Analysis of Freezing and Thawing Processes on Typical Underlying Surface in Permafrost Area of Tibet Plateau

Based on the soil observation data of Amdo station in Tibet plateau, the permafrost is divided into four stages by the daily maximum and minimun soil temperature at 10 cm depth: the thawing stage, the completely thawed stage, the freezing stage and the completely frozen stage. The difference between ground soil and air temperature (Ts-Ta) was analyzed and influence of freezing thawing process on the heat and moisture exchange was discussed by combining the freezing thawing action with the temperature, ground temperature, snow depth, relative humidity and precipitation data. The results show that: 1) Each layer of soil complete melting process from top to bottom before East Asian Monsoon. The freezing process completed from mid October to early December.The melting period is generally longer than the freezing period; 2) The soil humidity value area is concentrated in the rainy season, 10 cm above the depth is the humidity value area. The temperature gradient above the soil is greater than below. In the melting stage, the isothermal phase transition of soil temperature is maintained at 0℃ for a long time.At the same time, the diurnal variation of soil surface temperature was the largest in the whole year. The highest diurnal variation reached 22. 5 degrees centigrade. 3) The (Ts-Ta) reached its peak in the early June, then decreased gradually, reaches the minimum value in early January, the annual change rate of 0℃ to 12℃. The change of temperature is sensitive to relative humidity, precipitation and snow cover.