Association of two rare freezing rain events over the Jianghuai region with anomalous tropical sea-air conditions and stratospheric polar vortex

Based on the ECMWF ERA5 (European Centre for Medium-Range Weather Forecasts Reanalysis Version 5) atmospheric reanalysis dataset and daily precipitation data from the Climate Prediction Center (CPC) along with the extended reconstructed global monthly mean sea surface temperature (SST) from the National Oceanic and Atmospheric Administration (NOAA) during 1981—2024, two rare freezing rain events that occurred over Jianghuai region in early 2010 and 2024 (hereafter Event 1 and Event 2) are analyzed and compared. The analysis is focused on differences in observed characteristics, atmospheric circulation patterns and temperature stratification associated with the two freezing rain events. The association of the two events with tropical ocean-atmosphere forcings and stratospheric polar vortex anomalies as well as their possible effects are further explored. Results are as follows: (1) The range and duration of Event 2 are respectively larger and longer than that of Event 1. In Event 1, freezing rain mainly occurred over Huaibei region during the daytime of 10 February 2010, whereas in Event 2, it occurred on 20—21 February 2024 and extended southward, reaching the Yangtze River Basin. These two freezing rain events occurred under similar circulation patterns, which are characterized by the convergence of cold air from north and warm moist air transported by southwesterlies in front of the trough and the periphery of the Western Pacific subtropical high. The stable and sustained transport of warm moist air is crucial for the establishment and maintenance of the melting layer and the inversion layer during both events. (2) The concurrent El Niño event in the tropical Pacific is favorable for the establishment of a vertical temperature configuration with cooling in the middle levels and warming in the lower levels over the Jianghuai region. Meanwhile, the intensified western North Pacific anticyclone anomalies in the lower levels enhanced moisture transport, providing a large-scale background favorable for the two freezing rain events. (3) The deep convection associated with the active Madden-Julian Oscillation (MJO) over the Western Pacific triggered strong anomalous anticyclonic circulations over the Northwestern Pacific, providing abundant moisture transport to Jianghuai region for the occurrences of the two freezing rain events. This facilitated the establishment and maintenance of the melting layer and inversion layer in the lower levels. (4) In the early stages of the two freezing rain events, the sudden stratospheric warming (SSW) events resulted in the deformation and southward displacement of the stratospheric polar vortex to the Asia. This led to cold air outbreaks by modulating tropospheric circulations in the mid-high latitudes. The cold air strongly converged with warm and moist air over Jianghuai region. As a result, a stable and sustained inversion structure was established, which, accompanied by intense convergence of low-level horizontal wind fields and upward motion, facilitated the occurrence of the two freezing rain events. In general, during the two unexpectable freezing rain events over Jianghuai region, the joint effects of tropical ocean-atmosphere forcing and the deformation of the stratospheric polar vortex played a significant role in the occurrence of these events. Therefore, MJO and SSW events should be considered for short-term predictions of freezing rain processes in the Jianghuai region.