the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jul 2025
The impact of low-level jets on the power generated by offshore wind turbines
Abstract. Low-level jets (LLJ) are local maxima in the vertical wind speed profile. They are frequently observed at heights of approximately 50 m to 500 m above sea level in offshore regions. The influence of low-level jets on the power production and loads of wind turbines has not been researched thoroughly. In this paper we investigate the influence of low-level jets on wind turbine performance in an offshore wind farm. We derive vertical wind profiles up to heights of 350 m from lidar plan position indicator scans with different elevation angles at the wind farm Nordergründe in the German Bight, located approximately 15 km from the coast. We detect LLJs with a frequency of occurrence between 2.4 % to 22.6 %, based on different definitions used in literature at the observed location. We analyse their influence on the power production of the turbines using operational wind farm data. We observe a negative influence on power production and increased power fluctuations in low-level jet situations compared to situations with equal wind-veer-corrected rotor equivalent wind speed (REWS) but without LLJs. Further, we conduct aeroelastic simulations for a set of wind profiles with varying veer, shear, turbulence intensity and shape of the LLJ core. Increasing veer and shear both have a negative impact on the simulated power production, while the shape of a low-level jet only slightly alters the energy conversion process at the wind turbine for the same REWS. Thus, we conclude the main driver for the efficiency-lowering effect during the presence of low-level jets to be the combination of positive and negative shear, causing a high absolute shear across the rotor area as well as increased absolute veer.
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EC1: 'Comment on wes-2025-118', Etienne Cheynet, 14 Jul 2025
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Dear authors,
Thank you for the submission of the article. Please find below some feedback that may help strengthen the manuscript. Hopefully, you will find some of them useful
- The literature review appears to be incomplete or at least selective. The work of Gutierrez et al. [1], Murphy et al. [2], and others on low-level jets (LLJs) is noteworthy and may be directly relevant to the present study. The manuscript could better establish its novelty by more clearly identifying the knowledge gap relative to prior research. The current self-citation rate is approximately 20%, which further suggests that the literature review would benefit from broader coverage.
- The use of lidar PPI scanning for wind profiling is indeed an interesting component of the study. However, this technique has been applied in previous research in both wind engineering and meteorology. As such, it may not be considered fundamentally novel, contrary to what is suggested in the cover letter. See, for instance, references [3] and [4] for both recent and earlier applications of this scanning mode. Notably, Visich and Conan [4] also used PPI scanning to detect LLJs.
- The study reports large wind veer across the rotor, with Δθ ranging from 0 to 40 degrees. It may be worth clarifying whether values of ∣Δθ∣>20∘ are realistic under typical atmospheric conditions. For example, wind veer is rarely above 0.1°/m, even in stable conditions. For a turbine with a rotor diameter of 126 m, this would suggest a typical Δθ<15∘. If previous studies have reported larger directional shear in the atmospheric boundary layer, it would be helpful to cite them in support of the current findings.
- The reported frequency of LLJ occurrence under convective conditions appears higher than that found in previous studies, such as Wagner et al. [5]. An interpretation of these results in light of existing literature could help contextualise the findings.
References
[1] Gutierrez, W., Ruiz-Columbie, A., Tutkun, M., & Castillo, L. (2017). Impacts of the low-level jet's negative wind shear on the wind turbine. Wind Energy Science, 2(2), 533–545.
[2] Murphy, P., Lundquist, J. K., & Fleming, P. (2019). How wind speed shear and directional veer affect the power production of a megawatt-scale operational wind turbine. Wind Energy Science Discussions, 2019, 1–46.
[3] Goit, J. P., Yamaguchi, A., & Ishihara, T. (2020). Measurement and prediction of wind fields at an offshore site by scanning Doppler LiDAR and WRF. Atmosphere, 11(5), 442.
[4] Visich, A., & Conan, B. (2025). Measurement and analysis of high altitude wind profiles over the sea in a coastal zone using a scanning Doppler LiDAR: Application to offshore wind energy. Ocean Engineering, 325, 120749.
[5] Wagner, D., Steinfeld, G., Witha, B., Wurps, H., & Reuder, J. (2019). Low level jets over the southern North Sea. Meteorologische Zeitschrift, 28(5), 389–415. https://doi.org/10.1127/metz/2019/0948Citation: https://doi.org/10.5194/wes-2025-118-EC1 -
RC1: 'Comment on wes-2025-118', Anonymous Referee #1, 31 Jul 2025
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This manuscript presents a very comprehensive study of the impact of low-level jets (LLJs) on offshore wind turbine performance, effectively integrating lidar data, SCADA, and aeroelastic simulations with OpenFAST. The paper's key methodological strength is its use of a veer-corrected rotor equivalent wind speed (REWS) to compare LLJ and non-LLJ cases. This approach allows for a direct assessment of energy conversion efficiency by comparing scenarios with an equivalent energy flux, rather than relying on a simple hub-height wind speed.
The primary finding—that LLJ profiles tend to reduce power production and increase power fluctuations for the same REWS due to wind shear and veer—is an important contribution. This result, while in line with the established understanding that wind variability across the rotor reduces conversion efficiency, provides valuable new evidence from a real-world offshore environment. The work is well-executed and addresses a topic of significant relevance to the wind energy industry.
However, I have a few concerns related to the framing of the study's conclusions, which risks misinterpretation, and the justification for key methodological choices, particularly the LLJ detection algorithm.
Major Comments and Concerns
(a) My primary concern is that the wording used throughout the manuscript (including the title, abstract, and discussion) creates a strong impression that LLJs lead to a general decrease in turbine power (e.g., Title; Line 416; Line 441), not just a decrease in conversion efficiency. This framing undermines the well-known physical mechanisms of LLJs (e.g., inertial oscillation), which accelerate wind to super-geostrophic speeds, thereby increasing the total available wind energy and potentially increasing absolute power output compared to non-LLJ conditions. The depiction in Figure 2, while illustrative of profile shape, could also enhance this potentially false impression.
To rectify this, the authors should reframe the manuscript to focus explicitly on the impact of LLJs on the turbine's energy conversion efficiency. This is the true finding of the study. Consequently, the title should be revised to reflect this focus (e.g., "...on the power conversion efficiency of offshore wind turbines"). To test the hypothesis of overall power impact, the authors could perform a direct comparison of absolute power from SCADA data during LLJ episodes versus non-LLJ episodes on the same days (thus excluding non-LLJ extreme events, e.g. cyclones). Without this analysis, claims about overall power reduction are unsubstantiated.
(b) The study's conclusions are heavily dependent on the choice of the shear-based LLJ definition from Hallgren et al. (2023), which yields an occurrence frequency (22.6%) that is nearly an order of magnitude larger than other methods (e.g., Wagner, Kalverla), which are in closer agreement with one another (Table 5). The authors must provide a more robust justification for using this "odd one out" definition. The author should clarify how the shear is computed. Is it the maximum shear above and below the jet core? Given the sensitivity of shear computation to the variations and noise, which are known to be present in measurements, how are these factors treated?
Minor Comments and Suggestions
1. Line 172 (and Introduction): The concept of REWS is fundamental to this paper's methodology and novelty, yet it is not formally introduced until Section 2.4. REWS and its motivation should be introduced much earlier, in the Introduction (Section 1), to properly frame the study for the reader.
2. Line 170: To highlight its significance, consider changing the section title to better reflect the use of REWS, for example: "Performance Analysis Using an Equal Rotor Equivalent Wind Speed Framework."
3. Line 133: How is the wind direction estimated from the VAD algorithm? This assumption of a spatially homogeneous wind direction is critical, as the lidar scans cover a range of nearly 10 km. A brief discussion of the validity and potential uncertainty of this assumption is needed.
4. Lines 149-152: The vertical profile is derived from multiple low-angle scans. It is unclear where the resulting vertical profile is horizontally located relative to the turbine. An illustration depicting the scan geometry and the effective location of the final wind profile would be extremely helpful for clarity.
5. Figure 2: As mentioned in the major comments, this figure risks creating a false impression. This is a perfect opportunity to visually demonstrate the paper's core concept: including a non-LLJ (e.g., logarithmic) profile that has the exact same REWS. This would clarify that the study is about the shape of the profile, not the absolute magnitude of the wind.
6. Figure 13: This figure is insightful but could be improved. Is it possible to add a color code or different markers to the data points that relates back to the specific profile characteristics shown in Figure 5? This would help the reader understand the source of the scatter in the LLJ results.
7. Section 2.5: The simulations confirm the general trend of reduced performance. However, as seen in Figure 13, there appear to be a few intriguing cases where the turbine performance for LLJ profiles is better than for the logarithmic or even uniform profiles with the same REWS. A deeper analysis and discussion of these specific cases could reveal valuable insights into turbine energy conversion under complex inflow. It also raises the question: could this be a limitation or artifact of the Blade Element Momentum (BEM) theory used in OpenFAST under such extreme shear?
Citation: https://doi.org/10.5194/wes-2025-118-RC1
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