Analysis of environment condition and supercell evolution for a rare severe hail weather event in Hainan

The conventional meteorological observation data, observations from intensive automatic weather stations and Doppler weather radar data at Haikou are used to analyze a rare severe hail weather event in Hainan on 20 March 2013. It is concluded that the convectively unstable atmospheric stratification, in which the dry and cold air in the middle level pile up the warm and humid air in the low-level, and the inversion in the low have supplied the beneficial conditions for the accumulation of instable energy. The moderate or strong vertical wind shears are advantageous to the development and continuance of severe convection. The convergence line caused by sea-land breeze and the topographic forcing effect are the main mechanism that triggers the low-trough type hail in Hainan. Four supercells were generated successively during this hail event. Two of them came from a mother’s echo and, one of them split into a right-moving supercell but the other into a left-moving supercell. A meso-anticyclone is found in the left-moving supercell, and the low-level weak echo region (WER) locates on the left backside of its moving direction. The righ-moving supercell was accompanied with a meso-cyclone, while the low-level WER was in the right backside of its moving direction. With the favorite heights of 0 ℃ and -20 ℃ levels, the hail early warning can be issued as the three-body scatter spike (TBSS) in base reflectivity are found, and the forecasts have 20-30 min ahead of normal at most. Before hails fall down to surface, the echo's top height of 55 dBz reach above the height of -20 ℃ level and the Vertically Integrated Liquid (VIL) was experienced a few jumps. When VIL reach 65 kg·m-2 in general, the hail weather occurred. The lead time comes up to three volume coverage patterns (VCP) time when issuing the hail warning as VIL reaches 60 kg·m-2. The hail event ends when VIL drops below 40 kg·m-2.