Comparison of Spatiotemporal Distribution and Occurrence Conditions of Large and Small Hail Events in Beijing-Tianjin-Hebei Region

Using hail records at national meteorological stations for 2014–2018, ERA-interim reanalysis data and Doppler weather radar data, the spatiotemporal distribution of hail events (HEs) in the Beijing-Tianjin-Hebei region is revealed, and the environmental conditions and hailstorm structures corresponding to large hail (diameter ≥ 20 mm) events (LHEs) and small hail (2 mm ≤ diameter < 20 mm) events (SHEs) are compared. It is found that although HEs may be more frequent in mountainous areas, most LHEs occur in the plains and near the foot of the mountains. HE frequency peaks in June and the average hailstone size is larger during May and June. According to daytime records, the HEs predominantly occur in the afternoon and evening, while LHE tends to be more in the evening. Comparison of environmental parameters suggests that relative to SHEs, LHEs tend to correspond to higher 2-m temperature, wetter lower layer, larger difference in RH between 925 and 500 hPa, greater unstable energy and stronger wind shear. Hailstorms associated with LHEs tend to feature greater mesoscale rotation velocity than those associated with SHEs. Hailstorms usually show rapid increase (RI) in vertically integrated liquid (VIL) before hailstones are observed. A significant difference between the hailstorms associated with LHEs and SHEs is that the former has obviously longer time interval between the end of VIL RI and the occurrence of hailfall, indicating that the large hail size benefits from the constant supply of liquid water and the hail can be lifted by updraft for long time.