NASA - National Aeronautics and Space Administration

Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data obtained over South Asia during eight premonsoon seasons (March–May) show that the precipitation is more convective in nature and more sensitive to synoptic forcing than during the monsoon. Over land areas, most rain falls from mediumsized systems (8500–35 000 km2 in horizontal area). In continental regions near the Himalayas, these mediumsized systems are favored by 500-mb trough conditions and are of two main types: 1) systems triggered by daytime heating over high terrain and growing to reach maximum size a few hours later and 2) systems triggered at night, as moist upstream flow is lifted over cold downslope flow from the mountains, and reaching maximum development upstream of the central and eastern Himalayas in the early morning hours. Systems triggered by similar mechanisms also account for the precipitation maxima in mountainous coastal regions, where the diurnal cycles are dominated by systems triggered in daytime over the higher coastal terrain. Medium-sized nocturnal systems are also found upstream of coastal mountain ranges. West-coastal precipitation systems over India and Myanmar are favored when low pressure systems occur over the upstream oceans, whereas Indian east-coastal systems occur when high pressure dominates over Bangladesh. Over the Bay of Bengal, the dominant systems are larger (.35 000 km2), and have large stratiform components. They occur in connection with depressions over the Bay and exhibit a weaker diurnal cycle.