Abstract:

Numerous studies have shown that the number of hailstorms in Shaanxi Province (northern China) has been significantly decreasing from the late 1990s to the present, but studies on the characteristics of hailstorm changes within more specific provincial regions are lacking. To investigate the hail occurrence patterns in Shaanxi Province and to better predict and prevent hail, this paper analyzes the characteristics of hail variability in Shaanxi Province on the basis of hail data recorded at 99 ground stations from 2000 to 2019 provided by the Meteorological Center of the Shaanxi Provincial Meteorological Bureau. For the analysis of different regions in the province, the stations were divided into three regions, namely northern Shaanxi (Yan’an and Yulin), central Guanzhou (Xi’an, Baoji, Xianyang, Tongchuan, and Weinan), and southern Shaanxi (Hanzhong, Ankang, and Shangluo) according to the administrative divisions of the cities where the stations are located. The climatic characteristics of hail in each region were analyzed separately. We found that the number of hailstorms in Shaanxi Province increases from south to north; a linear fit of elevation to the mean annual hailstorm number revealed a significantly positive correlation between elevation and mean annual hailstorm number at low elevations, with each 1 m increase in elevation within 1.5 km of elevation increasing the mean annual hailstorm number by 0.0007. Furthermore, hail occurrence in Shaanxi Province increases from south to north with seasonal differences; hailstorms in northern Shaanxi occur more frequently in the summer and less frequently in the winter. The region with the next-highest number of hailstorms is Guanzhong, and an average number of hailstorms occurs in southern Shaanxi. A 5 year sliding average of the interannual hail counts was fitted linearly, and results show a decreasing trend in the overall interannual hail counts in Shaanxi Province, with the fastest decline in northern Shaanxi, followed by Guanzhong; there was no significant interannual variation in southern Shaanxi. Using monthly averages of ERA5 parameters provided by the ECMWF and considering the physical mechanisms of the hail process, eight indicators were selected and fitted to the 5 year sliding average: (1) convective effective potential energy (CAPE); (2) the K-index; (3) 2 m temperature (i.e., air mass temperature 2 m from the ground); (4) 2 m dew point (i.e., air mass dew point temperature 2 m from the ground); (5) 0 ℃ layer height; (6) convective precipitation; (7) total precipitation; and (8) cloud base height. A linear fit was made to the hail counts; results show that CAPE played a dominant role in the interannual trend of hail in Shaanxi Province, and the K-index played a dominant role in the summer in northern Shaanxi and in the spring in Guanzhong. Furthermore, the 0 ℃ layer height played a dominant role in the trend of hail counts during the summer and autumn in Guanzhong and during the spring and summer in southern Shaanxi.

Key words: hail, interannual variation, CAPE, number of hail, Shaanxi