EMPIRICAL ORTHOGONAL FUNCTION-ANALYSIS OF HUMIDITY PROFILES OVER THE INDIAN-OCEAN AND AN ASSESSMENT OF THEIR RETRIEVABILITY USING SATELLITEMICROWAVE RADIOMETRY

The paper examines the variability of vertical humidity profiles over the Indian oceanic region using a set of 1200 radiosonde observations spanning 10 years (1982-1991). The examination is based upon the metho d of empirical orthogonal function (EOF) analysis. The first EOF expla ins 61 % of the total variance and the first three EOFs together accou nt for 85 % of the total variability. The first principal component is almost perfectly correlated with the total precipitable water (TPW) a nd the second one is well correlated with the ratio of boundary layer moisture and TPW. This fact and an inequality derived from the analysi s of the variances of individual terms of the EOF expansion of specifi c humidity are utilised to establish an algorithm for retrieving humid ity profile from satellite microwave measurement of TPW over the regio n of study. Power of the retrieval technique is demonstrated using 127 independent radiosonde measurements and by plotting the profiles of r ms error and bias. The method is found to be distinctly superior compa red to a power-law retrieval. A few examples of profile retrieval from satellite measurements of TPW have been checked against colocated rad iosonde measurements. Some examples of retrieval show that the method is uniquely able to capture the humidity variability in the boundary l ayer, particularly the high moisture loading in the monsoon season.