Do offshore wind farms weaken or enhance surface wind and wave fields?

Abstract

Over the North Sea, larger and larger part of the water surface is being covered by wind farms. Studies have shown consistent results regarding farm wake effects at hub height, characteristic of reduced wind speed and enhanced turbulence. Close to water surface, published studies using both measurements and modeling have suggested enhanced wind speeds sometimes, and reduced wind speeds some other times. Hence, this study investigates the research question: Do offshore wind farms weaken or enhance surface wind and wave fields?We use the mesoscale atmosphere-wave-wake coupled modeling system that consists of the Weather Research and Forecast (WRF) model, Spectral Wave Nearshore (SWAN) model with the wave boundary-layer model (Du et al. 2017, Fischereit et al. 2022). We use the Fitch Wind Farm Parameterization scheme (Fitch et al. 2012), with four coefficients for the advection of the wind farm-generated Turbulence Kinetic Energy (TKE): a = 1, 0.25, 0.1 and 0, corresponding to larger and larger TKE advection. The model is used together with flight measurements of wind fields upwind, above and downwind of offshore wind farms, collected during the project WIPAFF (Bärfuss et al. 2019, Lampert et al. 2020). We use two case studies, one following Bärfuss et al. (2021) (with fetch effect) and one following Larsén and Fischereit (2021) (without fetch effect). There is no evidence of generally enhanced surface winds and waves in the presence of wind farms. Enhanced surface winds and waves can however be generated numerically when using e.g. a = 1, as a result of numerical distribution of excessive TKE and momentum generated at hub height down to the surface. The study suggests that the wake effect is rather sensitive to the value of a, regarding both horizontal and vertical distribution from the hub height. Measurements are needed to understand the distribution of turbine-generated TKE and to help defining a- value for specific conditions.