A Novel Sampling Method for Satellite-Based Offshore Wind Resource Estimation

Synthetic aperture radar (SAR) measurements from satellites can be used to estimate the spatial wind speed variation offshore in great detail. The radar senses cm-scale roughness at the sea surface which can be translated to wind speed at the height 10 m using an empirical geophysical model function. A system has been setup at Risoe DTU for offshore wind retrievals from ENVISAT radar data. The resulting wind maps have a spatial resolution of ~500 m. If 70 or more overlapping satellite SAR scenes are available for a given area of interest it is possible to estimate the wind resource. A major advantage of satellite-based offshore wind resource assessment is the spatial information gained at high resolution from the satellite imagery. Limitations include the low sampling rate and the fixed acquisition times of satellite data. A new satellite scene is typically available every 3-4 days over a given site of interest and the acquisition takes place in the morning and evening only. At Risoe DTU we have utilized our ever-growing collection of wind maps from ENVISAT to compute wind statistics for the North Sea and the Baltic Sea. The reliability and the spatial coverage of the satellite-based wind climatology have improved gradually as more data were collected. The satellite scenes have been treated as random samples and weighted equally in our previous analyses. Here we introduce a novel sampling strategy based on the wind class methodology that is normally applied in numerical modeling of wind resources. The method is applied within a wind and solar resource assessment study for the United Arab Emirates funded by MASDAR and coordinated by UNEP. Thirty years of NCEP/NCAR reanalysis data are used to define approximately 100 geostrophic wind classes. These wind classes show climatologically representative large-scale meteorological conditions for the region of interest. The wind classes are used to make the most representative selection of satellite images from the ENVISAT image catalogue. A minimum of one satellite image is chosen per wind class. The frequency of occurrence of each wind class is used to weight the satellite-derived winds for the calculation of wind climatology. The applied selection and analysis of images should improve the reliability of wind climate estimates compared to random selection of images each given equal weighting.