# Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight
## 5 Results and Discussion
### LLJ Extent: Case Study for 2 June 2017  
On 2 June 2017, a day with southeasterly wind directions, water temperatures from 12 \({}^{\circ}\)C in the central German Bight up to 17 \({}^{\circ}\)C along the coastline, and air temperatures from 13 \({}^{\circ}\)C in the central German Bight up to 16 \({}^{\circ}\)C over land in northwestern Germany, an LLJ event was observed by four wind lidar systems and airborne measurements near the wind park Amrumbank West simultaneously (Figure 9). Figure 10 additionally shows profiles of a nocturnal LLJ which was established later that day at the locations Heliogland and St. Peter-Ording.  
An interplay of several mechanisms is responsible for the observed acceleration of wind speed below 300 m. The advection of relatively warm air that has been heated up on its way above the warmer land surface over the cold North Sea and relatively cold near-surface air resulted in surface-based temperature inversion with cold air masses below and warmer air masses above, which constitutes a stably stratified atmosphere. As known from the nocturnal LLJ, a temperature inversion decouples friction forces and supports the development of an LLJ. Additionally, the transition between land to open sea is associated with a dramatic change in surface roughness, resulting in low surface friction forces over the open water causing higher wind speeds. In contrast, the warm land and the cold ocean would expect a sea-breaze circulation that counteracts the observed wind direction.  
A deeper look into the spatial extent of the LLJ is provided by the use of WRF simulations.  
WRF simulations were able to reproduce airborne measurements by a Dornier-128 research aircraft [52; 53] (Figure 11) of a stable stratification above land during the morning, which is persistent above the sea surface at 09:15 UTC (Figure 12). Associated with the temperature inversion, a belt of increased wind speed is represented during the time of the observations, confirming the different wind speed observations (Figure 12). The  
Figure 8: LLJ properties depending on the gradient of virtual potential temperature as an indicator for stability. (**upper left**) Maximum wind speed, (**upper right**) core height, (**lower left**) vertical wind shear, (**lower right**) wind direction at core hight. The radiosonde ascents at midnight are indicated by a blue symbol, the ascents at noon by a red symbol. The range of neutral atmospheric stratification is given in yellow.  
Figure 11: Flight track of the research aircraft on 2 June 2017. The coloured parts of the flight track indicate the location of the vertical profiles of wind speed and potential temperature that were used for model validation (Figure 12). The main purpose of the flight was to examine the flow situation around Amrumbank West wind farm.  
Figure 10: LLJ event on 2 June 2017 late evening, measured at the lidar sites Heligoland and St. Peter-Ording.  
Figure 12: WRF simulations and airborne observations on 2 June 2017 at 09:15 showing: (**a**) vertical profile wind speed, (**b**) potential temperature, and (**c**) water vapour mixing ratio.  
Figure 13: _Cont._  
Therefore, the overall extent of the LLJ above the German Bight can be estimated as at least 110 km in the north-south direction (from Wilhelmshaven to Amrumbank West) and 130 km in the east-west direction (from St. Peter-Ording to FINO1) from observational data. This is in agreement with observations above land, where LLJs can extend several hundreds of kilometres in length and width [67]. WRF simulations even suggest a larger extent of up to 170 km in the north-south direction and of at least 260 km in the east-west direction (see wind speed at 100 m altitude in Figure 13).