Reliability of ERA5 Reanalysis Data for Wind Resource Assessment: A Comparison against Tall Towers

The reliability of ERA5 reanalyses for directly predicting wind resources and energy productionhas been assessed against observations from six tall towers installed over very heterogeneoussites around the world. Scores were acceptable at the FINO3 (Germany) offshore platform forboth wind speed (bias within 1%, r = 0.95?0.96) and capacity factor (CF, at worst biased by 6.70%)and at the flat and sea-level site of Cabauw (Netherlands) for both wind speed (bias within 7%,r = 0.93?0.94) and CF (bias within 6.82%). Conversely, due to the ERA5 limited resolution (~31 km),large under-predictions were found at the Boulder (US) and Ghoroghchi (Iran) mountain sites, andlarge over-predictions were found at theWallaby Creek (Australia) forested site. Therefore, usingERA5 in place of higher-resolution regional reanalysis products or numerical weather predictionmodels should be avoided when addressing sites with high variation of topography and, in particular,land use. ERA5 scores at the Humansdorp (South Africa) coastal location were generally acceptable,at least for wind speed (bias of 14%, r = 0.84) if not for CF (biased by 20.84%). However, due to theinherent sea-land discontinuity resulting in large differences in both surface roughness and solarirradiation (and thus stability conditions), a particular caution should be paid when applying ERA5over coastal locations.