The Response of Precipitation to Initial Soil Moisture over the Tibetan Plateau: Respective Effects of Boundary Layer Vertical Heat and Vapor Diffusions

This study investigates the influences of initial soil moisture perturbations over the Tibetan Plateau (TP) on precipitation simulation, and the respective effects of boundary layer vertical diffusion for heat (K_h) and vapor (K_q). Results indicate that the responses of K_h and K_q to soil moisture are significant mainly in the daytime. Wetter land surface corresponds to weaker vertical diffusion, which not only strengthens thermal forcing and dynamic lifting in the lower atmosphere, but also encourages air saturation near the top of the boundary layer to prevent the environmental dry air entrainment/invasion, thus leading to more convection and precipitation. Due to the responses of heating to dry or wet land surface are heterogeneous over the TP, wetter land surface would also be conducive to the southeastward propagation of precipitation due to the enhanced contrast between the cold in the northwestern TP and the warm in the southeastern TP.

When K_h and K_q vary synchronously, they have competitive effects on precipitation intensity by influencing the relative importance of water vapor and atmospheric stability conditions in the boundary layer. Adjusting the relation between K_h and K_q would deactivate the competitive effects. Specifically, stronger K_h but weaker K_q would alleviate the overestimated precipitation by inhibiting vertical transport of water vapor to the top of the boundary layer and attenuating convective instability in the boundary layer. Results illuminate the importance of boundary layer vertical diffusion in the relation between soil moisture and precipitation.