the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 May 2024
Meteorological Impacts of Offshore Wind Turbines as Simulated in the Weather Research and Forecasting Model
Abstract. Offshore wind energy projects are currently in development off the east coast of the United States and will likely influence the local meteorology of the region. Wind power production and other commercial uses in this area are related to atmospheric conditions, and so it is important to understand how future wind plants will change the local meteorology. We compared one year of simulations from the Weather Research and Forecasting model with and without wind farms incorporated, focusing on the lease area south of Massachusetts and Rhode Island. We assessed changes in wind speeds, 2 m temperature, surface heat flux, turbulent kinetic energy, and boundary layer height during different stability classifications and ambient wind speeds over the entire year. Because the wake behavior may be a function of boundary-layer stability, in this paper, we also present a machine learning algorithm to quantify the area and distance of the wake generated by the wind plant. This analysis enables us to identify the relationship between wake extent and boundary-layer height. Hub-height wind speed is reduced within and downwind of the wind plant, with the strongest impacts occurring during stable conditions and faster wind speeds. Wind speeds at 10 m increase within the wind plant area during stable conditions. Differences in 2 m temperatures and surface heat fluxes are small, but are largest during stable conditions and strong wind speeds. Turbulence kinetic energy (TKE) increases within the lease area with increasing wind speeds at both the surface and at hub height. At hub height, TKE increases do not depend on stability, but at the surface, TKE increases most during unstable conditions as the turbulence injected at hub height is mixed down to the surface. Boundary-layer heights increase within the wind plant, and decrease slightly downwind during stable conditions. Deeper boundary-layer heights, exceeding 100 m, tend to correlate with smaller wake areas and distances, though other factors likely also play a role in determining the extent of the wind farm wake.
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RC1: 'Comment on wes-2024-53', Anonymous Referee #1, 10 Jun 2024
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The manuscript presents an additional analysis of a subset of the simulations conducted previously by the team and published in Rosencrans et al. (2024). The current analysis uses only the simulations that were conducted with 100% of the added TKE by the turbines available, which was the default value in the Fitch parameterization in older versions of the WRF. The default value in new versions of the WRF is 25%, not 100%. There is no justification in the paper as to why the value of 25% was not used.
By contrast, it is known that using 100% causes an overestimation of added TKE at the grid cells with the turbines. For example:
- Eriksson et al. (2015) report excessive TKE by about a factor of 2 (their Figures 15 and 19);
- Abkar and Porté-Agel (2015) find that TKE is overestimated by 50%-200% using the 100% factor (their Figure 5);
- Pan and Archer (2018) find that using the 100% value causes an overestimation of at least a factor of two in several WRF simulations of commercial-scale offshore wind farms (their Fig. 6);
- Archer et al. (2022) propose the 25% value because using 100% causes an overestimation by up to 300% in TKE (their Figure 6, case 4).
As the current manuscript uses the 100% value, which is unjustified and likely to cause incorrect distributions of heat and momentum fluxes, the manuscript as is should not be published. The team needs to rerun the simulations using the 25% value or, at the very least, rerun the simulations with the 25% value for selected months (say one per season) and present a sensitivity analysis of the results.
Minor issues
- It is unclear why the runs named “d” were separated out and studied here. The wind speed (0-3 m/s) is below the cut-in wind speed of the turbines, thus there is no power extraction in these runs. The results should look identical to those of the NoWF, but they do not, possibly because some turbines may be experiencing wind speeds locally that are above the cut-in. Nonetheless, I do not see any value in the analysis of these results.
- Figure 4: do not show QKE but TKE. QKE is only used inside the WRF model but TKE is the well-known, physically-meaningful variable of interest. Same for the discussion in Section 4.4, focus on the real variable TKE, not on the WRF-specific QKE.
- Eq. 1: The flux should be the virtual heat flux, not just the heat flux, typo perhaps. Also, I assume this is calculated from the WRF output fields, but it is unclear which level(s) was chosen for the mean theta_v.
- Remove the sentence at line 225: your results are indeed the same as those of Rosencrans et al. (2024), you can’t say that they are confirming them.
- Explain why there is a slight increase in 10-m wind speed at the wind farms in stable conditions. What mechanism can cause this unusual finding? Is it possible that it has to do with the excessive TKE from the 100% coefficient?
- Line 294: is the downward heat flux is reduced, thus less heat comes down to the surface from the layer above it, how come “more heat is transferred to the surface”?
- Figure 13: the square patterns are concerning … if they are truly due to the fact that there are more turbines in the grid cells along the lines (line 321), then why are we not seeing a similar pattern in the wind speed deficits, only in the added TKE?
- All figures: add units.
Citation: https://doi.org/10.5194/wes-2024-53-RC1 -
AC1: 'Reply on RC1', Nicola Bodini, 10 Jun 2024
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Please see our responses in the attached document.
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RC2: 'Comment on wes-2024-53', Anonymous Referee #2, 17 Jun 2024
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Review of WES 2024 53
Meteorological Impacts of Offshore Wind Turbines as Simulated in the Weather Research and Forecasting Model
Daphne Quint et al
General comments:
This article explores the impact on a range of meteorological fields by the presence of large wind farm cluster off the USA east coasts using the WRF mesoscale model and using the WFP wind farm parameterization, using the 100% added TKE option.
Overall I find the article outlines a repeatable methodical approach and describes results, but lacks clarity on the motivation for the investigation and lacks discussion on the limitations of the method. What conclusions are to be drawn beyond describing the response of a model (WRF) in these “no wind farm” and “with wind farm” simulations? What research question is being asked? What is the hypothesis being tested? Please revise to address this.
The choice of one WRF set-up, one WRF wind farm parameterization, and one setting for the added TKE option, is a severe limitation of the article. It means that the whole paper becomes a description of model results, rather than focussing on what might actually happen in nature itself.
The manuscript needs to be revised to include a comprehensive discussion of the limitations of WRF-WFP, and what that might mean for the given results. Please include more justification for the model set-up, for example, why only a one year simulation? How might a longer period or different year impact the results?
I think there is a lack of physical mechanisms, and where mechanisms are conjectured, no model fields are used to back these up (see specific comments).
The paper several times states where results confirm what is already published, as a reader I would like more clarity on what are the most novel parts of the study and what led to these novel parts being of interest for investigation. Please revise to address this.
Latter sections seem a bit rushed. Adding to the limitation discussion, it would be good to include what would be good further studies to pursue, and what might be an approach to the difficult question of validation. Please revise to address this.
Specific comments:
L16: “exceeding 100 m” -> “exceeding 1000 m”?
L52: “extreme scale” , suggest changing this term. “Extreme” 10 years ago is not “extreme” today.
L81: The sentence “determine … how .. influence the local environment”, it should be reformulated to say this is modelled local environment being investigated, not the actual environment in nature.
L107: Please detail more about what is meant by “the model produced unrealistic wind speeds, … “. Please describe and state what it is that is unrealistic.
L96 and Table 1: Why was this period chosen?
L94 and Figure 1: Why was the domain chosen as it is? What is the reason for the far eastward extent?
Figure 3: It is strange to have a caption referring to a later caption.
L136: In the description of the BLH definitions, what happens in transitions from one stability condition to another, is there a discontinuity in the BLH? Could the authors use a sentence or to to justify the use of the approach of Olson et al (2019) for this analysis. What is the most relevant BLH determination for a wind farm do the authors think or recommend?
Table 2: It might be better to have “region 1” and “region 2” also part of the wind speed column in this table, to remind the reader of the reasoning behind the wind speed partitioning.
L162: I am a bit wary about this statement about the “tight coupling” because it suggests that everything can be explained by atmospheric stability, but there may be very important other aspects of the profile, and these might be overlooked by this approach. Please expand on the justification of the approach.
L166: “a leveling of the power production”, I think a better term here would be “the rated power production being reached and not increasing further”.
L167: “To isolate” , again similar to the L162 comment. It is not just wind speed that is varying, even though you keep stability and direction within a certain band. Please discuss other things in the profile that might vary, given this constraint on stability and direction.
L175: Why is 1 m/s deficit chosen as the measure of a wake? Why not other measures, such as relative deficit? What are the advantages of this measure, what is the impact of different wind speeds (NWF simulations) on this wake definition?
L178: Please explain why there are “not contiguous” wind speed perturbations, could they be related to the wake? How do you discount that there may be a distant response to the wind farm, perhaps oscillation in wake above and below the 1 m/s threshold that has been chosen.
L192: “ill defined” wakes. This seems a bit subjective to me, perhaps wakes are not neat and tidy as we might expect. Please justify. And is the 15.2% of hours with “ill defined” wakes not quite a significant share of the time?
L211: Please can the authors explain why the wake is compared in wind speed across the different stability classes? Is the mean wind speed the same for the different stability classes, if not, the difference in wake deficit can be partly due to this effect.
L214: The authors write “”due to increased mixing from aloft”, but this statement is not argued with data from the model, but appears to be more like a hypothesis for a possible, and plausible mechanism. Please justify the statement or rephrase it.
L219: It would help the reader to refer to region 3 next to the “above 11 m/s”.
L225-228: Does this effect also show when wind speeds are in the range 15 m/s - 25 m/s where the thrust is dropping significantly? See Fig 2b.
L233: Same question as above.
L240: “reduced more”, more than what? Does the deficit increase, or does the absolute wind speed reduce? It reads more like the latter, but I think it is the former.
L251: Please quantify “increase slightly”.
L260: Temperature increases by “around 0.05 degrees”. Is this significant?
L261-263: Are these statements conjecture or justified by model fields of fluxes? Please reformulate so it is clearer.
L289: The heading “heat flux”, please clarify what kind of heat flux is being looked at. Surface heat flux, vertical heat flux, sensible heat flux, etc, etc.
L407: The use of the word “promote” infers a causal relationship, is that what is meant?
Citation: https://doi.org/10.5194/wes-2024-53-RC2 -
CC1: 'Comment on wes-2024-53', Andrea Hahmann, 28 Jun 2024
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The comment was uploaded in the form of a supplement: https://wes.copernicus.org/preprints/wes-2024-53/wes-2024-53-CC1-supplement.pdfDisclaimer: this community comment is written by an individual and does not necessarily reflect the opinion of their employer.
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