Impact of Planetary Boundary Layer and Cloud Microphysics on the Sensitivity of Monsoon Precipitation Using a Gray‐Zone Regional Model.

This study explores how the Indian Summer Monsoon rainfall simulations are affected by the mathematical representations of the planetary boundary and microphysics parameterizations at a gray zone resolution. The monsoon is simulated with the Weather Research and Forecast model at a 9 km horizontal resolution over South–East Asia for three different years, chosen to represent early, normal, and delayed monsoon onset. Changing the boundary layer from a hybrid to a local scheme dramatically impacts the monsoon simulation, reducing around 40% in total rainfall. In contrast, changing the microphysics scheme has a less pronounced impact (5%) on the precipitation but still induces regional variations. To assess how physical parameterization changes affect South Asia's rainfall, we evaluate their impact on both the energy and moisture budgets over the subcontinent. It is revealed that local changes in evaporation do not directly drive changes in precipitation. Instead, changes in the regional distribution of the energy sources and sinks modify the atmospheric circulation, which affects the distribution of rainfall. In particular, the boundary layer changes can substantially increase the latent heat flux, strengthening the monsoon. In the tropics, overturning circulation that exports energy is tied to a net inflow of water. Thus, the intensification of the monsoonal circulation results in an enhanced water inflow and increased rainfall. Plain Language Summary: The high resolution atmospheric model can produce realistic monsoon simulations, and such simulations are highly sensitive to the physical parameterizations used in the model. Current research reveals how cloud microphysics and planetary boundary layer can severely affect the precipitation patterns over India. In this study, a monsoon is simulated using the Weather Research and Forecast model at a 9 km horizontal resolution over south Asia during 2007, 2008, and 2015, representing an early, normal, and delayed monsoon onset. It has been found that changing the boundary layer scheme from hybrid to local scheme has a dramatic impact on simulation, with a reduction of around 40% in total rainfall. Simulations with a hybrid scheme capture most of the circulations and precipitations patterns, but the local scheme leads to an overall weaker monsoonal strength. Different microphysics parameterizations show similar total rainfall distributions (5% variations) but exhibit regional variabilities. Key Points: Weather Research Forecast (WRF) can accurately reproduce the monsoon precipitation distribution but exhibit sensitivity to physical processesChanges to planetary boundary layer parameterizations cause around 40% change in seasonal precipitationIncreased heating intensifies the overturning circulation, exports more energy, and thus import more rainfall [ABSTRACT FROM AUTHOR]