Abstract
This paper compares the daily solar irradiation available at surface estimated by the MERRA (Modern-Era Retrospective Analysis for Research and Applications) re-analysis of the NASA and the ERA-Interim re-analysis of the European Center for Medium-range Weather Forecasts (ECMWF) against qualified ground measurements made in stations located in Europe, Africa and Atlantic Ocean. Using the clearness index, also known as atmospheric transmissivity or transmittance, this study evidences that the re-analyses often predict clear sky conditions while actual conditions are cloudy. The opposite is also true though less pronounced: actual clear sky conditions are predicted as cloudy. This overestimation of occurrence of clear sky conditions leads to an overestimation of the irradiation and clearness index by MERRA. The overall overestimation is less pronounced for ERA-Interim because the overestimation observed in clear sky conditions is counter-balanced by underestimation in cloudy conditions. The squared correlation coefficient for clearness index ranges between 0.38 and 0.53, showing that a very large part of the variability in irradiation is not captured by the re-analyses. Within an irradiation homogeneous area, the variability of the bias, root mean square error and correlation coefficient are surprisingly large. MERRA and ERA-Interim should only be used in solar energy with proper understanding of the limitations and uncertainties. In regions where clouds are rare, e.g. North Africa, MERRA or ERA-Interim may be used to provide a gross estimate of monthly or yearly irradiation. Satellite-derived data sets offer less uncertainty and should be preferred.
Highlights
The solar radiation impinging at ground level is an essential variable in solar energy
If only the clearest conditions are retained by selecting the largest KT for each station, the bias for the clear sky conditions is less in surface solar irradiance or irradiation (SSI) for MERRA (59 J cmÀ2) and is negative for ERA-Interim (À33 J cmÀ2)
The major finding in this study is that the MERRA and ERAInterim re-analyses often predict clear sky conditions while actual conditions are cloudy
Summary
The solar radiation impinging at ground level is an essential variable in solar energy. It is often called surface solar irradiance or irradiation (SSI) in solar energy, solar flux or solar exposure when dealing with measurements, and downwelling shortwave flux, or downwelling surface shortwave flux in numerical weather modelling. The present article deals with the surface daily solar irradiation, i.e. the energy received per surface unit during a day. It is computed by a radiative transfer model and depends on the representation of the whole set of radiatively active variables of the atmospheric column above the point. Of interest here are the MERRA (Modern-Era Retrospective Analysis for Research and Applications) re-analysis proposed by the NASA Global Modeling and Assimilation Office and the ERA-Interim re-analysis of the ECMWF (European Center for Medium-range Weather Forecasts)
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