引发暴雨天气的中尺度低涡的数值研究

A numerical study on a mesoscale vortex producing a heavy rainfall event

2008年7月17—19日发生在山东的大到暴雨天气是由“海鸥”台风和副热带高压共同向山东输送水汽，与弱冷空气相互作用造成的。对流层低层的中尺度低涡是暴雨天气的直接制造者。利用常规观测资料和中尺度模式WRF（Weather Research and Forecasting）的模拟资料对该中尺度低涡的结构及形成机制进行了分析研究。结果表明，数值模拟可以清楚地捕捉到中尺度低涡东移过程中有新的涡旋中心形成，并与原来的涡旋中心合并的过程，而不是简单的沿切变线东移。中尺度低涡形成在增温增湿明显、上升运动为主的对流区内；中尺度低涡形成后其中心转为下沉运动，对流区东移，降水区位于低涡的东北和东南象限。中尺度低涡上空近地面层的冷池、600～400hPa的弱冷空气堆、900～850hPa的弱风区及高低空急流耦合发展是中尺度低涡形成和发展阶段的重要特征。中尺度低涡减弱阶段，下沉运动变强，低空急流和高空出流都明显减弱。涡度方程的收支表明，对流层低层的散度项、倾侧项及对流层中层的水平平流项和铅直输送项是正涡度的主要贡献者。中低层的水平辐合、涡度由低层向高层的垂直输送都有利于中尺度低涡的形成和发展。倾侧项对中尺度低涡的形成也有重要贡献。中尺度低涡形成后期，低层辐合、高层辐散及垂直输送的减弱导致正涡度制造的减弱，从而使中尺度低涡减弱。

A heavy rainfall event, which was mainly caused by typhoon Haiou, subtropical high and weak cold air,occurred in Shandong Province on 17--19 August 2008. A vortex in the low-troposphere is the maker of the heavy rainfall event. The structure and mechanism of the mesoscale vortex were studied by using routine observational data and the simulated data from mesoscaie model WRF （Weath- er Research and Forecasting）. The results show that the eastward movement of the mesoscale vortex was accompanied by a process that a new vortex center came into being and combined with the old vor- tex instead of a simple movement along the shear line. The mesoscale vortex formed in the convection area with obvious temperature rising, deep saturated layer and rising motion. After the formation of the mesoscale vortex, downdraft dominated the center of the vortex, convection area moved to its east part and the rainfall area lay in its northeast and southeast quadrants. The cold pool near boundary layer, dry intrusion between 600 hPa and 400 hPa, weak wind area between 900 hPa and 850 hPa as well as the coupling between low level jets （LLJ） and upper level jets are important characteristics of the mesoscale vortex during its generating and developing phases. During the fading stage of the mesoscale vortex, downdraft became strong and deep, and LLJ and the outflow of upper level weakened obviously. The a- nalysis of vorticity equation suggests that divergence term and tilting term in the low-troposphere, hori- zontal advection term and convection term in the mid-troposphere are the main contributors for the posi- tive vorticity. The horizontal convergence in low levels and the transportation of vorticity from low lev- els to high levels are beneficial for the formation and development of mesoscale vortex. The increase of positive vorticity in low levels is partly caused by the transformation from horizontal vorticity to vertical vorticity. The tilting term is very important for the formation of mesoscale vortex. In the later phase of its formation, the weakening of the low level convergence,high level divergence and the vertical trans- portation led to the weakness of positive vorticity, and thus the weakness of the mesoscale vortex.