Low-level jets in the North and Baltic seas: Mesoscale model sensitivity and climatology using WRF V4.2.1

Low-level jets (LLJs), characterized by wind speed maxima in the lower part of the atmospheric boundary layer, play a crucial role in shaping wind resource availability, particularly as modern wind turbines reach heights exceeding 200 m. Understanding the climatology of LLJs is essential for optimizing wind energy assessments in offshore environments. We leverage wind measurements from lidars and a mast at five sites in the North and Baltic seas to evaluate the ability of the Weather Research and Forecasting (WRF) model and the widely used reanalysis ERA5 to characterize LLJs and use the optimal WRF setup to generate a detailed 5-year climatology. We test the sensitivity of LLJ representation to key WRF model configurations, including grid spacing, vertical resolution, surface layer (SL), and planetary boundary layer (PBL) parameterizations. Results reveal that LLJ representation strongly depends on the PBL scheme, with LLJ frequency varying by more than a factor of 3 across configurations. For example, the Mellor-Yamada-Janjic (MYJ) scheme favored LLJ formation, while Yonsei University (YSU), BouLac (BL), and Mellor-Yamada-Nakanishi-Niino (MYNN) 2.5 (with bl_mynn_mixlengthCombining double low line0) were less prone. The best-performing setup employed scale-Aware subgrid mixing (km_optCombining double low line5; 3DTKE), accurately capturing LLJ occurrence rates, intensity, and vertical profiles. In contrast, ERA5 significantly underestimated LLJ frequency and failed to resolve key features, highlighting its limitations for detailed LLJ analysis. The 5-year LLJ climatology provides new insights into the spatial and seasonal distribution of LLJs, offering valuable guidance for offshore wind resource assessment and planning in the region. 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 m, 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, where LLJs contribute to up to 30% of the wind capacity. In spring and summer, strong coastal gradients are observed in jet timing, height, and direction, particularly along eastern shorelines. Strong variations in the mean duration are also seen, with the longest-lasting jets occurring in the Southern Bight.