Synoptic Control and Surface Conditions of Hail Events Over the Beijing Metropolitan Region in Warm Seasons

The fine spatial characteristics of hail events and their associated environmental conditions across the Beijing metropolitan region (BMR) under distinct circulation patterns are examined, using direct hail observation from quality-controlled disaster dataset, L-band rawinsondes, surface automated weather stations and global reanalysis data during the warm season of 2011–2021. Four dominant circulation patterns: the northwest flow in front of ridge (NW), straight westerly flow (STW), cold vortex (CV) and pre-trough (PreT) have been identified with an objective classification method. Results show that weak synoptic forcing (NW/STW) favours localised hail events despite high convective available potential energy; while strong synoptic forcing patterns (CV/PreT) promote extensive hail events, especially for those with significant vertical wind shear. Two primary hail hotspots emerge, the northern mountains and urban core, with distinct environmental controls. The terrain–flow interaction and the relatively thinner melting layer favour hail events over the northern mountains in all circulation patterns. In contrast, urban hail distribution exhibits synoptic dependence: with weak west to southwest low-level flows and mid- to high-tropospheric cold air brought by strong northwesterly (NW/CV), hail events tend to concentrate over the urban area with surface warmer air and organised wind convergence, which favour the initiation and enhancement of storms as they move southeastwards. With strong low-level southerly flow dominating BMR and warm mid- to high troposphere due to weak westerlies (STW/PreT), hailstorms tend to move eastwards, preferentially producing hail over the northern mountains through orographic lifting, rather than the urban region. These results suggest the modification of the urban environment and mountain–plain circulations on the distribution of hail under varying circulation patterns.