Regional, Seasonal, and Diurnal Variations of Extreme Convection in the South Asian Region

Abstract

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.

Variation in the orographic extreme rain events over the Meghalaya Hills in northeast India in the two halves of the twentieth century

“…2), are related to the short, intense convective downpours that occur more frequently between the end of March and May Goswami et al 2010). This progressive change in rainfall character, from more convective to less convective with a growing contribution of light rains, is already visible at the end of the summer monsoon season and is continued during the post-monsoon season (Romatschke et al 2010;Romatschke et al 2011b). At Cherrapunji, the changes in frequency of the rainfall pattern show a significant (z = −2.6) reduction in light rains, and a close-to-significant (z=1.8) increase in heavy rains, in the pre-monsoon season.…”

“…3) is the consequent lowering of the correlation with the increasing rainfall limit. The high correlation of 0.55 for light rainfall events between Cherrapunji and Shillong probably reflects the large stratiform rainfall systems which developed over the Meghalaya during the monsoon season (Romatschke et al 2011b;Goswami et al 2010;Romatschke et al 2010). These large systems, with irregularly embedded convective cells enhanced locally by orographic lifting, produce high rainfall within an approximately 40-km-wide zone (twice as wide as in the pre-monsoon season) at the southern part of the Meghalaya.…”

Evolution of precipitation and convective echo top heights observed by TRMM radar over the Indian Ocean during DYNAMO

“…The PR data sets used are the version 7 TRMM 2A25 and 2A23 products, which contain estimated near-surface rain rate, three-dimensional spatial distribution of reflectivity, and rain-type (convective, stratiform, and other) classification [Awaka et al, 1997]. Data were processed as in Houze et al [2007] and Romatschke et al [2010], and the data were mapped onto a 0.05°× 0.05°Cartesian grid with 0.25 km vertical spacing. The TRMM precipitation radar has a minimum detectable reflectivity factor of about 17 dBZ, so echo top heights are determined by the height of the 20 dBZ contour.…”

Seasonal and Diurnal Variations in Rainfall Characteristics over the Tropical Asian Monsoon Region Using TRMM-PR Data

“…Several previous studies have focused on seasonal change of diurnal convective systems over Malaysia and Japan (Oki and Mushiake 1994), the Tibetan Plateau (e.g., Fujinami and Yasunari 2001), South Asia (Kodama et al 2005;Romatschke et al 2010;Houze 2011a, 2011b), and some tropical regions (e.g., Biasutti et al 2012). The studies in South Asia found that convective rainfall systems are dominant in the pre-monsoon season.…”