Diagnostic analysis of a return-flow heavy snowfall in Beijing in 2022

To deeply understand the formation mechanisms of intense return-flow snowfall in North China during winter, this study analyzes the heavy snowfall event that occurred in Beijing on February 12–13, 2022, using ground observations, wind profiler data, and ERA5 reanalysis. Synoptic and dynamic diagnostic methods combined with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) particle trajectory model are employed to investigate the synoptic circulation configuration, Lagrangian particle trajectories, physical quantity field, and moist potential vorticity (MPV), while also identifying critical indicators with predictive value. The results show that the heavy snowfall resulted from the combined influence of a 500 hPa cold vortex, coupled upper- and lower-level jet streams, an 850 hPa low vortex, and a low-level easterly return flow. The low-level easterly flow over Beijing primarily originated from the northeast and was concentrated below approximately 1.5 km, whereas the southwesterly flow above 2.5 km mainly came from the southwest. Topography significantly modified the dynamical and thermodynamical characteristics of the southerly flow through blocking and forced lifting, thereby enhancing snowfall. The easterly return flow formed a near-surface “cold air cushion” that overlapped with a southwesterly warm and moist airstream around 700 hPa. Under strengthening southerly winds, frontogenesis was promoted, markedly intensifying the upward motion and snowfall intensity. The most intense snowfall occurred in the overlapping region with high absolute values of MPV components (MPV1 > 0 and MPV2 < 0). And the snowfall development was primarily driven by vigorous cyclonic vorticity generated through the slantwise ascent of warm, moist air over the cold cushion. Key indicators with forecasting significance include: persistent surface easterlies combined with accelerating southerly winds in the mid-to-lower troposphere indicating snowfall intensification; and the concurrent abrupt increases in the maximum of MPV1 and decreases in the minimum of MPV2 corresponding to precipitation enhancement. This study not only establishes a refined physical conceptual model for this return-flow snowfall event but also enriches the set of operational forecasting and warning indicators for intense winter return-flow snowfall in North China.