Abstract: Based on the hourly precipitation,real-time radiosonde data and the NCEP/NCAR(National Centers for Environmental Prediction/National Center for Atmospheric Research)reanalysis data with resolutions of 1°×1°,we analyzed the characteristics of critical circulation systems,vapor transportation,dynamics,moist potential vorticity and vertical helicity of a flash heavy rain event happening in Bayingolin Mongol Autonomous Prefecture on August 23 to 24,2016.The results indicate that the flash heavy rain occurs under the following circulation conditions:South Asia high with an eastern pattern increases dramatically and the long wave trough from Western Siberia to the Balkhash Lake moves eastward slowly.The rapid convergence of the wind direction and wind speed at the lower tropospheric level provides the power and vapor conditions for the flash heavy rain.There are three components for the water vapor sources at three rainstorm stages.One is the western water vapor transport produced by the convergence between the southward cold air guided by the northerly wind in front of the Ural ridge and the westerly flow;the second is the west-southern water vapor transport carried by the low vortex at the southwest side of Tibetan Plateau;the third is the southerly water vapor transport brought by the pacific subtropical high,in which 94.26% of water vapor transportation comes from the west and south airflow.At the early stage before the flash heavy rain occurred,a mesoscale vertical circulation cell forming over the east and west sides of the heavy rainfall area is the main dynamic mechanism for its occurrence.The vertical helicity evolution is closely related to the flash heavy rain.The appearance of heavy precipitation usually accompanies with the coupling between the negative vertical helicity value at the upper level and positive one at the lower level.Heavy rain often appears in the area with steep and dense θ_se lines.The downward spread of high moist potential vorticity at the upper tropospheric level triggers the release of potential instability energy,resulting in the occurrence of heavy rain.

Key words: Water vapor transport, Dynamical mechanism, Moist potential vorticity, Slantwise vorticity development, Vertical helicity