Circulation characteristics and causes of extreme low temperature and blizzard in Hunan in December 2018

Using multi-source data, an extreme snowstorm event and its circulation anomaly in Hunan Province on December in 2018 were analyzed. The results show that the snowstorm was a rare extreme weather event. The depth of snow cover in Nanxian, Yiyang and the lowest temperature in Linli, Changde, all set their records. The blocking high pressure in Ural Mountains and the south branch trough in the low latitudes are abnormally strong as well. With the surface center pressure of 1 070 hPa, the Siberian cold airmass is unusually powerful. The Western Pacific subtropical high is stronger, and positioned to the north and slightly westward than in normal year. At the same time, the subtropical front is strong, and the temperature gradient from north to south is above 18 ℃. These are the background field of the large-scale circulation that caused this event. In the early stage of snow enhancement, the function of frontogenesis has an obvious indicating effect. Favorable water vapor and convergence conditions for the intensification of snowfall are provided due to the intensification of 700 hPa southwest jet, the development of wet layer to more than 400 hPa, the coupling of high and low level jet and the passage of low trough at 700 hPa. The northeast-southwest cloud belt is inlaid with meso- β-scale cloud clusters of -20~-40 ℃, the continuous passage of which through Hunan is the direct cause of the extreme blizzard. The obvious "bull's eye" characteristic has shown in radial velocity field of radar echo. Established and maintained for nearly 14 hours, the deep southwest jet over 2.4 km becomes one of the main reasons for the persistence of heavy snowfall.