Abstract: A 10-layer moist mesoscale model has been employed to simulate a heavy rain case over the Yangtse valley during Meiyu season.The structures and evolutions of subsynoptic or medium scale systems are well simulated.Thp results are as follows:1.The low-level vorticity,the evolution of subsynoptic or medium scale systems are closely associated with heavy rain.Peaks of 3-h precipitation developed and moved along with the vorticity maximum center.2.The vorticity maximum at lower level intensified while it moved near or under the center of negative vorticity at the upper level,concurrently,the upward motion and the amount of precipitation Was amplified.3.A warm moist lid existed by the top of PBL over rainfall area the inversion of moisture was main feature of the lid,characterized by a maximun of.θseThe formation and mentainance of the lid were related to the transformation of warm and moist flow by south-west LLJ.