# Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight
## 5 Results and Discussion
### LLJ Dependence on Atmospheric Stability  
For the radiosonde data set, temperature and humidity data are available as well. Figure 7 shows the vertical gradient of the virtual potential temperature, which is an indicator of atmospheric stability, for the individual LLJ events. There is a trend that LLJ  
Figure 6: Total number of LLJ occurrence (10 min averages) in spring (green: March, April, May), summer (red: June, July, August), autumn (magenta: September, October, November) and winter (blue: December, January, February) at Heligoland (**upper figure**) and Norderney (**lower figure**) for each hour of the day in UTC based on wind lidar data.  
events observed around noon occurred mostly during stable conditions. At midnight, LLJ events are observed most frequently for neutral or even unstable atmospheric conditions.  
The LLJ parameters' relation to stability and others is investigated more in detail in Figure 8: The LLJ maximum wind speed varies between 2.5 and 17 m s\({}^{-1}\), with no clear dependence on stability. The LLJ core height varies between 70 and 350 m without a clear correlation with stability.  
Vertical wind shear is confined to the range of 0.3 to 0.6. This is significantly higher than previously reported values for coastal zones, e.g., a vertical wind shear coefficient of 0.11 was reported for neutral conditions for platforms in the Gulf of Mexico [65]. The impact of wind shear in the range of 0.1 to 0.4 on the probability of operation for wind turbines for a fixed Weibull distribution near the surface was investigated in [66]: A higher wind shear results in a higher probability of operation of wind turbines, in particular for turbines with high cut-in speed.  
The wind direction for neutral and unstable conditions is most frequently from north to east during the night. This can be explained by colder air masses above land which are advected to the warmer sea surface.  
During the daytime, stable conditions are observed in all wind directions. This is as expected for wind directions from the sector east to southwest if synoptic-scale flow dominates over local effects like sea breeze, as warmer air masses from land are transported above the colder water surface, leading to a surface-based temperature inversion. For wind from the sector west to northeast, this is unexpected. However, one has to take into account that for these wind directions, the radiosondes are transported from the island of Norderney towards the coast, where warmer temperatures are likely. The distance of the radiosondes from the launch site up to reaching an altitude of 500 m was typically within the radius of 3 km.  
Figure 7: LLJ events depending on the vertical gradient of the potential temperature based on radiosonde data from 9 August 2019 to 8 August 2020. The radiosonde ascents at midnight are indicated by a blue symbol, and the ascents around noon by a red symbol. The yellow horizontal bar indicates an (arbitrary) range of very small changes in virtual potential temperature, separating cases with clearly stable conditions (above) and clearly unstable conditions (below).