Cause analysis of rare continuous blizzard occurring in January 2011 in Hunan province

Rare and continuous blizzard occurred in January 2011 in Hunan province was analyzed comprehensively by using conventional observational data and NCEP reanalysis data. Conclusions are drawn that stability and continuance of Ural Mountains blocking high and south branch trough in the Bay of Bengal made a circulation characterized by "low in south and high in north". It was favorable for cold airflow and warm airflow to join up in the middle and lower reaches of the Yangtze River providing the beneficial large-scale circulation background for the occurrence of continuous blizzard. The establishment and reinforcement of southwest jet at 700 hPa supplied mass of water vapor and unstable energy to blizzard region, while continuous and powerful water vapor transport and convergence played a crucial role in the continuance and reinforcement of blizzard. From Tibetan Plateau to eastern China there were two obvious positive vorticities spread eastward orderly, which is propitious to the continuance and reinforcement of upward motion. During the blizzard, stronger frontogenesis maintained, quasi-stationary front was stable, as well as front zone strengthened gradually. The strongest stage of the front zone was consistent with the strongest stage of the blizzard, and the coupling of upper-level jet and front strengthened the upward motion over Hunan. This is the main reason which led to the continuance of blizzard. The dry layer originated by the prefrontal dry and cold air in the middle troposphere strengthened convective instability of the blizzard, while the post-frontal dry and cold air playing as a cold air cushion wedged into the space bellow warm and humid airflow to cause the frontogenesis and both the uplift and condensation of warm and humid air, which triggered the release of instable energy.