Long-term variations of estimated global solar radiation and the influencing factors in Hunan province, China during 1980-2013

Long-term variation of estimated global solar radiation (E (ga dagger")) and its relationship with total cloud cover (TCC), low cloud cover (LCC), water vapor content (WVC) and aerosol optical depth (AOD) were investigated based on the observations at 21 meteorological stations in Hunan province, China. Long-term variations of all variables were calculated for each station; the Mann-Kendall trend test was used to detect the significant level of temporal development trend for each variable; the Pearson correlation analysis was used to measure their linear relationships. Annual E (ga dagger") generally decreased at the rate of -2.11 x 10(-3) MJ m(-2) decade(-1) in Hunan province during 1980-2013. Seasonal mean E (ga dagger") decreased at the rate of -11.99 x 10(-3), -4.71 x 10(-3) and -4.51 x 10(-3) MJ m(-2) decade(-1) in summer, autumn and winter, respectively, while the increasing trend was observed in spring (15.74 x 10(-3) MJ m(-2) decade(-1)). The annual variation of E (ga dagger") in Hunan province was dominantly determined by the variations of AOD (0.33 x 10(-3) decade(-1)) and LCC (0.24 % p decade(-1)). But the spatial variation of E (ga dagger") in Hunan province was complex. All 21 stations were divided into four groups according to the long-term trends of E (ga dagger"), TCC, LCC, AOD and WVC. An increasing E (ga dagger") was observed at stations in group 1, which was determined by the variability of TCC. The variability of AOD and TCC might contribute to the increasing E (ga dagger") in group 2. There were decreasing trends of E (ga dagger") for the stations in group 3 and group 4, which were largely determined by the increases of AOD and LCC.