the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
On the peculiar polarimetric signatures backscattered by a still or quasi-still wind turbine acquired by an X-band radar in stare mode at high temporal resolution (64 ms): preliminary investigations
Marco Gabella
Martin Lainer
Daniel Wolfensberger
Jacopo Grazioli
Abstract. A still wind turbine (WT) observed with a fixed pointing radar antenna shows peculiar polarimetric signatures: during two minutes (from 17:08 to 17:10 UTC) of the first day (March 4, 2020) of the WT MeteoSwiss X-band radar campaign in Schaffhausen, the copolar correlation coefficient between the two orthogonal polarization states was persistently equal to 1. The reflectivity at vertical polarization was bounded between 38.5 and 41.5 dBz; however, the changes between two consecutive 64 ms values (retrieved by means of 128 transmitted pulses) were either 0 dBz or ±0.5 dBz. The 2-min median (mean) value was 40.0 (39.9) dBz over the 1875 echoes of this interval. The reflectivity at horizontal polarization was persistently equal to 56.5 dBz, which means no change exceeding ±0.25 dBz. The standard deviation (1874 degrees of freedom) of the differential phase shift, which in the absence of precipitation was, in fact, coincident with the dispersion of the differential backscattering phase shift, was as small as 3.0°. During two 10-min intervals (17:10–17:20 UTC and 17:30–17:40 UTC) the rotor has moved less than 1 revolution; however, this slow movement together with a change in blade angles and nacelle orientation was sufficient to cause large changes and significant variability in the polarimetric signatures, with two pairs of ZH consecutive values reaching the extreme of 78.5 dBz, which is the absolute reflectivity maximum reached in the whole campaign (March 4–21, 2020). Between 17:20 and 17:30 UTC, the rotor has accomplished 22.5 revolutions: the variability becomes smoother and softer in the central part of the interval (probably thanks to uniform rotor speed and “frozen” blade angles and nacelle orientation). It is desirable and recommended to extend this preliminary (32-minute) analysis (based on thirty thousand polarimetric measurables) to several other 10-min intervals with zero rotor speed.
Marco Gabella et al.
Status: final response (author comments only)
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RC1: 'CommeComment on "On the peculiar polarimetric signatures backscattered by a still or quasi-still wind turbine acquired by an X-band radar in stare mode at high temporal resolution (64 ms): preliminary investigations"nt on amt-2022-316', Anonymous Referee #1, 06 Jan 2023
As with the earlier paper from Lainer et al. (2021) it was a fun for me to read this manuscript. It is the first time I find my review to be cited in the next publication :-).
I strongly recommend to publish its content. Nevertheless, some improvements are necessary and among them are important issues.
The most important issue is the interpretation of the presented measurements. I do not see anything "peculiar". Let me describe my point of view a bit more detailed:
A WT is a scatterer that is neither small compared to the radar wavelength nor is it small compared to the diameter of the main lobe of the radar beam. It is - in general - not of constant shape but changing its properties with (i) nacelle orientation, (ii) rotor angle, and (iii) blade angle. The shape of the rotor blades even changes with (iv) wind speed, as the blades are bended by the wind. The echo "seen" by the radar further depends on the (v) elevation under which the radar "looks" at the WT and the exact (vi) height and (vii) horizontal position and (viii) the diameter of the radar beam at WT position. Furthermore, the (ix) position of the WT within the recent range gate of the radar has to be considered. --- There are more but minor dependencies that impact the echo from a WT, as the distance between radar and WT which is implicitly included in (v) to (ix) but further indicates how well the radar beam can be aproximated by a plane wave.
To give a more intuitive description I cite an engineer who once told me: Imagine the WT was coated with polished chromium and you light the WT with a spotlight. You see the reflections gliding over the surface of the WT, occurring and vanishing with the motion of the WT. At visible wavelengths the surface of a WT is mat but at radar wavelengths it appears to be glossy.
For the antenna we call the dependency on azimuth and elevation its directivity pattern. We know, the larger the antenna the stronger the (possible) gradients of the directivity pattern. For the scatterer the corresponding term is "differential scattering cross section." Which, in the end, is nothing else but the directivity pattern of a scatterer. The differential scattering cross section of a WT is at least(!) dependent on the nine parameters mentioned above (i to ix). As the WT is much larger than the radar antenna, we have to expect very strong gradients of the differential scattering cross section to occur.
The presented study investigates variation due to the first four parameters, keeping all radar related parameters constant. The stability of the echoes during periods where the WT is standing still (condition "a" in the discussion) indicates that WT and radar are very reliable. The variations of the echoes of different "type a" periods simply show the dependency on rotor angle and blade angle. As these two angles are random but constant the measured values are random but constant.
For a slow rotating rotor the experiment measured the differential scattering cross section at high resolution, mostly regarding rotor angle. We see all the extreme values. With increasing rotational speed (and constant temporal resolution) the angular resolution at which we see the crossection is reduced/coarsened. Thus the extreme values are smoothed out, everything looks smoother. This is immediately seen in the figures.
On the other hand: rotor speed is totally unimportant for an instantaneous (single) radar beam and its echo. The integration over several pulses (here 128) introduces changes in the echoes due to rotational speed.
There is nothing peculiar but the scattering cross section of a WT is complicated. So, please, shorten the title and remove the term "peculiar". (E.g.: "On the polarimetric backscatter of a still or quasi-still wind turbine.")
Dealing with the partially very precise time information is difficult and inconvenient. I propose to add two different indicating schemes:
1. Mark the four 10-min periods for which you have WT properties as I to IV in the figures. (Introducing e.g. black vertical markers at 17:10, 17:20, 17:30 and creating the four different "WT time steps".)
2. Mark those periods with comparable rotational speed and blade angles as indicated as a) through d) in the discussion by e.g. blue vertical markers and indicate the periods as a_1, a_2, a_3, b_1, and so on.
Most of the precise time indicators in the text could be replaced by these indications of time periods. The markers can occur in the figures 2 to 5. Figure 6 and 7 should then be assigned to the corresponding periods.
The authors expect the differential reflectivity to be close to 0 dB (line 435: "easier to understand"). If we recall that photographers use a polarizing filter to reduce reflections on (glossy) surfaces we know that reflections at (glossy) surfaces may introduce polarization effects. Especially, multiple reflections (internally, only from the WT) will cause strong polarization of the backscattered signal. (Review also Line 387 f.)
Minor remarks:The abstract shows already very detailed information which is not necessary. If the authors insist on having these details in the abstract, they should add the distance between radar and WT.
Gabella et al. (2008) (line 90), Gabella and Perona (1998) (line 92), and the book by Fabry (line 191) do not show up in the references. I did not check more entries but obviously the references have to be controlled.
In line 108 it needs to be 180 m x 180 m x 75 m.
Line 182: remove one "that"
Line 373: red curve in Fig. 2 (not inf)
Line 412: Shouldn't it be "It could have been caused"?
Line 430: The comma is falsely shifted to line 431.
Line 452: Remove "have"
Line 473f: Use Z_v as introduced in 2.3.1 and not ZV. (Same for ZH)
Citation: https://doi.org/10.5194/amt-2022-316-RC1 - AC1: 'Reply on RC1', Martin Lainer, 11 Apr 2023
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RC2: 'Comment on amt-2022-316', Anonymous Referee #2, 05 Mar 2023
General Comments
The topic of this study is both interesting and timely as the weather radar community seeks to find mitigating actions to cope with the increasing number and size of wind turbines. One of them is to understand how the wind turbines are seen in the observations and can this information be used to identify and remove the wind turbine echoes as simultaneously keeping the precipitation echoes. The polarimetric variables has generally proven to be a useful metric for the classification of clutter and there have not earlier been many studies of the polarimetric signatures, one comes to mind Hall et al. 2017, where the dual-pol variables are used to classify the wind turbine echoes at C-band with a fuzzy logic - based methodology.
I find this study important, and it provides new insights that can be used to develop classification algorithms. However, in my opinion, this manuscript and the presented research are not yet scientifically mature enough to be published, and I would recommend major changes. The authors acknowledge that these are preliminary investigations in the title, and I would encourage them to analyze their dataset more thoroughly to provide more conclusive results, including the second period of still wind turbines (March 19, 2020), which would strengthen their findings.
As noted by Anonymous Referee #1, the title is quite complex, and I agree that it could be shortened. Additionally, the use of the word "peculiar" to describe the polarimetric signatures is confusing, and I suggest using another adjective, such as "distinctive." Checking the Merriam-Webster dictionary the word peculiar is defined as characteristic of only one person, group, or thing: DISTINCTIVE or different from the usual or normal: ECCENTRIC, UNUSUAL. I assume the authors have meant the first interpretation, but I and I assume also the other Anonymous Referee #1 interpreted the second option and it was slightly confusing to read the manuscript. The language throughout the manuscript should be checked for any phrases or words that are more appropriate for spoken language than written language. While I am not a native speaker, I can provide some examples in the minor comments section.
In the detailed theoretical explanations or definitions, such as in section 2.3, the authors should pay attention to using precise definitions. For example, the authors should distinguish between radar reflectivity and reflectivity factor, which have different dimensions. I suggest providing an exact definition of these terms in the manuscript and then stating that the authors will use reflectivity to refer to reflectivity factor throughout the manuscript, as is common practice in the field.
Major Comments
As Anonymous Referee #1 noted, the abstract is way more detailed with the specific numbers. I would suggest rewriting the abstract by firstly providing a brief description of the measurement setup and then main conclusions without referring to specific periods.
The suggestions of Referee #1 were good to clarify the periods of interest, the authors should name them and indicate them in the figures. Referring to the chosen names in the text will make the manuscript easier to read.
Line 42: The authors state that research on polarimetric signatures of wind turbines is rare, which is true. However, it would be helpful to see a comparison with at least one other study, such as the one by Hall et al. 2017
In lines 80-100, a schematic picture showing how the radar is located in respect to the wind turbine with the distances and stated elevation angles would be beneficial.
As stated in general comments, especially the section 2.3.1 (lines 156-171) should be rephrased with correct terminology. The authors should be careful when using reflectivity and reflectivity factor, and "Log-transform" should be changed to e.g. "in logarithmic units." The lines 166 -171: The authors should clarify the explanation of the range of reflectivity values that can be measured with Meteo Swiss radars, as it is currently unclear and DN is not defined.
The authors should explain why they performed an extrapolation of 8 minutes in lines 251-253.
In lines 296-302, the authors should rephrase the section and add references to the correct figures.
Minor Comments
Lines 51 – 64: Section about the BS. I do see the need to explain BS in general, but I cannot really see how a wind turbine could be used for monitoring hardware due to its varying signatures, at least operationally, and now the section reads as to justify the campaign set up by using then wind turbines as BS. Maybe considering rephrasing this section.
Line 59: Clarify the meaning of "hit" in the context of the sentence, "However, since it is hit during the operational weather scan program…."
Lines 65 – 78: Rephrase this section and provide a brief description of the manuscript structure rather than providing detailed results.
Line 100: Consider changing the word "peculiar" when referring to the stare-mode strategy.
Line 116: Rephrase the title: "Wind turbine data and metadata collection: a very peculiar 40 min interval under detailed investigation."
Lines 121 – 123: When listing parameters, use "e.g." instead of "…", and write temperature with a lowercase letter.
Line 135: Rephrase "Unimportant if during almost 8 minutes no stare mode radar data are available" as it is unclear.
Line 139: Use a lowercase letter for "maximum."
Line 145: Rephrase the sentence to make it complete.
Check that equations are styled consistently throughout the manuscript.
Line 185: Remove "very" in "A very important…."
Line 191: Clarify if the numbering "e06.1" is referring to a chapter.
Lines 208 – 211: Rephrase the example of quantization of co-polar correlation coefficient as it seems redundant and not necessary. Remove the extra "use" in line 209.
Line 232: Clarify the meaning of "a standard deviation of 360°/1200.5 would be expected."
Lines 235-238: Rephrase and remove the detailed results in this section.
Line 242: Suggested to use "radar variables" instead of "backscattering properties."
Line 251: Remove "amazing" as it is not typically used in a scientific context.
Line 277: Remove "very" in "at the original (very high) temporal resolution."
Line 280: Remove "impressive" or replace it as it is not typically used in a scientific context.
Lines 307 and 336: Replace "remarkable" as it is not typically used in a scientific context.
Line 308: Replace "huge" as it is not typically used in a scientific context.
Line 371: Use a lowercase letter for "maximum."
Paragraph 421 – 433: Remove "very" statements and avoid using IF and THEN in capital letters in this section as it is not suitable for a scientific context, in my opinion.
In the Summary section, rephrase lines 472 – 477 without the exclamation marks and questions such as "How comes?" and "Well, because, to our great surprise."
In Figures and their corresponding captions (Figures 2. - 5.), could you clarify why "MAX" is written in capital letters while the other terms such as median, mode, and minimum are written in lowercase letters. I suggest to state somewhere in the text that the mean is not shown in these figures, since the text is often referring to mean.
Hall, W. et al. (2017), Offshore wind turbine clutter characteristics and identification in operational C-band weather radar measurements. Q.J.R. Meteorol. Soc., 143: 720-730. https://doi.org/10.1002/qj.2959
Citation: https://doi.org/10.5194/amt-2022-316-RC2 - AC2: 'Reply on RC2', Martin Lainer, 11 Apr 2023
Marco Gabella et al.
Marco Gabella et al.
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