Abstract: Based on conventional meteorological data and the 6-h FNL 1°×1° reanalysis data from National Centers for Environment Prediction (NCEP)/National Centers of Atmospheric Research (NCAR),we conducted the dynamic diagnosis analysis on a rainstorm event in Zhejiang province on June 20,2018,which includes the major circulation systems,isentropic potential vorticity,and vertical helicity.The results indicated that the major influencing systems for this rainstorm event are the low-level vortex shear at 850 hPa,weak cold air intruded in the middle troposphere,the South Asian Anticyclone and the Subtropical High at 200 hPa.Positive vertical helicity center at 700 hPa is a good indicator of the occurrence of rainstorm events.The evolution and form of isentropic potential vorticity are indicative of cold air activity.The convergence of airflow on both sides of the isentropic vorticity center is favorable for the development of surface low pressure.High-level isentropic potential vorticity corresponds well to the cold air activity,and precipitation areas are influenced greatly by the movement of areas with large isentropic potential vorticity values towards the south.

Key words: System coupling, Vertical helicity, Isentropic potential vorticity, Diagnostic analysis