Sensitivity of Monsoon Precipitation on Local Evaporation and Large-Scale Circulations Using Cloud-Permitting Model

Simulations with a high-resolution cloud permitting atmospheric model have been shown to accurately capture many features of the Indian Summer Monsoon (ISM) including the timing of the onset and the intra-seasonal variability. Such simulations are however highly sensitive to the physical configurations of the models. In particular, we investigate here how changes in the parameterization used for cloud microphysics (MP) and planetary boundary layer (PBL) can severely affect the precipitation patterns over India.
In this study, the ISM is simulated with the Weather Research and Forecast (WRF) model at a horizontal resolution of 9km over the Indian monsoon region (39 E – 111 E & EQ – 38 N) for three different years, i.e. 2007, 2008 and 2015, which are representative of an early, normal and delayed monsoon onset respectively. Two different PBL (ACM2 and MYNN) and MP (Thompson and WDM6) schemes are tested over the 3 years. It is shown that the choice for the PBL scheme has a dramatic impact on the ISM. Indeed, while simulations performed with ACM2 captures most of the circulations and precipitations patterns over India, simulations using MYNN, lead to a substantial reduction in rainfall and an overall weakening of the atmospheric circulation. In contrast, the impacts of the microphysical schemes are much less pronounced. Simulations with the Thompson scheme being better able to capture the rainfall over the Western Ghat regions and Arabian Sea, while the WDM6 schemes produced excess rainfall over Northeastern India and the Himalayan Foothills.
To assess how changes in physical parameterization affects the over rainfall over South Asia, we analyze both the energy and water budgets of the subcontinent. It is shown here that the changes in precipitation are not directly driven by local changes in evaporation. Rather, changes in regional distribution of the energy sources and sinks modify the atmospheric circulation, which in turns affects the distribution of rainfall. It is also found that increase heating in ACM2 intensifies the direct overturning circulation exports more energy and thus import more rainfall over this region following the direct forcing hypothesis compared to MYNN scheme. It is argued here that systematic study of the energy and water budget can provide important guidelines for understanding the sensitivity of precipitation patterns in regional climate models.