Coupling Atmosphere and Waves for Coastal Wind Turbine Design

Offshore wind farms in coastal areas are considered by the Danish government to contribute to the goal of having 50% of the energy consumption from renewable sources by 2025. Therefore, new coastal developments will take place in Danish areas. The impact of waves on atmosphere is most often described by roughness length, which is typically determined by the Charnock formulation. This simplification in many atmospheric models has been shown to bring bias in the estimation of the extreme wind. Some wave-dependent formulations have been reported to overestimate the drag coefficient and roughness, but new roughness formulations have been proposed to better estimate wave-wind interactions according to observations. In the present work, an assessment of several roughness descriptions is performed, and implications for coastal wind and wave modelling are studied. An atmospheric (WRF) and spectral wave model (MIKE 21 SW) are implemented for the North Sea in order to consider wave effects on roughness. The objective is to see the reaction of an atmospheric model to the water surface description through offline coupling. A comparison with three simplified roughness formulations embedded in WRF showed a 50% variation in roughness and 20% in wind, with the better formulation for wind leading degraded predictions of roughness compared with observations. The large estimates of roughness when using a 3rd generation wave model are evident offshore, while a roughness formulation based on wave age produces more realistic values. However, at a coastal site, both estimates were within the same range. The impact of roughness on the wave model is discussed in terms of an idealized case for fetch-limited wave growth.