# Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight
## 5 Results and Discussion
### Statistics on LLJ Occurrence  
For the site Heligoland, the data set of a full year from 24 March 2017 to 23 March 2018 is analysed here. LLJ profiles were recorded during 7.19% of the total time.  
For the site Norderney, the data set from 9 August 2019 to 8 August 2020 is analysed. LLJ profiles were recorded during 11.13% of the total time.  
Figure 5 shows the occurrence of LLJ events throughout the year based on lidar data, and for comparison also based on radiosonde data obtained in the same time period (only available at Norderney). For each day where an LLJ occurred, the altitude interval affected by the LLJ is marked by a vertical black line. This indicates the altitude with the upper boundary of the minimum wind speed and the lower boundary where the wind speed has the same value as the altitude of the minimum wind speed. It can be seen that for both sites, the distribution of LLJ events follows specific patterns of typically a few consecutive days. This is in agreement with the typical lifetime of synoptic circulation patterns which either enable or prevent the formation of LLJ by different atmospheric conditions [10] and was also observed in the one-year data set of Braunschweig, Northern Germany [27]. For both sites, most LLJs appeared in spring (March, April, May) and summer (June, July, August), with fewer in autumn (September, October, November) and even less in the winter months (December, January, February).  
The LLJ profiles identified by the radiosondes at 0 and 12 UTC at the site Norderney are provided in Figure 5 for the same time periods as the wind lidar observations. A similar  
Figure 4: Weibull distribution (\(k=2.12\) and \(A=10.91\,\mathrm{m\,s^{-1}}\)) **(left)** and wind rose **(right)** at Heligoland at \(120\,\mathrm{m}\) for the period 24 March 2017 to 23 March 2018 **(upper panels)**, and Weibull distribution (\(k=2.33\) and \(A=10.39\,\mathrm{m\,s^{-1}}\)) **(left)** and wind rose **(right)** at Norderney at \(120\,\mathrm{m}\) for the time period 9 August 2019 to 8 August 2020 **(lower panels)**.  
annual distribution is observed by the continuous wind lidar data from the same site. The annual LLJ distribution is comparable for both time periods.  
Figure 5: Occurrence and altitude distribution of the LLJ events averaged over each day that were observed by wind lidar at Heligoland from 24 March 2017 to 23 March 2018 (**first figure**), and determined  
by radiosonde profiles at Norderney from 24 March 2017 to 23 March 2018 (**second figure**), observed by wind lidar at Norderney from 9 August 2019 to 8 August 2020 (**third figure**), and by radiosonde profiles at Norderney from 9 August 2019 to 8 August 2020 (**lower figure**). The altitude interval affected by the LLJ is marked by a vertical black line. Maximum wind speed is marked by a black dot. Yellow areas show intervals with no valid data.  
Figure 6 shows the distribution of LLJ events over the day for Heligoland and Norderney. A minimum occurrence is visible between 9 and 14 UTC. LLJs occur most frequently during the nighttime. The diurnal distribution is similar for Norderney with a minimum LLJ occurrence between 10 and 14 UTC.  
The jet core height distribution of the LLJ maximum for Heligoland is located most frequently at an altitude of 120 m, thus within the altitude of the rotor blades. LLJ core heights have been identified between 40 m and 250 m in 86% of all cases. For Norderney, the altitude distribution is similar with the most frequent occurrence at an altitude of 110 m. The distribution of the maximum wind speed is similar for both cases as well. The maximum LLJ wind speed typically increases with core height. This feature has been observed for the radiosonde and continuous lidar data both at Heligoland and Norderney (not shown). This relationship has also been shown in [64].