A comprehensive evaluation and uncertainty quantification of offshore turbulence intensity from WRF mesoscale simulations

Abstract. At the beginning of each wind energy project, an as accurate as possible characterization of the site-specific wind resources is needed for any economic considerations of the project. Besides wind speed and direction, which can be measured easily and of which the representation in models has been investigated in various studies, information about wind turbulence suffers from a lack of confidence yet, although it is one of the key parameters that define the lifetime of wind turbines, and its characteristics are of high importance already in the planning phase. In this study, we analyze the feasibility of the Weather Research and Forecasting Model (WRF) to represent this turbulence in terms of turbulence intensity (TI) over a full year in the North Sea and quantify the range of uncertainties. The impact of the planetary boundary layer scheme used for model simulation on these uncertainties was investigated as well as the usage of model-resolved and subgrid-scale turbulence. Furthermore, results were evaluated for increasing spatial resolution in the model and stability impacts. It was found that TI values based on subgrid-scale turbulent kinetic energy (TKE) are significantly closer to observations than model-resolved TI (i.e. based on standard deviation of wind speed or wind speed components). As modelled TKE decreases with increasing resolution, the factor used in the TI calculation needs to be adapted to obtain comparable TI to observations. Nevertheless, independent of the planetary boundary layer scheme, resolution and test location in the North Sea, the lowest errors for TI are around 0.024 (RMSE) and 30 % (MAPE). For any further improvement either LES simulations or a kind of correction method like Measure-Correlate-Predict (MCP) becomes necessary.