| Lots of studies on mesoscale convective system in mei-yu front have been carried out, and some theories obtained. The purpose of this paper is to improve understanding about heavy rain in mei-yu front system. It is helpful to daily weather forecasting and research. Our job started with researching physical variables according a mei-yu process happening in Fujian province. It is necessary to study the mechanism about heavy rain leading to flood, which happened at min-jiang in Fujian province. A continuous mei-yu front heavy rainfall attacked Fujian from 13 to 27 June 2010. The heavy rainfall showed long time lasting and great intensity. These were rare. This paper was aiming at first rainfall period (from 14 to 15, June). The heavy rainstorms in middle-north and west parts of Fujian province were analyzed. Comprehensive analysis on circulation background, mesoscale convective system and physical variables was done to study the possible trigger mechanism about happening and developing of this heavy rainstorm.Firstly, the large-scale background model was summarized. Heavy rain occurred in northern edge of subtropical high and southern warm area of lower-shear. When heavy rain happened, the ridge line of subtropical high maintain on near 22°N. A pair of trough and ridge existed in middle-high latitude on 500 hPa. There was convergence of three flows on 850 hPa, which were warm and wet flow from Bengal bay, warm and wet flow from South China Sea and cold air from east road.Secondly, diagnosis about physical variables showed that the infiltration of cold advection from east road from middle-high and developing of lower jet excited the occurrence and development of convection in middle-lower, which resulted in very unstable stratification of atmosphere. Severe convection happened near the negative value center of△θse, energy front area of Ose and high CAPE value area. The development of upper air cold vortex droved the development of lower vortex, which triggering the happening of severe convection. In daily forecasting of severe rainfall, forecasters could only preliminarily considered the strengthen or weaken of jet just form wind and stream.But just the happening time of jet disturbance was very important to occurrence and developing of heavy rain.For studying the triggering role of jet disturbance to severe rainstorm, we caculated u and v disturbance quantity(u-u, v-v) and combined them in some areas. Analysing the combined disturbance of jet, it showed that the northwest wind disturbance in tropopause and divergence in top of severe convective storms strengthen development of vertical motion. And strong southwest wind disturbance in middle-lower troposphere strengthen vertical atmosphere unstable. Because of the development of lower jet, mass of warm and wet flow rised strongly with divergence in upper and sinking around clouds, which induced heavy rainfall occurrence and developing. According analysing the distribution of jet disturbance, the occurrence time and areas of heavy rainfall can be judged. It is helpful to forecasting of severe rainfall areas. Analysing vertical circulation in meridional profile showed that the jets in upper and lower and the establishment of secondary circulation coupled from them were beneficial to lower convergence and upper divergence. The existing of sinking flows in middle troposphere and intersection of some flows upon heavy rain areas caused the occurrence of severe convection.Both negative value of moist potential vorticity on 700 hPa and 850hPa showed good corresponding relation with severe rainfall areas. Analyze showed that there was good relationship between moist potential vorticity and the occurrence, development and motion of severe rainfall. It also showed that moist symmetric, convection and baroclinic instability had important roles to occurrence and development of severe rainfall system.Finally, we used WRF to simulate this heavy rain process. According to analyze the middle-lower environment background, the change tendency of rainfall distribution with time, subtle structure of rainfall triggering from cold air and change of physical variables, it showed that the simulation was quite reasonable in respect of circulation background and general distribution of heavy rainfall changing with time. When cold air from middle layer met warm and wet air from lower, moist unstable energy released. This made geopotential and convective unstable layer develop, and had triggering effect to warm areas heavy rains. Heavy rainfall time corresponded to the strengthening of warm and wet flow on lower. It also corresponded to distribution of 384Kθse on upper. The simulated vertical upward motion showed upward-downward in turn from lower to 200 hPa. The time suddenly increased of positive vorticity on lower and negative vorticity corresponded to heavy rainfall time.
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