Abstract

The monthly mean daily data of global solar radiation and sunshine hours for a period of 40 years (1961 - 2000) at 15 stations in Northwest China have been used to study their long-term, seasonal, and inter-annual variations. The variation of clearness index (K_T), relative sunshine (R_S) and the correlations have been analyzed. The results show that there are significant declining trends of both clearness index (~ 2.56%/decade) and relative sunshine duration (~ 1.36%/decade) with confidence level larger than 99% during 1961 to 2000 in Northwest China. The 40 years averaged clearness index and relative sunshine duration are higher than 0.40 and 40% at all 15 stations. In some remote sites such as Hami Dunhuang and Golmud, clearness index and relative sunshine duration are even higher than 0.60 and 70%, respectively. Linear and quadratic function fitting methods have been used to estimate the clearness index based on the measured sunshine duration data. According to the correlation coefficient (R), coefficient of determination (R²), Mean Bias Error (MBE), Root Mean Square Error (RMSE) and Mean Percentage Error (MPE), both methods could be employed in estimating global solar radiation of location that has the same geographical location information as Northwest China.

Key words: Global solar radiation, sunshine hours, clearness index, relative sunshine duration, Northwest China.