# Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight
## 1 Introduction
Temperature inversions are mainly the result of warm-air advection from the land above the colder sea surface, which typically occurs during the day and with wind directions from land to sea. Above the Baltic Sea, LLJ events related to stable stratifications are regularly observed in spring and summer, when the water temperature is generally lower than the air temperature advected from land to sea within any direction [29; 30]. The maximum wind shear of LLJs is stronger over land than above sea, associated with a weaker inversion over the sea [31]. Above the North Sea, the most frequent wind direction for the FINO1 (Forschungspalttform in Nord- und Ostsee) research platform has been reported as southwest [5; 32; 33; 34]. At FINO1, a one-year analysis of LLJ occurrence based on lidar data and measurements at the meteorological mast showed a high frequency of occurrence of 65% of all days with a mean wind speed at a jet core height of 11.8 m s\({}^{-1}\), a most frequent altitude of core height of around 250 m and most frequent wind direction at the jet core from northwest [8]. The most frequent core height, which is the altitude of the wind speed maximum, was observed at 200-220m. However, these numbers may be biased by the low data availability [8]. In contrast, at the meteorological mast IJmuiden tower, Dutch coast, LLJ events have been observed only during 12% of the time based on a 4-year lidar data set, covering the altitude range from 90 m to 315 m [35]. The most frequent stable stratifications have been reported in the months May to August [36]. During these periods, there is a high probability of the development of long-reaching wakes downwind of wind parks, which further modify the wind field for downstream wind parks, and have been observed by synthetic aperture radar (SAR) satellite images [4; 37; 38]. In stable stratifications, wakes with a length exceeding 20 km have been assessed from SAR data [39; 40]. As LLJs may be associated with these stable stratifications, they could influence the energy production of offshore wind farms located close to the coast. For numerical simulations, the LLJ phenomenon associated with very stable boundary layers and extraordinary wind shear distributions is a challenging scenario since turbulence is kept at unrealistic high levels in order to get other features in the model correct [41]. It is not yet included in the common engineering models used in wind energy to predict the power output for offshore wind parks, where only neutral boundary layer conditions are considered with a logarithmic wind profile (e.g., Openwind [42], WindFarmer [43]). Stable stratification leads to significantly longer wakes with a slower wind speed recovery compared with unstable conditions: The average wake length (wind speed has recovered to 95% of the freestream wind speed) under stable conditions exceeds 50 km, while under neutral/unstable conditions, the wake length amounts to 15 km [5]. Energy yield assessments and wake model validations have to put more emphasis on actual stability conditions, particularly for offshore wind farms. Both the wind speed reduction inside wind farms as well as the wake length in current engineering models are set for neutral conditions, but these are found to underestimate the wake effects for stable conditions [5]. The importance of considering wind profiles instead of wind measurements at hub height has already been recognized in many studies and currently has been included in the new revision of the IEC 61400-12-1 standard [44; 45; 46; 47; 48; 49]. Besides the positive effect of enhanced power production, the high wind shear associated with LLJs modifies oscillation frequencies and increases fatigue effects in the rotor blade and whole wind turbine structure [50]. On the other hand, the enhanced wind shear associated with LLJ may counteract wake effects [51].