Interdal-annual variation of freezing event and its relationship with atmospheric circulations in Hubei province

Based on the daily observation data from 16 meteorological stations and the NCEP/NCAR reanalyzed data from 1961 to 2010 in Hubei province, the distributions of the numbers of freezing day and its evolution characteristics were analyzed. The atmospheric circulation characters were discussed in more and less year of freezing day. The results indicate that the mean annual freezing day in winter is 8.2 days, and its spatial distribution is characterized with more in the west and less in the east and more in the north and less in the south, which is related to the topography of Hubei province. There are many mountains in the west of Hubei province and less river valley. The inter-decadal variation of freezing day is obvious in recent 50 years. Its spatial distribution changes with ladder shape from more in the west and less in the east to more in the north and less in the south, while temporal distribution is in a increasing trend firstly and then in a decreasing trend and in a increasing trend again in recent 5 years. There is an abrupt change of decreasing in the early of 1990s. Additionally, the pentad freezing day concentrates in the areas and the southern areas along the Yangtze River, especially in the fourth and fifth pentad of January. The high ridge in Ural Mountain and the East Asia trough strengthen (weaken) abnormally at 500 hPa in high latitude area of the Asia, and so are the central Asia trough over the east of the Caspian Sea in the middle and low latitude areas and the Indian-Burma trough. At the same time, Mongolia strong (weak) cold high controls the East Asia mainland on sea surface pressure field, and it makes the freezing day more (less) in Hubei province.