Biogeosciences Discuss., 8, 6775-6832, 2011
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
North American CO2 exchange: intercomparison of modeled estimates with results from a fine-scale atmospheric inversion
S. M. Gourdji1,8, K. L. Mueller1, V. Yadav1,7, D. N. Huntzinger1,9, A. E. Andrews2, M. Trudeau2, G. Petron2, T. Nehrkorn3, J. Eluszkiewicz3, J. Henderson3, D. Wen4, J. Lin4, M. Fischer5, C. Sweeney2, and A. M. Michalak1,6,7
1Department of Civil & Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA
2Global Monitoring Division, Earth System Research Laboratory, National Oceanic & Atmospheric Administration, Boulder, CO 80305, USA
3Atmospheric and Environmental Research Inc., Lexington, MA 02421, USA
4Department of Earth & Environmental Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada
5Lawrence Berkeley National Lab, Berkeley, CA 94720, USA
6Department of Atmospheric, Oceanic & Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA
7Department of Global Ecology, Carnegie Institution for Science, Stanford, CA 94305, USA
8Department of Environmental Earth System Science and Program on Food Security & the Environment, Stanford University, Stanford, CA 94305, USA
9School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff AZ, USA

Abstract. Robust estimates of regional-scale terrestrial CO2 exchange are needed to support carbon management policies and to improve the predictive ability of models representing carbon-climate feedbacks. Large discrepancies remain, however, both among and between CO2 flux estimates from atmospheric inverse models and terrestrial biosphere models. Improved atmospheric inverse models that provide robust estimates at sufficiently fine spatial scales could prove especially useful for monitoring efforts, while also serving as a validation tool for process-based assumptions in terrestrial biosphere models. A growing network of continental sites collecting continuous CO2 measurements provides the information needed to drive such models. This study presents results from a regional geostatistical inversion over North America for 2004, taking advantage of continuous data from the nine sites operational in that year, as well as available flask and aircraft observations. The approach does not require explicit prior flux estimates, resolves fluxes at finer spatiotemporal scales than previous North American inversion studies, and uses a Lagrangian transport model coupled with high-resolution winds (i.e. WRF-STILT) to resolve near-field influences around measurement locations. The estimated fluxes are used in an inter-comparison with other inversion studies and a suite of terrestrial biosphere model estimates collected through the North American Carbon Program Regional and Continental Interim Synthesis. Differences among inversions are found to be smallest in areas of the continent best-constrained by the atmospheric data, pointing to the value of an expanded measurement network. Aggregation errors in previous coarser-scale inversion studies are likely to explain a portion of the remaining spread. The spatial patterns from a geostatistical inversion that includes auxiliary environmental variables from the North American Regional Reanalysis were similar to those from the median of the biospheric model estimates during the growing season, but diverged more strongly in the dormant season. This could be due to a lack of sensitivity in the inversion during the dormant season, but may also point to a lack of skill in the biospheric models outside of the growing season, particularly in agricultural areas. For the annual continental budget, the boundary conditions used as an input into the inversions were seen to have a substantial impact on the estimated net flux, with a difference of ~0.8 PgC yr−1 associated with results using two different plausible sets of boundary conditions.

Citation: Gourdji, S. M., Mueller, K. L., Yadav, V., Huntzinger, D. N., Andrews, A. E., Trudeau, M., Petron, G., Nehrkorn, T., Eluszkiewicz, J., Henderson, J., Wen, D., Lin, J., Fischer, M., Sweeney, C., and Michalak, A. M.: North American CO2 exchange: intercomparison of modeled estimates with results from a fine-scale atmospheric inversion, Biogeosciences Discuss., 8, 6775-6832, doi:10.5194/bgd-8-6775-2011, 2011.
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