回流, 降雪, 轨迹追踪, 水汽输送

In order to improve the accuracy of return flow snowfall forecast in Beijing area, the three return flow snowfall cases occurring in Beijing area in February 2019 are analyzed by using the ERA5/GDAS reanalysis data and the HYSPLIT back trajectory model for the meteorological analysis and characterization of the water vapor transport. The results show that, for all the three cases, the water vapor content in the easterly flow is limited regardless of the typical or atypical return flow, and significant snowfall can be produced when there is a cooperation of the southerly water vapor channel. In the typical return flow case (Feb. 14), the high backward return flow and the Loop Inversion Trough cooperate, and the water vapor transport channel is deeper, which is conducive to snow. In the two atypical return flow cases (Feb. 6 and 12), the water vapor transport is concentrated in the lower layers of the atmosphere (below 850 hPa). The case of Feb.12 has a significant contribution from the southerly water vapor transport, and the neglect of the southerly water vapor transport is one of the reasons for the underreporting of snowfall on that day. The arrival time of water vapor transported to Beijing area by the easterly and southerly vapor channels both basically correspond to the time of significant snowfall during the daytime, and can be used as a key factor for forecasting return flow snowfall.

return flow, snowfall, trajectory tracking, water vapor transport