2004 Volume 82 Issue 6 Pages 1777-1792
Based on field observations, theoretical analyses, and numerical simulations, this study investigates the structure and the evolution of the atmospheric boundary layer (ABL) and convection over the Tibetan Plateau during the dry season. Both field observations and numerical simulations show that the convection over the plateau evolves from dry shallow convection in the morning to wet deep convection in the afternoon. The shallow convection is organized, and its major wavelength is controlled by mesoscale hills. The deep convection is not very regular. Both nonlinear scale interactions and latent heat release from convection may play significant roles in the development of the deep convection. However, the deep convection near mountains is related to an interactive process between mountain-valley circulations and rain evaporative cooling. The mountain-valley circulations in the afternoon can be either upslope or downslope.
The plateau ABL can extend to heights of almost 3 km above the ground surface, and is characterized by a well-mixed layer of potential temperature. The energy budget in the ABL indicates that the sensible heat is the dominant energy for sustaining the ABL growth, and radiations also play a significant role, but the rain evaporative cooling below the wet convection suppresses ABL development. The ABL evolution is strongly associated with the convective activities. The convection not only efficiently exchanges the quantities between the near-surface layer and the upper layer, but also enhances the air entrainment near the top of the ABL.
