A synoptic analysis of the unprecedented severe event of the consecutive cryogenic freezing rain in Hunan Province

Four consecutive large-scale disastrous weather processes of the cryogenic freezing rain and snow storm occurred in southern China from 10 January to 2 Feburary, 2008，due to the Ural blocking high residing stably over the Eurasian mid-high latitudes. In this paper, based on the multiple conventional and non-conventional observational data as well as the NCEP reanalysis data, the freezing rain distribution and evolution characteristics over the worst stricken areas of Hunan Province are analyzed in detail via diagnosing the anomalous atmosphere circulation and water vapor transport as well as some related synoptic factors. Moreover, the topographic effect on this event is discussed, and the causes of various precipitation formation, including the development mechanism responsible for the strong freezing rain, are investigated. The results show that the Ural blocking high, the western Pacific subtropical high and the deep trough over Indian and Burma are the key weather systems which led to the occurrence of the whole event. In the meantime, the abundant and continuous supplementation of water vapor is very favorable to enhancing the intensity and scale of the freezing rain. In addition, the plentiful supercooled water drops near the ground and the warm-type inversion layer imposed below the deep cold layer are possibly the key factors to maintaining such unprecedented weather processes. The formation and devoplement of the freezing rain can be attributed to the combined effects of a variety of meteorological factors. On the other hand, the daily average surface temperature, 700 hPa wind field, inversion temperature intensity and thickness of the cooling layer are useful to predict the freezing intensity. The thicker the melting layer and the deeper the underlying cooling layer with a lower averaged daily surface temperature, the more remarkable the development of the freezing rain was. The key points for the freezing prediction should be whether there are intense increases of temperature and humidity around 700 hPa as well as a strong temperature reduction below 850 hPa. The strong southwesterly jet at 700 hPa and the sustained northeasterly at 850 hPa greatly contributed to the inverse temperature reinforcement and the thickness growth of the cooling layer. Furthermore, the special topography and the complex terrain over Hunan Province also exerted great impacts on the occurrence and development of the freezing event.