SAR Observations of Offshore Windfarm Wakes

Abstract

Satellite synthetic aperture radar (SAR) is the only operational instrument providing information on near-surface ocean wind fields with coverage of up to several hundred kilometres and spatial resolutions below 100 m. SAR is independent of daylight and cloud conditions and therefore an interesting information source for the analysis of wakes behind offshore windfarms. In this chapter, an overview is given on the research that was done on this subject so far. In a first step, the basic measurement principle of SAR is explained, and typical imaging configurations and operation modes are presented. A summary is given of important past, present and future satellite SAR missions launched by various nations and organisations. In this context, the growing amount of data, which are available for analysis, and the existing efforts to ensure a continuity of SAR data acquisitions are pointed out. Both potentials and limitations of the system are discussed with a focus on offshore windfarm-related issues. Of particular importance is the fact that the basic quantity measured by SAR is the sea surface roughness. It is clear that the relationship between the roughness and wind speeds at higher levels is not straightforward, in particular in complex environments like the surroundings of offshore wind farms, and this will be discussed in some detail. Other complications can be caused by image features, which are actually related to oceanic processes like ocean current divergence. Finally, a basic limitation of SAR wind measurements is the fact that information about wind direction can only be obtained in a very indirect way and even that is not guaranteed. In practice, many users of SAR data therefore use additional data from numerical models or in situ stations. Approaches to estimate the wake length from SAR data are presented, and some derived results concerning the relationship between wake length and atmospheric stability are discussed. The discussion also includes some image features, which appear counterintuitive at first sight, like an apparent increase of surface roughness within about 10 km downstream offshore windparks observed on some SAR scenes. Additional applications of SAR data in the context of offshore windfarming, like the assessment of wind energy potential on larger spatial scales, are briefly addressed as well. As one major conclusion drawn in this chapter, it is strongly recommended to use SAR data in combination with other sources of information, like in situ data or numerical model simulations. Different options and challenges associated with data merging of this kind are discussed.