Comparative analysis of wet-cold compound extremes in Southern China and Northeastern China during the winter

Based on high-resolution gridded daily observations of 2-meter air temperature, relative humidity, and precipitation from Chinese meteorological stations during 1962-2022, this study objectively defines wet-cold compound extremes (WCCE) conducive to low-temperature freezing rain and snow weather, and further compared the WCCEs between northeastern China and south of the Yangtze River. The results indicate that: (1) WCCEs frequently occur over south of the Yangtze River, with an average frequency exceeding 30 days per year. Northeastern China is another hotspot of WCCEs, with an average occurrence of more than 10 days. (2) The WCCEs south of the Yangtze River are characterized by moderate temperature drop and high humidity, with an average duration of 5.2 days and affected area of approximately 300,000 km². In contrast, northeastern China experiences stronger low temperature and moderate humidity, with a shorter average duration of 3.3 days and smaller affected areas (383,000 km²). (3) The intensity and spatial extent of regional WCCE events south of the Yangtze River mainly exhibit interannual variability. By contrast, the intensity and spatial extent of regional WCCE events in northeastern China have increased by 100% and 43.5% over the past 30 years (1992-2021) relative to the preceding 30-year period (1962-1991), respectively. This intensification of WCCE is intimately associated with the increased variability of relative humidity in this region. The findings of this study are significant in improving our understanding of the differences in WCCE between southern and northern regions of China.