Spatiotemporal Variations of Snowfall Days over the Tibetan Plateau from 1981 to 2010

Snow is a vital component of alpine regions due to its large seasonal variations and distinctive physical properties, it greatly affects climate, hydrology, and ecology at regional and global scales. The Tibetan Plateau (TP), also called "the roof of the world", is the highest land and typical alpine region on the earth with over 4000 m above sea level. Snow on the TP is a vital fresh water source in the Himalayan region and downstream areas, and also a significant indicator of climate change from regional to global scales. Therefore, under global climate warming, the variation of snowfall days over the TP, which is defined as the number of days with snowfall (including sleet) weather at a station from 2000 BT to 2000 BT, is concerned by international scientific community and local people.Spatial and temporal variations of snowfall days over the TP are analyzed using snow observations from 94 meteorological stations for the period from 1981 to 2010, and the following results are obtained. Snowfall days over the TP are generally characterized by more snowfall days in the central and eastern alpine region, southern slope of the Himalayan range and the Qilian mountains in the northeast, and less snowfall days in valleys in south and the Qaidam Basin in the north. The number of averaged annual snowfall days for the TP is 60 days, of which 45% occurs in spring, 28% in winter, 22% in autumn and 5% in summer, respectively. There is a very significant decreasing trend in mean annual snowfall days from 1981 to 2010 over the TP with a rate of 10.5 days per decade. The abrupt climate change of snowfall days from more to less occurs around 1997 and in the later 10 years, annual snowfall days decrease obviously especially in 2010, which is an anomalous snowless year. Intra-annual distribution of snowfall days over the TP is characterized by double-peak type. The first peak occurs during transition period of atmospheric circulation from winter to summer and the second peak takes place in transition period of atmospheric circulations from summer to winter with less peak value. During the transition time of atmospheric circulation, low value weather system along with upward air movement and temperature and humidity conditions in the upper air is favorable for snowfall over the TP. A very significant correlation exists between decrease in snowfall days and increase in temperature from 1981 to 2010 over the TP. Under global warming and more rapid temperature increasing on the TP, the rainfall previously in snow form converts into liquid rainfall, which may significantly contribute to the decrease of snowfall days over the TP in the recent 30 years.