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 the understanding of the role of the Western Ghats in shaping regional 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 is used to identify the role of topography. A No Latent Heating simulation is used to understand the role of latent heating. The combined effects of topography and latent heating are examined in a No Mountain No Latent Heating simulation. The impact of sea surface temperature (SST) on precipitation is also explored by adding an idealized warm pool to realistic SST forcing.

Convection simulated in the WRF model agrees with TRMM and MODIS satellite products. These sensitivity simulations are used to quantify the changes of the precipitation, e.g., the mean rate, spatial distribution, and diurnal cycle. The control simulation shows rainfall maximum over the Western Ghats and over 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, convection still occurs along the western coast. In the No Latent Heating simulation, there is shallow stratiform cloud over the crest of the Western Ghats. WRF simulates drizzle from this cloud. This stratiform precipitation disappears in the No Mountain No Latent Heating simulation.

The Western Ghats enhances local orographic convection and produces a stronger “rain shadow” in the downwind continent. The upwind offshore convection over the Arabian Sea depletes water vapor fluxes and CAPE, reducing coastal convection downwind. Stronger offshore convection is associated with higher SSTs in the Arabian Sea, suggested by both observation and sensitivity simulation.

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.