Characteristics of circulation anomalies of two continuous low temperature rain and snow freezing weather processes in eastern China in February 2024

Two persistent low temperature, rainfall/snowfall and freezing processes occurred in the central and eastern regions of China from January 1-4 and February 19-24, 2024, which are mainly characterized by precipitation of wide range, long duration, extreme rainfall amount, and complex precipitation phase, and combined with significantly low temperature. These events brought serious impacts on the traffic during the Spring Festival. Using observation data from the National Meteorological Information Center, NCEP/NCAR reanalysis data, this work analyzed the characteristics of the large-scale circulation anomaly to reveal the reasons for the northward movement of Western Pacific subtropical high and the stratospheric precursor signal of the strong cold air. The results show that: (1) The situation of "low in the west and high in the east" at 500 hPa over the Chinese mainland was conducive to the occurrence of sustained precipitation in the central and eastern regions. During the first process (from January 31 to February 6, 2024), there was no obvious blocking near the Ural Mountains. The second process (from February 19 to February 25, 2024) was sustained by the development of blocking high in the north of Lake Baikal. During both processes, the West Pacific subtropical high was stronger and further north than usual. Combined with the southern trough, it was conducive to the transport of warm and humid air to the central and eastern regions of China. The cold air at the lower level moved south and the warm and humid air climbed along the cold pad at the lower level, providing important dynamic lifting and temperature stratification conditions for a wide range of rain and snow. (2) The main source of water vapor for the first process was the abnormal westerly wind transport from the north of the Bay of Bengal. For the second process, the abnormal water vapor transport from the northwest Pacific contributed greater, with a stronger water vapor transport flux intensity. (3) The northward deviation of the West Pacific subtropical high was possibly influenced by the Rossby wave energy in the middle and upper troposphere from West Africa to Northern Europe propagating downstream in the middle latitudes and subtropical regions. (4) The descent of the stratospheric polar vortex occurring around the 3rd pentad of February 2024 could be used as a precursor signal for the cold wave activity of the second process. The descent of the stratospheric polar vortex was ahead of the troposphere, providing a new basis for forecasting the cold air activities in China during winter.