Exploring the coastal effects relevant for offshore wind farming using the space borne synthetic aperture radar data in the German Bight

In the coastal zone complex atmospheric processes such as momentum and heat fluxes are  caused by large differences between the land and the sea. The smoother sea surface leads to wind speeds, which are usually higher over the ocean than over land. In addition, there are complicated effects caused by temperature gradients in the ocean due to water depth variations.  This study focuses on the investigation of the change in the horizontal wind field and the atmospheric stability between the coast and up to 200 km offshore.

The wind resources at 10 m height are assessed from synthetic aperture radar (SAR) data acquired by the satellites Sentinel1A/B over the German Bight within the period of 2017-2020 with a focus on offshore wind directions. The satellite data provide information on sea surface roughness, which can be linked to near surface wind speed.  Information on the air-sea thermal components is  provided by model data from the German weather service (DWD).

The SAR data  show a significant increase of wind speed offshore in most cases. Increasing wind speeds between land and sea over fetch distances of 70 km and more are often detected. The increase δu in horizontal wind speed between offshore and the coast exceeds 2.5 m/s in average. Furthermore, the estimated atmospheric stability shows an impact on the wind speed gradients. The thermal stability appears to dictate the distance over which the wind increases. Strong thermal stability tends to influence the horizontal wind gradient by increasing the fetch distance over more than 100 km. In the context of offshore wind farming, the potential effects of these horizontal wind gradients on the wind power will be discussed.