Abstract

Offshore wind farms are usually located remote from meteorological stations. Except from the sonic anemometers mounted at the nacelles of the wind turbines according to industrial standards, the access to wind measurements in the farm and its immediate surroundings is therefore limited. With the deployment of clusters of wind farms in the European Seas, the modification of the local wind and the generation of a turbulent wake by one farm can influence the power production of the neighbouring farm. Spaceborne Synthetic Aperture Radar (SAR) is an ideal instrument to observe such effects due to its high spatial resolution and large coverage. SAR can image a cluster of wind farms at the same time. The wind farm turbulent wake is manifested in the SAR intensity images as dark elongated areas downwind of a wind farm while the surrounding ocean areas appear brighter. Good agreement between images from SAR and Doppler radar measurements has recently been reported for the wake of the Westermost Rough wind farm in cases without stable atmospheric stratification. Inspired by this first of its kind comparison of ground-based Doppler radar and SAR images of wind farm wakes, we compare in this work SAR images and simulations of the two interacting wakes of the Sheringham Shoal and Dudgeon wind farms at the British East coast. The wakes are simulated by the Weather Research and Forecasting (WRF) code, a state-of-the-art regional meteorological code which has a simple built-in wind farm model. The normalized radar cross-section (NRCS) is estimated from the Sentinel-1 IW GRD products by averaging the intensity pixels to the output resolution of the model (i.e. 500 m).

Introduction

Sentinel-1 is an operational C-band SAR earth observation mission consisting of two satellites in polar-orbit. It is developed by the European Space Agency (ESA) for the Copernicus initiative and has a free and open data policy (www.copernicus.eu). SAR satellites do not depend on illumination and can penetrate clouds, thus can generate frequent observations over large areas. Satellite SAR ocean images are basically a "fingerprint" of the lower atmosphere on to the ocean surface, and to first order a measure of surface stress. Thus, traditional SAR wind is "wind" modified to be consistent with the surface stress, i.e., equivalent neutral wind. A neutral transfer coefficient that can be used to convert the satellite wind to kinematic surface stress for assimilation into numerical weather prediction models have been developed (Bourassa et.al., 2019).

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