Characteristics of Atmospheric Stratification and Melting Effect of Severe Hail Events Over Southern China

Based on conventional L-band sounding, automatic weather station data and ERA-Interim 0.125°×0.125° 6h reanalysis data, we researched 23 heavy hail events (hail diameter ≥ 20mm ) selected according the grade of hail in China (GB/T 27957-2011) and occurring in Southern China from 2004 to 2017, mainly analyzed its characteristics of atmospheric stratifications, melting effect quantificationally, and set up a physical parameters model to distinguish heavy hail. The results are as follows: (1) vertical stratification of heavy hail events with characteristic of upper dry and lower moist is more evident than that of small hail (hail diameter ≥ 5mm and <20mm) events, and vertical potential instability is mainly triggered by upper dry and lower moist rather than upper cold and lower warm. (2) The ratio of H-/H+ (cold could/warm could) can be used to distinguish heavy hail and small hail, and the ratio above 1.6/1 is one of the conditions to forecast heavy hail. (3) Compared with small hail events, the height of maximum thermal buoyancy of heavy hail is higher than the height of -5℃, which helps hail embryo enter efficient growth layer (-10℃～-30℃), driving heavy hail growth. The maximum thermal buoyancy ≥4℃ is a key threshold to distinguish heavy hail and small hail. (4) based on statistics and comparative analysis of this paper, a physical parameters model was set up, including △Td85 ≥46℃, T-Td≥15℃ at 500hPa, the minimum T-Td≤2℃ between 700hPa and 1000hPa, H-/H+≥1.6/1, the intensity of maximum thermal buoyancy ≥ 4℃, and the height of maximum thermal buoyancy > the height of -5℃, which favor heavy hail generation.