Synoptic Climate Settings and Moisture Supply for the Extreme Heavy Snowfall in Northern China during 6-8 November 2021

A record-breaking extreme heavy snowfall (EHS) event hit northern China during 6-8 November 2021, with two maximum snowfall centers in North China (NC) and Northeast China (NEC), which inflicted severe socioeconomic impacts. This paper compares the differences in the synoptic processes and moisture supply associated with the EHS event in NC and NEC, as well as the atmospheric circulation anomalies before the event, to provide a reference for better prediction and forecasting of EHS in northern China. Synoptic analyses show that a positively tilted, inverted 500-hPa trough channeled cold-air outbreaks into NC, while dynamic updrafts along the front below the trough promoted moisture convergence over this region. In NEC, the dynamic updraft south of the frontogenesis region firstly triggered a low-level Yellow-Bohai Sea cyclone, which then converged with the 500-hPa trough to ultimately form an NEC cold vortex. Calculation of the vorticity tendency indicates that absolute vorticity advection was a better indicator than absolute vorticity divergence for the movement of the trough/ridge at the synoptic scale. Moreover, NOAA's HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model results reveal that the moisture for the EHS over NC mainly originated from the mid-to-low levels over the Asian-African region and the Eurasian mid-to-high latitudes, accounting for 32% and 31%, respectively. In contrast, the source of water vapor for the EHS over NEC was mainly the Eurasian mid-to-high latitudes and East Asia, with contributions of 38% and 28%, respectively. The findings of this study shed some fresh light on the distinctive contributions of different moisture sources to local precipitation. Further analyses of the atmospheric circulation anomalies in October reveal that a phase shift in the Arctic Oscillation related to the weakening of the polar vortex could have served as a useful indicator for the cold-air outbreaks in this EHS event.

Automated summary

A record-breaking extreme heavy snowfall event occurred in northern China from 6-8 November 2021, causing severe socioeconomic impacts. This study compares the synoptic processes and moisture supply differences between the North China and Northeast China regions during this event, aiming to provide insights for better prediction and forecasting of extreme heavy snowfalls in northern China. The findings highlight the distinctive contributions of different moisture sources to local precipitation, as well as the potential role of the Arctic Oscillation in indicating cold-air outbreaks during this event.