| Based on data of the general observational, remote sensing and the numerical simulation, it is statistically analyzed some features of mesoscale low vortex in low levels related with heavy rain during Meiyu season in 1998～2005, which includes the its source region, moving path, life cycle, scale and related convection. Protracted the source land and moving path chart of mesoscale vortex, which could differentiate scale and convection. Based on that, we selected some cases to do synthesized analysis and diagnoses, for research the synoptic position and environmental condition, that is in favor of the generation of mesoscale vortex. Used the high resolution data output from WRF model, to diagnose the vortexes through analyzed the second-order or nonlinear vertical shear of basic flow fields and the dry air intrusion, and to post the mechanism of the mesoscale vortex's generation and development and how to effect the rainfall. The major results showed:(1) The source region of the mesoscale vortexes in lower reaches of the Yangtze River was located in the area of Dabie Mountain in Anhui province, the horizontal scales were 100～400km, vertical scales were in the layer of 1000700hPa. There existed two movement paths, one was moving northeastward and entered into the sea in the north part of Shandong province, the other was moving southeastward and entered into the sea between the south part of Jiangsu province and in the north part of Zhejiang province. The life cycle in the land generally persisted less than 48 hours. About 70% of the vortexes excited one or more the mesoscale convective systems and generated heavy rains. The heavy rains generally occurred in the south or southeast section of the vortexes, it is related with the warm moisture stream in lower levels. The further analysis of synoptic situations and surrounding conditions in which the vortexes formed show that over 90% of the vortexes formed in the bottom or the front of the trough, the velocity advection transfer in the front of the trough is one of the necessary conditions. The jet allocation in high and low levels, the vapor transfer in low levels, of played the important role, the front in low level and the topographic feature play important role for low vortexes generation(2) On the bases of the analysis on the 20 process of rainstorm aroused by eddy occurred over the mid and lower reaches of the Yangtze River during Meiyu, most basic flow fields of the rainstorms aroused by meso-scale eddy have a second-order or nonlinear vertical shear. This is exactly the physical sources of vortex Rossby wave. Thus the vortex Rossby wave is greatly significant to the physical mechanism of meso-scale eddy system, including its creation, development and movement. According to the research, when the second-order vertical shear of base flow Uzz>0, coupled with the steering flow in 200 hPa is comparatively strong, the eddy move to ENE; when the second-order vertical shear of base flow Uzz<0, and the jet of low-middle atmosphere is stronger than the upper atmosphere, that is the absolute value of Uzz is very small, the eddy move to ESE; when the second-order vertical shear of base flow Uzz< 0, and the jet of low-middle atmosphere is more stronger than upper atmosphere, the eddy move to SW; when the jet of middle atmosphere is more stronger than low-upper atmosphere, the eddy move to NNW.(3) Using the mesoscale numerical model WRF(v2.2), it is simulated a heavy rain event of low vortex in Meiyu front in Jianghuai area during the period from 1200UTC June 21 to 1200UTC June 23,2003. Based on the model output data with high spatial and temporal resolution, the dry air intrusion and its effect on the development of vortex are analyzed. The results show that the dry air with high potential vorticity in the upper troposphere intruded into the lower layer, it promoted the development of cyclone and convection, and subsequently enhanced the precipitation.
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