Rainstorm in the East Airflow of Typhoon Periphery and Its Isentropic Potential Vorticity Character

After the No.0414 typhoon named as \!Yunna\makes landfall on Zhejiang Province it rapidly weakenedinto typhoon depression.The rainstorm appeared in the east airflow area of the north of the typhoon depression 48 h later.The 850,700 and 500 hPa heights of middle and lower levels in the rainstorm area had no convergence system then and the stronger cool air had affected on the area before rainstorm occurring the potential instability.So both the dynamical lifting and potential instability conditions didn′t make are unfavourable to the rainstorm occurring.In order to prove up the formative reason of rainstorm under this case,using the routine message and the 1°×1° reanalysis data of NCEP,the rainstorm occurring condition and the development and its isentropic potential vorticity character are analyzed.The results show that a declining westward updraft from bottom to top forms in the rainstorm area because the inverted trough in the middle level of the typhoon depression moves more rapidly than low level.And there were symmetric instability in the upper air of the rainstorm area.Inclined updraft causes symmetric instability energy to release and make the declining convection development.These content the dynamical and unstable conditions of rainstorm occurring.The water vapor transporting in the typhoon periphery provided abundant water vapor conditionsfor heavy rain birth.The analysis of isentropic potential vorticity shows that high value potential vorticity's band has better corresponding relationship to the precipitation area.The emergence of positive potential vorticity anomaly center has an indicative meaning for rainstorm′s occurring and developing.