Offshore meteorology for multi-mega-watt turbines

To achieve precise wind resource assessments, to calculate loads and wakes as well as for reliable short-term wind power forecasts, the vertical wind profile above the sea has to be modelled with high accuracy for tip heights up to 160m. For these purposes, it is crucial to consider the special meteorological characteristics of the marine atmospheric boundary layer. Continuing our work in the EU-project ANEMOS [1], we analysed marine wind speed profiles that were measured at the two met masts Horns Rev (62m high) and FINO1 (103m) in the North Sea. We found pronounced effects of thermal stratification and of the influence of the land-sea transition. In many situations, the wind shear is significantly higher than expected with standard approaches in mesoscale models. Nevertheless, the numerical analysis of the marine wind field above the North Sea from the German Weather Service seems to provide a good assessment of wind speeds at 103m height. For an improved simulation of the vertical wind speed profiles, we developed a new analytic model of marine wind velocity profiles. In particular, the flux of momentum through the Ekman layers of the atmosphere and the sea is described by a common wave boundary layer. The good agreement between our theoretical profiles and observations at Horns Rev and FINO1 support the basic assumption of our model that the atmospheric Ekman layer begins at 10 to 30m height above the sea surface.