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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 10 | Copyright
Atmos. Chem. Phys., 18, 7189-7215, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 24 May 2018

Research article | 24 May 2018

The impact of transport model differences on CO2 surface flux estimates from OCO-2 retrievals of column average CO2

Sourish Basu1,2, David F. Baker1,3, Frédéric Chevallier4, Prabir K. Patra5, Junjie Liu6, and John B. Miller1 Sourish Basu et al.
  • 1NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, CO, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 3Cooperative Institute for Research in the Atmosphere, Colorado State University, Ft. Collins, CO, USA
  • 4LSCE-CEA-UVSQ-CNRS, Orme des Merisiers, Gif-sur-Yvette, France
  • 5RCGC/IACE/ACMPT, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan
  • 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Abstract. We estimate the uncertainty of CO2 flux estimates in atmospheric inversions stemming from differences between different global transport models. Using a set of observing system simulation experiments (OSSEs), we estimate this uncertainty as represented by the spread between five different state-of-the-art global transport models (ACTM, LMDZ, GEOS-Chem, PCTM and TM5), for both traditional in situ CO2 inversions and inversions of XCO2 estimates from the Orbiting Carbon Observatory 2 (OCO-2). We find that, in the absence of relative biases between in situ CO2 and OCO-2 XCO2, OCO-2 estimates of terrestrial flux for TRANSCOM-scale land regions can be more robust to transport model differences than corresponding in situ CO2 inversions. This is due to a combination of the increased spatial coverage of OCO-2 samples and the total column nature of OCO-2 estimates. We separate the two effects by constructing hypothetical in situ networks with the coverage of OCO-2 but with only near-surface samples. We also find that the transport-driven uncertainty in fluxes is comparable between well-sampled northern temperate regions and poorly sampled tropical regions. Furthermore, we find that spatiotemporal differences in sampling, such as between OCO-2 land and ocean soundings, coupled with imperfect transport, can produce differences in flux estimates that are larger than flux uncertainties due to transport model differences. This highlights the need for sampling with as complete a spatial and temporal coverage as possible (e.g., using both land and ocean retrievals together for OCO-2) to minimize the impact of selective sampling. Finally, our annual and monthly estimates of transport-driven uncertainties can be used to evaluate the robustness of conclusions drawn from real OCO-2 and in situ CO2 inversions.

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Short summary
CO2 measurements from the global surface network and CO2 estimates from satellites such as the Orbiting Carbon Observatory 2 (OCO-2) are currently used to quantify the surface sources and sinks of CO2, using what we know about atmospheric transport of gases. In this work, we quantify the uncertainties in those surface source/sink estimates that stem from errors in our atmospheric transport models, using an observing system simulation experiment (OSSE).
CO2 measurements from the global surface network and CO2 estimates from satellites such as the...