SATELLITE OBSERVED ATMOSPHERIC WATER VAPOUR FIELDS DURING THE INDIAN SUMMER
MONSOON :
ANALYSIS AND SENSITIVITY STUDIES
by
CHANDRA MOHAN KISHTAWAL
Centre for Atmospheric Sciences
THESIS SUBMITTED
IN FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
to the
INDIAN INSTITUTE OF TECHNOLOGY, DELHI
June 1994
CERTIFICATE
This is to certify that the thesis entitled ' SATELLITE OBSERVED ATMOSPHERIC WATER VAPOUR FIELDS DURING THE INDIAN SUMMER MONSOON : ANALYSIS AND SENSITIVITY STUDIES' being submitted by CHANDRA MOHAN KISHTAWAL for the award of the degree of DOCTOR OF PHILOSOPHY is a record of the original bonafied 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 the award of any degree/diploma.
(Dr. O.P. Sharma ) Centre For Atmosphei Indian Institute of Tec Hauz Khas, New Delh
(Dr. M.S. Narayanan)
Meteorology and Oceanography Group Space Applications Centre
(Indian Space Research Organisation) AHMEDABAD - 380053
Acknowledgements
My sincere thanks are due to my supervisor, Dr. M S Narayanan, who took personal interest and encouraged me to take up the present research work. At all the stages of my research he kindly provided me necessary guidance and motiva- tion for the completion of my thesis. I owe a great deal to him for the care he took to improve my computational and writing skills.
I am especially thankful to Dr. 0 P Sharma who, despite knowing my shortcom- ings, constantly encouraged me and taught me how to see light through clouds.
His sporting spirit and friendly attitude was an stimulating experience for me. From him I learnt that a research work is like an adventure, which is full of challenges, uncertainties and a lot of excitement !
My dissertation was kindly previewed by Dr. P C Pandey, and Dr. P C Joshi Scientist, MOG, whose pertinent suggestions helped me a lot in improving my dissertation.
I would like to express my sincere appreciation to my senior colleague Dr.
P. K.PaI who provided many valuable suggestions and guidelines at all stages of my research work. I owe my sincere thanks to Dr. Sujit Basu from whom I learnt some valuable lessons in mathematics.
I would like to thank all my MOG colleagues who provided, me much needed encouragement and from whom I have learnt a lot. I would like to mention the names of Mr. W.J. Prakash and Mr. Rajkumar for helping me in the preparation of my thesis and also Mrs. Sathyabhama and Mr. H.I. Andharia for providing me uninterrupted computational facilities for my research work. My one year stay in IIT-Delhi was an unforgettable experience. I am especially thankful to my IIT-D friends Sangeeta Agrawal , Ashish Mitra and Mukul Teweri for their kind support and help.
I don't have words to express my gratitude to my parents who incurred a great deal of hardship for the upliftment of their children. May God give me enough strength for living upto their expectations.
Finally I gratefully acknowledge the continuing patience,encouragement and support of my dear wife Anju who cheerfully and unselfishly contributed to boost my morale in tough times.
C CM. k I
51-1TAWAL)ABSTRACT
The study of atmospheric water vapour and its variability is of crucial importance because of the influence of water vapour on a number of geophysical processes. Also a precise knowledge of water vapour and its vertical distributions is prerequisite to successful numerical weather prediction (NWP) especially in the tropics. The origin, transport and variability of water vapour is critical for determining the rainfall during the Indian summer mon- soon which affects the life and economy of India and its people. Until recently the scarcity of meteorological observations over the Indian Ocean has limited the study of water vapour fields during monsoon, but now data from different spaceborne observing systems offer a new opportunity to observe and describe these systems. Due to large data coverage, reasonable spatial and temporal resolution and representability, the space borne systems are the only logical choice for the study of large scale moisture fields. However, even at present the moisture information retrieved from these __ systems is far from complete and different sensors suffer from their own
limitations.
In the present thesis, the applicability of satellite observations of water vapour in diagnostic analysis during the Indian summer monsoon is explored.
' Numerical experiments have been carried out to assess the sensitivity of monsoon flow to the variations in water vapour fields.
In Chapter-2 of the present thesis NOAA/HIRS derived moisture fields during different phases of two normal and two poor monsoon years have been analysed. Seasonal mean fields of moisture in low (surface to 700 mb) and middle layer (700-500 mb) show a dry anomaly over the Arabia subcontinent
and wet anomaly over the Bay of Bengal during good monsoon years while anomalies show an opposite trend during the poor monsoon years. The zonal and meridional propagation of low frequency oscillation in satellite derived moisture fields is also examined. Fluctuations in the 30-50 day range are found to exhibit longer time period and high amplitudes in the poor monsoon years while in good monsoon years these fluctuations are of small time period - small amplitude type. It was observed that the difference of zonally and monthly averaged precipitable water over the Bay of Bengal and the Arabian Sea , termed as the Differential Moisture Signal (DMS) is an indicator of total Indian rainfall, excluding orographically influenced regions. In the same chapter , NOAA/HIRS derived moisture fields have been used to compute surface level moisture transport for the monsoon seasons of 1980,1981 and 1984. The global relationship between monthly mean surface-level humidity and precipitable water (Liu, 1986) has been applied for the computation of surface level humidity from satellite derived total precipitable water (PW).
Monthly mean analysed wind fields at surface level were provided by Florida State University and were used for moisture flux computations. The analysis indicates net positive surface-level moisture flux divergence over the Arabian sea and net negative moisture flux divergence over the Bay of Bengal throughout the monsoon season. It has also been noticed that evaporation over the Arabian sea is a variable quantity and forms a significant part of the net moisture budget over the Arabian Sea. The relative contribution of cross- equatorial flux and evaporation from the Arabian Sea has been studied for all three years.
Microwave instrument SSM/I onboard DMSP satellite provides measurements of precipitable water in all weather conditions with reasonable
accuracy. In Chapter-3 the applicability of these observations over the Indian Ocean were examined by comparing the evolution of moisture fields observed by SSM/I with those provided by ECMWF global analysis model, for the pre- onset phase of 1988 monsoon. A significant positive bias in SSM/I measure- ments of PW was noticed over the regions of high convective activity. A simple 'approach based on the linear regression analysis of a large number of ship radiosonde observations over the Arabian Sea is suggested to estimate the horizontal moisture fluxes in terms of satellite derived lower tropospheric wind velocities and total precipitable water. In the same chapter the variability of vertical humidity profiles over the Indian Ocean have been examined using 1200 ship radiosonde observations spanning ten years. The examination is based upon the method of empirical orthogonal function (EOF) analysis. The first EOF explains 61 % of the total variance and the first three EOFs together account for 85% of the total variability. The first principal component is almost perfectly correlated with the total precipitable water (PW) and the second one is well correlated with the ratio of boundary layer moisture and PW. This fact and an inequality derived from the analysis of the variance of individual terms of the EOF expansion of specific humidity are utilised to establish an algorithm for retrieving humidity profiles from satellite microwave measurements of PW over the region of study. The method is found to be distinctly superior compared to a power-law retrieval, which is the most commonly used form of structure function for retrieving humidity profiles in NWP models. In a case study, assimilation of SSM/I observed PW fields in a general circulation model indicates some improvement in rainfall prediction over convective regions.
In Chapter-4 the sensitivity of numerical prediction of monsoon to
variations in water vapour fields has been examined by conducting some experiments with a general circulation model. Newtonian relaxation method is adopted to subject the model atmosphere under sustained moisture anomalies.
The impact of negative variations of moisture was seen as a divergent circulationtion anomaly, while that of the positive variation was seen as a stronger convergent anomaly. Although the humidity fields display a resilient behavior, and relax back to normal patterns (1-3 days) after the forcing terms in humidity equation are withdrawn, the circulation anomalies created by the moisture variation keep growing. A feedback between positive mois- ture anomalies and low level convergence exists, which is terminated in the absence of external forgings of water vapour.
