Toward quantifying turbulent vertical airflow and sensible heat flux in tall forest canopies using fiber-optic distributed temperature sensing
- 1University of Bayreuth, Bayreuth, Germany
- 2Bayreuth Center of Ecology and Environmental Research, Bayreuth, Germany
- 1University of Bayreuth, Bayreuth, Germany
- 2Bayreuth Center of Ecology and Environmental Research, Bayreuth, Germany
Abstract. The paper presents a set of Fiber Optic Distributed Temperature Sensing (FODS) experiments to expand the existing microstructure approach for horizontal turbulent wind direction by adding measurements of turbulent vertical wind speed and direction, as well as turbulent sensible heat flux.We address the observational challenge to isolate and quantify the weaker vertical turbulent motions from the much stronger mean advective flow signals. In the first part of this study, we test the ability of a cylindrical shroud to reduce the horizontal wind speed while keeping the vertical wind speed unaltered. Shroud experiments were performed using two sonic anemometers and two pressure ports in an open experimental area over short grass, with one observing system located inside the cylindrical shroud, but without any fiber-optic (FO) cables. The flow statistics were compared across shroud configurations of different shapes, colors, rigidity, and porosity. The white insect screen shroud with the rigid structure and 0.6 m diameter was identified as the most promising setup in which the correlation of flow properties between shrouded and unshrouded systems is maximized, and the RMSE was significantly lower. The optimum shroud setup reduces the horizontal wind standard deviation by 35 %, has a coefficient of determination of 0.972 for vertical wind standard deviations, and a RMSE less than 0.018 ms-1 when comparing the shrouded to the unshrouded setup. Spectral analysis showed a fixed ratio of spectral energy reduction in the low frequencies, e.g., > 2 s, for temperature and wind components, momentum, and sensible heat flux. Unlike low frequencies, the ratios decrease exponentially in the high frequencies, which means the shroud dampens the high-frequency eddies with a time scale < 6 s, considering both spectra and cospectra together. In the second part, the optimum shroud configuration was installed around a heated fiber-optic cable with attached microstructures in a forest to validate our findings, but the analysis revealed a failure to isolate the magnitude and sign of the vertical wind perturbations from FODS. However, concurrent observations from an unshrouded part of the FODS sensor in the weak-wind subcanopy of the forest (12–17 m above ground level) yielded meaningful measurements of the vertical motions from coherent structures with distinct sweep and ejection phases. These signals allow for detecting the turbulent vertical airflow at least 60 % of the time, and 71 % when conditional sampling was applied. Comparison with vertical wind perturbations from sonic anemometry resulted in correlation coefficients of 0.35 and 0.36, which increased to 0.53 and 0.62 for conditional sampling. Evaluating the first direct sensible heat fluxes from FODS against those from the classic eddy covariance using sonic anemometry yielded an encouraging agreement in both magnitude and temporal variability for selected periods. This observational achievement is an important step toward developing a FODS-based flux sensor capable of resolving heat flux continuously across spatial and temporal scales.
Mohammad Abdoli et al.
Status: closed
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RC1: 'Comment on amt-2022-210', Anonymous Referee #1, 27 Oct 2022
This work consists of one of the first attempts to determine the structure of turbulence in forest areas using fiber-optic distributed temperature sensing against the traditional eddy covariance method. In my opinion, the technique is promising and, in the future, it can help in research involving turbulent exchanges between the forest and the atmosphere, considering that the temperature field is probably what controls these exchanges in these environments. I believe the paper has scientific merits and is within the scope of the journal.
- AC1: 'Reply on RC1', Mohammad Abdoli, 31 Oct 2022
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RC2: 'Comment on amt-2022-210', Anonymous Referee #2, 29 Oct 2022
This paper deals with a new technique to measure the turbulence inside canopis using a new technology. The authors described the idea/new instrument, put it at the their lab (outside field) for adjustments and then installed it inside a forest. They did not find good/nice results with the new apparatus and concluded that it has to be improved. Attached there are many others smallcomments/suggestions.
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AC2: 'Reply on RC2', Mohammad Abdoli, 12 Nov 2022
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-210/amt-2022-210-AC2-supplement.pdf
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AC2: 'Reply on RC2', Mohammad Abdoli, 12 Nov 2022
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RC3: 'Comment on amt-2022-210', Anonymous Referee #3, 04 Nov 2022
The authors present a very challenging approach to measure wind and sensible heat flux with fiber optic distributed temperature sensing. Unfortunately, the results of the study are not so promising as it appeared that the shroud did not work well to measure vertical wind speed. I appreciate the authors braveness to submit (partly) failed experiments. Also failed experiments can help the community to learn. Having said this, I think the current manuscript needs major revisions as it seems more a good draft then a full paper (yet). Problem statement and method section miss essential information and certain choices are not well explained. Additionally, I think that also many of the figures can be improved with less abbreviations and make them more self-explanatory (e.g., by giving the figure titles as 'setup 1, setup 2'. this would also reduce the caption lenght). In the attached pdf I commented in detail. Here I only indicate my main comments.
- The outline of the study is that the authors first investigate several shroud configurations on a grass field (EBG). The 'best' shroud is then later used in a follow-up experiment in a forest. However, in the method section there is barely any information on the different setups and why shroud color, mesh size, rigidity or shape would affect the measurements. What were the design criteria. This part should be extended and improved.
- Why are not all the shroud experiments (EBG) compared to the sonic (thus also setup 1 and 2)? Now the benchmark is the 'unshrouded' FODS measurements, which is also an experimental method. I would benchmarkt the shrouds to the sonic as this is likely closer to the truth.
- Base the first test, the authors pick 'the best shroud' setup to apply it in a forest. Only surprising change, is that 'suddenly' the shroud lenght is increased. While from study 1 the authors could have learnd that dimensions matter for the wind direction. This is in my view a major shortcomming of this paper.
- Despite the admitted 'failure' of the forest experiments, the authors still show the initial plan to calculate the sensible heat flux. But what is the value of this, once the wind speed measurements are not correct?
- The reference list contains 34 references, from which 16 are from the own research group. This is almost 50%! I highly recommend to put the study into a more broad context. Many other groups also worked in this study field, including groups that also work with FODS. In the attachement I added some suggestions.
I hope my comments help to improve the manuscript.
-
AC3: 'Reply on RC3', Mohammad Abdoli, 12 Nov 2022
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-210/amt-2022-210-AC3-supplement.pdf
Status: closed
-
RC1: 'Comment on amt-2022-210', Anonymous Referee #1, 27 Oct 2022
This work consists of one of the first attempts to determine the structure of turbulence in forest areas using fiber-optic distributed temperature sensing against the traditional eddy covariance method. In my opinion, the technique is promising and, in the future, it can help in research involving turbulent exchanges between the forest and the atmosphere, considering that the temperature field is probably what controls these exchanges in these environments. I believe the paper has scientific merits and is within the scope of the journal.
- AC1: 'Reply on RC1', Mohammad Abdoli, 31 Oct 2022
-
RC2: 'Comment on amt-2022-210', Anonymous Referee #2, 29 Oct 2022
This paper deals with a new technique to measure the turbulence inside canopis using a new technology. The authors described the idea/new instrument, put it at the their lab (outside field) for adjustments and then installed it inside a forest. They did not find good/nice results with the new apparatus and concluded that it has to be improved. Attached there are many others smallcomments/suggestions.
-
AC2: 'Reply on RC2', Mohammad Abdoli, 12 Nov 2022
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-210/amt-2022-210-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Mohammad Abdoli, 12 Nov 2022
-
RC3: 'Comment on amt-2022-210', Anonymous Referee #3, 04 Nov 2022
The authors present a very challenging approach to measure wind and sensible heat flux with fiber optic distributed temperature sensing. Unfortunately, the results of the study are not so promising as it appeared that the shroud did not work well to measure vertical wind speed. I appreciate the authors braveness to submit (partly) failed experiments. Also failed experiments can help the community to learn. Having said this, I think the current manuscript needs major revisions as it seems more a good draft then a full paper (yet). Problem statement and method section miss essential information and certain choices are not well explained. Additionally, I think that also many of the figures can be improved with less abbreviations and make them more self-explanatory (e.g., by giving the figure titles as 'setup 1, setup 2'. this would also reduce the caption lenght). In the attached pdf I commented in detail. Here I only indicate my main comments.
- The outline of the study is that the authors first investigate several shroud configurations on a grass field (EBG). The 'best' shroud is then later used in a follow-up experiment in a forest. However, in the method section there is barely any information on the different setups and why shroud color, mesh size, rigidity or shape would affect the measurements. What were the design criteria. This part should be extended and improved.
- Why are not all the shroud experiments (EBG) compared to the sonic (thus also setup 1 and 2)? Now the benchmark is the 'unshrouded' FODS measurements, which is also an experimental method. I would benchmarkt the shrouds to the sonic as this is likely closer to the truth.
- Base the first test, the authors pick 'the best shroud' setup to apply it in a forest. Only surprising change, is that 'suddenly' the shroud lenght is increased. While from study 1 the authors could have learnd that dimensions matter for the wind direction. This is in my view a major shortcomming of this paper.
- Despite the admitted 'failure' of the forest experiments, the authors still show the initial plan to calculate the sensible heat flux. But what is the value of this, once the wind speed measurements are not correct?
- The reference list contains 34 references, from which 16 are from the own research group. This is almost 50%! I highly recommend to put the study into a more broad context. Many other groups also worked in this study field, including groups that also work with FODS. In the attachement I added some suggestions.
I hope my comments help to improve the manuscript.
-
AC3: 'Reply on RC3', Mohammad Abdoli, 12 Nov 2022
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2022-210/amt-2022-210-AC3-supplement.pdf
Mohammad Abdoli et al.
Mohammad Abdoli et al.
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