the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evaluating the consistency between OCO-2 and OCO-3 XCO2 estimates derived from the NASA ACOS version 10 retrieval algorithm
Thomas E. Taylor
Christopher W. O'Dell
David Baker
Carol Bruegge
Albert Chang
Lars Chapsky
Abhishek Chatterjee
Cecilia Cheng
Frédéric Chevallier
David Crisp
Lan Dang
Brian Drouin
Annmarie Eldering
Liang Feng
Brendan Fisher
Dejian Fu
Michael Gunson
Vance Haemmerle
Graziela R. Keller
Matthäus Kiel
Thomas Kurosu
Alyn Lambert
Joshua Laughner
Richard Lee
Junjie Liu
Lucas Mandrake
Yuliya Marchetti
Gregory McGarragh
Aronne Merrelli
Robert R. Nelson
Greg Osterman
Fabiano Oyafuso
Paul I. Palmer
Vivienne H. Payne
Robert Rosenberg
Peter Somkuti
Gary Spiers
Cathy To
Paul O. Wennberg
Shanshan Yu
Jia Zong
Abstract. The version 10 (v10) Atmospheric Carbon Observations from Space (ACOS) Level 2 Full Physics (L2FP) retrieval algorithm has been applied to multi-year records of observations from NASA's Orbiting Carbon Observatory -2 and -3 sensors (OCO-2 and OCO-3, respectively) to provide estimates of the carbon dioxide (CO2) column-averaged dry-air mole fraction (XCO2). In this study, a number of improvements to the ACOS v10 L2FP algorithm are described. The post-processing quality filtering and bias correction of the XCO2 estimates against multiple truth proxies are also discussed. The OCO v10 data volumes and XCO2 estimates from the two sensors for the time period August 2019 through February 2022 are compared, highlighting differences in spatiotemporal sampling, but demonstrating broad agreement between the two sensors where they overlap in time and space. A number of evaluation sources applied to both sensors suggest they are broadly similar in data and error characteristics. Mean OCO-3 differences relative to collocated OCO-2 data are approximately 0.2 ppm and −0.3 ppm for land and ocean observations, respectively. Comparison of XCO2 estimates to collocated Total Carbon Column Observing Network (TCCON) measurements show root mean squared errors (RMSE) of approximately 0.8 ppm and 0.9 ppm for OCO-2 and OCO-3, respectively. An evaluation against XCO2 fields derived from atmospheric inversion systems that assimilated only near-surface CO2 observations, i.e., did not assimilate satellite CO2 measurements, yielded RMSEs of 1.0 ppm and 1.1 ppm for OCO-2 and OCO-3, respectively. Evaluation of errors in small areas, as well as biases across land-ocean crossings, also show encouraging results, for each sensor and in their agreement. Taken together, our results demonstrate a broad consistency of OCO-2 and OCO-3 XCO2 measurements, suggesting they may be used together for scientific analyses.
Thomas E. Taylor et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2022-329', Anonymous Referee #2, 08 Mar 2023
This paper evaluates of the consistency between OCO-2 and OCO-3 xCO2 (column-averaged dry air mole fraction) retrievals using the ACOS v10 algorithm. The authors describe updates in the ACOS v10 algorithm as applied to the two instruments, as well as the retrieval data flow from pre-processers to posterior quality filtering and bias correction. Data volume and xCO2 retrievals are compared between the OCO-2 and OCO-3. The satellite retrievals are also compared with truth proxies including ground-based TCCON network and global models that assimilates only near-surface CO2 observations. Additionally, the authors also attempt to characterize retrieval errors over small areas and from retrievals across coastal lines. From these analyses, the authors conclude that OCO-2 and OCO-3 ACOS v10 xCO2 retrievals are broadly consistent and can be used together in science studies. Overall, this is a comprehensive study on the quality of xCO2 retrievals from OCO-2/-3 and should be of interest to readers of Atmos. Meas. Tech. The paper is well-organized, well-written and the figures are of good quality. The analysis methods used in the paper are mature and well-established. I would recommend that the paper be published in Atmos. Meas. Tech after minor revisions.
Specific comments
The use of TCCON as truth proxy for retrieval evaluation does appear to be circular, as indicated by the authors. Have some other data sources (e.g., from airborne campaigns) been considered as truth proxies?
The authors discuss the updates from ACOS v8/v9 to v10, and I’m wondering what are the impacts of these updates in terms of retrieval quality? Can the authors compare, for example, the RMSE of ACOS v8 vs. TCCON with that of ACOS v10 vs. TCCON?
Line 9: how do you interpret the differences between OCO-2 and OCO-3 in the context of precision/bias estimates against truth proxies? Do the OCO-2/-3 differences reflect or represent systematic errors or certain components of the systematic errors?
Line 100: spatial scale for precision and accuracy requirements?
Line 125: version 10 here refers to the L1B algorithm? This is a bit confusing. Is this part of level 1 or level 2?
Lines 163-164 - specify the variability of the bias in the two versions?
Table 2: it appears that Table 2 is not mentioned in text.
Line 252: how is initial median calculated (from the six models in Table 4)? Line 253 mentions “four models”.
Line 259: how is model/TCCON profile converted to 20 layers of ACOS profiles?
Table 7: what is considered truth xCO2 for SAA?
Line 290: course should be coarse?
Line 290: logarithm is taken for AODfine as well?
Line 322: FIn should be In.
Figure 3: Y axis spacing is not linear?
Section 4 – I’m wondering if Section 4 can be moved to the Appendix?
Figure 4 – Can you show the correlation between OCO-2 and OCO-3?
Figure 8 and Section 6.1 – what is the typical variability of individual soundings during each overpass? Do the comparisons change if the size of the domain is changed?
Line 482: course should be coarse?
Line 484: Table 4 and Table 5 list six models, not four.
Figure 9: the title of right column in the figure is a bit confusing.
Figure 10 and line 514: northward propagation is not that obvious in the plot (especially Figure 10b), perhaps mark it in the figure?
Figure 11: describe in the caption what each data point represents in the scatter plots.
Citation: https://doi.org/10.5194/amt-2022-329-RC1 -
RC2: 'Comment on amt-2022-329', Anonymous Referee #1, 09 Mar 2023
Manuscript “Evaluating the consistency between OCO-2 and OCO-3 XCO2 estimates derived from the NASA ACOS version 10 retrieval algorithm” from Taylor et al., 2023 (https://amt.copernicus.org/preprints/amt-2022-329/), presents an overview of the NASA ACOS algorithm version 10 XCO2 data products from OCO-2 and OCO-3 including algorithm descriptions (focussing on updates) and comparisons. The paper covers an important topic appropriate for Atmos. Meas. Tech. (AMT) and is very well written. It will be a very useful reference for those working with OCO-2 and OCO-3 data. I recommend to publish this paper in AMT after the mostly minor comments listed below have been considered by the authors for the revised version of this paper.
Abstract line 15: “show encouraging results”: Encouraging for what? Please modify this sentence if possible, using a more quantitative statement.
Relevant for several figures: Explain at one place or in the corresponding figure caption what the “mu”, “sigma”, and “N” values stand for (and other parameters, e.g., “G”).
Page 2, line 42: I would have expected that random error is mainly due to instrument noise but this most important factor is missing here.
Page 3 line 50: Please add “:” at the end of the first sentence.
Page 3, line 51: “December 2019”: In the Abstract (line 6) August 2019 is listed as first month. Please check and correct, if necessary.
Page 4, line 96: move “e.g.” into brackets.
Page 6, line 178: Reference to Crisp paper should be in brackets after ATBD. There are also several other places with format related reference issues, please check and correct.
Page 10, line 264: How is the "variability" / “real variability” defined in this case? This is important for Figures 11 and 12, too. Please provide more details in the Figure 11 and 12 figure captions.
Page 10, line 268: Please add the definition of the pressure weighting function (it is the pressure difference of / over a layer divided by surface pressure?).
Page 10, line 268: I assume that the CO2 averaging kernel is (essentially) the change of the retrieved XCO2 divided by the change of the true XCO2 (effectively obtained by perturbing the CO2 profile). Not clear what is meant here with “normalized”?
Page 12, line 291: Have the bias correction coefficients obtained using only the good (i.e., quality flagged data with QF=0) data or using all data?
Table 7: I recommend to always take the same number of digits after the decimal point.
Table 7: Unit of N: maybe better write "(10^6 soundings)"?
Table 7: Unit of Fraction Good QF: either in the header of write it for each number individually.
Table 7: Why is the Fraction Good QF of the models as truth proxy always smaller than for the others (e.g. 44% compared to 67% and 69% of the other proxies)?Table 7: What is the reason to mention the average of all three truth proxies? Aren't they different by default and not really comparable?
Page 15, line 314 following: "The same is not true": Maybe correct this to "This is different".
Page 16, line 320: Section B -> Appendix B.
Page 16, line 321: FIn -> In.
Page 16, line 330: Please replace “M” by million (if this is what is meant here).
Page 17, line 362 following: "while OCO-3 has a sinusoidal-like pattern of higher/lower densities over mid-latitude land": Why is this the case?
Figure 1: The country and continent boarders are too thick. It is difficult to see the data over land. Please use thinner lines. This is also true for Figs. 4, 5, 9 and A4.
Figure 1: What is the difference between white and grey? Please add this info to the figure caption.
Figure 2: The meaning of N is not explained. I assume it is the total number of "good quality" measurements?
Figure 2: I would have assumed that Land Glint has less "Good Quality" pixels than Land Nadir. Why is this not the case? In addition, the percentage number is relative to the total number of "Good Quality" flagged data. Shouldn't this be relative to all measurements of the respective category, i.e. how many Ocean Glint are "good" of all Ocean Glint measurements? I think this would be better comparable between panel a and b?Page 17, line 351: Using “Figure” instead of “Fig.” at the beginning of a sentence.
Page 18, line 366: Explain JEM-EF.
Page 20, line 393 following: "thereby forcing the models to rely more on the prior spatial variations": What does this mean? What are "prior spatial variations" and why do models for OCO-2 rely on the high latitudes? Do you mean deriving global flux estimates using both instruments?
Page 20, line 406 following: I think you describe the mean values (mu values?) in Fig. 6, but are the given standard deviations (sigma?) a measure for uncertainty? If yes, the mean values accounting for the uncertainties are much closer to each other.
Figure 4: Do positive values mean underestimation of uncorrected data? Please add explanation.
Figure 6 caption: Are "Ocean" and "Land" observation modes? Do you mean glint over ocean and nadir over land?
Figure 6 (a): Is there a reason for the nearly 2 ppm land ocean difference at northern latitudes?
Page 23, line 436 following: "Only overpasses with at least one hundred good quality-flagged OCO soundings were retained.": Does this mean, at least one hundred good quality soundings for one overpass within the 2.5x5.0 box?
Page 24, line 440: "(includes land-nadir, land-glint, and land-target)": This explanation should be added to the description of the previous figures, too, where "land" and "ocean" observation are mentioned.
Page 26, line 491: The bias statements are only true over the oceans.
Page 27: line 512 following: I don't understand this sentence. Do you mean that the signal changes sign at the equator starting from February 2020?
Page 27, line 513 following: Where does this northward propagation come from?
Page 28, lines 524-526: But isn't this the same metric as in Fig. 9 where you wrote that this is not the difference between the OCO instruments, but a measure how far from the MMM they are?
Page 28, line 537: Is the “theoretical error” the one that is reported as uncertainty in the XCO2 Level 2 product files?
Page 28, line 533: "land": maybe write land-nadir?
Page 28, line 537: "it is highly correlated": What is the correlation coefficient?
Page 28, line 537: "values around 43%": Isn't a discrepancy of 43 % a large difference?
Figure 10: Why is there a double "Delta" letter in the captions of (c) and (f)? Maybe you can adapt these titles to match with Fig. 9 right column?
Figures 11 and 12: using sigma for "variability" and the standard deviation in the figure is pretty confusing. Maybe correct the sigma in the captions by "var."?
Figures 11 and 12: Is the "uncertainty" in the title of panels (b) and (d) the variability?
Figures 11 and 12: I don't understand how panel b is generated from the data shown in panel a. I would have expected many more dots (about 136356).
Figures 11 and 12: What is the unit of "sigma XCO2" in panels a and c of Fig. 11 and 12?Page 30, line 540: Where does the value of 1 sigma come from?
Page 30, line 546: Would you expect the "uncertainty" to be on the one-one-line?
Page 31, line 553: "land and ocean": Are they all in glint or all in nadir mode?
Page 31, line 562: Please replace “k” by kilo (if this is what is meant here).
Page 31, line 574: Please explain TSIS-SIM.
Page 34, line 640: Please replace “y” by years (if this is what is meant here).
Figure A1: Caption: “4 of 8”: Numbering starts with 1, or?
Page 42, line 749: Please replace “m” by minutes (if this is what is meant here).
Page 42, line 766: “Researchers are urged to use these files …”. Please add this important recommendation to Section “Data availability” to minimize the risk to overlook it.
Figure B3: Definition of delta_XCO2: Is it OCO-3 minus OCO-2? Please add this information.
Figure B5: Caption: Please add that a difference is shown and not absolute XCO2 values.
Page 46, line 790: Add after Panel (a): “of Fig. B6”.
Page 47, line 818: What is the “light-weight per-day format”? Is this the format of the “Lite files”? If yes then please use the term Lite files.
Typos:
Page 13, line 303: Variance: I think sigma_1 and sigma_2 need to be squared.
Page 16, line 321: Fin -> In.
Page 21, line 409: “10 day” -> “10 days”
Figure 10, caption: Add space before “gridded”.
Page 26, line 481: course --> coarse?
Page 36, line 676: /gt
Page 42, line 741: add comma after B3.
Citation: https://doi.org/10.5194/amt-2022-329-RC2
Thomas E. Taylor et al.
Thomas E. Taylor et al.
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