SNOW COVER AND DEPTH OF FREEZE-THAW ON THE TIBETAN PLATEAU: A CASE STUDY FROM 1997 TO 1998

An increase in snow cover on the Tibetan Plateau has been observed over the past 50 years. The frequency of snow disasters on the Plateau has also increased. Much of the Qinghai-Xizang (Tibetan) Plateau is underlain by permafrost. Some observation sites of the GEWEX Asian Monsoon Experiment (GAME)-Tibet are located in the Naqu-Anduo area, and have been operating since 1997. A data set obtained from the GAME-Tibet measurement network was used to examine variations in the active layer, with emphasis on the heavy snow cover in 1997/1998. Analysis in this study is focused on air and soil temperature, soil moisture, freeze-thaw depths, and snow depth in years of normal (1998/1999) and abnormally heavy (1997/1998) snow cover. Results indicate that anomalously deep snow cover influenced the pattern of soil freeze-thaw depth through its control over ground temperature. Although air temperature was lower than normal during this period, ground temperatures were appreciably warmer under the heavy snow cover, which retarded frost penetration at depth. Snow cover can also contribute to the cooling process through its high albedo. Because the anomalously heavy snow load lasted well beyond the normal date of meltout, soil thaw developed later than usual during the summer of 1998. Interannual variations in snow depth have significant implications for the maintenance of Tibetan permafrost.