青藏高原地表位涡密度强迫对2008年1月中国南方降水过程的影响Ⅱ:数值模拟

青藏高原东坡近地表的辐合能够增加地表的位涡密度（PVD）。高耸的青藏高原与等熵面相切, 其东坡近地表增加的位涡密度成为等熵面边界上的位涡密度强迫源。利用IAP/LASG FAMIL全球大气环流模式探究青藏高原地表位涡密度增长对2008年初中国华南地区雨雪灾害天气形成的影响。首先与观测资料对比发现, 通过张弛逼近动力初始化, 在参照试验中, 模式能够比较合理地再现青藏高原东部的地表位涡密度增长和1月24-27日中国华南的大气环流场及降水场。而在高原的地表位涡密度增长减弱的敏感性试验中高原下游区域特别是华南沿海、广西到山东一带的降水明显减小甚至消失。对结果的分析表明:青藏高原区域的地表位涡密度增长在低空能够增强中国华南沿海地区的南风和水汽输送以及负的绝对涡度平流输送；在高空, 高原上产生的正的位涡密度沿西风环流向下游输送, 形成高层正的绝对涡度平流。从而在高原下游形成绝对涡度平流随高度增强的大尺度环流背景, 有利于上升运动发展。同时, 高原地表位涡密度增长在低空所激发的气旋式环流增加了华南的水汽输送, 最终激发了华南极端降水的产生。该个例模拟的结果证实了青藏高原东部的地表位涡密度强迫激发其下游极端天气发生的一种新机制。 

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.