Diagnosis of a Regional Thundersnow in Beijing, Tianjin and Hebei in February 2024

The generation mechanism of extreme snowfall and convective snowfall in Beijing-Tianjin-Hebei region of China from 20 February to 21 February in 2024 are analyzed with high-resolution observations and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis data (ERA5) with a 0.25° by 0.25° spatial resolution. Results show that the extreme snowfall occurs under the background of North China backflow snowfall circulation, and 500-700 hPa warm and moist air at altitudes of 500 hPa to 700 hPa rises over a layer of cool, wet air, resulting in the formation of snow. The backflow cold air is deep, the northeast wind extends from ground to 850 hPa height, and the maximum wind speed reaches 10 m·s^-1. At the same time, 500-700 hPa southwest air flow is deep and strong, and 700 hPa southwest low-level jet stream center intensity reaches 32 m·s^-1. The positive anomaly of meridional wind in the middle layer and the negative anomaly of zonal wind in the lower layer both exceeds 10 times the standard deviation during snowfall, while the strong southerly wind provides ample water vapor. In this event, the total atmospheric precipitable water in North China significantly exceeds the annual average for February. Extremely favorable water vapor conditions, combined with appropriate circulation patterns, leads to severe snowstorms. In addition, the radar map displays characteristics of mixed precipitation echoes, with reflectivity factors appearing in bands and patchy areas that exceed 30 dBZ. On the satellite cloud image, the corresponding cloud top brightness temperature is below -60 ℃. During the same period, the maximum snowfall recorded in 1 h reaches 5.9 mm, surpassing the historical extreme value in February since the station is established. Further analysis reveals that high wind speeds contribute to an increase in vertical wind shear between the mid and lower levels. Before the heavy snowfall occurred, the vertical wind shear between four different levels of the lower atmosphere increases continuously, with the maximum vertical wind shear reaching 35 m·s^-1 between 150 m and 4 km. The lateral friction creates eddies that generated dynamic energy and conditional symmetric instability energy. At the same time, the frontogenesis function at the height of 700 hPa is positive, leading to vertical circulation generated by frontogenesis. For moist atmospheres above 700 hPa, the upper branch of the frontal secondary circulation triggers the release of unstable energy, resulting in the formation of elevated convection. Phenomenon results in observed convection, lightning, and short periods of heavy snowfall.