Mean wind and turbulence profiles over the ocean with roughness saturation

This paper reviews measurements of wind profiles in the atmospheric boundary layer in strong wind (thermally neutral) conditions over open water and the ocean, and the variation of the roughness parameters with mean wind speed. Based on the wind data recorded on the coast of the island of FrØya (Norway) in the 1980s, and dropwindsonde profiles in hurricanes, the paper shows that 'capping', or saturation, of the surface drag coefficient becomes apparent at a mean wind speed at 10 m height of about 25 m/s. Wind speed models used in the offshore industries were investigated, (the ISO model, the API 'tropical cyclone' model and the IEC model). The ISO model, although based on good quality data from FrØya, does not allow for the saturation of the roughness above about 25-30 m/s, even though that was apparent in the FrØya data. 'Capping' of the aerodynamic roughness length for wind speeds greater than 28 m/s is represented appropriately in the API 'tropical cyclone' model, and hence the model represents the mean wind properties reasonably well in severe tropical cyclone conditions. However, the turbulence intensities in the API 'tropical cyclone' model, based on over-land measurements (ESDU), are overpredicted for winds over the ocean, at heights above 20 m. The IEC models are entirely based on over-land measurements, and hence are not representative of over-water conditions such as those required for offshore wind farms. New model profiles for over-ocean strong winds are proposed for wind speeds up to hurricane strength, based on the ISO profiles, but with capping of the surface drag coefficient at a value of 0.0025, at a mean wind speed at 10m height of 25 m/s. The proposed turbulence intensity model is also a revision of the ISO profile, also with capping above 25 m/s. The proposed model profiles are in better general agreement with recorded data in strong winds than those currently specified in international standards, and are applicable to all wind speeds in synoptic-scale events, including those in tropical cyclones, typhoons and hurricanes. As well as the FrØya data, the revised strong-wind models are supported by measurements from Atlantic hurricanes, gales in the North Sea, landfalling typhoons in Japan and Cyclone 'Yasi' in Queensland, Australia.