the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Nov 2023
Assessment of the Webb-Pearman-Leuning Correction Method for Estimating CO2 Flux in a Tropical Coastal Sea
Abstract. CO2 fluxes in coastal waters are vital for the global carbon cycle. The Eddy Covariance technique was used with open-path gas analyzers to estimate CO2 fluxes. However, these analyzers can lead to overestimation due to water vapor and temperature effects, and the Webb-Pearman-Leuning (WPL) correction method was applied to improve the accuracy of the estimated CO2 flux. This study investigates the application of the WPL correction method on CO2 flux measurements over a tropical coastal sea location. The analysis reveals that the CO2 flux in the coastal waters mainly functions as a sink, with the diel cycle showing smaller flux magnitudes during the day and increased uptake during the night. The application of the WPL correction can result in sign changes of CO2 flux, indicating a shift from a CO2 sink to a CO2 source. These sign changes occur frequently, particularly during afternoon hours, and can significantly impact the implications regarding carbon exchange. The WPL correction parameters, especially those related to temperature and water vapor fluctuations, play crucial roles in influencing the CO2 flux variations. The decrease in dry air molar density and increased vertical wind speed within the correction related to water vapor fluctuations are the major reasons for the sign change of the CO2 flux. Based on the quality flagging of the WPL correction, the non-sign change CO2 fluxes are predominantly considered reliable data, while most of the sign change fluxes should be specially checked.
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RC1: 'Comment on egusphere-2023-2383', Anonymous Referee #1, 19 Nov 2023
This paper evaluates the Webb-Pearman-Leuning (WPL) correction for eddy covariance (EC) measurements of CO2 fluxes. The WPL correction is an important technique and crucial for the accuracy of flux measurements. The manuscript is in general well organized and presented. My biggest concern of this study, however, is that the results of WPL correction (Figs. 3-7), albeit rather detailed, is solely the difference between fluxes “with” and “without” the WPL correction, which lacks the “ground truth” to be compared with. In other words, even the authors have detailed knowledge of the sign changes, and the dominant terms in WPL correction, there is no way to assess if the WPL correction actually improves the accuracy of CO2 measurements or makes it worse. To justify the fundamental significance and scientific merit of this study, it needs a reliable third-party in-situ or remotely sensed CO2 flux dataset, independent of the EC tower used in this study, to properly “assess” the accuracy and reliability of the WPL corrections. Otherwise, the current study is a sheer sensitivity analysis of the WPL and its dependence on climatological conditions, which can be performed without actual CO2 flux measurements.
Specific comments
- Figure 1: it will be good to have photo(s) of the actual EC tower and/or map of topography at the measurement site.
- Equation 1: the correction of (kinematic) sensible heat flux term, should it be potential temperature instead of temperature anomaly, though the difference is small at the sea level?
- Section 3.1, the 1st paragraph, how is Fc,0 computed? Is that the “raw” CO2 flux? It needs to be clearly defined. Also, is it “diel” or “diurnal” cycle?
- Figure 2: plots (a) and (b), the region circumscribed by the dashed lines, representing standard errors (standard deviations?), can be shaded for better clarity. Also the measurement uncertainty for the sensible heat flux (Fig. 2c) should also be shown. In addition, as the vapor flux (latent heat) is also included in the WPL correction, it is also recommended to be shown in this figure.
- Figure 2: the caption states that all plots are “climatological” variation, so it is understood that the diurnal cycle is averaged over the entire year (January to December 2016). This should be clarified and explicitly stated.
- Figure 2d can be grouped with Fig. 3 to show the results and analysis of the discrepancy.
Citation: https://doi.org/10.5194/egusphere-2023-2383-RC1 - AC1: 'Reply to RC1', Muhammad Fikri Sigid, 12 Dec 2023
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RC2: 'Comment on egusphere-2023-2383', Anonymous Referee #2, 01 Feb 2024
Sigid and others quantify the impact of the WPL correction on eddy covariance measurements using an open path sensor over a coastal sea near the shore. I read the manuscript with interest but feel that unfortunately the framing of the manuscript misses the mark. This is because the WPL correction either should be applied to measured fluxes in open path systems – because it is required to satisfy the mass balance – or it should not in closed path systems with adequate pressure and temperature dampening because not applying it satisfies the mass balance. Studying its impacts serves little purpose because one is studying the consequences of balancing mass or not, which is not of interest.
There may be some important technical notes to be made over a warm sea dominated by latent heat fluxes where the WPL terms will be quite small, and it is interesting that they impact CO2 fluxes, but this may be more of a curiosity. Perhaps worthy of very brief mention if only to emphasize the importance of WPL correction for open path systems. (As an aside, I have heard the argument that open path eddy covariance shouldn’t be applied over open water, but from the materials presented I was unable to ascertain why.)
For these reasons the manuscript should be rejected in its present form as it explains – in quite a bit of detail – the consequences of not balancing mass. It misses an enormous opportunity to explain the mechanisms that underlie the observed fluxes, especially the interesting results that CO2 uptake is greater at night (this was unexpected for me, and I am curious to know why), seasonal patterns in flux, the potential influence of different currents and water movements on flux, and how fluxes may or may not be changing over time. Reframing the manuscript to focus on the causes of observations after applying the WPL term would make it interesting and help the community understand this unique system.
As minor comments I’m not sure why so many wind directions were removed from the analysis; was this due to the impact of the tower? It seemed a bit extreme and perhaps unnecessary to remove so many datapoints. The manuscript is also overly verbose; any word and sentence that isn’t necessary to explain key findings should be removed. Focusing the study on science rather than required technical corrections will result in a valuable contribution to the literature.
Citation: https://doi.org/10.5194/egusphere-2023-2383-RC2 - AC2: 'Reply on RC2', Muhammad Fikri Sigid, 23 Feb 2024
Status: closed
-
RC1: 'Comment on egusphere-2023-2383', Anonymous Referee #1, 19 Nov 2023
This paper evaluates the Webb-Pearman-Leuning (WPL) correction for eddy covariance (EC) measurements of CO2 fluxes. The WPL correction is an important technique and crucial for the accuracy of flux measurements. The manuscript is in general well organized and presented. My biggest concern of this study, however, is that the results of WPL correction (Figs. 3-7), albeit rather detailed, is solely the difference between fluxes “with” and “without” the WPL correction, which lacks the “ground truth” to be compared with. In other words, even the authors have detailed knowledge of the sign changes, and the dominant terms in WPL correction, there is no way to assess if the WPL correction actually improves the accuracy of CO2 measurements or makes it worse. To justify the fundamental significance and scientific merit of this study, it needs a reliable third-party in-situ or remotely sensed CO2 flux dataset, independent of the EC tower used in this study, to properly “assess” the accuracy and reliability of the WPL corrections. Otherwise, the current study is a sheer sensitivity analysis of the WPL and its dependence on climatological conditions, which can be performed without actual CO2 flux measurements.
Specific comments
- Figure 1: it will be good to have photo(s) of the actual EC tower and/or map of topography at the measurement site.
- Equation 1: the correction of (kinematic) sensible heat flux term, should it be potential temperature instead of temperature anomaly, though the difference is small at the sea level?
- Section 3.1, the 1st paragraph, how is Fc,0 computed? Is that the “raw” CO2 flux? It needs to be clearly defined. Also, is it “diel” or “diurnal” cycle?
- Figure 2: plots (a) and (b), the region circumscribed by the dashed lines, representing standard errors (standard deviations?), can be shaded for better clarity. Also the measurement uncertainty for the sensible heat flux (Fig. 2c) should also be shown. In addition, as the vapor flux (latent heat) is also included in the WPL correction, it is also recommended to be shown in this figure.
- Figure 2: the caption states that all plots are “climatological” variation, so it is understood that the diurnal cycle is averaged over the entire year (January to December 2016). This should be clarified and explicitly stated.
- Figure 2d can be grouped with Fig. 3 to show the results and analysis of the discrepancy.
Citation: https://doi.org/10.5194/egusphere-2023-2383-RC1 - AC1: 'Reply to RC1', Muhammad Fikri Sigid, 12 Dec 2023
-
RC2: 'Comment on egusphere-2023-2383', Anonymous Referee #2, 01 Feb 2024
Sigid and others quantify the impact of the WPL correction on eddy covariance measurements using an open path sensor over a coastal sea near the shore. I read the manuscript with interest but feel that unfortunately the framing of the manuscript misses the mark. This is because the WPL correction either should be applied to measured fluxes in open path systems – because it is required to satisfy the mass balance – or it should not in closed path systems with adequate pressure and temperature dampening because not applying it satisfies the mass balance. Studying its impacts serves little purpose because one is studying the consequences of balancing mass or not, which is not of interest.
There may be some important technical notes to be made over a warm sea dominated by latent heat fluxes where the WPL terms will be quite small, and it is interesting that they impact CO2 fluxes, but this may be more of a curiosity. Perhaps worthy of very brief mention if only to emphasize the importance of WPL correction for open path systems. (As an aside, I have heard the argument that open path eddy covariance shouldn’t be applied over open water, but from the materials presented I was unable to ascertain why.)
For these reasons the manuscript should be rejected in its present form as it explains – in quite a bit of detail – the consequences of not balancing mass. It misses an enormous opportunity to explain the mechanisms that underlie the observed fluxes, especially the interesting results that CO2 uptake is greater at night (this was unexpected for me, and I am curious to know why), seasonal patterns in flux, the potential influence of different currents and water movements on flux, and how fluxes may or may not be changing over time. Reframing the manuscript to focus on the causes of observations after applying the WPL term would make it interesting and help the community understand this unique system.
As minor comments I’m not sure why so many wind directions were removed from the analysis; was this due to the impact of the tower? It seemed a bit extreme and perhaps unnecessary to remove so many datapoints. The manuscript is also overly verbose; any word and sentence that isn’t necessary to explain key findings should be removed. Focusing the study on science rather than required technical corrections will result in a valuable contribution to the literature.
Citation: https://doi.org/10.5194/egusphere-2023-2383-RC2 - AC2: 'Reply on RC2', Muhammad Fikri Sigid, 23 Feb 2024
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