A dynamical downscaling of ERA-Interim in the North Sea using WRF with a 3km grid-for wind resource applications

Large offshore wind energy projects are being planned and installed in the North Sea, and there is an urgent demand for high-resolution atmospheric statistics to assess potential power production and revenue. Meteorological observations are too sparse to obtain those statistics, and global reanalyses like ERA-Interim have a resolution too coarse in space and time to capture important small-scale and terrain-driven features of the atmospheric flow. We therefore dynamically downscale ERA-Interim with the mesoscale model Weather Research and Forecasting to a 3 km grid to capture those unresolved features, for the period 1999-2008. The large-scale flow is conditioned by spectral nudging, and we make use of observation nudging towards QuikSCAT near-surface winds. The downscaling results in 100m wind-speed distributions and mean wind speeds, which are closer to the observations than ERA-Interim, while the accuracy in terms of root-mean-square error decreases. The observation nudging partially counteracts this latter effect, improving the root-mean-square error of wind speed and direction by 0.5 m s(-1) and similar to 10 degrees, respectively. We also introduce the power skill score, specifically designed to evaluate model performance within wind resource mapping. The power skill score confirms that the dynamical downscaling improves the distribution of wind speed in ranges where high accuracy is important for wind resource assessment. Copyright (C) 2016 John Wiley & Sons, Ltd.