Characterizing Coastal Turbulence and Wind Speed Gradients Using an Anemometer Array to Improve Offshore Wind Energy Assessment

Abstract. As interest and investment in offshore wind farms increase, it becomes essential to better understand how interactions at the air-sea interface impact wind and turbulence in the marine boundary layer. Central to this effort is understanding the role of waves, especially on the relatively shallow continental shelves where most wind farms are located and waves are shaped by bathymetry. To address this, we analysed hundreds of hours of high frequency wind speeds measured by an anemometer array on a coastal tower from 14 to 26 m above the mean water surface. Wind speed gradients diverge substantially from established values, and the boundary layer was found non-constant ~60 % of the time. This means that the assumptions required for applying Monin Obukhov Similarity Theory cannot be met, so wind speeds aloft cannot be accurately predicted using this canonical methodology. Wind speed gradients were highest during alongshore winds and lowest during onshore. This occurred because waves refract and shoal parallel to shore which increases surface roughness independent of wind. Onshore winds cross the waves and create strong wind–wave coupling, while alongshore winds move along the waves, weakening the coupling and reducing effective roughness. Changes in the roughness then alters the wind speed which propagated through the boundary layer impacting winds aloft. The results of this study should be used to inform wind farm siting to optimize energy yield.