Temporal and spatial distribution of strong precipitation days over the Tibetan Plateau

By using of the daily precipitation and temperature data of 48 stations from 1961 to 2008 in Qinghai and Tibet,the spatial and temporal characteristics of strong precipitation days during the winter(November to next March) and the summer half year(May to September) over the Tibetan Plateau are discussed by means of linear trend analysis,Morlet wavelet and Mann-Kendall analysis etc statistical analysis methods.Through the contrastive analysis of the precipitation characteristics over the plateau and plain area with latitude,more than 5 mm of daily snowfall and 25 mm of daily precipitation are used as the critical values of heavy precipitation during the winter and summer half year respectively according to the actual situation of precipitation over the plateau.The results show that probabilities of heavy rainfall resulting are lager in the areas which have more rainfall.The spatial distribution of heavy rainfall days is very similar to total rain over the Plateau,which decreases from southeast to northwest during the summer half year,while declines from the center of the Plateau to the surrounding during the winter half year.In the southeast of the Plateau,although the snow days and snow is few,the heavy rainfall days account for a higher proportion of the total snowfall days.Heavy precipitation is mainly concentrated in early July to mid-August(early November to mid-late March) during the summer(winter) half year.Heavy rainfall days in mid-June increased slowly,and in late July they are the most,then,after mid-August reduce rapidly.Additionally,the heavy precipitation during the summer(winter) half year has obvious interannual oscillation with quasi-six years(5-6 years) and inter-decadal oscillation with quasi-10-11-year(quasi-15-year).During the summer half year,the space variation tendency of heavy rainfall days is uneven.It mainly increases(decreases) in the northeast(southeast) of Qinghai and in the west(east) of Tibet.During the winter half year in addition to the Yarlung Zangbo River and individual regions show decreasing trends,most parts of the Plateau are significantly increased.In the recent 48 years,heavy rainfall days show weak downward trend,while weak rainfall days show upward trend during the summer half year.Because of that the rise tendency of heavy snowfall(days) is much stronger than weak snowfall(days),the rise tendency of snowfall is mainly affected by heavy snowfall during the winter half year over the Plateau.