# Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight
## 1 Introduction  
Wind energy is a fundamental base for power generation in Germany. In particular offshore wind farms contributed 24.7 TWh, which is around 4.3% of the overall power consumption in Germany in 2019 [1]. As wind energy represents a highly variable source of energy, it is of interest to have accurate predictions of wind resources and to quantify effects that enhance or decrease power production. Especially in the wind speed ranges between the cut-in speed of around 4 m s\({}^{-1}\) and rated speed of typically 10 m s\({}^{-1}\), small changes in wind speed are important as the power output of wind turbines is proportional to the cube of the wind speed. For the effect of sea breeze with an increase of wind speed in this wind speed range and short temporal duration, the effect on offshore wind power production is not negligible; Ref. [2] gives a value of 10% wind energy enhancement on an exemplary day due to the presence of the coastal jet, depending on the sea-breeze type and coastline characteristics. Therefore, a major research effort is currently being undertaken to improve the understanding and prediction of the offshore wind field [3], and include in particular effects such as wind farm wakes [4, 5, 6], the blockage effect [7], and low-level jets [8]. A maximum in the vertical profile of horizontal wind speed within the atmospheric boundary layer (ABL) frequently occurs in combination with stable stratifications andtemperature inversions, which decouple the surface friction force from layers above the inversion [9; 10]. This phenomenon is referred to as low-level jet (LLJ), with various definitions in the literature concerning the strength of the enhanced wind speed and its origin. It has been observed for decades worldwide, above land, in coastal areas, and above sea [9; 11; 12; 13], and investigated in numerical studies, e.g., [14; 15; 16; 17]. With more installations of wind turbines at increasing altitudes, the LLJ gets into the focus for energy harvesting. For specific places with a pronounced LLJ occurrence, like in the Great Plains east of the Rocky Mountains, the phenomenon is already taken into account for wind energy applications [18; 19]. In contrast to Europe, there is a sloping terrain in the Great Plains with increasing altitude towards the west, which results in frequent LLJs with higher wind speed [20]. The main mechanism of LLJ formation is the development of a surface-temperature inversion, leading to the decoupling of surface friction from the atmosphere above the temperature inversion, and resulting in inertia oscillations [21]. Above land, such a surface-based inversion typically develops by radiative cooling during cloud-free nights [10], enabling LLJ formation. Other conditions contributing to the formation of LLJs are the land-sea breeze [2; 22], the monsoon [23], frontal passages [14], temperature gradients induced by sloping terrain [24], and general orography [25]. In the Northern German Plain and the Netherlands above land, several studies show the frequency of occurrence of LLJ events in a range from 20 to 52% of all nights [10; 26; 27], with more LLJ nights in summer associated with strong temperature inversions [27], and differences due to definitions. The most frequent core height was observed at 200-220 m for Braunschweig, Germany [27], and the maximum wind speed was most frequently in the range of 4-9 m s\({}^{-1}\). Closer to the coast, at Cabauw, the Netherlands, the most frequent core height was at lower altitudes (140-260 m) and the maximum wind speed was higher (6-10 m s\({}^{-1}\)). A study at the midlatitude site Julich in western Germany showed a mean core height of 375 m and a median maximum wind speed of 8.8 m s\({}^{-1}\), with a similar diurnal distribution of LLJ events like in Braunschweig and an overall LLJ occurrence of 13% for a 4-year data set [28]. Above water, the diurnal cycle of the ABL is different due to the larger heat capacity of the water surface and less strong cooling during nighttime.