Study on air-sea interaction processes over the Indian seas during different epochs of Asian summer monsoon

STUDY ON AIR-SEA INTERACTION PROCESSES OVER THE INDIAN SEAS DURING DIFFERENT EPOCHS

OF ASIAN SUMMER MONSOON

by

MOHAN KUMAR N.

Centre for Atmospheric Sciences

submitted

in fulfilment of the requirements of the degree of DOCTOR OF PHILOSOPHY

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI INDIA

JULY. 1991

CERTIFICATE

This is to certify that the thesis entitled - A Study On Air-Sea Interaction Processes Over The Indian Seas During Different Epochs of Asian Summer Monsoon - being submitted by Mohan Kumar. N for the award of the degree of DOCTOR OF PHILOSOPHY, is a record of the original bonafide research work carried out by him. He has worked under our joint guidance and supervision and has fulfilled the requirements for the submission of this thesis. The results presented in this thesis have not been submitted in part or full to any other University or Institute for award of any degree or diploma.

„--Ac4x1 a4xo, lAr04„..2

(U.C. MOHANTY) (P.C. SINHA)

Principal Scientific Officer Chief Scientific Officer Centre for Atmospheric Centre for Atmospheric

Sciences ' Sciences

Indian Institute of Technology Indian Institute Of Technology

New Delhi New Delhi

ACKNOWLEDGEMENTS

I express my deepest sense of gratitude to Dr. U. C.

Mohanty and Dr. P. C. Sinha for their valuable guidance and constant encouragement throughout the course of this work.

I acknowledge Prof. Abraham H Dort for kindly providing the Comprehensive Ocean Atmosphere Data Set analysed at GFDL, USA.

My thanks are due to the India Meteorological Department for providing the MONEX- 79 dataset.

I gratefully acknowledge the help extended to me by the faculty members and staff of CAS, IIT, Delhi.

I express my sincere thanks to Dr. T. Venugopal for his suggestions and help during the course of this work. Many friends have helped me during my stay at the centre, I wish to express my cordial thanks to all of them in particular Dr.

Alexander John, Dr. K. J. Ramesh and Mr. K. V. J. Potty.

I am extremely grateful to the Director, NPOL, Cocnin and my colleagues in the Ocean Science Group for their support during the course of this work.

I thank Mr. Aravind Swain and Mr. Ashok for taking extreme care in drafting the manuscript.

ABSTRACT

In recent years the air-sea interaction studies have assumed significance owing to their applications in climatic studies.

The studies of air-sea interaction processes over the tropical Indian seas are meager due to inadequate observations. Since the large scale monsoon flow is a result of the positive feedback from the ocean to atmosphere, a proper understanding of the ocean-atmosphere coupling is essential to document the variabilities and the intensity of the monsoon. The influence of the surface marine meteorological parameters especially sea surface temperature has been investigated by a number of researchers. However, these study could not firmly establish the role of SST in the monsoon variability as they revealed conflicting results. In the present study attempt has been made to document the synoptic and the climatic scale variabilities of air-sea interface fluxes of heat and moisture in relation to the monsoon activity.

Results on the interlink between surface fluxes and large scale convective activity lead to a close look in to the intensity of the marine boundary layer (MBL) and transfer of fluxes through it. Therefore, study of the marine boundary layer characteristics in terms of intensity and exchange processes over the tropical Indian seas is also important in the monsoon meteorology.

This is due to the fact that in the processes of transport of momentum, heat and moisture fluxes from the oceanic surface to

the free convective zone, the MBL plays an important role. In view of the above fact, a study on the structure of marine boundary layer is examined in the frame work of a diagnostic (steady state one dimensional model) and time dependant model.

The height of marine boundary layer and turbulent kinetic energy which is the measure of the intensity of turbulence and turbulent eddy exchange coefficient which transport fluxes across the marine boundary layer are examined during different epochs of the summer monsoon.

These studies clearly demonstrate a positive feedback of upper oceanic surface fluxes and large scale convective activity in relation to summer monsoon. Further, development of a coupled upper layer-PBL convective model is desirable to look in to the air-sea interaction in the light of monsoon activity. Such a study in turn will lead to a better understanding on the Asian summer monsoon.

The objective of this thesis is to carry out a detailed study of the surface marine boundary layer processes over the Indian seas during different epochs of summer monsoon on a synoptic and climatic time scale. A detailed modelling study of the turbulent and mean structure of the monsoon boundary layer is also attempted.

CONTENTS

Page No.

ABSTRACT

CHAPTER I INTRODUCTION 1.1 Introduction

1.2 Field experiments conducted to study air-sea interaction processes

1.2.1 Review of air-sea interaction processes over the Indian seas

1.2.2 Studies on marine boundary layer over the Indian seas

1.3 Objectives of the present study CHAPTER II OCEANIC HEAT BUDGET ESTIMATION ON A

SYNOPTIC TIME SCALE

2.1 Introduction 23

2.2 Data set used for the study 25 2.2.1 Classification of Phases 26 2.3 Computational Procedure 27 2.3.1 Computation of shortwave radiative flux 28 2.3.2 Computation of longwave flux 34 2.3.3 Computation of Sensible and Latent

heat fluxes 38

2.4 Results and Discussions 40 2.4.1 Estimation of Shortwave flux 40

1

3

8

16

20

2.4.2 Estimation of Longwave flux 45 2.4.3 Variability of surface marine meteorological

fields and the components of oceanic heat budget."

CHAPTER III CLIMATOLOGICAL VARIABILITIES OF SURFACE MARINE METEOROLOGICAL FIELDS DURING FLOOD, NORMAL AND DROUGHT YEARS

3.1 Introduction 54

3.1.1 Studies on heat budget estimates over the

Pacific and Atlantic Oceans 55 3.1.2 Heat budget estimates over the Indian Ocean 56

3.2 Data 57

3.3 Method of Computation 59 3.3.1 Estimation of incoming solar radiative

flux Q

R 60

3.3.2 Estimation of effective outgoing longwave 62 radiation Q

B

3.3.3 Computation of sensible and Latent heat 62 fluxes (Q

H and Q E)

3.4 Results and Discussions 64

3.4.1 Validity of Data 65

3.4.2 Validity of the method used for the estimation of oceanic heat budget 66 3.4.3 Variabilities of surface marine meteorological

parameters and heat budget components

during flood, normal and drought years 69

3.4.4 Difference fields 75

CHAPTER IV SURFACE MARINE BOUNDARY LAYER CHARACTERISTICS OVER THE INDIAN SEAS DURING DIFFERENT EPOCHS THE SUMMER MONSOON : STEADY STATE MODEL

4.1 Introduction

78

4.1.1 Characteristics of PBL over the

Indian Seas

81

4.2 Method of computations

84

85

91

92

4.3 Data

97

4.4 Model Outputs

98

4.5 Results and Discussions

99

OVER THE INDIAN SEAS DURING DIFFERENT EPOCHS OF SUMMER MONSOON : TIME

DEPENDENT MODEL

5.1 Introduction

113

5.2 Model Description

114

114

117

119

5.4 Data

121

5.5 Results and Discussions

122 127

REFERENCES