Use of TOPEX altimeter for the study of diurnal and spatial distribution of southwest monsoon rainfall over the Bay of Bengal and the Arabian Sea

Abstract

In this paper, the rain detection capability of the dual-frequency (Ku and C band) radar altimeter onboard, the non-sun-synchronous TOPEX/Poseidon (T/P) satellite is exploited to study the diurnal variability of rainfall over Indian oceanic regions during the southwest monsoon season. The study is done using three consecutive years (1993, 1994 and 1995) of T/P altimeter data. Based on the difference of normalized backscattered coefficient, Δ σ o (C–Ku band), the T/P satellite observations are categorized into three different classes of “no rain,” “low rain,” and “high rain.” The data is further divided into 12 time intervals of 2 h each, starting from 0000 to 2400 Z. The uneven distribution of observations in each time interval due to orbit characteristics of T/P satellite is removed. The histograms of rain events are plotted for the Bay of Bengal and the Arabian Sea to study diurnal and regional variability of rain events. The variation of rainfall intensity, within “high-rain” category, over the two regions is also studied. The results showed that there is no consistent diurnal variability of rainfall exist over the Arabian Sea and the Bay of Bengal regions from year to year. However, the 3-year composite data shows more rain events over the Arabian Sea at early morning hours between 0000 and 0200 GMT. This is verified by concurrently available TOPEX Microwave Radiometer (TMR) observations of rain events. The intensity of rain rate also does not show any marked diurnal variability. The probability of rain events is significantly high over the Bay of Bengal region compared to the Arabian Sea region. This is also verified with TMR-based analysis. Further, interannual variability of rain events and amount over the two regions from Δ σ o-based analysis is also discussed in association with interannual variations in the monsoon activities over these two regions.