Circulation classification and environmental parameter characteristics of nighttime hail weather in Kunming City from June to August

To investigate the characteristics of nighttime hail activity in summer in Kunming, this study analyzes the spatiotemporal distribution of nighttime hail and the movement directions of hail clouds based on Kunming C-band weather radar data, hail disaster records, and ERA5 reanalysis data in July–August from 2010 to 2023. The T-mode principal component analysis (PCT) method is applied to objectively classify the circulation patterns at 500 hPa and analyze the associated environmental parameters. The results show that: (1) There were 73 nighttime hail days from June to August during 2010–2023 in Kunming. Hail cloud movements were primarily westerly (39.4%), followed by easterly (31.6%), northerly (13.2%), stationary (10.5%), and southerly (5.3%). (2) The optimal classification of the 500 hPa geopotential height at 20:00 (BJT, the same below) on nighttime hail days resulted in six types. The primary circulation types are the Bay of Bengal trough pattern (30.1%), the Indian low pattern (20.5%), and the Bay of Bengal-South China Sea double low pattern (21.9%). The first two types exhibit shear and convergence of low-level westerlies over central Yunnan, with hail clouds mostly coming from the west and north. These patterns are characterized by sufficient atmospheric energy, with median of available potential energy (CAPE) in July reaching 754.7 J·kg⁻¹ and 940.8 J·kg⁻¹, respectively. The temperature lapse rate of the middle and lower layers can exceed 15℃, indicating favorable unstable conditions, but resulting in relatively slight hail disasters. (3) Under the Bay of Bengal–South China Sea double low pattern, hail clouds tend to originate from the northeast. The convergence of easterlies and westerlies at 700 hPa promotes saturation and condensation of moist air (specific humidity≥12.0 g·kg⁻¹), enhancing local convective activity and lowering the height of key layers. Hail events under this pattern mainly occur in July–August and are associated with more severe disasters. This findings provides a reference for hail potential forecasting and disaster warning in Kunming.