Estimation of All-Sky Solar Irradiance Components Over Rugged Terrain Using Satellite and Reanalysis Data: The Tibetan Plateau Experiment

Accurate knowledge of the at-surface solar irradiance (SSI) is essential to retrieving surface and atmospheric properties using satellite measurements of back-scattered and reflected radiance. The latter is affected by surface-atmosphere interactions, including the effects of terrain. The SSI is affected by the same processes. This study proposes a method to estimate the components of instantaneous surface solar irradiance: direct, isotropic and circumsolar diffuse, and terrain irradiance, which is expected to improve the simultaneous retrieval of Aerosol Optical Depth (AOD) and surface reflectance. The method takes into account the coupled effects of topography and atmosphere by combining parametrization and the Look-Up Table (LUT) approaches. The method was applied to rugged terrain over the Tibetan Plateau using MODIS atmosphere and surface data, ERA5 reanalysis data, CALIOP aerosol data and a Digital Elevation Model (DEM). The results showed that the SSI estimates were in satisfactory agreement with ground observations at four stations over the Tibetan Plateau in 2018 with R ² values of 0.61, 0.44, 0.41, and 0.49, respectively, and RMSE of 205.7 W/m ² , 176.9 W/m ² , 186.0 W/m ² , and 201.2 W/m ² , respectively. Estimations of the diffuse irradiance were evaluated separately against the only available in-situ observations at the Dali Station and the results were better than our SSI estimates with R ² , RMSE, and relative BIAS being 0.71, 94.98 W/m ² , and 31%, respectively. The isotropic and circumsolar diffuse irradiance accounted for 37.57% and 7.68% of the total annual SSI respectively, while diffuse irradiance accounted for 46.48% of the total annual SSI. Under clear skies, every 0.1 increase in AOD caused about a 35 W/m ² increase in diffuse irradiance and a decrease of about 25 W/m ² of SSI.