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.