Temporal and spatial variations of convection in South Asia are analyzed using eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data and NCEP reanalysis fields. To identify the most extreme convective features, three types of radar echo structures are defined: deep convective cores (contiguous 3D convective echo $40 dBZ extending $10 km in height) represent the most vertically penetrative convection, wide convective cores (contiguous convective $40 dBZ echo over a horizontal area $1000 km2) indicate wide regions of intense multicellular convection, and broad stratiform regions (stratiform echo contiguous over an area $50 000 km2) mark the mesoscale convective systems that have developed the most robust stratiform regions.
The preferred locations of deep convective cores change markedly from India’s east coast in the premonsoon to the western Himalayan foothills in the monsoon. They form preferentially in the evening and over land as near-surface moist flow is capped by dry air aloft. Continental wide convective cores show a similar behavior with an additional nocturnal peak during the monsoon along the Himalayan foothills that is associated with convergence of downslope flow from the Himalayas with moist monsoonal winds at the foothills. The oceanic wide convective cores have a relatively weak diurnal cycle with a midday maximum. Convective systems exhibiting broad stratiform regions occur primarily in the rainiest season and regions— during the monsoon, over the ocean upstream of coastal mountains. Their diurnal patterns are similar to those of the wide convective cores.