Evaluation of Models for the Vertical Extrapolation of Wind Speed Measurements at Offshore Sites

Monin-Obukhov theory predicts the well-known log-linear form of the vertical wind speed profile. Two parameters, namely the aerodynamic surface roughness length and the Monin-Obukhov-length, are needed to predict the vertical wind speed profile from a measurement at one height. Different models to estimate these parameters for conditions important for offshore wind energy utilisation are discussed and tested: Four models for the surface roughness and three methods to derive the Monin-Obukov-length from measurements are compared. They have been tested with data from the offshore field measurement Rødsand by extrapolating the measured 10 m wind speed to 50 m height and comparing it with the measured 50 m wind speed. The mean wind speed at 50 m height is under-predicted by 2% to 5% when extrapolated from a 10 m measurement. For the sea surface roughness it has been found that the simplest approach, a constant roughness, gave the best result. From the models to derive the Monin-Obukhov-length the bulk method performed best. It has been shown that this is due to shortcomings of the description of the stability influence in the stable regime.