Estimation of Hourly Solar Radiation at the Surface under Cloudless Conditions on the Tibetan Plateau Using a Simple Radiation Model

In this study, the clear sky hourly global and net solar irradiances at thesurface determined using SUNFLUX, a simple parameterization scheme,for three stations (Gaize, Naqu, and Lhasa) on the Tibetan Plateau wereevaluated against observation data. Our modeled results agree well withobservations. The correlation coefficients between modeled and observedvalues were >0.99 for all three stations. The relative error of modeledresults, in average was <7%, and the root-mean-square variance was<27 W m^-2. The solar irradiances in the radiation model were slightly overestimatedcompared with observation data; there were at least two likely causes.First, the radiative effects of aerosols were not included in the radiationmodel. Second, solar irradiances determined by thermopile pyranometersinclude a thermal offset error that causes solar radiation to be slightlyunderestimated. The solar radiation absorbed by the ozone and water vapor was estimated. Theresults show that monthly mean solar radiation absorbed by the ozone is<2% of the global solar radiation (<14 W m^-2). Solar radiationabsorbed by water vapor is stronger in summer than in winter. The maximumamount of monthly mean solar radiation absorbed by water vapor can be up to13% of the global solar radiation (95 W m^-2). This indicates thatwater vapor measurements with high precision are very important for precisedetermination of solar radiation.