Abstract:According to hail observation data based on 80 weather stations in Gansu Province and the NCEP/NCAR data with resolution of 1°×1°, spatio temporal characteristics of hail events during 1974-2013 in Gansu are analyzed by the methods of statistical analysis and physical diagnosis. The results show that hail occurs frequently in plateau and mountain regions but rarely in valley, basin, and Gobi desert. Annual mean of hail days is 0.05-9 days with three largest values in plateau and mountains, which indicates that the altitude plays an important role in the formation of hail in Gansu. The seasonal cycle shows that hail falls earliest in March, latest in November, and mainly from May to August. The hail days present an evident decreasing trend during the recent 40 years. Hail days decrease rapidly by 0.5 d/10 yr on average in the whole province since 21st century after a high incidence during the seventh-eighth decades of last century. The decreasing trend can also be found in hail days from May to August. However, its interannual and interdecadal variations are greatly different. Except in Hexi area, the annual means of hail days in the other four regions undergoes an abrupt change of decline. Comparison analysis of years with more and less hail days in May over Gansu reveals that: As hail is a product of severe convection, the most significant character in more hail years is the positive abnormal convective energy, which can be more than two times of the normal value. After that, environmental temperature and humidity level in the more hail years develop into favorable conditions for a series of physical processes of hail formation in cumulonimbus, growing and melting before the hail falls to the ground. For example, there is an evident temperature trough at 500 hPa with temperature of 2℃ below normal value, and meanwhile, humidity increases in low level and decreases in middle level. Such condition is beneficial to form the unstable “upper dry and lower wet layer”. The 0℃ layer (3900-4500 km) is favorable for hail increase and the -20℃ layer (6400-7000 km) is helpful for hail’s falling. In addition, the vortex at 200 hPa guarantees a strong upward stream to support the increase of hail.