Underestimation of strong wind speeds offshore in ERA5: evidence, discussion, and correction

Abstract. Offshore Wind power plants have become an important element of the European electrical grid. Studies of metocean site conditions (wind, sea state, currents, water levels) form a key input to the design of these large infrastructure projects. Such studies heavily rely on reanalysis datasets which provide decades-long model time series over large areas. In this article, we address a known deficiency of one these reanalysis datasets, ERA5, namely that it underestimates strong wind speeds offshore. For doing so, comparisons are made against CFSR/CFSv2 reanalyses as well as high quality wind energy specific in-situ measurements from floating LiDAR systems. The ERA5 surface drag formulation and its sea state dependency are analysed in detail, the conditions of the bias identified, and a correction method is suggested. The article concludes with proposing practical and simple ways to incorporate publicly available, high-quality wind energy measurement datasets in air-sea interaction studies alongside legacy measurements such as met buoys.