Investigation of low-level jets and their impacts on wind turbine performance in the southern North Sea using Doppler lidars

Low-level jets (LLJs) are fast-moving air streams in the lower part of the troposphere, characterized by wind maxima and wind shear typically occurring at the same level as wind turbine rotors. Technological advances have enabled the design of taller and more efficient wind turbines, making LLJs at higher altitudes potentially significant for their performance. Evaluating LLJ characteristics and understanding their formation mechanisms is essential for accurately assessing turbine loads and power production.

In this context, three years of wind profiles obtained every 15 minutes from two Doppler lidars installed in Dunkirk, a coastal city in northern France, were used to detect LLJs up to 1,500 m. The study focused on assessing the frequency and main characteristics of LLJs in the region and identifying their formation mechanisms. Additionally, the study aimed to evaluate the impact of these jets on wind turbines, especially given the rapid development of offshore installations.

Results indicate that LLJs are a common atmospheric phenomenon, occurring 15 % of the time, predominantly on the nights of the spring and summer seasons. This suggests that frictional decoupling due to radiative cooling is a key factor in LLJ formation. However, the city's coastal location induces additional formation mechanisms driven by the land-sea thermal gradient and the proximity of the English Channel.

The results demonstrated that these jets impact wind turbines since 38 % of the LLJ cores are located in the rotor layer of the most commonly installed offshore wind turbines. However, LLJs are not necessarily beneficial for their power production as the high wind speeds they imply are confined to a relatively thin layer, while the wind outside of this layer exhibits relatively lower velocities. Jet shear has a minimal impact on these turbines since it is similar to the shear observed in non-jet conditions. Indeed, these turbines are mainly located within the surface layer, where ground-induced shear is predominant. On the contrary, future wind turbines will be more impacted by LLJs due to larger rotor sizes and will experience greater negative shear, leading to significant loads on their blades.