Comparative Analysis of the Two Unusual Freezing Rain Events in Northeast China

There were two unusual freezing rain events in Northeast China during 17–20 November 2020 (Process 1), and 7–11 November 2021 (Process 2), which caused substantial damage in Jilin and Heilongjiang provinces. These two events were diagnosed and analyzed using the NCEP/NCAR and EC-ERA5 reanalysis data, surface observations, and sounding data. The analysis suggests that the developing, northward-moving Jiang–Huai cyclones are the key influencing systems on the surface for both events. The region corresponding to freezing rain is located in the intensive belt of the isobaric line, which is on the side of the cold air side and north of the warm front close to the ground. Furthermore, there are different processes for the formation of freezing rain. In Process 1, a “cold wedge” is caused by the downturn of surface air temperatures, followed by the warm air from the cyclone climbing over the “cold wedge,” while the warm front is maintained at 850 hPa. In Process 2, a substantial amount of warm, moist air is transported northward with the upturn of surface air temperatures and the development of a warm tongue at 850 hPa. The formation of the freezing rain is caused by the warm moist air descending onto the cold underlying surface. Both these freezing rain processes occurred with favorable vapor conditions, namely, strong upward winds and a front zone. A “cold–warm–cold” vertical temperature stratification is developed, resulting in both the melting layer and temperature inversion layer being located in the lower atmosphere and the freezing layer close to the surface. The typical “ice melting” mechanism characterizes most of the freezing rain in North China. The height of the top of the temperature inversion layer in Process 1 is higher than that in Process 2. Additionally, the intensity and thickness of the temperature inversion layer, as well as the duration of the temperature inversion layer, are also larger in Process 1. These differences in the temperature inversion layers for the two processes lead to the observed differences in ice accretion thickness on electric wires. The topography has some influence on freezing rain. The 3-D model for freezing rain weather in Northeast China is provided after the diagnostic analysis.