 C. Upper-Troposphere Circulation 38

B. Responses in the Upper Troposphere 79

Divergence Patterns on 2 July 41

C. Relation between the Mid-Troposphere Circulation and West Coast Weather 80

E. Clouds and Weather 42

The Trough Axes and West Coast Rainfall 81 Circulation Changes in the Mid-Troposphere 81 Preactive Period 83 Active Period 83 Inactive Period 83 T I R O S Satellite Observations of the Arabian Sea Cyclone 84

Composite Representations of the Active Period

50

A . Data and Procedures 50 B. Distribution of Wind, Pressure, Temperature, and Moisture 50 C. Low-Level Circulation 60 D. Distribution of Divergence, Vorticity, and Vertical Motion 60

T I R O S V Observations of 4 and 5 July 1962 84

E. Divergence Cross Sections 60

T I R O S V I I Observations of 12 and 13 August 1964 84

F. Vorticity Cross Sections 67

D. Destruction of the Arabian Sea Cyclone 84

G. Vertical Motion and Rainfall Cross Sections 67

The Polar Trough Hypothesis 84

H. A Model of Clouds, Vertical Motion, and Temperature 69

VIII.

The Inactive Period A. Mid-Troposphere Circulation 70

79

A. The Role of Surface Disturbances 79

D. Divergence, Vorticity, and Vertical Motion 38

VII.

Other Aspects of the Summer Monsoon System

Ventilation of the Arabian Sea Cyclone 89

70

X.

Concluding Remarks

90

NOTES AND REFERENCES

93

B. Low-Level Circulation 73 C. Upper-Troposphere Circulation 73 D. Weather Changes 73

Abstract A detailed study of an Arabian Sea summer monsoon system has become possible because of frequent and accurate upper air observations that were recorded between 26 June and 10 July 1963 by research aircraft of the U. S. Weather Bureau Research Flight Facility and the Woods Hole Oceanographic Institution. These unique data, particularly for the northeast Arabian Sea region, provide, with conventional observations, a three-dimensional data coverage from the surface to 14 km on several days of an active Arabian Sea monsoon. Part 1 presents a discussion of the long-term-mean, three-dimensional structure of the summer monsoon circulation over the Arabian Sea and India. A general review of the early investigations and proposed hypotheses reveals the difficulties that are encountered in attempting to explain the vagaries and complexities of the summer monsoon regime. The procedures used to incorporate and analyze the great variety of observations obtained during the International Indian Ocean Expedition are also given, together with a summary of the types of observations recorded on research flights. Part 2 presents specific details of the kinematic and thermo-dynamic structure of a particular mid-tropospheric cyclone — the principal activators of heavy rains over the northeast Arabian Sea and western India. This cyclone had its beginning in the mid-troposphere over the northeast Arabian Sea during the preactive monsoon period of June 1963; it matured quickly, and remained nearly stationary and apparently anchored to the subcontinent throughout the active monsoon period. After 12 days of heavy rains the cyclone system decayed, the weather improved, and an inactive or relatively dry monsoon period commenced. This monsoon system exhibited remarkably stable characteristics, which are exemplified by composite representations and a

Abstract

(continued)

model of observed and derived meteorological parameters. TIROS weather satellite photographs, which pictorially display the cloudiness associated with the Arabian Sea cyclone system, reveal the changing cloud structure as it progressed through its life cycle. Although the entire atmosphere over the subcontinent responds to changes in the summer monsoon regime, the response of the mid-troposphere is quickest, and most direct, to the changes in the circulation and moisture content. Part 3 presents a discussion, in terms of case histories, of the direct role of the mid-troposphere circulation and its influence on the weather over the west coast of India and interaction with the low and high troposphere. Some examples are presented of TIROS photographs which visually portray the various stages in development and decay of mid-tropospheric cyclones over the Arabian Sea and subcontinent which occurred during 1962 and 1964.

PART Î:

Introduction

The entire summer monsoon system of South Asia affects the lives and economies of more of the world's population than does any other weather regime on earth. The vagaries of the summer monsoon weather over India are well known and have had a profound effect on her people. For example, in 1899 the monsoon rains failed to appear over a major portion of that country. Although some provinces in the northeast part of India, and Burma, did experience rainfall that was slightly above normal, nearly all other provinces suffered extreme drought, and rainfall from June to September was as much as 60 to 80 per cent below normal. In contrast, very heavy rains fell over most of the country in 1917, causing extensive flooding, but, again, in the very next year, 1918, drought conditions occurred everywhere except in the extreme northeast section. The Indian summer monsoon is characterized by a southwest flow in the lower troposphere below 400 mb and an easterly flow in the upper troposphere above 300 mb. This general circulation pattern persists from May through September of each year. Periodically, the summer monsoon circulation strengthens, and prolonged heavy rains occur over a large portion of India; at other times, wind speeds decrease over the Arabian Sea and very little rain falls over the western and central parts. Now that atmospheric soundings are being simultaneously made to very high levels over South Asia, there is little doubt that for a complete explanation of the summer monsoon the middle- and high-tropospheric circulations as well as the lowlevel flow should be taken into consideration. The juxtapositions and the variations in intensity of these circulations from day to day determine whether the summer monsoon over any part of India will remain inactive or become active. In this monograph an active period of the summer monsoon is considered as one in which extensive

cloud cover and rains develop after the low-level southwest current has been established. Rain, commencing to fall, spreads over wide areas of the west coast, often to interior regions, with 24-hour amounts generally exceeding 3 cm. Before and after such periods, rainfall is light, and the southwest flow continues to prevail in the low-levels, but wind speeds over the Arabian Sea are lower and directions more variable. Such periods before the first active monsoon are referred to as premonsoon; those between active periods as breaks. I. CLIMATOLOGY OF THE SUMMER MONSOON Prior to 1958, when Ramakrishnan et al. (3) published their report on the climatology of the upper air over India, very little detailed information had been available, in the form of published atlases, as to the temperature, pressure, and winds over that area. In the report, wind analyses were based principally on pibal data which, in the streamline patterns, reflected a bias toward fairweather conditions. Radio wind data at levels above cloud bases had been available for about six stations in the northern part of India for the period from 1950 to 1956 — the period when the data were compiled for the climatic study by these authors. After 1956, thirteen uniformly distributed rawinsonde stations were put into operation. Hence, the first tasks undertaken by staff members of the International Meteorological Center (IMC) at Bombay in 1963 were: to collect all the available upper-wind data over the Indian Ocean and surrounding continents; to compute mean winds; and to construct a preliminary atlas of streamlines of the mean resultant winds for all standard pressure levels. The resultant atlas, compiled in 1964 by Raman and Dixit (4), has provided useful background reference for both daily analyses at IMC and for research over the entire International Indian Ocean Expedition (IIOE) region.

4

STRUCTURE

OF

AN

ARABIAN

SEA

SUMMER

MONSOON

SYSTEM

The following paragraphs present a discussion of the climatological circulation for July for the surface to 900 m layer and for the 500 mb and 200 mb levels. The analyses, taken from the atlas by Raman and Dixit, serve as references for analytical comparison and climatic background to the material covered in this monograph. The streamline patterns represent the mean atmospheric flow patterns for July in the low-level southwest current, the mid-tropospheric current, and the persistent upper-level tropical easterlies. A northsouth, mean vertical cross section near 73E for July has been constructed from mean temperatures and resultant winds to provide vertical continuity between the standard levels. In Parts 1 and 2 of this monograph reference to specific locations or geographical areas is made by their regional names (see Fig. 1). A. Mean Circulations for July (Fig. 2)

FIGURE

1:

G e o g r a p h i c locator m a p ; h a t c h e d area outlines that por-

tion of the H i m a l a y a n

mountain

range which stands above 4

kilo-

meters. R a w i n s o n d e data u s e d in the north-south vertical c r o s s sect i o n s are taken f r o m the s t a t i o n s with n u m b e r s . S c h e m a t i c

repre-

s e n t a t i o n s of the direction of the m e a n s u r f a c e layer air flow for July are s h o w n by the b r o a d - b a n d e d a r r o w s labelled A r a b i a n S e a and Bay branch.

branch

Following the northward march of the sun during the Northern Hemisphere spring, heat lows begin to form in May over continental regions surrounding the Arabian Sea. A trough of low pressure is formed and extends from Somalia northward across Arabia and into the principal heat low which is located over West Pakistan; here, the lowpressure belt extends southeastward into central India. The mean locations of the most persistent of these lows surrounding the Arabian Sea are represented by the streamline vortices in Figure

INTRODUCTION

2A. By the end of May, the heat lows have become well established and the southwest monsoon flow has commenced to spread northward over the Arabian Sea, India, and the Bay of Bengal. During July the mean 500 mb pressure-height and streamline patterns (Fig. 2B) reflect a trough over the northeast part of the Arabian Sea and central India, between 15N and 20N. However, the trough is not as pronounced in the mean 500 mb pressure-height field as it is in the mean wind field. At times, intense mid-tropospheric cyclones develop between 700 mb and 500 mb in the trough over the northeast Arabian Sea, and on these occasions the Arabian Sea monsoon is most active. At other times, anticyclonic flow dominates the midtroposphere over the Arabian Sea region delineated by the cyclonic-turning streamlines shown in Figure 2B. These situations are associated with relatively dry periods over the west coast. Two other mid-tropospheric features of note are the subtropical ridge line and the Bay of Bengal cyclones. In general, the subtropical ridge, which is found between 700 mb and 300 mb over the surface heat lows, is indicative of the outflow which compensates for low-level inflow. Heat transported upward over the heated subcontinent contributes significantly to the maintenance of the subtropical ridge between 25N and 30N (Fig. 2C). North of 25N the reversal in flow above 700 mb helps to establish and to maintain a shear zone in the mid-troposphere between easterlies and

5

F I G U R E 2: Kinematic analyses of mean resultant winds for July at: (A) 900 m, (B) 500 mb, and (C) 200 mb. Winds plotted over the Arabian Sea in Charts (B) and (C) are mean resultant winds determined from aircraft reports. Solid lines are streamlines and dashed lines are isotachs labelled in knots. The heavy dashed line on Chart (C) denotes the axis of the cross section of the mean resultant winds and mean temperatures for stations whose identifying number is given at the top of Chart (D) and on Figure 1 for this and all subsequent cross sections. The solid lines on Chart (D) give isotachs of total wind speed labelled in knots; dashed lines are isotherms in centigrade; and the heavy broken lines separate easterly and westerly winds. In Chart (D) mean temperatures in centigrade are plotted for July above the solid dots, and wind directions (first three digits) and wind speeds (last two digits) are plotted below. The letters E and W indicate centers of m a x i m u m easterlies and westerlies, respectively.

6

STRUCTURE

OF

AN

ARABIAN

SEA

SUMMER

MONSOON

westerlies. Warm-core cyclones often develop over the Bay of Bengal in July, extending to 300 mb. Usually, the mid-tropospheric portion of these systems remains over the east coast (see Fig. 2B) while the low-level portions first drift northwest, and then usually stagnate over the Ganges Valley. From day to day, wind directions in the high troposphere south of the subtropical ridge line vary little from east, but at times there are marked changes in the speed field. The primary jet stream in the easterlies forms in May and, until October, remains between 200 mb and 100 mb with a mean position near 5N. Secondary speed maxima form above 200 mb in the easterlies over the central Deccan and northeast Arabian Sea when cyclonic activity is intense in the mid-tropospheric trough. At times, the primary and secondary easterly jets merge over the south Deccan near 15N. The plane of the north-south cross section (Fig. 2D) is nearly perpendicular to the flow in both the lower and upper troposphere. Below 700 mb, near 30N, warm temperatures and weak winds are associated with the heat lows, and above 700 mb the weak winds are associated with the subtropical anticyclone. At approximately the 400 mb level, south of 25N, a zone of weak, variable winds separates the high-level easterlies from the lowlevel westerlies. The two cores of maximum easterlies have mean resultant wind speeds of about 70 kts in the primary jet at 175 mb, and of more than 70 kts in the secondary jet at 100 mb. In the mean, the maximum monsoon westerlies are found below the secondary jet in the lower troposphere. North of 35N the southern edge of the polar westerly maximum shows up; the mean center of this maximum lies near 40N. On almost any day in July cross sections along 73E will have the same general features as shown in Figure 2D. Day-to-day variations in position and intensity of the easterly and westerly maxima do occur; but the features described are persistent and reveal the general steady-state characteristics of the summer monsoon circulations.

SYSTEM

supply of rainfall from June to October, sooner or later a very large portion of western India is directly affected by drought and crop failure. Fifty years ago, a drought in one year signalled the approach of a famine in the next. But today, though droughts still occur, famine is rare. When the Arabian Sea monsoon is active, it replenishes the water reserves needed to carry India and her people through the next several months of nearly cloudless skies. Recent studies show that the Arabian Sea branch can be further divided into two separate currents. One part of the main low-level current flows northward across the equator between 45E and 60E and then, turning northeast, flows over the Arabian Sea, crossing the Konkan coast north of 15N. The southern current breaks away near 10N over the Arabian Sea and flows eastward across the south Malabar coast to join the Bay of Bengal branch in the vicinity of Ceylon. The daily surface charts indicate great steadiness in the direction and speed of the low-level winds over the Arabian Sea. An elongated speed maximum is always present near the east coast of Arabia in association with the large thermal gradient between land and sea (6). On occasion, over the central and eastern parts of the Arabian Sea, marked variations in the wind speeds occur over periods of several days. Detailed analyses reveal no well-organized downstream propagations of speed maxima or minima, as are found in the northeast trades of the North Pacific and, probably, in the southeast trades of the Indian Ocean. C. Divergence of the Mean Resultant Winds (Fig. 3)

The divergence and convergence patterns associated with the heat lows and the mid-tropospheric trough are the dominant features which are closely related to the weather over the sub-

B. Branches of the Monsoon Current (Fig. 1)

In the vicinity of the subcontinent of India, the monsoon current in the lower 5 km consists of two main branches: The Bay of Bengal branch, which influences the weather over the northeast part of India, the Bay of Bengal, Burma, and Malaya; and the Arabian Sea branch, which greatly influences the weather over the west, central, and northwest parts of India. If the Arabian Sea branch is inactive, i.e., fails to bring the needed

FIGURE 3:

Divergence

analyses

of the

mean

resultant

winds for:

(A) Surface to 900 m, (B) 700 mb, (C) 500 mb, and (D) 200 mb for July. The divergence values, given in units of 10"E sec-', were computed by a triangle method, similar to that described by Bellamy (5)', using mean resultant winds from rawin stations over India and from the mean streamlines at selected grid points over the Arabian Sea and Bay of Bengal. The letter C denotes a center of convergence, and D, a center of divergence. Dashed lines on Chart (C) are isohyetal for July

labelled

pattern.

in cm day-'

and

represent

the

mean,

gross

rainfall

8

STRUCTURE

OF AN

ARABIAN

SEA

SUMMER

MONSOON

continent in July. The mean monthly divergence values are about one order of magnitude less than the divergence values on any particular day in July. Over most of the subcontinent and adjacent water areas, the total divergence is zero. Convergence extends from the surface to above 500 mb between 15N and 25N from the northeast Arabian Sea across the Deccan to the Orissa coast. The mean centers of convergence are associated with the mid-tropospheric trough and with the cyclonic circulations which form over the Konkan coast and in the Bay of Bengal. The mean distribution of rainfall for July shows agreement with the mean low-level convergence patterns over the Deccan and the east coast. Above 300 mb the easterlies are divergent over the midtropospheric trough. The divergence in the easterlies compensates to a large extent the inflow and rising motion associated with the mid-tropospheric trough. The level of non-divergence at 20N is near 300 mb. Convergence in the lower troposphere from West Pakistan southward into the Ganges Valley is associated with the cyclonic-turning and rising air in the heat lows, but from 700 mb to near 300 mb, divergence is associated with the anticyclonicturning flow above the heat lows. Over the heat low in West Pakistan there are two levels of nondivergence; one near 850 mb and the other near 400 mb. Divergence associated with relatively strong winds over Kathiawar is a significant feature in the lower troposphere over the northeast Arabian Sea. This divergence combined with strong subsidence on the southern periphery of the heat lows results in many extremely hot and rainless days over northwest India and West Pakistan during July. Although divergence characterizes the lowlevel flow south of Kathiaivar and along the Konkan coast in the mean, some of the heaviest rains frequently occur along the Konkan coast. This apparent anomaly suggests that the principal rainproducing systems are to be found above the surface layer. Convergence is found in the mean low-level flow of the southern branch of the Arabian Sea monsoon along the Malabar coast and over the eastern Arabian Sea. The complex rainfall distribution along the Malabar coast is related to the convergent nature of low-level flow, orographic lifting of moist air over the Western Ghats, and cyclone development within the mid-tropospheric trough over the Konkan coast. While low-level con-

SYSTEM

vergence and frequent rains are characteristic of the Malabar coastal area, the divergence over the south Deccan below 700 mb results from west winds with mean resultant speeds of more than 20 kts. Relatively little rain falls over the south Deccan and Coromandel coast in July. In the middle troposphere there is very little divergence in the mean over south India, but between 300 mb and 200 mb, the upper-tropospheric easterlies are strongly divergent. The level of non-divergence is above 200 mb. High values of convergence associated with maximum easterlies above 200 mb probably compensate for the divergence below 200 mb. II. EARLY MONSOON DESCRIPTIONS

Prior to 1950, the literature concerning the Indian summer monsoon consisted primarily of descriptions of seasonal variations in rainfall, temperature, and cloud cover, as related to surface pressure patterns. Even before the nineteenth century it was recognized that the summer monsoon circulation was closely tied to the annual northward march of the sun. In 1628, Edmund Halley, the English scientist, writing in the Philosophical Transactions of the Royal Society, proposed that the monsoons of South Asia result from differential heating of land and sea surfaces. In 1921 Simpson (7) supported this hypothesis, but also emphasized the role of the Himalayas as an orographic barrier which forced the moist monsoon air upward, causing cloudiness and rainfall. This idea, popular as it was for many decades, did not explain the large variations in amounts of clouds and precipitation over west and central regions of the subcontinent. Before the radio wind observations became available, pibal wind data were sparse for the midand upper-troposphere for the summer rainy season, and hence the analyses of upper-level winds were biased towards fair-weather conditions; meteorologists had to be content with explaining the state of the atmosphere over India in terms of surface weather patterns and a few pibals. Variations in the intensity of the monsoons, as marked by changes in rainfall amounts, were explained by the movement of surface depressions into and along the "seasonal low-pressure trough." It was universally accepted that once the equatorial lowpressure trough shifted northward over the subcontinent, the onset of the monsoon rains was imminent. This concept still has some degree of popularity.

INTRODUCTION

III. RECENT MONSOON INVESTIGATIONS Research during the past 10 to 15 years has revealed that the establishment of the summer heat lows, or of the seasonal low-pressure trough, although a necessary condition, is certainly not always a sufficient one in bringing about the active monsoon weather conditions and heavy rains. The devastating droughts of 1899 and 1918 dramatize this fact. Over the years, many detailed studies have been made of the surface synoptic charts associated with extreme weather conditions of the summer monsoon; yet, until very recently, the true nature of this large-scale circulation has been practically unknown. With the exception of a few investigations made over very limited regions of South Asia, the lack of detailed and unbroken records of wind data f o r the upper atmosphere has precluded the possibility of thoroughly investigating major aspects of the Asian monsoons. Today, surface weather charts are still useful f o r describing surface conditions, but, in reflecting changing monsoon conditions, they are no more revealing than they were fifty years ago. However, recent innovations such as 24-hour pressure-change analyses and surface temperature anomaly charts, when combined with upper-air analyses, provide useful diagnostic adjuncts to the tropical meteorologist's conventional tools. The advent of an active rawinsonde program in India in 1953 brought on increasing efforts to solve some of the mysteries of the upperatmospheric circulations and associated weather patterns of the summer monsoon. Subsequently, published descriptions have become increasingly complex. Since 1955, a great deal has been written about the monsoons over South Asia, particularly by meteorologists in India. One of the bestconsolidated references on this subject (as it applies to India) is Monsoons of the World (8), a collection of papers published by the India Meteorological Department and presented at a symposium on monsoons in N e w Delhi during February 1958; the variety of descriptions and hypotheses presented is indeed impressive. Several workers attributed the onset of the rainy season and variations in rainfall within that period to development of an easterly jet and a westward progression of easterly wave perturbations at levels near 100 mb (9); part of this postulate presupposed intensification of the subtropical anticyclone over the heated Tibetan Plateau. Others considered the movement, location, and in-

9

tensity of the subtropical anticyclone in the upper troposphere to be the prime factors in creating active or inactive monsoon conditions (10, 11, 12, 13,1U) • These workers placed primary emphasis on the high troposphere. Still others (15) emphasized the external influences on the mid-troposphere and reported that the year-to-year variations in the beginning or "burst" of the monsoon rains were directly influenced by large-scale troughs in the westerlies. Still others considered that low-level synoptic events provided the monsoon activators—through combined actions among the heat lows, the orographic lifting of moist air over the Western Ghats which parallel the west coast of India, and the pulsations in the low-level basic southwest current (16, 17). In addition, a proposal has been made (18) that pulsations within the southeast trades in the south Indian Ocean propagate northward across the equator into the southwest monsoon current of the Arabian Sea and ultimately bring about increases in rainfall along the west coast. Occasionally, too, emphasis has been placed on equatorial eddies, south of the subcontinent and near the equator, as significant though littleunderstood contributors to the changes in the monsoon current. In recent years, there has been a growing interest in and awareness of the influence of the mid-tropospheric circulations over south Asia and associated divergence patterns south of the subtropical ridge (19, 20).

IV. DATA AND A N A L Y T I C A L PROCEDURES A. Observations

A i r c r a f t of the U. S. Weather Bureau Research Flight Facility ( R F F ) and the Woods Hole Oceanographic Institution ( W H O I ) reconnoitered large regions of the Arabian Sea from very low levels to as high as 14 km during 63 separate weather missions f r o m 15 May to 11 July 1963. Thirty-one research missions were flown by R F F and W H O I aircraft between 26 June and 10 July. Details concerning the participating aircraft, days of operation, and type of observations are summarized in T A B L E 1. The R F F (DC-6) flights were usually made simultaneously over the same track, one near 500 m, and the other, near 6 km (the 500 mb level). Occasionally, both the R F F and W H O I flew low-level missions on the same days (see T A B L E 1), though in different areas. Meteorological data recorded on magnetic tape by airborne computers or obtained from films of the

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