Abstract:With intensive detecting data, the observational characteristics of meso-scale weather system triggering heavy rainfall over the middle valleys of Changjiang River is analyzed. The Results show that the interaction of multi scale synoptic systems leads to heavy rainfall, and several β scale convective systems moving along α-scale shear directly result in heavy rainfall. Several meso scale heavy rainfall masses come into being with the forming and developing of convective system and move along the active route of the convective system. The physical image of the β scale convective system can be described as follows. Air flow in the northeast and southwest of the shear converged into the shear and ascend and joined into the convective system, then the air current rose in the foreside of (or along) the θ_se front and rotated clockwise with a height under the effect of warn advection and diverge as southwest air flow at the height of 7km and subside at the range of 400～500km, and lastly a meso scale vertical circulation formed by the θ_se front with convective cloud in the ascending branch of the circulation.