Validation of Sentinel-1 SAR Wind Products with Measurements from Buoys and Lidars

Sentinel-1 Synthetic Aperture Radar (SAR) is a multi-purpose monitoring satellite suite that, among many applications, provides sea surface wind speeds at high spatial resolution. The overall aim of the study is to quantify the accuracy of the SAR wind products from Copernicus Ocean Wind, called OCN OWI, and from the Technical University of Denmark (DTU) Department of Wind and Energy Systems’ product called DTU SAR. Both products serve as a basis for offshore wind resource mapping for offshore wind energy planning. With the growth in offshore wind farms, offshore wind resource information is highly relevant. However, a comparison between the two products is lacking. This study fills this gap by presenting a comprehensive validation of the two Sentinel-1 wind speed products using wind speed measurements from 18 weather buoys and 10 floating wind lidars in the European Seas. It is the first time a comprehensive wind lidar data set has been used for SAR wind validation. Key findings: OCN OWI vs. lidar (buoy) shows R2 = 0.93 (0.84), root mean square error (RMSE) = 1.18 m/s (1.61 m/s), mean absolute error (MAE) = 0.86 m/s (1.24 m/s), and bias = -0.5 m/s (-0.6 m/s). DTU SAR vs. lidar (buoy) shows R2 = 0.88 (0.84), RMSE = 1.3 m/s (1.6 m/s), MAE = 0.92 m/s (1.22 m/s), and bias = 0.02 m/s (-0.04 m/s). OCN OWI provides a filtered data set, and validation vs. lidar shows R2 = 0.95 and RMSE = 0.88 m/s; however, at the expense of discarding more than 50% of all data. The lidar vs. SAR wind speed statistics outperformed the buoy comparison statistics for all metrics studied. The 3 km Norwegian reanalysis (NORA3) wind speeds vs. lidar (buoy) show RMSE = 1.27 m/s (1.64 m/s) and bias = -0.01 m/s (-0.43 m/s). Lidar wind speed data are more accurate than buoy data and give a more trustworthy validation of SAR wind speed and model wind speeds than buoy data. Lidar data are recommended for validation studies on Geophysical Model Functions on SAR winds. Satellite Global Precipitation Measurement (GPM) Integrated Multi-satellitE Retrievals (IMERG) are collected at the buoy and lidar sites for comparison of SAR-based wind speed accuracy during precipitation. SAR and NORA3 show consistently higher RMSE values vs. buoy and lidar data with increasing precipitation and higher mean wind speeds at higher precipitation rates, but no systematic bias. Creating a precipitation flag for Sentinel-1 SAR-based winds would reduce the number of available samples and potentially lead to biased estimates of the wind resource. Vertical wind profiles at lidar locations are compared to SAR-based wind profile extrapolation, including stability correction.