the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Oct 2023
The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5
Christoffer Hallgren
Heiner Körnich
Stefan Ivanell
Ville Vakkari
Erik Sahlée
Abstract. The change of wind direction with height (the directional shear) affects both the power production from a wind turbine, wake effects and aerodynamic loading. In this study, a climatology of the relative occurrence of strong directional shear over Scandinavia is created using 43 years of hourly ERA5 data covering the height range of a modern wind turbine and at wind speeds of operation. It is shown that strong directional shear (≥15° over the rotor) is occurring 20–30 % of the time over land and 10–25 % of the time over the extended Baltic Sea. The height of the atmospheric boundary-layer and the wind speed at hub height are identified as the most important predictors for strong directional shear, with low boundary-layer heights and weak winds being the main causes. Associated with this, a strong land–sea seasonality is observed. Furthermore, ERA5 is validated against lidar soundings from two coastal sites, both indicating a major underestimation in the distribution of the directional shear in ERA5. Especially in strongly stratified boundary-layers ERA5 struggles, with 25 % of the data having errors exceeding 24° and 28° for Östergarnsholm and Utö respectively.
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Christoffer Hallgren et al.
Status: open (extended)
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RC1: 'Comment on wes-2023-129', Anonymous Referee #1, 27 Oct 2023
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The highlight of the paper is not clear. From the concluding remark, what I could read is ERA5 data is not appropriate to assess the wind direction shear. I totally agree with this conclusion, but then, what is the meaning of the analysis carried out in this manuscript?
Or if the authors try to say it is appropriate to use ERA5 for directional shear assessment, then, I have strong concern on this point. The directional shear is strongly affected by smaller scale phenomena which can be resolved by ERA5.
I do not think that simple (major/minor) revision of this manuscript can answer my concerns. I strongly recommend the authors to reconsider the structure of the paper clarifying the objective with enough justification of the methodology for the objective before submitting the manuscript again.
Followings are some more detailed points that also should be considered when rewriting the manuscript.
Page 7 Figure 2: What is the intention to show this figure. Do the authors want to discuss the difference between two measurement sites? Or discuss the appropriateness to use ERA5 data by comparing with lidar measurement? Better to modify the figure clarifying the intention of the figure.
Page 13 (Figure 5): The authors showed that BLH is the most important parameter to determine the directional shear, and LWT and profile type are not so important. However, in the later sections, the authors discuss the effect of LWT and profile type on the maximum directional shear. I do not see any clear logic in this discussion.
Page 7 (section 2.3): The definition of the “Maximum directional shear” is little unclear. Better to use equations to define this value. In addition, is there any existing studies which uses this definition, or is it an “invention” of the authors? In either case, this should be clarified. In the former case, better to refer appropriate documents. In latter case, the authors need more explanation and justification.
Page 17 (Section 3.4, Figure 8, Table 2, Validation of the ERA5 for maximum directional shear): If it is a “validation”, more quantitative discussion is needed. Why do the authors use a contingency table and avoid direct comparison of wind direction or maximum directional shear value? How much is the bias? How much is the RMSE? More straight forwardly, what is the accuracy of wind direction estimation of ERA5 data ?
Page 21 (Figure 10): The profile type “LLM” is missing in ERA5 data. I assume this is because of the low resolution of ERA5 data. But then, what is the meaning of this analysis? In addition, the information on the frequency of occurrence is missing making it difficult to understand which profile type is dominant and important.
In addition, the definition of these profile type are not very common, and would like the authors to show example shape of each profile for the sake of readers.
Citation: https://doi.org/10.5194/wes-2023-129-RC1
Christoffer Hallgren et al.
Christoffer Hallgren et al.
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