Impacts of surface potential vorticity density forcing over the Tibetan Plateau on the evolution of precipitation over southern China in January 2008. Part Ⅰ: Data analysis

The external source/sink of potential vorticity (PV) is the original driving force for atmospheric circulation. The relationship between surface PV generation and surface PV density (PVD) forcing is discussed in details, as well as the characteristics and application of the PVD equation in different coordinate systems. In addition, by taking the extremely low temperature and icy weather over southern China in January 2008 as an example, a new mechanism concerning the influence of surface PVD forcing over the eastern flank of the Tibetan Plateau on precipitation over southern China is investigated. The generated surface PVD forcing propagated eastward in the middle troposphere, and the associated positive zonal absolute vorticity advection resulted in the increase of cyclonic relative vorticity downstream. Therefore, ascending air flow and convergence in the lower troposphere developed, which resulted in the development of the southerly winds. The development of the southerly winds favored the increase of the negative meridional absolute vorticity advection, which led to the large circulation background over southern China that the absolute vorticity advection increased with the increasing height. Then, the ascending air flow enhanced, which strengthened the southerly winds and the vertical gradient of the absolute vorticity advection between the lower and middle troposphere in turn. Therefore, the development of the ascending motion, together with the abundant water vapor transported by the southerly winds, led to the formation of the severe precipitation over southern China.