Variation Trend of Direct and Diffuse Radiation in China over Recent 50 Years

Using the direct and diffuse solar radiation data during 1961—2009 from 14 stations and during 1961—1991 from 49 stations in China, and the method of linear regression (taken time as an independent variable), wavelet analysis and Mann-Kendall statistical test, we analyze the characteristics of the direct and diffusion radiation from different aspects including decadal variation, long-term seasonal change, inter-decadal anomaly, cycle and abrupt change. The results are as follows. 1) The spatial and temporal distribution of the climatic trend for the direct and diffuse radiation are revealed by using the normalized climatic tendency coefficient. At the same time, these results are comparable. 2) The direct solar and diffuse radiation in China show decreasing trend, but there are many differences in region, inter-annual and seasonal trend. 3) Based on the characteristics of the geographical distribution on observation stations and according to the climate zone, the classification and the statistical analysis of the percentage of site occupation the long-term changes of the radiation can be clearly shown to have the characteristics of area and local difference. 4) Using wavelet and Mann-Kendall test method, the statistical results show that the radiations in different regions have different periods and abrupt change. 5)The long-term variations of the direct and diffuse radiation in most big cities turn to decrease more obviously than in other cities. Some. Other studies show that such a climatic phenomenon may be caused by environmental factor change of urbanization related to the difference in development speed between cities.