Abstract
High-accuracy wind data for coastal regions is needed today, e.g., for the assessment of wind resources. Synthetic Aperture Radar (SAR) is the only satellite borne sensor that has enough resolution to resolve wind speeds closer than 10 km to shore but the Geophysical Model Functions (GMF) used for SAR wind retrieval are not fully validated here. Ground based scanning light detection and ranging (LiDAR) offer high horizontal resolution wind velocity measurements with high accuracy, also in the coastal zone. This study, for the first time, examines accuracies of SAR wind retrievals at 10 m height with respect to the distance to shore by validation against scanning LiDARs. Comparison of 15 Sentinel-1A wind retrievals using the GMF called C-band model 5.N (CMOD5.N) versus LiDARs show good agreement. It is found, when nondimenionalising with a reference point, that wind speed reductions are between 4% and 8% from 3 km to 1 km from shore. Findings indicate that SAR wind retrievals give reliable wind speed measurements as close as 1 km to the shore. Comparisons of SAR winds versus two different LiDAR configurations yield root mean square error (RMSE) of 1.31 ms − 1 and 1.42 ms − 1 for spatially averaged wind speeds.
Highlights
Synthetic Aperture Radar (SAR) systems are remote sensing devices that measure radar backscatter signals from the Earth’s surface to infer geophysical parameters
Three cases are presented in more detail to illustrate possible challenges when comparing SAR and scanning light detection and ranging (LiDAR) wind speeds (Figure 7)
Sentinel-1A SAR wind retrievals with CMOD5.N from the coastline up to 5 km offshore have been compared to scanning LiDAR wind measurements in the Northern Sea on the Danish West Coast, for the first time showing how LiDAR and SAR measure typical wind speed reduction when approaching the coast from offshore
Summary
Synthetic Aperture Radar (SAR) systems are remote sensing devices that measure radar backscatter signals from the Earth’s surface to infer geophysical parameters. Over the ocean they receive backscatter primarily from centimeter-scale Bragg waves. SAR wind fields at those resolutions will represent the spatial variability of the wind [2]. These measurements are very suited for applications that need high spatial resolution over a large area, e.g., assessment of wind resources necessary before building offshore wind farms
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