Characteristics of Precipitating Convective Systems in the South Asian Monsoon

Abstract

Eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data show how convective systems of different types contribute to precipitation of the South Asian monsoon. The main factor determining the amount of precipitation coming from a specific system is its horizontal size. Convective intensity and/or number of embedded convective cells further enhance its precipitation production. The precipitation of the monsoon is concentrated in three mountainous regions: the Himalayas and coastal ranges of western India and Myanmar. Along the western Himalayas, precipitation falls mainly from small, but highly convective systems. Farther east along the foothills, systems are more stratiform. These small and medium systems form during the day, as the monsoon flow is forced upslope. Nighttime cooling leads to downslope flow and triggers medium-sized systems at lower elevations. At the mountainous western coasts of India and Myanmar, small and medium systems are present throughout the day, as an orographic response to the southwesterly flow, with a slight superimposed diurnal cycle. Medium systems are favored over the eastern parts of the Arabian Sea and large systems are favored over the Bay of Bengal when an enhanced midlevel cyclonic circulation occurs over the northern parts of these regions. The systems forming upstream of coastal mountains over the Bay of Bengal are larger than those over the Arabian Sea, probably because of the moister conditions over the bay. The large systems over the bay exhibit a pronounced diurnal cycle, with systems forming near midnight and maximizing in midday.

Intra-seasonal variability of cloud amount over the Indian subcontinent during the monsoon season as observed by TRMM precipitation radar

“…As the system organization increases, the stratiform fraction increases, as does the general spread and amount of rainfall (in phase with the increase in the stratiform cloudiness). Romatschke and Houze (2011) also observed similar characteristics over the Indian monsoon region from TRMM observations, and inferred that the convective (stratiform) fraction decreases (increases) with the increasing system size. Recent studies (e.g., Lin et al, 2004;Abhik et al, 2012) using TRMM observational and NWP model reanalysis data have also documented the lead-lag relationship of convective and stratiform rainfall buildup at the onset of the ISM active phase.…”

“…on the WCI, the convective systems form windward of the terrain of Western-Ghat Mountain as an orographic response to the southwesterly monsoon flow, which develops inward of coast line. Earlier observational studies indicated that the ISM cloudiness over this region is mostly convective (Romatschke and Houze, 2011;Grossman and Durran, 1984). These studies noted that since the convection over this region is non-extreme in nature, they do not develop large stratiform outflows.…”

“…Since these phases are spread over nine years, the diurnal bias in the sampling is also another likely source of error (Negri et al, 2002 andBowman et al, 2005). We expect the benefits of having such a high-resolution dataset to outweigh the disadvantage of some statistical fluctuations since the horizontal gradients and temporal scale of precipitation over this region during the monsoon season, are probably much larger than the sampling errors (Anders et al, 2006;Romatschke and Houze, 2011). Also Schumacher and Houze (2003) and Bell and Kundu (2000) have suggested that 0.6 m/year rain accumulation threshold gives less noisy results.…”

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

“…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.…”

“…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.…”