Study On The Classification Of Snow Cover And Influencing Factors In West China

Based on the observed data of snow cover days, daily temperature, daily precipitation, daily highest and lowest temperatures from 232 weather stations covering the period of 1951～2004 and daily snow depth data of SMMR and SSM/I from 1980 to 2004 in the west region of 105°E China, classification of snow cover in West China is analyzed by using interannual variability of snow cover days method and snow cover days method. Difference and credibility of two kinds of snow cover data are discussed, a more applicable method on classification of snow cover in West China is chosen. The main factors that would throw impacts on different snow cover types are investigated based on the result from it, so as to provide the basis for studying the mechanism of snow cover changes and estimating its trend. The main conclusions are as follows:(1) By using the observed data of stations and interannual variability of snow cover days method, the regions of stable snow cover, regular unstable snow cover and irregular and unstable snow cover have been classificated. North Xinjiang, Tianshan Mountain and high altitude mountains areas of East Tibetan Plateau are regions of stable snow cover. South and East Xinjiang Basin around, Hexi Corridor, North Qinghai Province, area from the west of Tibetan Plateau to Nyainqentanggula Range and South Tibet Valley are regular unstable snow cover area. Regions covering Tarim and Turpan Basin, Ha Soon Gobi, Badan Jaran Desert, Tibet and Sichuan Provinces border and the most area from Chengdu to Kunming are irregular and unstable snow cover areas. Regions without snow cover include Sichuan Basin and South Yunnan Province. Regions of stable snow cover standing in East Tibetan Plateau have a considerable interdecadal change. In the 1980s, the boundary between the areas with and without snow cover advanced to the most south position. (2) The greatest variances of snow cover are found in the bordering region of two different types of snow cover areas, their interannual variability tend to appear the most value, and classification of them may be easy to change. It is found that, after abrupt change of climate in 1987, the snow cover areas, their types are different with before, the interannual variability of them almost increase, indicating decrease stability of the snow cover in these areas.(3) Based on satellite remote sensing data, regardless of which method to use, the snow cover types are much the same as the results from station observed data in West China. Compared with the observed data of station, the satellite remote sensing data shows a little more snow cover days in West China, so the classification of snow cover based on satellite remote sensing data shows the larger regions of stable snow cover on Tibetan Plateau than the result from station observed data by using the snow cover days method. North Mountain, West Sichuan Plateau and Yunnan Province are the different types of snow cover regions. Using interannual variability of snow cover days method, the results of satellite remote sensing obviously reflect terrain features in West China, but also well describe the conditions of snow cover in areas without recording of observed data.Considering the classification of snow cover by using station observed data and satellite remote sensing data, the result shows that interannual variability of snow cover days method is much better than snow cover days method on classification of snow cover in West China.(4) The autumn, winter and spring snow cover days is negative correlation to the temperature and positive correlation to the precipitation in West China. The main factors influence changes of snow cover days are temperature, precipitation and temperature from winter to spring in the regions of stable and regular unstable snow cover. In autumn and winter, temperature has more significant effect on changes of snow cover days than precipitation does in the regions of irregular unstable snow cover area. Furthermore, the highest temperature has more important impact on snow cover days.(5) Certain atmospheric circulation can provide temperature and precipitation conditions for the formation of the snow. In the positive anomaly snow cover years, both the European trough and the Ural ridge move easterly and deepen evidently, the northwest current in front of the ridge leads cold air move to the south. The northerly wind from Central Siberian to China promotes the cold air to West China at the same time. Vapor from the west coast of Arabian Sea flow around the Tibetan Plateau, the warm and cold air meet in West China, and produce snow at low temperature. But, in negative anomaly snow cover years are actually opposite with them.