The effect of low density over the “roof of the world” Tibetan Plateau on the triggering of convection

Abstract. We study the relationships between convective characteristics and air density over the Tibetan Plateau (TP) from the perspective of both climate statistics and large eddy simulation (LES). First, based on climate data, we found that there is stronger thermal turbulence and higher frequency of low cloud formation for the same surface relative humidity over the eastern and central TP compared with the eastern monsoon region of China. Second, we focus on the dynamical and thermal structure of the atmospheric boundary layer (ABL) with low air density. With the same surface heat flux, a decrease in air density enhances the buoyancy flux, which increases the ABL depth and moisture transport from the subcloud layer into the cloud layer. With the same low cloud cover for different air densities, the greater ABL depth for lower air density means that the average mixed-layer relative humidity with higher air density will be greater than that with low air density. Results from a subcloud convective velocity scaling scheme were compared with LES results, which indicated that the original fixed parameter values in this scheme may not adequate in case of lower relative humidity and weaker thermal turbulence in the subcloud layer.