The Comparative Analysis Of Persistent Heavy Rainfall Events In West And East Sichuan Basin

Based on the conventional observation data, reanalysis data from NCEP/NCAR, satellite data(TBB),trajectory tracking model of HYSPLIT and so on, the characteristics of circulation background, source of cold air and water vapor, convective system over Tibetan Plateau (TP) and southwest vortex are analyzed on the two persistent heavy rainfall in western Sichuan basin and eastern Sichuan basin. Then, WRF model simulation forced by diurnal cycle boundary conditions successfully reproduces diurnal variation in Sichuan Basin, to analyzes the reason of the precipitation diurnal variation. At last, the sensitivity test pointed out that the surface heat flux of TP on the influence of the precipitation in Sichuan Basin. The conclusions are summarized below:1.The differences of circulation background and diurnal variation of the convective activity:results show that in the western basin event, the location of high-level jet at 200 hPa was norther, the subtropical high was norther and stronger, the westerly trough and southwest vortex were westerner than the eastern basin event; the convective activities in western Sichuan plateau and basin have significant diurnal variation:from the afternoon to early evening in western Sichuan plateau, from the late evening to the next early morning in Sichuan basin, the difference is the eastward propagation of convective system from the edge of TP merged with the clusters over western basin in the western basin event, but for the eastern basin event, convective system weakened as it moved eastward, there was no merging process.2.The cold air and water vapor transport of western basin event and eastern basin event are significantly different:cold air in the middle troposphere came from central Asia and western edge of the TP, water vapor in low level was transported by southwest airflow from the bay of Bengal in the western basin event; cold air came from mid-latitude at the early stage of event, while during the later period, the westerly flow in the middle troposphere mainly came from Bengal and southern TP, water vapor in low level was transported by southeast airflow that from South China Sea in the eastern basin event.3.The reasons for the diurnal variation of precipitation:with upward movement in the thermal circulation、low level jet、southwest vortex strengthened during the night, the heavy rainfall over Sichuan Basin always occurs in the evening. The differences are that western basin precipitation mainly from the beginning of the 15UTC, greatly influenced by the easterly wind in low-level, while eastern basin rainfall mainly from the beginning of the 18UTC, influenced by the westerly wind in low-level.4.The sensitivity test that closed the TP surface heat flux showed:decreased precipitation over main of TP, increased rainfall over eastern Sichuan Basin, the rainfall area moved eastward. In the sensitivity test, the South Asia high weaker slightly and intensity of westerly trough was weaker, but the 700 hPa vortex in eastern basin enhanced, and the severe vertical wind shear was in favor of cyclone development over eastern Sichuan Basin, leading to precipitation over eastern basin strengthened.