Comparison of two severe low-temperature snowstorm and ice freezing events in China: Role of Eurasian mid-high latitude circulation patterns

摘要

A severe low-temperature snowstorm and ice freezing event occurred in China in early 2018. This event was similar to that occurred in early 2008 but with a weaker intensity and salient difference in location and extent. The reason for this discrepancy was investigated in this study using station data and atmospheric reanalysis data. The results reveal that two different circulation patterns (i.e., zonal dipole pattern and meridional dipole pattern) over the mid-high latitudes of Eurasia played an important role. The zonal dipole pattern, which is closely related to the 2018 event, is characterized by positive anomalies over the Ural Mountains and negative anomalies over Lake Baikal in the 500 hPa geopotential height field. This pattern can cause intense low temperatures in northern China through its influence on the anomalous cyclone over Northeast Asia. The meridional dipole pattern, which is highly associated with the 2008 event, is characterized by positive anomalies over Siberia and negative anomalies over Asia in the 500 hPa geopotential height field. This pattern corresponds to a strong East Asian winter monsoon (EAWM) and can drive cold air to move further southward, causing nearly nationwide chilly weather, particularly in northwestern China and south of the Yangtze River valley. The two patterns, combined with the strengthened western Pacific subtropical high (WPSH), have a synergistic role in the occurrence of snowfall. However, these patterns have weak correlations with the number of icy days but provide favourable conditions for ice freezing events.