Triggering effects of large topography and boundary layer turbulence on convection over the Tibetan Plateau

In this study, we analyze the diurnal variations and formation mechanisms of low clouds at different elevations. We further discuss whether there exists a triggering mechanism for convection over the Tibetan Plateau (TP) and whether there is an association among low air density, strong turbulence, and ubiquitous popcorn-like cumulus clouds. The buoyancy term (BT) and shearterm (ST) over the TP are significantly greater than those at low elevations, which is favorable for the formation of an increasing planetary boundary layer height (PBLH) and also plays a key role in the convective activities in the lower troposphere. From the viewpoint of global effects,the triggering of convection by boundary layer dynamics is analyzed over the TP, but also in the Northern Hemisphere over the Rocky Mountains. It is foundthat ST and BT are strong over both high-elevation regions. The strong thermal turbulence and large-scale ascending motion jointly result inobvious positive values of PBLH-LCL (lifting condensation level) under low relative humidity (RH) conditions over the TP. The obvious large-scale subsidence on both sides of the Rocky Mountains, especially the western side, leads to inversion above the PBL and lower RH within the PBL, which further leads to negative values ofPBLH-LCL and decreased low cloud cover (LCC) in most parts of the Rocky Mountains. The slightly greater-than-zero PBLH-LCL corresponds spatially to increased LCC in the partial regions of the central Rocky Mountains. Thus, less LCC is generated at the Rocky Mountains compared to the TP.

Automated summary

In this study, the diurnal variations and formation mechanisms of low clouds at different elevations are analyzed. The existence of a triggering mechanism for convection over the Tibetan Plateau (TP) and the association among low air density, strong turbulence, and ubiquitous popcorn-like cumulus clouds are further discussed. The findings suggest that the buoyancy term (BT) and shear term (ST) over the TP play a significant role in the convective activities in the lower troposphere and contribute to the formation of an increasing planetary boundary layer height (PBLH).