the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2024
A Novel Assessment of the Vertical Velocity Correction for Non-orthogonal Sonic Anemometers
Abstract. Non-orthogonal sonic anemometers are used extensively in flux networks and biomicrometeorological research. Previous studies have hypothesized potential underestimation of the vertical velocity turbulent perturbations, necessitating correction to increase flux measurements by approximately 10 %, while some studies have refuted that any correction is needed. Those studies have used cross comparisons between sonic anemometers and numerical simulations. Here we propose a method that yields a correction factor for vertical velocity that requires only a single sonic anemometer in situ but requires some assumptions and adequate fetch at a sufficient distance above roughness elements where surface similarity is valid. Correction factors could be important in adjusting flux network and other flux data, as well as assessing the energy budget closure that is used as one of the flux data quality measures. The correction factor is confirmed in one field experiment and comparison between a CSAT3 and RMY 81000VRE, but it does not work well for the more complex form factors shown in a field comparison of an IRGAson and a CSAT3a.
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CC1: 'Comment on amt-2024-152', Gerhard Peters, 25 Oct 2024
4 comments:
1
In Table 1 there are 4 entries on Metek sonics (USA-1 and uSonic-3) with contradicting correction factors. Small correction factors were found by Mauder and Zeeman, 2017 (M), and Beyrich et al. 2002 (B). Large correction factors were allegedly found by Horst et al., 2015 (H), and Pena et al., 2019 (P). While M and B evaluated the standard products provided by the sonic, the last two citations are misleading:
- H investigated the implemented flow distortion algorithm theoretically and confirmed it basically. The cited numbers in Table 1 (22 – 32%) are not “vertical velocity correction factors” but the expected maximum shadow for the case of flow along a sound path. This is quite a different object.
- P examined the flow distortion algorithm implemented in USA-1 sonics in field experiments with the result of satisfying quality. The cited number is related to the impact of the algorithm. By the way, they used similar symmetry properties of turbulence parameters as in this paper, in a well-defined restricted regime, to check the soundness of the standard sonic outputs.
2
A flow distortion correction should be conceptually superior to a gross correction of sigma_w using turbulence symmetry properties, because the applicability of these symmetries depend on quite restrictive assumptions.
3
The basic assumption that mainly the vertical wind component is subject to transducer shadowing while the horizontal components are more or less unaffected may be challenged. Whether or not this assumption is valid depends on the nature of the wind vector variation. For clarity we may consider two extreme mechanisms:
- The wind inclination varies, while the speed is constant. In this case an angle dependent shadowing can (not necessarily) lead to a truthful variation of the horizontal component while the variation of vertical component is affected.
- The speed varies, while the inclination is constant. In this case we expect in first order a linear shadow effect on the speed, and hence on the speed variation. In this case all components of the wind vector are affected by the same factor.
The reality is of course a mixture of 1 and 2.
4
Multipath sonics were developed in the meantime in order to avoid the issue addressed.
Citation: https://doi.org/10.5194/amt-2024-152-CC1 -
RC1: 'Comment on amt-2024-152', Anonymous Referee #1, 30 Oct 2024
The manuscript presents an interesting investigation to derive a simple, yet effective, method to correct instrumental errors from sonic anemometers in the measurement of the vertical velocity component. The manuscript is well-structured and written, but I have some major queries related to the methodology and to which extent this can be applied. Please find them below.
- Table 1: It is worth reporting the method used to derive the correction factor in the notes column, dividing sensor-to-sensor comparisons from sensors-to-model, wind tunnel from outdoor experiments, and specifying which conditions have led to different estimations of the correcting factor for the same anemometer.
- The determination of the near-neutral stability range should account also for the horizontal wind components and not only the vertical ones. Also, if the horizontal wind components are not distorted as claimed, they should identify the near-neutral range more precisely. I was also expecting the near-neutral stability range across z/L=0, like having |L|<500 or |z/L|<0.05 as it is mostly observed in the literature, but that seems not the case. Is this a characteristic of the sites? Does it affect your correction method or limit the applicability to this specific location?
- Given the different site's data are collected from, did you apply a single pre-processing technique (like for despiking, computation of fluctuations and covariances, etc.)? Does this technique involve EddyPro like in the case of the CSAT+IRGAson eddy covariance station? Do you expect the preprocessing differences, if any, can be responsible for the success or unsuccess of the correction method? What about the different sampling rates?
- One of the assumptions of the similarity theory requires the flow to be in a steady state, at least statistically. How did you ensure that? Can unsteadiness be an additional source of error for the vertical velocity?
- Given the results in Fig. 5, how robust is your correction to different averaging periods?
Citation: https://doi.org/10.5194/amt-2024-152-RC1 -
CC2: 'Comment on amt-2024-152', Ebba Dellwik, 30 Oct 2024
It is really nice to see a new solid study regarding sonic anemometer accuracy, reviving the important topic of flow distortion corrections.
As a follow-up to the comment by Gerhard Peters, I would like to clarify that we, in our study (Pena et al 2019), suggested a different way to judge whether the sonic anemometer observations were affected by flow distortion/transducer shadowing errors without comparison to other instruments. Our idea was to study the ratio of the power spectral density in the inertial subrange for the observations. Based on theory, this ratio should be 4/3 for the w/u velocity component ratio. By testing the Metek USA1 sonic with and without the manufacturer's correction we found a strong difference; and that, by including the flow distortion correction, the ratio was close to 4/3. We also tested the CSAT sonic anemometer with and without flow distortion corrections and found that the ratios were consistently different from 4/3, which would indicate that the instrument was still in need of a correction. Different from your study, we argued that a disagreement with the 4/3 ratio could not be used directly for correcting the fluxes and variances, since the error likely is split between the horizontal and vertical components.
I think it would be interesting if you elaborated on the relative pros and cons of these methods and also commented on the different interpretations.
Best regards
Ebba Dellwik
Citation: https://doi.org/10.5194/amt-2024-152-CC2 -
RC2: 'Comment on amt-2024-152', Anonymous Referee #2, 31 Oct 2024
General comments: As other recent papers have tried to quantify the bias in the vertical wind component of non-orthogonal sonic anemometer configurations this paper, using empirical evidence on the nature and structure of turbulence statistics (i.e. 𝛔w/u*) demonstrates how correction factors for various sonic anemometer types can be derived. Certain assumptions have to be made regarding representativeness and homogeneity of the landcover where these observations were obtained, as well as the normal suite of conditions that are satisfied in order to obtain the results presented here.
Specific Comments:
- In the filtering of the ratio, were there any mean wind speed thresholds or filters that were used in the determination of the correction factors? Typically, the lower the wind speed, the more variable the wind direction can become and the determination of u* can become more noisy.
- Is there some sense of the uncertainty of 1.25 as the ratio? I have seen reported values ranging from 1.21 to 1.3 in various papers. What is the sensitivity of the magnitude of this ratio in the determination of the correction factors? Also, once the correction factor was determined, this should then be used to correct the raw value of u*, which of course will affect the determination of the stability parameter z/L since L is a function of u*3. For example, a 10% error in u* will affect the magnitude of the Monin-Obukhov Length by over 30%.
- In Table 3, it would be very helpful to add statistical uncertainty values or confidence limits for the various correction factors.
Citation: https://doi.org/10.5194/amt-2024-152-RC2
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