垂直向基流二次切变对梅雨锋中尺度低涡暴雨系统的影响

The effect of the second-order vertical shear of the basic flow on meso- scale severe rainstorms in the meiyu front

β中尺度低涡是引发长江中下游地区梅雨锋暴雨的主要中尺度天气系统之一,采用实况统计与数值模拟相结合的方法,对1999—2005年长江中下游地区梅雨期间23个低涡暴雨过程进行分析得出:暴雨一般都发生在中低层低涡南侧的西南急流里,急流的强度和位置直接影响降水落区和强度,低涡所激发的涡旋Rossby波在急流里传播时引发不稳定,产生强降水。根据基本流场风速二次切变理论,进一步研究表明:大部分低涡降水区基本流场都存在二次切变或者非线性切变,而这种情况正是涡旋Rossby波产生的物理根源。当垂直向基流风速二次切变U_(ZZ)>0,且高层200 hPa附近引导气流比较强时,低涡移向东北偏东;当垂直向基流风速二次切变U_(ZZ)<0,且中层急流比上下层略强,即U_(ZZ)绝对数值很小,低涡移向东南偏东;当垂直向基流风速二次切变U_(ZZ)<0,且中低层急流相对于高层急流很强的时候,低涡移向西南向;当垂直向基流在中层的急流很强,上下急流不明显时,低涡移向西或西偏北。因此,垂直向基流风速二次切变是影响梅雨锋中尺度低涡路径的关键因子,这一结论对于梅雨期间低涡暴雨落区预报有很好的指示意义。

Meso-βscale low vortex is one of the main weather systems to trigger mesoscale meiyu front rainstorm in the middle and lower reaches of the Yangtze River.Based on the observations and the numerical weather simulations using the WRF,23 rainstorms caused by meso-fi scale low vortex in the middle and lower reaches of the Yangtze River from 1999 to 2005 are analyzed.The results show that most of meiyu front rainstorms occurred in the southwestly jet which usually located at the south of meso-βscale low vortex, and that the location and intensity of the southwestly jet may directly influence the location and intensity of rainfall.The instability triggered by the votex Rossby wave which is caused by a low vortex can result in heavy precipitation.Further study according to the theory of the second-order vertical shear theory of basic current wind shows that most of the basic flow fields of rainstorms are associated with a second-order or nonlinear vertical shear,which is just the physical source of vortex Rossby wave.Furthermore,it is found that when the second-order vertical shear of base flow UZZ〉0 with the steering flow at 200 hPa strong,the vortex moves toward the ENE;when the second-order vertical shear of base flow UZZ〈0 with the jet of the middle atmosphere stronger than that of both the lower and upper atmosphere（i.e.the absolute value of UZZ is very small）,the vortex moves toward the ESE;when the second-order vertical shear of base flow UZZ〈0 with the jet of the lower-middle atmosphere stronger than that of the upper atmosphere,the vortex moves toward the SW;and when the jet of the middle atmosphere is stronger than that of both the lower and upper atmosphere,the vortex moves toward the NNW.Therefore,the second-order vertical shear is one of the key factors to influence the track of a mesoscale low vortex in Meiyu front,which may help to improve vortex rainstorm forecasting during meiyu period.