Impacts of a Shallow Convection Scheme on Kilometer-Scale Atmospheric Simulations over the Tibetan Plateau

Satellite observations show that cumulus is one of the dominant cloud types in the summer over the Tibetan Plateau (TP), indicating prevalent shallow convection (ShCn). However, the impacts of ShCn parameterization on simulations of near-surface atmospheric variables and land-atmosphere interaction over the TP remain largely unknown. This study conducts simulations at 5-km grid-spacing with the Weather Research and Forecasting (WRF) model for the TP in July and August of three years. The result with/without the University of Washington (UW) ShCn scheme is evaluated against in situ observations. The evaluation shows that the ShCn scheme improves the simulations of 2-m air temperature, relative humidity, precipitation amount, and the diurnal cycle of precipitation. It also reduces light rainfall, implying its potential to lessen the general bias of “too much light rain” in climate models. Furthermore, the simulation with the scheme shows that ShCn brings moist air from the planetary boundary layer (PBL) into the free atmosphere, generally lowering and delaying the accumulation of convective available potential energy, reducing precipitation amount and soil moisture. This air exchange also reduces cloud water content and enhances surface net radiation, strengthening sensible heat flux and evaporation. This, in turn, enables the PBL to grow through both air entrainment and surface sensible heating, leading to warmer and drier low-level profiles. These impacts are most pronounced in the interior of the TP, where strong surface sensible heat and low-level convergence trigger convection, but the lack of water vapor inhibits deep convection.