Diurnal variation of cloudiness over South Asia using satellite observations

[ABSTRACT]The climatological features of the diurnal cycle of cloudiness were investigated over three convective centers of interest over South Asia: over the Himalayas; north India and the Bay of Bengal (BOB). Three hourly blackbody brightness temperature (TBB) data from the METEOSAT-5 and rainfall data (3B42) from the Tropical Rainfall Measuring Mission (TRMM) were utilized. Analysis was concentrated on pre-monsoon and the summer monsoon season. Two methods were used to detect cloudiness: one method counted cloud clusters (CC) and utilized cloud tracking technique to identify time clusters (TC); and the other method computed cloud cover frequency (CCF). CC was defined as a contiguous area of pixels with TBB lower than the threshold of 219 K. The CCF was derived using 235 K as the threshold. From the analysis, it was noted that there were many CC over north India in the pre-monsoon season especially during the late afternoon hours. But deep convective activity was weaker over the region during the summer monsoon season. Bimodal diurnal cycle in CC occurrence was noted over the Himalayas during the summer monsoon season. It was noted that the peak convective activity occurred at 6 LT for the Bay of Bengal during both the summer monsoon and the pre-monsoon season. The life cycle of cloud clusters during summer monsoon season was also investigated. The diurnal cycle varied with location. There appeared a gradual delay in the preferred time of initiation, attainment of maximum area and dissipation as one progresses northward from BOB region through north India to the Himalayas.