HelioClim-3 (HC3) is a database providing time series of the surface downwelling solar irradiance that are computed from images of the Meteosat satellites. This paper presents the validation results of the hourly global horizontal irradiance (GHI) and direct normal irradiance (DNI), i.e., beam irradiance at normal incidence, of versions four and five of HC3 at seven Egyptian sites. The validation is performed for all-sky conditions, as well as cloud-free conditions. Both versions of HC3 provide similar OPEN ACCESS Remote Sens. 2015, 7 9270 performances whatever the conditions. Another comparison is made with the estimates provided by the McClear database that is restricted to cloud-free conditions. All databases capture well the temporal variability of the GHI in all conditions, McClear being superior for cloud-free cases. In cloud-free conditions for the GHI, the relative root mean square error (RMSE) are fairly similar, ranging from 6% to 15%; both HC3 databases exhibit a smaller bias than McClear. McClear offers an overall better performance for the cloud-free DNI estimates. For all-sky conditions, the relative RMSE for GHI ranges from 10% to 22%, except one station, while, for the DNI, the results are not so good for the two stations with DNI measurements.
International audienceMcClear, a fast model based on a radiative transfer solver, exploits the atmospheric properties provided by the EU-funded MACC project (Monitoring Atmospheric Composition and Climate) to estimate the surface downwelling solar irradiances for cloud-free instances. This article presents the first validation of the McClear model for the specific climate of the United Arab Emirates where skies are frequently cloud-free but turbid. McClear accurately estimates the global horizontal irradiance measured every 10 min at seven sites. The bias ranges from -9 W m-2 (-1% of the mean observed irradiance) to +35 W m-2 (+6%). The root mean square error (RMSE) ranges from 22 W m-2 (4%) to 47 W m-2 (8%) and the coefficient of determination ranges from 0.980 to 0.990. Estimates of the direct irradiance at normal incidence exhibit an underestimation that is attributed to the overestimation of the aerosol optical depth in the MACC data set and not accounting for the circumsolar radiation in McClear. The corresponding bias ranges from -57 W m-2 (-8%) to +6 W m-2 (+1%). The RMSE ranges from 62 W m-2 (9%) to 87 W m-2 (13%) and the coefficient of determination ranges from 0.830 to 0.863. When compared to two other models in the literature, McClear is better able to capture the temporal variability of the direct irradiance at normal incidence. The validation results remain comparable for the global horizontal irradiance
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