The influence of coastal topography on the South Asian Monsoon

Recent high-resolution precipitation observations from the Tropical Rainfall Measuring Mission (TRMM) have exemplified narrow, extremely heavy precipitation zones near and upstream of the coastal mountain ranges in South Asia (the Western Ghats in India and the Arakan Mountains in Myanmar). These narrow precipitation zones, and associated downstream rain shadows, play an important role in the ecoclimatological stability of the region, while modulating atmospheric heating and water and energy cycles of the South Asian Monsoon. While the role of the Tibetan Plateau has been shown by Yanai and others to be an important driver for the large scale circulation of the South Asian Monsoon, these smaller mesoscale mountain ranges may also play an important role in driving onshore moisture flows through wind-terrain-heating feedbacks.

In this study, we will examine the spatiotemporal variability of precipitation in these zones through use of data from the TRMM 3B42 3 hourly precipitation retrieval. Data from the TRMM precipitation radar (PR) will be used to characterize the horizontal and vertical structure of precipitation and latent heating. It is found that a significant fraction of the precipitation on the slopes of the Western Ghats tends to fall from isolated convection that has limited vertical extent, while precipitation over the slopes of coastal mountains in Myanmar tends to be deeper in vertical extent despite both regions exuding similar monsoon total precipitation. The dynamical environment of these two regions will be contrasted from reanalysis data, focusing on (1) lower tropospheric moist static stability, Froude number, vertical wind shear, and critical level variations, and (2) the implications of these contrasting convective regimes (tropospheric moistening and drying) on the broader monsoon circulation.