Abstract
Abstract This study examines the progress made by two new reanalyses in the estimation of surface irradiance: ERA5, the new global reanalysis from the ECMWF, and COSMO-REA6, the regional reanalysis from the DWD for Europe. Daily global horizontal irradiance data were evaluated with 41 BSRN stations worldwide, 294 stations in Europe, and two satellite-derived products (NSRDB and SARAH). ERA5 achieves a moderate positive bias worldwide and in Europe of +4.05 W / m 2 and +4.54 W / m 2 respectively, which entails a reduction in the average bias ranging from 50% to 75% compared to ERA-Interim and MERRA-2. This makes ERA5 comparable with satellite-derived products in terms of the mean bias in most inland stations, but ERA5 results degrade in coastal areas and mountains. The bias of ERA5 varies with the cloudiness, overestimating under cloudy conditions and slightly underestimating under clear-skies, which suggests a poor prediction of cloud patterns and leads to larger absolute errors than that of satellite-based products. In Europe, the regional COSMO-REA6 underestimates in most stations (MBE = −5.29 W / m 2 ) showing the largest deviations under clear-sky conditions, which is most likely caused by the aerosol climatology used. Above 45°N the magnitude of the bias and absolute error of COSMO-REA6 are similar to ERA5 while it outperforms ERA5 in the coastal areas due to its high-resolution grid (6.2 km). We conclude that ERA5 and COSMO-REA6 have reduced the gap between reanalysis and satellite-based data, but further development is required in the prediction of clouds while the spatial grid of ERA5 (31 km) remains inadequate for places with high variability of surface irradiance (coasts and mountains). Satellite-based data should be still used when available, but having in mind their limitations, ERA5 is a valid alternative for situations in which satellite-based data are missing (polar regions and gaps in times series) while COSMO-REA6 complements ERA5 in Central and Northern Europe mitigating the limitations of ERA5 in coastal areas.
Highlights
Different methods have been developed to estimate surface irradiance in the absence of ground records (Urraca et al, 2017c)
ERA5 has a mean positive bias of +4.54 W/m2 worldwide, which represents a reduction in the mean bias error (MBE) of around the 50% compared to ERA-Interim (MBE = +10.05 W/m2) and Modern-Era Retrospective Analysis for Research and Applications (MERRA)-2 (MBE = +11.34 W/m2)
High positive biases are still obtained in North America, coastal areas and small islands, such is the case of the extreme bias of +36.22 W/m2 obtained for Kwajalein (KWA), a small atoll of 16 km2 in the Marshall Islands
Summary
Different methods have been developed to estimate surface irradiance in the absence of ground records (Urraca et al, 2017c). Satellitebased models using images from geostationary satellites are the most extended approach (Sengupta et al, 2015) nowadays They provide gridded datasets of surface irradiance since the 1980s (Polo et al, 2016), with hourly or higher time resolutions and spatial resolutions down to few km. Atmospheric reanalysis is an alternative that produces long-term irradiance data with global coverage (including the poles), intra-daily time resolutions, spatial resolutions around 30–80 km and no missing values. They are usually distributed at no cost and include a large number of weather parameters besides surface irradiance, making them an attractive option to assess surface irradiance. The quality of irradiance data from reanalysis is generally lower than that of satellite-based products (Bojanowski et al, 2014; Urraca et al, 2017b)
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