Evaluation and Bias Correction of the ERA5 Reanalysis for Wind and Solar Energy Applications

The applicability of the ERA5 reanalysis for estimating wind and solar energy generation over the contiguous United States is evaluated using wind speed and irradiance variables from multiple observational data sets.  After converting ERA5 and observed meteorological variables into wind power and solar power, comparisons demonstrate that significant errors in the ERA5 reanalysis exist limiting its direct applicability for a wind and solar energy analysis.  Overall, ERA5-derived solar power is biased high, while ERA5-derived wind power is biased low.  Errors for the shortest duration, most extreme solar negative anomaly events are found to be statistically reasonably well represented in the ERA5, when completely overcast conditions occur in both ERA5 and observations.  Longer duration events on weekly to monthly timescales, which include partially cloudy days or a mix of cloud conditions, have ERA5-derived solar power errors as large as 40%.  ERA5-derived solar power errors are found to have consistent characteristics across the CONUS region.  The negative bias errors in the ERA5 windspeeds and wind power are largely consistent across the central and northwestern US, and offshore, while the eastern US has an overall small net bias.  For weekly to monthly timescales, the uncorrected ERA5-derived wind power errors approach 50%.  Corrections to the ERA5 are derived using a quantile-quantile method for solar power, and linear regression of wind speed for wind power.  These corrections greatly reduce the ERA5 errors, including for extreme events associated with wind and solar energy droughts, that will be most challenging for electric grid operation, while also avoiding potential over-inflation of the reanalysis variability resulting from differences between point-measurements and the temporally and spatially smoother reanalysis values.