Direct Comparison of Spaceborne SAR and On-Site LIDAR Measurements of Small-Scale Wind Variations around Offshore Wind Farms

Abstract

The increasing popularity of renewable energy resources has promoted the construction of offshore wind farms, particularly in the German Bight. While the wind farm layout consists of an array of large turbines, the interrelation of wind turbine wakes with the remaining array is of substantial interest. The downstream spatial evolution of turbulent wakes is very complex and depends on manifold parameters such as wind speed, wind direction and ambient atmospheric stability conditions. Besides the wake properties of single turbines, the interrelation of adjacent wind farms is a major subject of interest. To complement and validate existing numerical models, corresponding observations with high spatial resolution are needed. Over the past four decades, spaceborne remote sensing of sea surface winds with microwave sensors has developed into a mature technology. It is now playing an important role in Earth Observation (EO) due to the capability to measure with high spatial coverage and almost unaffected by weather conditions. Hence, the observations are commonly used for the improvement of numerical weather prediction (NWP). While scatterometers yield only coarse-resolution data, synthetic aperture radar (SAR) has proven to measure local wind related sea surface roughness with both, high resolution and wide coverage. On the other hand, earth-bound remote sensing techniques have evolved significantly. Especially Doppler LiDAR has become an important tool for offshore wind energy related research in the last years. Ground-based LiDAR windscanners enable to measure vertical wind profiles or horizontal planar scans of the radial wind speed within a range of several kilometres. While both methods have been successfully validated against meteorological models and in-situ point measurements, we present a direct comparison of platform based LiDAR measurements and high resolution 2D sea surface wind fields derived from spaceborne SAR. The analysis is further supported by a comparison with data from the numerical meteorological model COSMO-DE run by the German Weather Service (DWD). Our contribution will contain a comparison of a series of simultaneous wind field observations around the offshore wind farms “alpha ventus” and “Riffgat” in the North Sea taken with TerraSAR-X, Sentinel-1 and two scanning long range LiDAR systems. Both techniques are compared with a special focus on small scale variations in wind fields both, upstream and downstream from the wind facilities. Methods that have been developed to account for different measuring heights and scan duration of the two independent datasets will be presented and discussed regarding the applicability for turbulent atmospheric conditions predominant in the wake structures downstream of the wind turbines. Our initial results indicate that high resolution SAR-based wind fields are well-suited to identify and quantify small scale wind variations and therefore represent a valuable complementary data source for numerical models of offshore wind farms and NWP in general.