Comparison of two extreme rainfall/snowfall and freezing weather events during the Spring Festival transportation period in 2024

Abstract

From January 31 to February 6 (P1, hereinafter) and from February 19 to February 25, 2024 (P2, hereinafter), China experienced two large-scale and sustained low temperature, rainfall/snowfall, and freezing weather events, which seriously affected the Spring Festival transportation. This paper compared the two events using multi-source observations and ERA5 reanalysis data, with a particular focus on the mechanisms of the differences in precipitation intensity and the phases between rainfall, snowfall, and freezing rain in southern China. The results are as follows. (1) Both events exhibit extreme features in total precipitation and are accompanied by complex phase changes, with the range and the intensity of the freezing rain being the largest since 2009. Compared to P2, the intensity of freezing rain and the snow depth in P1 are greater. However, the range of freezing rain, the precipitation amount, and the convective intensity in P2 are greater. (2) Both the circulation patterns of the two events are affected by the high-altitude trough and low-level cold air. The southwest jet stream in front of the trough and the periphery of the western Pacific subtropical high (WPSH) is stable and significantly stronger than normal, providing sustained and abundant water vapor. The ascent of warm-humid air over a cold cushion and the horizontal convergence of low-level winds are crucial dynamic mechanisms for snowfall and rainfall. Besides, the stable maintenance of the inversion layer and the melting layer is the key to the occurrence of freezing rain and the changes in the precipitation phase. (3) Different mechanisms also exist in the two events. In P1, the air temperature in the freezing layer is between -4~0 ℃ in most areas, which is higher than that in P2 and is more conducive to the formation of freezing rain. In P2, the inversion zone is wide but moves more quickly, with a larger range while weaker freezing rain. In P2, the WPSH is located further northwest, favoring the strengthening and northward shift of the southwest warm-humid jet. Meanwhile, the strong cold wave also leads to a stronger cold cushion. The interaction between the cold and warm airs is conducive to the development of upward movement and the enhancement of precipitation. The longer duration of the melting layer leads to more cloud water content, but the temperature in the freezing layer is lower, which is beneficial for the formation of ice particles or wet snow.

Keywords

• severe snowfall
• freezing rain
• inversion layer
• precipitation phase
• western pacific subtropical high