Low-level jets in the southern North Sea: implications for wind turbine performance using Doppler lidar observations

Abstract. Accurate knowledge of wind conditions experienced by wind turbines is essential to assess their performance. Among these conditions, low-level jets (LLJs) are important to consider since they have a direct impact on wind turbines, as their cores are frequently located within the rotor layer. In this context, this study investigates the characteristics of LLJs and their formation mechanisms using 3.3 years of long-range Doppler lidar measurements obtained at Dunkerque, a coastal city on the southern North Sea. In addition, data from an ultrasonic anemometer and from the ERA5 reanalyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) were used to determine the conditions favoring LLJ occurrence. The analysis revealed that LLJs were present in 15.6 % of the 117,411 measured wind profiles. The average jet core speed was 8.4 m s⁻¹ , with a mean core height of 267 m. LLJs were more frequent during nighttime, especially in spring and summer. These characteristics were consistent with those obtained at other sites in the North Sea region, with some differences attributable to the location of Dunkerque on the coast near the Dover Strait. This position introduced additional formation mechanisms for LLJs, including land–sea thermal gradients and wind channeling in the English Channel. The impact of LLJs on wind turbines of varying dimensions was then assessed for both energy production and structural loads. For conventional turbines, with a hub height around 100 m, LLJs counter-intuitively tend to decrease power production at high wind speeds. Conversely, for more recent and future wind turbines, LLJs will improve power production in all conditions. The increase in turbine size will also greatly reduce their exposure to detrimental wind shear conditions, both in terms of speed and direction shear.