the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Uncertainties in OCO-2 satellite retrievals of XCO2 limit diagnosis of transport model simulation uncertainty
Abstract. Estimating regional CO2 sources and sinks is challenging due to limited data and uncertainties in transport models. Orbiting Carbon Observatory-2 (OCO-2) overcomes measurement limits, providing CO2 variations beyond in-situ networks. This study analyses altitude-wise model-observation CO2 differences from surface to upper troposphere using aircraft observations from ATom, Amazon, and CONTRAIL campaigns over OCO-2 total column CO2 (XCO2) sampling location to characterise sources of uncertainty in MIROC4-ACTM. We show model aligns better with ATom tropospheric columns (0.03 ± 0.03 ppm) than OCO-2 XCO2 (0.2 ± 0.5 ppm), especially over oceans, highlighting the need for expanded profile measurements to characterise errors robustly. Altitude-wise comparisons reveal this differences primarily occur in the lower troposphere (0–2 km), likely due to ACTM's near-surface land CO2 flux errors. In contrast, ACTM better matches aircraft CO2 in the middle (2–5 km) and upper (5–8 km) troposphere, likely due to accurate large-scale transport representation. Over the Amazon, CO2 differences with aircraft and OCO-2 differ, likely due to a lack of regional surface sites for inversion and insufficient high-altitude profile (~4 km) not representative of XCO2. Over Asian megacity airports, which are significant emission hotspots, the model shows a large negative difference with CONTRAIL than OCO-2. This discrepancy likely hints that MIROC4-ACTM is unable to capture urban fossil CO2 emission signals at airports due to coarse resolution (~2.8° x 2.8°) and higher resolution of OCO-2 limits ability to fully capture actual emission footprints.
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Status: open (until 13 Mar 2025)
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RC1: 'Comment on egusphere-2024-3976', Anonymous Referee #1, 11 Feb 2025
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Das et al. analyzed differences between MIROC-ACTM simulated atmospheric CO2 profiles and observations from aircraft and OCO-2. While the paper's title claims to focus on understanding how OCO-2 satellite retrieval uncertainties limit the diagnosis of transport model simulation uncertainty, the actual analysis presented does not address this question. The paper is poorly written with unclear reasoning and numerous grammatical errors. The quality does not satisfy ACP standards. I do not recommend this paper for publication based on the following major concerns:
Scope and Focus:
- Analyzing transport uncertainty requires examining tracer (e.g., CO2) vertical and meridional profiles. However, satellite retrieval algorithms have known limitations in resolving vertical details from XCO2 measurements. Using satellite vertical CO2 retrievals to analyze transport uncertainty is fundamentally limited. A meaningful study should examine how biases in vertical profiles derived from OCO-2 XCO2 could affect transport analysis (e.g., vertical mixing or PBL mixing). The manuscript fails to address these fundamental aspects of transport uncertainty analysis.
Methodological Issues:
- Analyzing model or data biases always requires establishing a 'ground truth'. This paper compares MIROC-ACTM simulated atmospheric CO2 profiles with observations from aircraft and OCO-2. However, it is not clear what is considered as the truth and what is being analyzed. The authors keep switching between these three models/products to evaluate the other two. In some cases, the authors consider MIROC-ACTM model simulation as the truth, which has certain limitations. My understanding is that the model output is the forward transport of a posterior flux, which was derived using only surface station data. However, the column-averaged model data might still be biased due to transport errors of the parent transport model and any additional bias from the inversion setup. It is fine to establish this model output as the truth, but the authors need to show that the column-average could represent the observed column-average.
- The method description part lacks many details. For example, how was the column-average calculated? How do you compare XCO2 with aircraft column-averages, considering they have different spatial coverage? Overall, the sampling strategy comparisons between different observation types need more careful consideration. The criteria for comparing different observational datasets need better justification. The statistical treatment of uncertainties requires more rigor. For example, what do you mean by variability? When you say there is a difference, is it significant?
Writing and Organization:
- The manuscript suffers from severe organizational and writing issues that significantly impair its readability and scientific credibility. The logical flow is persistently problematic, with paragraphs lacking clear structure and transitions, making it extremely difficult to follow the authors' reasoning. Most sections and paragraphs lack topic sentences to guide readers through the argument progression. The paper's overall organization is confusing, with little logical connection between sections, making it challenging to understand how different components of the analysis relate to each other. The manuscript is also filled with grammatical errors, suggesting it has not been properly proofread. The writing style lacks scientific rigor and precision - many statements are made without proper justification or clear explanation, and technical terms are used inconsistently. Here are a few examples (I cannot list all instances due to the extensive number of vague presentations and grammatical errors):
- L242: "lesser" is incorrect usage
- L244: "Negative difference" is vague; better to specify low or high
- L249: The bias referenced is undefined
- L250: "some regions" is too vague
- L269: "CO2_in-situ" is vague, as aircraft data could also be in-situ measurements
- L278-280: The focus of this sentence is unclear
- L282: "The vast part of the region" - which region?
- L349-350: "These differences are..." - this statement lacks supporting analysis
- L378-379: "This essentially..." - this statement lacks supporting evidence
- L384: "Needs further …" is an incomplete sentence
- L425-429: This content belongs in the introduction/motivation section
The paper requires thorough revision by a native English speaker and substantial reorganization to meet the standards expected of scientific publications.
Citation: https://doi.org/10.5194/egusphere-2024-3976-RC1
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