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

Meso β scale 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 β 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 β scale 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 U ZZ >0 with the steering flow at 200 hPa strong, the vortex moves toward the ENE; when the secondorder vertical shear of base flow U ZZ <0 with the jet of the middle atmosphere stronger than that of both the lower and upper atmosphere (i.e. the absolute value of U ZZ is very small), the vortex moves toward the ESE; when the second-order vertical shear of base flow U ZZ <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.