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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/amt-2016-175
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
21 Jul 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Measurement Techniques (AMT) and is expected to appear here in due course.
Evaluation And Attribution Of OCO-2 XCO2 Uncertainties
John Worden1, Gary Doran1, Susan Kulawik2, Annmarie Eldering1, David Crisp1, Chris Frankenberg3,1, Christian O'Dell4, and Kevin Bowman1 1Jet Propulsion Laboratory / California Institute for Technology
2BAERI research
3California Institute for Technology
4Colorado State University
Abstract. Evaluating and attributing uncertainties in total column atmospheric CO2 measurements (XCO2) from the OCO-2 instrument is critical for testing hypotheses related to the underlying processes controlling XCO2 and for developing quality flags needed to choose those measurements that are usable for carbon cycle science. Here we test the reported uncertainties of Version 7 OCO-2 XCO2 measurements by examining variations of the XCO2 measurements and their calculated uncertainties within small regions (~ 100 km x 10.5 km) in which CO2 variability is expected to be small relative to variations imparted by noise or interferences. Over 39 000 of these “small neighborhoods” comprised of approximately 190 observations per neighborhood are used for this analysis. We find that a typical ocean measurement should have a precision and accuracy of 0.35 and 0.24 ppm respectively for calculated precisions larger than ~ 0.25 ppm. These values are approximately consistent with the calculated errors of 0.33 and 0.14 ppm for the noise and interference error (assuming that the accuracy is bounded by the calculated interference error). The actual precision for ocean data becomes worse as the signal-to-noise increases or the calculated precision decreases below 0.25 ppm for reasons that not well understood. A typical land measurement (both nadir and glint) is found to have a precision and accuracy of approximately 0.75 ppm and 0.65 ppm respectively as compared to the calculated precision and accuracy of approximately 0.36 ppm and 0.2 ppm. However, this precision includes the effects of synoptic variability in the total column that could be as high as 0.5 ppm during the summer drawdown period. The accuracy is likely related to interferences such as aerosols or surface albedo and is a lower bound as it is evaluated by comparing gradients in OCO-2 estimates of XCO2 to expected gradients across the region and not by direct comparison to well-calibrated XCO2 measurements from the ground network.

Citation: Worden, J., Doran, G., Kulawik, S., Eldering, A., Crisp, D., Frankenberg, C., O'Dell, C., and Bowman, K.: Evaluation And Attribution Of OCO-2 XCO2 Uncertainties, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2016-175, in review, 2016.
John Worden et al.
John Worden et al.

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Short summary
This paper evaluates the uncertainties of the total column carbon dioxide (XCO2) measurements from the NASA OCO-2 instrument by comparing observed variations in small geographical regions to the calculated uncertainties of the data within this region. In general we find that the XCO2 uncertainties are consistent with calculated values of approximately 0.2 ppm over the ocean whereas variations over land likely have larger uncertainties of at least ~ 0.7 ppm.
This paper evaluates the uncertainties of the total column carbon dioxide (XCO2) measurements...
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