Abstract: Based on conventional meteorological observations in the upper air and at the ground,observational data from a Doppler Radar,and National Centers for Environmental Prediction (NCEP) reanalysis data,we comparatively analyzed the characteristics of formation,maintenance,and development of two hail events that occurred successively in September 2018 in Dandong.The results indicated that low-level wind shear lines,strong vertical wind shears,and convergence lifting are favorable for the occurrence of hail events.The intensity of hail events is greatly influenced by the intruding way and strength,the establishment of low-level jets,and vertical wind shears.The two hail events are caused by a Northeast Cold Vortex (NCV) and an upper-level cold trough,respectively.On September 6,the turn of transverse trough leads to the fast movement of cold air masses that are supplemented from the rear of the cold vortex,and the southwest jets at 850 hPa transport plenty of water vapor,which enhanced the hail event compared with that on September 3.The radar echoes show distinct features during the two hail events.On September 3,a convective storm from multiple monomer lines gradually turns into an arched echo when it moves towards the east,with a clear headwind area on the radial velocity map.While on September 6,several super monomers are triggered along the ground convergence line and show the train effect movement.The radial velocity map appears to be blurred and shows a higher Vertical Integrated Liguid water content (VIL).

Key words: Hail, Radar echo, Multi-scale, Northeast Cold Vortex (NCV)