The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5

Hallgren, Christoffer; Körnich, Heiner; Ivanell, Stefan; Vakkari, Ville; Sahlée, Erik

doi:https://doi.org/10.5194/wes-2023-129

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https://doi.org/10.5194/wes-2023-129

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16 Oct 2023

| 16 Oct 2023

Status: this preprint has been withdrawn by the authors.

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, and 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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Received: 26 Sep 2023 – Discussion started: 16 Oct 2023

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Christoffer Hallgren, Heiner Körnich, Stefan Ivanell, Ville Vakkari, and Erik Sahlée

Interactive discussion

Status: closed

RC1:
'Comment on wes-2023-129', Anonymous Referee #1, 27 Oct 2023

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
- AC1: 'Reply on RC1', Christoffer Hallgren, 04 Feb 2024
  
  Dear reviewer,
  Thank you for your time and review of our manuscript, we agree with your comments. Restructuring of the manuscript is needed and we also think that the manuscript would benefit from a more in-depth analysis and validation. We have several ideas on how to do this. However, the first author of the manuscript is leaving academia in just a few weeks, and thus we have asked the editor for an open extension of the deadline. We need to re-distribute the work within the group which will take time.
  We hope for your kindest consideration with the situation at hand.
  On behalf of all co-authors,
  Christoffer Hallgren
  
  Citation: https://doi.org/10.5194/wes-2023-129-AC1
RC2:
'Comment on wes-2023-129', Anonymous Referee #2, 08 Dec 2023

The manuscript provides insight into the occurrence and environmental influences on directional shear across the rotor-swept area using observations and ERA5 for locations in the Baltic Sea. The authors have chosen a relevant and important characteristic to evaluate, especially as wind turbine rotor diameters continue to increase. Additionally, given the widespread employment of ERA5 for wind resource assessment and as an input to higher-resolution models, the validation of ERA5 performance for directional shear is welcome. The graphics are clear and very nicely arranged.
I recommend restructuring the results section of the manuscript to begin with (and perhaps enhance) Sections 3.3 (observations of directional shear) and 3.4 (validation of ERA5 for directional shear). These sections, particularly 3.4, are very well explored and provide the greatest value to the audience.
Chronologically, it makes sense to begin the results section with the observation-based results (3.3), then use them to validate ERA5 for the temporal period covered by the observations (3.4), and then proceed with the 43-year climatology (3.1) and predictor study (3.2) if ERA5 was to validate well, which, in the case of directional shear over the authors’ study area, it does not appear to do so. So, while I found the ERA5-based climatology and predictor study to be interesting, particularly the latter, I question whether these analyses should be included.
I feel that there is still great value in this manuscript for sharing the observational results and performance evaluation of ERA5. A few detailed suggestions follow, some of which may become irrelevant if the authors choose to restructure the manuscript.
Line 1: Is there a word missing after “affects both the?” Suggest rewording to something like “affects two major influences on power production, wake effects and aerodynamic loading.”
Line 112: It would be helpful to cite and discuss previous validations of ERA5’s BLH here, such as Madonna et al. (2021) and Guo et al. (2021).
Line 221: Same point as above. How well do you expect ERA5 to perform in representation of sea level pressure and subsequent categorization of synoptic conditions?
Line 241: Are the ERA5 points offshore or land-based?
Line 257: I suggest rewording this sentence, and others at regular intervals throughout the ERA5-based results section, to remind the audience that ERA5 is implying these findings as opposed to them observationally occurring. I feel that it is important, given that ERA5 is not performing strongly in directional representation versus observations, to regularly remind the reader that these results are model-based and may not be indicative of actual conditions. Line 310 does a wonderful job at including such a comparison.
Section 4: I recommend relating your findings back to impacts on wind power production.
References
Guo, J., et al.: Investigation of near-global daytime boundary layer height using high-resolution radiosondes: first results and comparison with ERA5, MERRA-2, JRA-55, and NCEP-2 reanalyses, Atmospheric Chemistry and Physics, 21(22), 17079-17097, https://doi.org/10.5194/acp-21-17079-2021, 2021.
Madonna, F., et al.: Assessment of Trends and Uncertainties in the Atmospheric Boundary Layer Height Estimated Using Radiosounding Observations over Europe, Atmosphere, 12(3), 301, https://doi.org/10.3390/atmos12030301, 2021.

Citation: https://doi.org/10.5194/wes-2023-129-RC2
- AC2: 'Reply on RC2', Christoffer Hallgren, 04 Feb 2024
  
  Dear reviewer,
  Thank you for your time and review of our manuscript, we agree with your comments. Restructuring of the manuscript is needed and we also think that the manuscript would benefit from a more in-depth analysis and validation. We have several ideas on how to do this. However, the first author of the manuscript is leaving academia in just a few weeks, and thus we have asked the editor for an open extension of the deadline. We need to re-distribute the work within the group which will take time.
  We hope for your kindest consideration with the situation at hand.
  On behalf of all co-authors,
  Christoffer Hallgren
  
  Citation: https://doi.org/10.5194/wes-2023-129-AC2
AC3: 'Comment on wes-2023-129', Erik Sahlée, 20 May 2024

Dear reviewer,
Given the circumstance that the primary author has departed from academia, additional time is required to refine these findings. Therefore, we must currently withdraw our existing manuscript with the intention of resubmitting it, showcasing the improved results in the future.
Kind regards,
Erik Sahlée

Citation: https://doi.org/10.5194/wes-2023-129-AC3

Interactive discussion

Status: closed

RC1:
'Comment on wes-2023-129', Anonymous Referee #1, 27 Oct 2023

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
- AC1: 'Reply on RC1', Christoffer Hallgren, 04 Feb 2024
  
  Dear reviewer,
  Thank you for your time and review of our manuscript, we agree with your comments. Restructuring of the manuscript is needed and we also think that the manuscript would benefit from a more in-depth analysis and validation. We have several ideas on how to do this. However, the first author of the manuscript is leaving academia in just a few weeks, and thus we have asked the editor for an open extension of the deadline. We need to re-distribute the work within the group which will take time.
  We hope for your kindest consideration with the situation at hand.
  On behalf of all co-authors,
  Christoffer Hallgren
  
  Citation: https://doi.org/10.5194/wes-2023-129-AC1
RC2:
'Comment on wes-2023-129', Anonymous Referee #2, 08 Dec 2023

The manuscript provides insight into the occurrence and environmental influences on directional shear across the rotor-swept area using observations and ERA5 for locations in the Baltic Sea. The authors have chosen a relevant and important characteristic to evaluate, especially as wind turbine rotor diameters continue to increase. Additionally, given the widespread employment of ERA5 for wind resource assessment and as an input to higher-resolution models, the validation of ERA5 performance for directional shear is welcome. The graphics are clear and very nicely arranged.
I recommend restructuring the results section of the manuscript to begin with (and perhaps enhance) Sections 3.3 (observations of directional shear) and 3.4 (validation of ERA5 for directional shear). These sections, particularly 3.4, are very well explored and provide the greatest value to the audience.
Chronologically, it makes sense to begin the results section with the observation-based results (3.3), then use them to validate ERA5 for the temporal period covered by the observations (3.4), and then proceed with the 43-year climatology (3.1) and predictor study (3.2) if ERA5 was to validate well, which, in the case of directional shear over the authors’ study area, it does not appear to do so. So, while I found the ERA5-based climatology and predictor study to be interesting, particularly the latter, I question whether these analyses should be included.
I feel that there is still great value in this manuscript for sharing the observational results and performance evaluation of ERA5. A few detailed suggestions follow, some of which may become irrelevant if the authors choose to restructure the manuscript.
Line 1: Is there a word missing after “affects both the?” Suggest rewording to something like “affects two major influences on power production, wake effects and aerodynamic loading.”
Line 112: It would be helpful to cite and discuss previous validations of ERA5’s BLH here, such as Madonna et al. (2021) and Guo et al. (2021).
Line 221: Same point as above. How well do you expect ERA5 to perform in representation of sea level pressure and subsequent categorization of synoptic conditions?
Line 241: Are the ERA5 points offshore or land-based?
Line 257: I suggest rewording this sentence, and others at regular intervals throughout the ERA5-based results section, to remind the audience that ERA5 is implying these findings as opposed to them observationally occurring. I feel that it is important, given that ERA5 is not performing strongly in directional representation versus observations, to regularly remind the reader that these results are model-based and may not be indicative of actual conditions. Line 310 does a wonderful job at including such a comparison.
Section 4: I recommend relating your findings back to impacts on wind power production.
References
Guo, J., et al.: Investigation of near-global daytime boundary layer height using high-resolution radiosondes: first results and comparison with ERA5, MERRA-2, JRA-55, and NCEP-2 reanalyses, Atmospheric Chemistry and Physics, 21(22), 17079-17097, https://doi.org/10.5194/acp-21-17079-2021, 2021.
Madonna, F., et al.: Assessment of Trends and Uncertainties in the Atmospheric Boundary Layer Height Estimated Using Radiosounding Observations over Europe, Atmosphere, 12(3), 301, https://doi.org/10.3390/atmos12030301, 2021.

Citation: https://doi.org/10.5194/wes-2023-129-RC2
- AC2: 'Reply on RC2', Christoffer Hallgren, 04 Feb 2024
  
  Dear reviewer,
  Thank you for your time and review of our manuscript, we agree with your comments. Restructuring of the manuscript is needed and we also think that the manuscript would benefit from a more in-depth analysis and validation. We have several ideas on how to do this. However, the first author of the manuscript is leaving academia in just a few weeks, and thus we have asked the editor for an open extension of the deadline. We need to re-distribute the work within the group which will take time.
  We hope for your kindest consideration with the situation at hand.
  On behalf of all co-authors,
  Christoffer Hallgren
  
  Citation: https://doi.org/10.5194/wes-2023-129-AC2
AC3: 'Comment on wes-2023-129', Erik Sahlée, 20 May 2024

Dear reviewer,
Given the circumstance that the primary author has departed from academia, additional time is required to refine these findings. Therefore, we must currently withdraw our existing manuscript with the intention of resubmitting it, showcasing the improved results in the future.
Kind regards,
Erik Sahlée

Citation: https://doi.org/10.5194/wes-2023-129-AC3

Christoffer Hallgren, Heiner Körnich, Stefan Ivanell, Ville Vakkari, and Erik Sahlée

Supplement

https://doi.org/10.5194/wes-2023-129-supplement

Christoffer Hallgren, Heiner Körnich, Stefan Ivanell, Ville Vakkari, and Erik Sahlée

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Oct 2023	132	57	6	195
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Feb 2024	38	11	4	53
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Short summary

Sometimes, the wind changes direction between the bottom and top part of a wind turbine. This affects both the power production and the loads on the turbine. In this study, a climatology of pronounced changes in wind direction across the rotor is created, focusing on Scandinavia. The weather conditions responsible for these changes in wind direction are investigated and the climatology is compared to measurements from two coastal sites, indicating an underestimation by the climatology.


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