Characteristics of the ratios of snow, rain and sleet to precipitation on the Qinghai-Tibet Plateau during 1961-2014

Precipitation in different types has great influence not only on water resources and distribution of annual precipitation in cold regions, but also on the thermal regimes of frozen ground. The long-term variations of the ratios of snow, rain and sleet to precipitation were analyzed in this study. The 44 meteorological stations were selected, which are located on the Qinghai-Tibet Plateau (QTP) including permafrost and seasonal frozen ground regions. The results indicate that the monthly snow/precipitation ratio in permafrost regions is far higher than that in seasonal frozen ground regions, but the monthly rain/precipitation ratio in permafrost regions is lower than that in seasonal frozen ground regions, and the monthly variation of sleet/precipitation ratio is indistinctive. The annual ratios of snow and sleet to precipitation show decreasing trends in both regions, and annual rain/precipitation ratio shows an increasing tendency. The dropping magnitude of annual snow/precipitation ratio in seasonal frozen ground regions is larger than that in permafrost regions. The maximum and minimum annual snow/precipitation ratios in both regions occur in winter and summer, respectively, and the second is in spring, instead, the maximum and minimum annual rain/precipitation ratios appear in summer and winter, respectively. The spatial features of the ratios of snow and rain to precipitation are largely opposite, that is, the low snow/precipitation ratio stations are usually associated with the high rain/precipitation ratio, and prominent seasonal diversities of three ratios can be found from their spatial patterns. In addition, the decreasing (increasing) tendency of snow (rain)/precipitation ratio in spring and autumn is more significant than that in winter and summer, and these stations of snow/precipitation ratio with downward trends are mainly located at the edge of the QTP. The ratios of snow and sleet to precipitation will gradually decrease (increase), and the rain/precipitation ratio will increase (decrease) with the air temperature (altitude) rises, respectively, while there are significant seasonal discrepancies. Moreover, the altitude zone showing remarkable variations of the ratios of snow and rain to precipitation ranges from 3000 to 4000 m a.s.l, especially in spring and summer.