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

Submitted as: research article 06 Feb 2019

Submitted as: research article | 06 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Prior biosphere model impact on global terrestrial CO2 fluxes estimated from OCO-2 retrievals

Sajeev Philip1,2, Matthew S. Johnson1, Christopher Potter1, Vanessa Genovesse3,1, David F. Baker4,5, Katherine D. Haynes6, Daven K. Henze7, Junjie Liu8, and Benjamin Poulter9 Sajeev Philip et al.
  • 1NASA Ames Research Center, Moffett Field, CA 94035, USA
  • 2NASA Postdoctoral Program administered by Universities Space Research Association, Columbia, MD 21046, USA
  • 3California State University, Monterey Bay, CA 93955, USA
  • 4NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, CO 80305-3337, USA
  • 5Cooperative Institute for Research in the Atmosphere, Colorado State University, Ft. Collins, CO 80521, USA
  • 6Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA
  • 7Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA
  • 8Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
  • 9NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Abstract. This study assesses the impact of different state-of-the-science global biospheric CO2 flux models, when applied as prior information, on inverse modeling top-down estimates of terrestrial CO2 fluxes obtained when assimilating Orbiting Carbon Observatory 2 (OCO-2) observations. This is done with a series of Observing System Simulation Experiments (OSSEs) using synthetic CO2 column-average dry air mole fraction (XCO2) retrievals sampled at the OCO-2 satellite spatio-temporal frequency. The OSSEs used the four-dimensional variational (4D-Var) assimilation system with the GEOS-Chem global chemical transport model (CTM) to estimate CO2 net ecosystem exchange (NEE) fluxes using synthetic OCO-2 observations. The impact of biosphere models in inverse model estimates of NEE is quantified by conducting OSSEs using the NASA-CASA, CASA-GFED, SiB-4 and LPJ models as prior estimates and using NEE from the multi-model ensemble mean of the Multiscale Synthesis and Terrestrial Model Intercomparison Project as the truth. Results show that the assimilation of simulated XCO2 retrievals at OCO-2 observing modes over land results in posterior NEE estimates which generally reproduce true NEE globally and over terrestrial TransCom-3 regions that are well-sampled. However, we find larger spread among posterior NEE estimates, when using different prior NEE fluxes, in regions and seasons that have limited OCO-2 observational coverage and a large range in bottom-up NEE fluxes. Posterior NEE estimates had seasonally-averaged posterior NEE standard deviation (SD) of ~ 10 % to ~ 50 % of the multi-model-mean NEE for different TransCom-3 land regions with significant NEE fluxes (regions/seasons with a NEE flux ≥ 0.5 PgC yr−1). On a global average, the seasonally-averaged residual impact of the prior model NEE assumption on posterior NEE spread is ~ 10–20 % of the posterior NEE mean. Additional OCO-2 OSSE simulations demonstrate that posterior NEE estimates are also sensitive to the assumed prior NEE flux uncertainty statistics, with spread in posterior NEE estimates similar to those when using variable prior model NEE fluxes. In fact, the sensitivity of posterior NEE estimates to prior error statistics was larger compared to prior flux values in some regions/times of the Tropics and Southern Hemisphere where sufficient OCO-2 data was available and large differences between the prior and truth were evident. Overall, even with the availability of dense OCO-2 data, noticeable residual differences (up to ~ 20–30 % globally and 50 % regionally) in posterior NEE flux estimates remain that were caused by the choice of prior model flux values and the specification of prior flux uncertainties.

Sajeev Philip et al.
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Status: final response (author comments only)
Status: final response (author comments only)
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Sajeev Philip et al.
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Short summary
This research was conducted to quantify the impact of different prior global biosphere models on the estimate of terrestrial CO2 fluxes when assimilating Orbiting Carbon Observatory-2 (OCO-2) satellite observations. To determine the prior model impact, we apply Observing System Simulation Experiments (OSSEs). Even with the substantial spatio-temporal coverage of OCO-2 data, residual differences in posterior CO2 flux estimates remain due to the choice of prior flux mean and uncertainties.
This research was conducted to quantify the impact of different prior global biosphere models on...