Comparing abnormalities in onshore  and offshore vertical wind profiles

Møller, Mathias; Domagalski, Piotr; Sætran, Lars Roar

doi:https://doi.org/10.5194/wes-5-391-2020

Articles | Volume 5, issue 1

https://doi.org/10.5194/wes-5-391-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Wind Energy Science Conference 2019

https://doi.org/10.5194/wes-5-391-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 5, issue 1

Research article

|

30 Mar 2020

Research article |

| 30 Mar 2020

Comparing abnormalities in onshore and offshore vertical wind profiles

Mathias Møller, Piotr Domagalski, and Lars Roar Sætran

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Final revised paper (published on 30 Mar 2020)
Preprint (discussion started on 30 Jul 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review for WES-2019-40', Anonymous Referee #1, 19 Aug 2019
- AC1: 'Reply to Anonymous Referee 1', Mathias Møller, 04 Nov 2019
RC2: 'Review of wes-2019-40', Anonymous Referee #2, 18 Oct 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Mathias Møller on behalf of the Authors (02 Dec 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (06 Dec 2019) by Sandrine Aubrun

RR by Anonymous Referee #1 (16 Dec 2019)

RR by Anonymous Referee #3 (04 Feb 2020)

Suggestions for revision or reasons for rejection

This paper presents an analysis of the wind speed vertical profiles observed at seven sites located in Northern Europe. Three of these sites are offshore (in the Baltic Sea) and 4 are onshore among which 3 are at less than 2 km from the coast. The goal is to characterize the deviation of the 10-minute averaged wind profile profiles from the shape expected applying the Monin-Obukhov similarity theory (MOST). An analysis of the differences between offshore and onshore sites is presented, as well as the correlations between these deviations, the wind speed at 100 m height, and the stability. Thus, the authors address an important question regarding the applicability of MOST on different kind of sites, which is particularly relevant for wind energy applications and wind engineering in general.

General comments
My general comments concern:
The representativeness of the onshore sites. Three sites are coastal and one is located in a forested region. It could have been interesting to include an onshore flat and open site in the analysis, which could have been a kind of reference regarding the applicability of MOST.
The methodology which has been applied, which consists in detecting local maximums or minimums in the wind speed profiles. These maximums or minimums are characterized by their number, without any consideration of their magnitude. Very weak inflections in the wind speed profile are not very significant (taking into account the uncertainties and the representativeness of the measurements) and will have no impact on wind turbines, and more generally won’t be relevant in wind engineering. A filtering could have been applied on the data set in order to retain only significant inflections in the analysis. The new paragraph on “inflection severity” answers partially to this weakness by providing the occurrence probability as a function of the inflection magnitude (for 1-inflection profiles only). But it shows that imposing a minimum of 0.2 m/s for this magnitude would remove a large part (up to 70-75% for some sites) of the detected inflections. I would suggest to take that into account in the conclusions and in the abstract. For example, I think that the proportion of inflected profiles which is mentioned for offshore sites (65-75%) is a bit misleading, due to the high occurrence of unstable cases for these sites, for which the inflections are most of the time not really significant and have probably no practical impact.
However this paper can still be considered as a useful contribution to the characterization of the wind vertical profiles.

Specific comments
Abstract: I don’t think we can say that “the occurrence of local maxima scales inversely to the roughness length”. This occurrence results from a combination of different physical processes among which other parameters (especially the stability) play an important role. Page 15 line 4 the authors mentioned that “the onshore occurrence of abnormal profiles is found to scale inversely with the distance to shore”, which is a different conclusion.
Paragraph 3.2: I am surprised that no direction sector was excluded from the analysis for the 3 FINO masts. For example, it is explained in paragraph 3.1.4 that for the FINO3 mast, only the anemometer installed in the direction 345° was used for consistency between vertical levels. Therefore a sector filtering should have been applied, especially regarding the very weak inflections of wind speed which are considered in the analysis.
Paragraph 3.3: Please mention the vertical levels that have been used to evaluate the stability. As mentioned by the authors, the distribution of stability was found by Argyle and Watson (2014) as very sensitive to the measuring heights used. In their paper, the occurrence of very unstable conditions was found to vary between about 38% to 70% depending on these heights. Thus it is difficult to state that there is a reasonable agreement between the two papers on this occurrence.
Page 19 lines 15-17: the sentence is not clear: if I understand correctly, the stability is not associated to the inflection height itself but to the time stamp corresponding to the occurrence of this inflection.
Page 20 line 11: the height at which the wind speed is considered should be mentioned
Page 21 lines 10-13: the sentence is not clear, please rephrase it.
Page 24 line 11: the decrease in very unstable conditions appears clearly for Skipheia, but is not so clear for Hovsore.

Technical corrections
p. 5 l. 12: I would suggest to define Nmax in a general way without reference to a Python function.
p. 11 l. 23: “an” should be replaced by “and”
p. 12 l. 21: “accuraccy should be replaced by “accuracy”
p. 17 l. 3: “the results of FINO1” should be replaced by “the results of FINO1 are”
p. 18 l. 1: “confirm that offshore sites to a larger degree experience local maximum in the wind profile than onshore sites” should be replaced by “confirm that offshore sites experience local maximum in the wind profile to a larger degree than onshore sites”
p. 19 l. 4: “the cause of this is result” should be replaced by “the cause of this result”
p. 21 l. 10: “specifically the the” should be replaced by “specifically the”
p. 24 in the figure 10 caption : “not shown” instead of “not show”
p. 26 l. 7: “over several height measurement” should be replaced by “over several measurement heights”
p. 27 l. 5: “the difference in speed at“ should be replaced by “the difference between the wind speed at”
P. 28 l. 7: If the paragraph “Wind direction of inflected profiles” is removed, please remove the title of this paragraph as well.
P. 28 l. 11: “has” should be replaced by “have”
p. 28 l. 13: “was” should be replaced by “were”
p.28 l. 27: I suggest to replace “spectrum” by “range”

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ED: Publish subject to minor revisions (review by editor) (05 Feb 2020) by Sandrine Aubrun

AR by Mathias Møller on behalf of the Authors (12 Feb 2020) Manuscript

ED: Publish as is (18 Feb 2020) by Sandrine Aubrun

ED: Publish as is (20 Feb 2020) by Carlo L. Bottasso (Chief editor)

AR by Mathias Møller on behalf of the Authors (28 Feb 2020) Manuscript

Short summary

This work has analyzed historical data of 10 min averaged wind speed measurements to investigate the accuracy of the commonly used equations for describing the wind velocity as a function of the height above ground. The results of analyzing data from a wide range of sites show that the common equations do not sufficiently describe certain physical phenomena, especially at offshore and coastal locations. The results imply that there is a need for more advanced models.