Analysis of an excessive heavy rain event occurred at Tianjin and the eastern part of North China from July 29 to 30 in 2011 was performed with the conventional observation data, intensive automatic weather station observations, satellite images, Doppler-Radar data, mi-crowave radiometer data and NCEP 6-hourly reanalysis data with resolution 1° × 1° . The results indicate that the mesoscale vortex quickly strengthening and moving northwards in favorable circulation background is main weather system led to this event. The enhancement of posi-tive vorticity advection in front of upper-trough results in the reducing of air pressure in the low-level and the development of mesoscale vor-tex, and the center of vortex is moving to absolute value of negative allobaric maximum gradient. Analysis on vertical structure of mesoscale vortex shows that, with the development of ramp airflow within vortex, convergence in the low-level is reinforced and vortex is a consistent vertical ascending movement inside. The maximum convergence center corresponds with positive vorticity center and both centers are located below 900 hPa. With strengthening of southeast flow in the low-level, the easterly winds inflow in the right front side of the vortex strengthens significantly, which not only provides favorable dynamic conditions but also sufficient water vapor for formation and continuance of heavy rain. Strong echo height of the precipitation is lower, and both hook echoes differed from the typical ones and mesoscale cyclones develop in the low-level causing severe precipitation and short-time gale. In addition, the change of both vapor density and liquid water content corresponds better to the precipitation, and the vapor density and liquid water content in the low-level increase rapidly in 15-30 min before severe precip-itation start, which provides a reference for forecasting and early warning of severe precipitation.% 利用常规观测资料与加密自动站资料、卫星云图、多普勒雷达与微波辐射计资料以及NCEP/NCAR 1°×1°再分析资料，对2011年7月29—30日发生在天津及华北东部的大暴雨过程进行了分析。结果表明：在有利的环流背景下，迅速发展北上的低空中尺度涡旋是此次大暴雨的直接影响系统。高空槽前正涡度平流加强，造成低层减压、中尺度涡旋发展；涡旋中心向负变压梯度绝对值最大方向移动。分析中尺度涡旋垂直结构表明，开始时涡旋内斜升气流明显，随其发展，低层辐合明显加强、涡旋内转为一致的垂直上升运动，且最大辐合中心与正涡度中心相对应，均位于900 hPa以下。随低层东南气流加强，涡旋右前侧偏东入流显著加强，其不但为涡旋发展提供了有利的动力条件，也为暴雨发生和维持提供了充足的水汽。此次降水的强回波高度较低，钩状回波及中尺度气旋均在低层发展，造成强降水和短时大风，有别于典型钩状回波。水汽密度及液态水含量变化与降水对应非常好，强降水前15～30 min低层大气水汽密度和液态水含量会迅速增大，这可为强降水提前预报预警提供参考依据。
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