Airborne measurements for investigating offshore wind farm wakes and modifications of sea state – benefits and limitations

Abstract. In the framework of the two large wind energy research projects WIPAFF and X-Wakes, crewed airborne measurements have been performed in and around wind farm clusters of the German Bight to investigate offshore wind farm wakes and associated sea-state modifications. These flights offer high spatial flexibility. Routes can be adapted in real time to wind direction, stability, wake extent, and features of interest, providing complementary coverage to fixed ground-based instruments and remote sensing systems. Aircraft observations achieve high vertical resolution in the order of several centimetres and allow simultaneous measurements of wind speed, turbulence, thermodynamic variables, air–sea fluxes, and sea surface characteristics. This enables a detailed description of wake structure, wake recovery, and the influence of atmospheric stability, as well as the interaction of multiple wakes across scales of tens to hundreds of kilometres. Airborne measurements also provide a direct link between atmospheric changes and sea-surface modifications, such as altered roughness or wave patterns, and supply valuable data for evaluating simulations and improving parameterizations used in wind farm modelling. When combined with satellite remote sensing, they help bridge the gap between high-resolution local observations and large-area coverage. A central limitation of aircraft campaigns is their restricted temporal coverage. Flights are episodic and sample evolving atmospheric conditions over finite time periods, which complicates the comparison with satellite snapshots and limits the ability to derive long-term statistics. As small wind farm effects that are in the order of the natural variability of the background flow are easily masked by natural variability in the marine boundary layer, effects such as global blockage are difficult to isolate using aircraft data alone. Overall, aircraft-based observations offer unique strengths when integrated with other measurement systems and modelling tools, despite inherent temporal constraints. This article summarizes which effects benefit from the analysis of airborne data sets, and shows examples where they helped to improve the understanding of the interaction of wind farms, atmosphere and sea state significantly.