Characteristics of Different Intensity Precipitation in Flood Season over the Tibetan Plateau

Based on the daily precipitation data of 25 stations in Tibet from 1973 to 2009,rainy days with different level precipitation in flood season over the Tibetan Plateau were analyzed using the methods of linear trend and Morlet wavelet transform. Results show that it was mainly the light and moderate rain over the Tibetan Plateau. The light rainfall was almost equal to the moderate rainfall but the heavy rainfall was less than 10% of the total rainfall. The spatial distribution of light rainfall days was very similar to that of the total rainfall days over the Plateau,which decreased from the downstream of the Yarlung Zangbo River to surrounding regions,and rainfall was the least over the northwest of the Plateau. While the moderate and the heavy rain days declined from Yarlung Zangbo River basin to the north and south sides. The moderate and the heavy rainfall days presented an apparent seasonal change,which concentrated mainly in late June to mid-August. The total and light rainfall days has decadal cycle with quasi-12-13-year. The light ( moderate) rainfall days showed a decreasing ( increasing) trend . But the total and the heavy rainfall days showed no significant increasing or reducing trend. The trend of rainfall days was quite different in spatial distribution,the total and light rainfall days decreased mainly in Yarlung Zangbo River and the eastern Plateau,while increased in the southern Plateau. In most parts of the Plateau moderate and heavy rainfall days mainly increased. Moderate rain days reduced in the southern marginal areas and heavy rain days reduced in the eastern Plateau.