2008年6月20—21日一次β中尺度切变线、低涡降水机制研究

利用WRF中尺度数值模式，模拟2008年6月20-21日江淮一次β中尺度切变线、低涡降水过程。分析发现：低层大尺度的0．5×10 -6m2·S-1·K·kg-1的大值位涡为切变线暴雨提供了背景场，在其南部边缘，低层的切变辐合及云水形成的非绝热加热，导致了正位涡的增长，使低层正涡度加大引起降水加强。低层的正位涡通过上升运动向上传递，导致了高层位涡正异常，高层位涡的正异常又可导致低层的气旋性涡度进一步加大并使降水加大；β中尺度低涡的生成与大别山地形关系不大，主要是由对流层高层正位涡异常引起，但是低涡的维持及降水与大别山的地形坡度密切相关，当地形平坦时，不利于低涡维持和降水加强，当具有大别山的地形坡度时，不论山脉的高低都有利于低涡维持和降水加强。

A Study on the Precipitation Mechanism of Meso-β-scale Shear Line and Low Votex during 20 and 21 June 2008

By using the WRF mesoscale numerical model, a simulation is performed to study the meso- β-scale shear line and low vortex of a heavy rainfall event, which occurred during 20 and 21 June 2008 in Jianghuai area. The results showed that low-level large-scale potential vorticity, which was more than 0.5 × 10 - 6m2 . s-1 . K . kg-1 ,provided a background field for the heavy rain induced by the shear line. On its southern edge, the non-adiabatic heating which came from the low-level shear covergence and cloud water led to the growth of the low-level positive potential vorticity, which brought greater precipitation as a result. At the same time, the low-level positive potential vorticity passed upwards through the upward motion, resulting in a high-level potential vorticity positive anomaly, which would lead to further increase of low-level cyclonic vorticity and thus enhanced the precipitation again. The generation of the meso-β-scale vortex had little to do with the terrain but was mainly caused by the potential vorticity positive anomalies at the upper troposphere. However, the maintenance of vortex and precipitation were closely related to the terrain slope of Dabieshan Mountain. Flat terrain is not conducive to the maintenance of the vortex and precipitation. But when there is a slope, either a high mountain or a low one, it is conducive to the maintenance of low vortex and precipitation.