Low-level jets in the North and Baltic Seas: Mesoscale Model Sensitivity and Climatology

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Abstract. Low-level jets (LLJs) are wind speed maxima in the lower part of the atmospheric boundary layer. Accurately accounting for these mesoscale phenomena in wind resource assessment is increasingly important as the height of wind turbines continues to grow. During LLJ events, wind speeds increase, leading to a general increase in power output. We utilize wind measurements from LiDARs and a mast for five sites in the North and Baltic Seas to assess the quality of the WRF model simulations for LLJ characterization. We also investigate the benefits of using the WRF model simulations compared to the widely used ERA5 reanalysis. In the WRF model simulations, we vary the grid spacing, vertical resolution, and the planetary boundary layer and land surface schemes, parameters we deemed most likely to have a substantial impact. The model’s performance is evaluated based on its ability to replicate observed distributions of LLJs and relevant associated characteristics, such as the shear and veer across the rotor-plane of typical large offshore wind turbines (30 m to 300 m). Finally, we generate a five-year LLJ climatology based on using the best-evaluated model configuration. The modeling results show a strong dependency of the LLJ representation and the associated wind profiles on the WRF model configuration and that relying on ERA5 for LLJ characterization is insufficient. For example, the LLJ rate-of-occurrence varied by up to a factor of three or more between some WRF model runs. The simulation using the optimized model configuration more accurately reflects the frequency, intensity, and vertical extension of LLJs, as confirmed by LiDAR data. In the North and Baltic Seas, LLJs occur along the western sea basins around 10–15 % of the time, with average jet heights between 140–220 meters, which are well within the height of operation of modern wind turbines. The most LLJ-prone region is east of southern Sweden, especially during spring and summer. These mesoscale phenomena contribute to up to 30 % of the wind capacity in some areas in this season. Analysis of the five-year modeled LLJ climatology in the North and Southern Baltic Seas gives insights into the physical mechanisms that create them.

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