Spectrum properties of near-surface wind field derived from SAR in the offshore windfarm areas in the German Bight

Abstract:
Characteristics of the wakes behind windfarms, such as the wake length and velocity deficit are inferred from nearsurface wind fields derived from SAR. The estimation of the relative wind speed deficit downstream the windfarms from wind field from SAR data is still challenging due to mainly the absence of reference or undisturbed wind speed at the exact location of the wake and also the spatial variation of the background field. A new methodology for the estimation of the velocity deficit is performed. It is based on a filter technique that takes into consideration the wind direction, geometry of the wake and the wind speed in the neighbouring areas. Horizontal wavenumber spectra of wind are also analysed regarding the atmospheric stability conditions given by the insitu measurement platform FINO1. The wavelength examined is typically 0.6-100 km. The spectra are averaged in accordance with stability over time and wind direction. The spectral slope of the wind field for all stability conditions are generally close to k(exp −5/3). However, the spectral power for unstable conditions is higher than the spectral power for stable conditions.