The Formation Of Positive Vorticity Resource Over The Tibetan Plateau And Its Impact On Meiyu Rainfall In Eastern China

Diagnostic analyses and numerical studies with NCEP reanalysis and conventional observational data are performed to investigate the mechanism of genesis and eastward motion of positive vorticity over Tibetan Plateau, and its impact on Meiyu rainfall. Some significant facts are revealed as follows:There is a positive vorticity resource over main Tibetan Plateau, which is connected with eastern China by a positive vorticity teansfer channel during Meiyu season in the summer. The positive vorticity centers (systems) shift from the plateau to eastern China successively, along the channel and almost each of them can trigger a severe rainstorm in Maiyu belt. Comparing with majority of Tibetan vortices which will disappear when it move out of the eastern plateau, while positive vorticity centers can keep on moving eastward to increase the Meiyu rainfall. This is also the clue for Meiyu accurate forecast.There are two reasons for the plateau becomes a positive vorticity source. First is the plateau warming in the summer season and the second is the westerly perturbation over the plateau. Cyclonic circulation over plateau would develop when the potential vorticity perturbations in higher level interact with the potential temperature perturbations in lower layer. It is different from the Hoskins potential vorticity theory to explain the positive potential vorticity abnormal in upper level and the positive feedback of cyclone in lower layer which the plateau is a strong warm source that the heating effect is stronger than the warm air advection. The superadiabatic state is frequently appears over the plateau, it would be favorable to the development of the cyclonic circulation. Owing to the thickness of the atmosphere over the plateau is thinner than the Rossby Penetrative Hight Hr=fL/N (about 10 km), it will make (1) the impact of the westerly perturbation over 300 hPa on near ground circulation easily. (2) the interaction between upper level and lower layer strongly, (3) the source of positive vorticity confined by topography over the plateau strictly.Plateau vortices are the typical system of positive vorticity. The vortex column would be strengthening when it moving to the eastern side of the plateau if it is overlapped or merged with another vortex column. Usually vortex column would decrease when it move away from the plateau. On the other hand, the plateau vortex could be increase the intensity when a westerly trough approaching itself.The calculation of the diabatic heating of the plateau vortex indicates that thesensitive heating is in advance to the latent heating. The diabatic heating especially the latent heating would be increased strongly when the vortex developing and shifting eastward. The diabatic heating decreased when the vortex moving away from the plateau. The numerical simulation demonstrate that formation of the plateau vortex strongly depend upon the sensitive heating flux and latent heating release. The up-ward latent heat transfer plays an important role to the strengthening of the plateau vortex when it moves to the east part of the plateau.The study on the budget of the vorticity and energy in different stages of the plateau vortices shows that thermal and dynamic effect of the plateau would inflate the air column. It is favorable to the ascending motion and transferring the vortex momentum upward from the near ground layer as well as the convergence in lower and mid layer. This process would transfer the available potential energy into eddy kinetic energy and convert the mean kinetic energy into eddy kinetic energy. On the other hand, the formation/development and maintenance of the plateau vortex would have strong influence from the upper level divergence/vorticity advection and the horizontal flux divergence of eddy kinetic energy associated with a upper level westerly trough or other perturbation.There are three manners of plateau positive vorticity center shifting eastward to affect the Meiyu rainfall along Yangtze River basin: (a) remote influences: Plateau vortex would be favorable to induce t...