Investigation of wind farm impacts on surface waves using coupled numerical simulations

A numerical investigation is carried out to understand the effect of offshore wind farm wakes on the surface wind and hence water waves. Two cases with the presence of wind farm wake effects from the literature are revisited, mainly due to two reasons: (1) the availability of various atmospheric and wave measurements; (2) one case with fetch effect and one without fetch effect. The coupled modeling system includes the atmospheric model WRF and ocean wave model SWAN, with the Wave Boundary Layer model implemented in SWAN. The wind farm wake effects are modeled using the Fitch Wind Farm Parameterization, where we used four coefficients of the parameter α to adjust the turbulent kinetic energy (TKE) coefficient (relevant to the magnitude and hence indirect advection of turbine-generated TKE: α=1, 0.25, 0.1 and 0). For the two cases, measurements suggest reduced wind speed and wave height under the wind farm wake effect. The modeling results are consistent, except for when α=1 is used. Using the currently standard value of α=1 results in excessive turbine-generated turbulence transported to the surface. This leads to enhanced surface winds and wave height; as this contradicts the measurements, we conclude that it is a numerical artifact. The study points to the importance of future research on a more accurate description of the horizontal and vertical transport of the turbine-generated TKE in WRF.