CONVECTIVE ACTIVITIES IN THE ATMOSPHERIC BOUNDARY LAYER OVER THE PLATEAU IN WESTERN CHINA

The convective phenomena in the atmospheric boundary layer on the Tibetan plateau are analyzed based on data Doppler sodars, the radiosonde system whit high resolution, and satellite images during the second field experiment of atmospheric sciences on the Tibetan Plateau in 1998. Strong convective activities were detected by sodars. The vertical velocity in the thermal plume exceeds 1 m/s. The time scales of the convective plumes are about 1.4 hours and they appear as organized mesoscale and small-scale turbulent motions. The physical mechanisms of the developing and maintaining of strong convection in the plateau boundary layer can be described as following. The strong convective activities are related with the intense total radiation in the plateau area, the strong baroclinity in the plateau boundary layer induced by the inhomogeneous heat status of the underlying surface caused by the complicated terrain, and the advection effects in the plateau boundary layer. With these effects, strong organized large eddies can occur in the boundary layer. The thermal plumes formed from the eddies develop upward and can be combined to from large thermal convective cells. Some convective cells can be combined with convective cloud clusters, in which the full convective mixture occurs when these eddies exceed the condensation level. The latent heat during the condensation process can help the cloud clusters to develop further, and the cloud clusters can become larger cloud groups, which can be seen in the satellite images.