The influence of surface potential vorticity density forcing over the Tibetan Plateau in the 2008 winter storm. Part Ⅱ: Numerical simulation

The surface air convergence on the eastern flank of the Tibetan Plateau (TP) can increase the in situ surface potential vorticity density (PVD). Since the elevated TP intersects with the isentropic surfaces in the lower troposphere, the increased PVD on its eastern flank thus forms a PVD forcing source on the intersected isentropic surfaces in the boundary. The influence of surface PVD forcing over the TP region on the extreme winter storm over southeastern China in 2008 is investigated based on numerical experiments by using the atmospheric circulation model FAMIL. Compared with observations, the simulation results show that, by using a nudging method to assimilate observational data into the initial flow, this model can reasonably reproduce the distribution of precipitation, atmospheric circulation and PVD propagation over and downstream of the TP region during the winter storm period. In order to investigate the impact of the increased surface PVD over the TP region on the extreme precipitation in southeastern China, a sensitivity experiment has been designed by reducing the surface PVD over the TP region. Comparison of the sensitivity experiment results with those counterparts in the control experiment shows that the precipitation over the downstream area decreases in the sensitivity experiment, especially in southeastern China. The rain band from Guangxi province to Shandong province almost disappears. It is demonstrated that in the lower troposphere, the increase of surface PVD over the TP region can generate anomalous cyclonic circulation over southern China, which plays an important role in increasing southerly winds and water vapor transport in the area; it also increases the northward negative absolute vorticity advection. In the upper layers, the surface PVD generated in the eastern TP region can propagate on isentropic surface along westerly winds and results in positive absolute vorticity advection in the downstream. Consequently, due to the development of both ascending motion and water vapor transport downstream of the TP, extreme precipitation event occurs over southern China. Thereby, a new mechanism concerning the influence of the increased surface PVD over the eastern TP slope on the extreme weather event occurring over eastern China is revealed.