Climatic Change and Stability of Total Cloud Amount over the Qinghai-Tibetan Plateau

The climate change and stability of total cloud amount over the Qinghai-Tibetan Plateau since recent 40 years is analyzed based on the monthly total cloud amount data observed by 80 meteorological stations during the period from January 1961 to December 2000.The result shows that the total cloud amount over the plateau decreases from the southeastern part to the northwestern part of the plateau.The stability of cloud amount in summer is higher than that in winter and higher in the eastern part than that in the western part.Therefore,the total cloud amount is high and stable over the headwaters of the rivers,eastern part of the Qinghai-Tibetan Plateau,western part of the Sichuan Plateau,Gannan Plateau and the Qilian Mountains;the monthly change of total cloud amount is significant over the southern and western parts of the Qinghai-Tibetan Plateau,where the cloud amount is the highest in summer.The cloud amount is high and its fluctuation is small over the eastern part of the Qinghai-Tibetan Plateau,Qilian Mountains and headwaters of the rivers in the central part of the plateau during the period from March to September,and its maximum occurs in June.The spatial extent of significant durative of the total cloud amount is large,but its duration is short and occurs mainly during the periods from July to August and from May to June.In annual variation,the total cloud amount is in a decrease trend. The variation of the total cloud amount is slow and has been in a slight increase trend over the headwaters of the rivers and the Qilian Mountains since the 1990s.Seasonally,the total cloud amount increases over the northwestern part but decreases over the southeastern part of the plateau in winter,spring and summer,it,however,decreases over the northwestern part but increases in the southeastern part in autumn.