The Development Mechanism for Tibetan Plateau Warm Vortices

The low-level cyclonic vortices which form over the Tibetan Plateau in the summer monsoon season are major rain-producing systems and have the potential to trigger cyclogenesis on the lee side when they move off the plateau. Two cases of the plateau vortices which occurred in July 1979 are studied. The characteristics of the vertical structure in their developing stage, and the circulation condition favorable for the eastward movement in the mature stage are diagnosed and presented by use of FGGE IIIb datasets. Numerical simulations with real data were performed using the GFDL limited-area mesoscale simulation model. Results suggest that the latent heating is an essential driving force for the development of the vortices studied here. The analysis of a continuous CISK model with a basic state resembling that actually observed over the summer plateau shows that the predicted unstable mode has a preferred scale, growth rate and vertical structure, all of which are qualitatively comparable to observations. The instability in the plateau environment is mainly attributed to 1) the relatively shallow vertical extent of heating located in the upper troposphere in which the heat capacity of the air column per unit surface area is relatively small; 2) the dramatic reduction of the static stability due to surface sensible heat flux; and 3) the significant increase of moisture content in the plateau boundary layer due to surface evaporation and monsoon transport of water vapor. Most of these favorable conditions are referred to as the dynamic and thermal effects of the elevated plateau terrain. In this sense, the development of the plateau vortices during the rainy season may be regarded as resulting from the interaction between the lane-sca1e circulation and the plateau topographic effect and from the release of convective latent heat.