On the Diurnal Cycle of Rainfall Near the Western Ghats during the Indian Summer Monsoon: Influences of the Low-Level Atmosphere and Land Surface

During the Indian Summer Monsoon, moisture-laden low-level westerly flow brings heavy rainfall to western India, due in part to the presence of the Western Ghats (WG). Though most of this precipitation is orographically generated, the land surface itself, and meteorology of the boundary layer play a secondary but important role. In this study, we show the influence of the low-level atmosphere and land surface on the diurnal cycle of precipitation near the WG through analysis of high-resolution convection-resolving simulations. The Weather Research and Forecasting model (WRF), coupled with the Noah LSM, was run continuously for August and September of 2008, 2009, and 2010 at a grid spacing of 5 km, with initial and lateral boundary conditions provided by the ECMWF ERA-Interim. We find that the model-generated precipitation compares remarkably well with the high-resolution gridded gauge-based rainfall dataset of Pai et al. (2014), when looking at “seasonal” daily rainfall statistics such as mean rainfall, standard deviation of daily rainfall, and frequency of rainy days, heavy rain days, and very heavy rain days. We also compare probability density functions (PDFs) of simulated rainfall against PDFs of the Pai et al. (2014) dataset for various regions near the WG to ensure WRF reproduces precipitation accurately. We find a wet bias in model-generated precipitation in the lee of the WG.

Output from the WRF-ARW at time increments of 30 minutes allows for analysis of temporal patterns of rainfall and insights into governing mechanisms. Analysis of rainfall rates over these three monsoon seasons reveals a clear diurnal pattern along the escarpment of maximum rates in the early afternoon. Timing of maximum precipitation rates translates into later afternoon and evening hours eastward in the lee of the WG. Just offshore, an early morning maximum in rainfall is observed. We link the observed patterns of the diurnal cycle of rainfall with land surface variables such as latent heat flux, sensible heat flux, and soil moisture, as well as low-level meteorological parameters such as low-level wind speed, depth of the planetary boundary layer, Convective Available Potential Energy (CAPE), and Convective Inhibition (CIN). The most pronounced diurnal cycles are found to exist in the lee of the WG, where the land surface and low-level meteorological parameters play a stronger role, particularly when synoptic forcing is weak. In conclusion, we compare our findings with observed diurnal cycles of precipitation over more inland regions of western India from other studies.