Impacts of the Western Ghats on Regional Precipitation in the Summer Monsoon

Orographic precipitation over the Western Ghats is the major source of the precipitation along the western coast of India. The deep convection over the mountains also influences the large-scale summer monsoon circulation and the upper-level gravity waves. To advance our understanding of the role of the Western Ghats in shaping precipitation, we conduct a series of high resolution cloud-resolving simulations using the Weather Research and Forecasting (WRF) model. Besides a control simulation, sensitivity simulations are conducted to explore the role of different physical forcing. A no-mountain simulation, with all terrain removed, is used to identify the role of topography. A no-latent-heating simulation, in which the latent heating of convection is disabled, is used to understand the role of latent heating.

These cloud-resolving simulations are used to quantify the impacts of the Western Ghats on the precipitation, e.g., the mean rate, frequency, intensity, spatial pattern, and diurnal cycle, over Western Ghats, the upstream Arabian Sea, and the downstream Indian peninsula. The control simulation shows rainfall maximum over the Western Ghats and the Arabian Sea about 600 km offshore west of the coast. The deep convection reaches above the level of 200 hPa. In the no-mountain simulation, the mean rainfall rate is reduced along the Western Ghats compared with the control simulation, while the rainfall maximum over the Arabian Sea show little change.

Over the upstream offshore region, the observed wind reversal from low-level westerlies to upper-level easterlies is captured in the model. We hypothesize that the orographic precipitation over Western Ghats is related to the low-level westerlies and the water vapor flux from upstream. The simulated convective available potential energy (CAPE) and zonal water vapor flux decrease geographically when approaching the continent, perhaps due to offshore moist convection. The model also reproduces the dry region to the east of the Western Ghats, even in the no-mountain simulation. We hypothesize that the land surface fluxes influence the instability and therefore the precipitation.

This Western Ghats study is an initiative toward a broader inter-comparison with other tropical mountain regions, including West Africa, East Africa, and Myanmar. These comparisons aim to improve our understanding of convection over the tropical mountains from a global perspective.