Analysis on circulation characteristics of Meiyu front rainstorm in the middle and lower reaches of Yangtze River in 2011

Using the objective analysis data and observed rainfall, the large-scale circulation characteristics and the frontogenetical conditions of Meiyu front of the two intense rainstorm processes in the middle and lower reaches of Yangtze River in June 2011 are comparatively analyzed.The results areas follows:(1)the strength of two-ridge-one-trough on 500 hPa in middle and high latitudes regions was stronger than that in average years.Sustainable meridional circulation on high latitude provided the required cold air for Meiyu front rainstorm.The frequent intersection between cold air and the southwesterly flow from the Bay of Bengal was advantageous to the frontogenesis of Meiyu front and large-scale intense rainstorm. (2) The strength of westerly and easterly jets was obviously stronger than that in average years.(3) The southwesterly flow from the Bay of Bengal converged with the southeasterly flow which was on the southern periphery of the western Pacific subtropical high and formed the strong southerly flow providing sufficient water vapor for Meiyu front rainstorm.The periods of water vapor convergence corresponded to four intense rainstorm processes. (4) The precipitation characteristics and frontogenetical conditions of Meiyu front between the two intense rainstorm processes were obviously different.The joint action of cold and warm air caused the frontogenesis of Meiyu front in the former,whereas in the latter there was no cold air,and the precipitation was caused by stationary front wave.