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

为提高北京地区回流降雪预报的准确率, 利用ERA5/GDAS全球格点再分析资料和HYSPLIT轨迹追踪模式, 对2019年2月发生在北京地区的3次回流降雪过程进行天气学分析和水汽输送特征分析。结果表明, 在3次回流降雪个例中, 偏东气流中的水汽含量有限, 当出现偏南水汽通道的配合时, 可产生明显的降雪。在典型回流形势(2月14日)下, 高后回流与河套倒槽配合, 回流水汽输送通道更为深厚, 利于降雪; 在非典型回流形势(2月6日和12日)下, 回流水汽输送集中于大气的低层(850 hPa以下)。2月12日南支气流水汽输送贡献较大, 对南支气流水汽输送的忽视是导致2月12日降雪漏报的原因之一。3次回流降雪个例在偏东和偏南路径上水汽输送至北京地区的时间与白天显著降雪的时段基本上对应, 可作为预报回流降雪的关键因子。

回流, 降雪, 轨迹追踪, 水汽输送