Terrestrial ecosystem carbon flux estimated using GOSAT  and OCO-2 XCO<sub>2</sub> retrievals

Wang, Hengmao; Jiang, Fei; Wang, Jun; Ju, Weimin; Chen, Jing M.

doi:https://doi.org/10.5194/acp-19-12067-2019

Articles | Volume 19, issue 18

https://doi.org/10.5194/acp-19-12067-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

The 10th International Carbon Dioxide Conference (ICDC10)...

https://doi.org/10.5194/acp-19-12067-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 18

Research article

|

27 Sep 2019

Research article |

| 27 Sep 2019

Terrestrial ecosystem carbon flux estimated using GOSAT and OCO-2 XCO₂ retrievals

Hengmao Wang, Fei Jiang, Jun Wang, Weimin Ju, and Jing M. Chen

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Final revised paper (published on 27 Sep 2019)
Preprint (discussion started on 03 Dec 2018)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'comment on paper by H. Wang et al', Anonymous Referee #1, 31 Dec 2018
- AC1: 'Author responses to reviewer 1', Fei Jiang, 25 Apr 2019
RC2: 'Interesting and well-done exercise, but the analysis is incomplete which prevents any conclusion', Anonymous Referee #2, 02 Jan 2019
- AC2: 'Author responses to reviewer 2', Fei Jiang, 25 Apr 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Fei Jiang on behalf of the Authors (26 Apr 2019) Manuscript

ED: Referee Nomination & Report Request started (19 May 2019) by Christoph Gerbig

RR by Anonymous Referee #2 (29 May 2019)

Suggestions for revision or reasons for rejection

I was pleased to receive the response of the authors to my comments, and I appreciate the extra effort that was done to address my concerns. I think the manuscript has improved substantially in this revision, but not yet enough for publication in ACP. The manuscript needs quite a few small modifications of errors, typos, and mistakes in at least one figure. It can also profit from rewriting and reordering parts of the text. And before publication can proceed, the authors need to consider the terms of use of the ObsPack data that they downloaded, as currently they did not comply with them. Please find below my additional comments on the new manuscript presented.

Main comments:

The use of the ObsPack instead of the flasks is in principle a good idea. I do not understand why the authors decided to download a carbontracker obspack though, as the website that explains this product (https://www.esrl.noaa.gov/gmd/ccgg/obspack/release_notes.html#obspack_co2_1_CARBONTRACKER) explicitly suggests not to use these files as primary source of data for inversions, but instead to get the latest real obspack from the website. It also reminds the user explicitly to comply with the terms of use of these data in a study, which means that (1) all data providers need to be contacted before publication to explain the use of their data and to agree on the way to acknowledge them, (2) a citation to the dataset through its DOI must be included in the text. I repeated this Fair Use Statement in my comments below. Without complying with these rules, the current manuscript should not be published.

Although I like the introduction of the poor-man’s inversion, the description in the methods section seems incorrect to me, and I find the way it is integrated into the study not very strong. This comes from the choice to use it as an extra inverse solution from the beginning, and to discuss its flux results alongside that of the other inversions. But the poor-man’s inversion can only be used to look at the global total flux (which it matches by design), and to look at the distribution of CO₂ mixing ratios and XCO2 values across the globe. This it should follow reasonably well, thus setting a benchmark to beat for real inverse solutions. Currently, the label “benchmark” is used throughout the text including that of “benchmark inversion” which is confusing: the flux result of this poor-man’s method is the one thing one should *not* put much emphasis on, especially not below the global total scale. It is therefore also no use to show its regional flux solution in Table 2 and in Fig 4, nor be discussed in Section 4.2 in my opinion.

It is a bit awkward that the reader is first learning a lot about GoSAT to OCO-2 flux differences and how their regional budgets differ in great detail in Section 4.2, but only later in Section 4.3 learns that the OCO-2 inversion is not very trustworthy and is not able to reproduce the atmospheric XCO2 and surface CO₂ better than the poor-man’s inversion (which can be called a benchmark in this context). So in fact, all I read earlier becomes then in a sense irrelevant. Please consider bringing the assessment of the quality of the inversions forward in the manuscript, so that the flux analysis that comes afterwards can focus more on the relevant part of the study (GoSAT and in-situ inverse results). OCO-2 can then be still discussed, but only to indicate whether GoSAT satellite results are corroborated or not by OCO-2.

Abstract: I think that the text does not summarize so well the main findings anymore, and should be rewritten. The main message should focus on the posterior fluxes compared to the in-situ inversion, and not comparing the two satellites to the prior. Then, one can highlight that the main difference on the largest scale is the latitudinal distribution of land sinks, with the satellites suggesting a smaller Boreal and Tropical sink, combined with larger temperate sinks in both the NH and SH. However, OCO-2 and GoSAT generally do not agree on which continent contains the smaller or larger sinks. Also, the comparison of the simulated surface mixing ratios and XCO2 columns shows that only GoSAT and the in-situ inversion perform better than a poor-man’s solution that closes the annual global mass balance of CO₂. This puts the usefulness of the OCO-2 retrieval product used here into question.

List of remarks:

page 1, line 15 “benchmark inversion”: I would refer to the latter as a poor-man’s inversion in which only the global CO₂ growth rate is projected onto the land biosphere, to be used as a benchmark for the simulated atmospheric CO₂ distributions of the real inversions.

page 1, line 22: “more consistent with …” simply say that the GoSAT-based inversion seems to best capture the observed global CO₂ growth rate.

Page 2, line 29: it is worth to say explicitly that the OCO-2 retrieval you used here seems unfit for inverse modeling, but that later versions seem to perform better (Chevallier et al., 2019, ACPD). I also urge the authors to focus their future efforts on the later retrieval products from OCO-2.

Page 4, line 84: please do not use “benchmark inversion” to label this flux product, but explain the purpose of this approach better.

Page 6, line 128: This is where my main comment comes into play. The Fair Use Statement given in the readme file of the Obspack you downloaded was:

# ObsPack Fair Use Statement
#
# This cooperative data product is made freely available to the scientific community and is intended to stimulate and support carbon cycle modeling studies. We rely on the ethics and integrity of the user to assure that each contributing national and university laboratory receives fair credit for their work. Fair credit will depend on the nature of the work and the requirements of the institutions involved.
# Your use of this data product implies an agreement to contact each contributing laboratory for data sets used to discuss the nature of the work and the appropriate level of acknowledgement. If this product is essential to the work, or if an important result or conclusion depends on this product, co-authorship may be appropriate. This should be discussed with the appropriate data providers at an early stage in the work. Contacting the data providers is not optional; if you use this data product, you must contact the applicable data providers. To help you meet your obligation, the data product includes an e-mail distribution list of all data providers.
# This data product must be obtained directly from the ObsPack Data Portal at www.esrl.noaa.gov/gmd/ccgg/obspack/ and may not be re-distributed. In addition to the conditions of fair use as stated above, users must also include the ObsPack product citation in any publication or presentation using the product. The required citation is included in every data product and in the automated e-mail sent to the user during product download.

Page 7, line 147: insert “area” between shaded and shows

Page 9, line 196: This is yet another reference “CO₂ trend” to the poor-man’s inversion. Pleas try to introduce it better, and use it consistently please.

Page 11, line 232: descripted = described

Page 12, line 249: I do not understand this formula and I wonder if a mistake was made. piror = prior (typo). But why do you add something proportional to the prior flux uncertainty, instead of proportional to GPP? And why do you need trial-and-error to determine the scaling factor k? This is not the same approach as taken by Chevallier, whom you cite for this approach.

Page 12, line 260: “inverted global carbon budgets” please remove “inverted”

Page 12, line 263: “benchmark inversion” please rewrite

Page 16, line 316: In my opinion the benchmark inversion is not very useful here, as its regional flux simply reflects global GPP and not a piece of information derived from the data like in the actual inversions. I suggest to remove it here, and in Fig 4.

Page 17, line 321: “close to the benchmark result”: by writing this, you suggest to the reader that it is a good thing for the inversions to be close to the benchmark. But for continental fluxes this is not true at all, and this is why I think this gives the wrong message when put into the figure/text/table.

Page 20, line 380: Why is this section here, and not part of Section 4.3 where once again a comparison to TCCON is presented? And why are the other two results (in situ and benchmark) not shown? I think it would help to group these results together.

Page 21, line 392, space missing and typo in “to0.59 pm”

Page 21, line 409: Please make clear that this is not surprising because part of these evaluation data were used in the inversion in that case

Page 22, line 426: litter = little

Page 23, line 436: “ground XCO2 observations”, please simply write “We use data from 13 TCCON sites to…”

Page 23, line 436: Please make clear that also here the comparison is not fully independent: the TCCON data were used in the bias correction scheme of at least OCO-2 (I don’t know about GoSAT but I suspect the same there).

Page 23, line 437: into = onto

Page 24, line 461: The fact that only the in-situ inversion beats the benchmark on all 4 numbers should be mentioned in the text.

Page 25, Figure 7: There seems to be an error in the figure: the bars for benchmark and in-situ are exactly the same for all sites. Please check and fix this.

Page 26, line 490: I would not say that OCO-2 could improve the modeling of CO₂ concentrations: your poor-man’s inversion shows that you can achieve better results by simply scaling your fluxes to match the global growth rate of CO₂.

Page 26, line 492: “bench inversion” incorrect

Page 26, line 495 “GOAST” typo

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RR by Anonymous Referee #1 (18 Jun 2019)

Suggestions for revision or reasons for rejection

General comments.
The aim of the study is to provide a comparison of simulated CO2 concentrations estimated with inverse model using OCO-2 and GOSAT retrievals for the year 2015. The questions of interest to broader audience are differences in the amplitudes of respective flux corrections and their spatial distributions, as well as how well the optimized simulations agree with the observed concentrations by surface flask and TCCON networks. Authors discuss the spatial variability of the satellite data biases with respect to TCCON data and the biases in inverse model estimated concentrations with respect to surface flask and TCCON data. Based on comparison of the estimated fluxes to benchmark inversion and posterior fits to ground-based observational data not used in inversion, authors conclude that use of GOSAT data in inversion results in better fit to observations than OCO-2 data. The manuscript is been resubmitted and was substantially revised with respect to earlier version. Presentation of the material is clear and has improved over initial submission, so manuscript can be accepted after minor revisions. The text should also be checked for orthographical errors.

Detailed comments
L74 Authors consider if “current OCO-2 observations have a greater potential than GOSAT …”. It would be useful to note what would be the reasons affecting usefulness of OCO-2 or GOSAT? Did they mean spatially and temporally varying biases?

L454 Although results of the analysis support the conclusions, it should be noted that the study period of 2015 is characterized by strong El-Nino and the spatial distribution of fluxes typical for more common non El-Nino years, appears disturbed in the El-Nino year.

L456 The uncertainty/bias in TCCON retrievals is cited, but evidence for the bias is not shown/discussed. Suggest to add some reference(s) on TCCON biases.

L490 Analysis of the posterior fit to surface flask observations in Southern and North hemispheres indicates there are biases in GOSAT and OCO-2, and those are changing in different directions. Comparison to TCCON show the retrieval bias difference between GOSAT and OCO-2 are in order of 1 ppm. It is worth noting that, while observed mean difference with TCCON calls for correction of the GOSAT and OCO-2 data, based on mean deviation from TCCON, it was not done in this study, as opposed to some other inverse modeling studies.

Suggested technical corrections
L68 Suggest to change “constrain the surface carbon flux inversion” to “constrain the surface carbon fluxes”
L250 Fprior mistyped
L256 suggest changing “by multiply by” to “by multiplying by”
L276 ‘In situ’ to ‘in situ’
L377 abbreviated TCCON station names (such as ‘Bial’) should be explained somewhere in the text.
L392 in “0.34 to0.59” space is missing
L408 in “0.93ppm” space is missing
L415 Instead of Figure 7, Figure 6 should be referred here, as comparisons to TCCON is on Line 447.

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ED: Reconsider after major revisions (19 Jun 2019) by Christoph Gerbig

AR by Fei Jiang on behalf of the Authors (31 Jul 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Aug 2019) by Christoph Gerbig

RR by Anonymous Referee #1 (17 Aug 2019)

RR by Anonymous Referee #2 (28 Aug 2019)

ED: Publish subject to technical corrections (28 Aug 2019) by Christoph Gerbig

AR by Fei Jiang on behalf of the Authors (02 Sep 2019) Author's response Manuscript

Short summary

The differences in inverted global and regional carbon fluxes from GOSAT and OCO-2 XCO₂ from 1 January to 31 December 2015 are studied. We find significant differences for inverted terrestrial carbon fluxes on both global and regional scales. Overall, GOSAT XCO₂ has a better performance than OCO-2, and GOSAT data can effectively improve carbon flux estimates in the Northern Hemisphere, while OCO-2 data, with the specific version used in this study, show only slight improvement.

Terrestrial ecosystem carbon flux estimated using GOSAT and OCO-2 XCO2 retrievals

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Terrestrial ecosystem carbon flux estimated using GOSAT and OCO-2 XCO₂ retrievals