Atmos. Chem. Phys. Discuss., 13, 5769-5804, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Regional inversion of CO2 ecosystem fluxes from atmospheric measurements: reliability of the uncertainty estimates
G. Broquet1, F. Chevallier1, F.-M. Bréon1, M. Alemanno2, F. Apadula3, S. Hammer4, L. Haszpra5, F. Meinhardt6, J. Necki7, S. Piacentino8, M. Ramonet1, M. Schmidt1, R. L. Thompson9,*, A. T. Vermeulen10, C. Yver1, and P. Ciais1
1Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, UMR8212, IPSL, Gif-sur-Yvette, France
2Servizio Meteorologico dell'Aeronautica Militare Italiana, Centro Aeronautica Militare di Montagna, Monte Cimone/Sestola, Italy
3Research on Energy Systems, RSE, Environment and Sustainable Development Department, Milano, Italy
4Universität Heidelberg, Institut für Umweltphysik, Heidelberg, Germany
5Hungarian Meteorological Service, Budapest, Hungary
6Federal Environmental Agency, Kirchzarten, Germany
7AGH University of Science and Technology, Kraków, Poland
8ENEA, Laboratory for Earth Observations and Analyses, Palermo, Italia
9Max Planck Institute for Biogeochemistry, Jena, Germany
10ECN-Energy research Centre of the Netherlands, EEE-EA, Petten, The Netherlands
*now at: NILU, Norwegian Institute for Air Research, Kjeller, Norway

Abstract. The Bayesian framework of CO2 flux inversions permits estimates of the retrieved flux uncertainties. Here, the reliability of these theoretical estimates is studied through a comparison against the misfits between the inverted fluxes and independent measurements of the CO2 Net Ecosystem Exchange (NEE) made by the eddy covariance technique at local (few hectares) scale. Regional inversions at 0.5° resolution are applied for the western European domain where ~ 50 eddy covariance sites are operated. These inversions are conducted for a 6-yr period (2002–2007). They use a mesoscale atmospheric transport model, a prior estimate of the NEE from a terrestrial ecosystem model and rely on the variational assimilation of in situ continuous measurements of CO2 atmospheric mole fractions. The misfits averaged over monthly periods and over the whole domain, are in good agreement with the theoretical uncertainties for prior (respectively inverted) NEE, with positive chi-square tests for the variance at the 2% (respectively 20%) significance levels, despite the scale mismatch and the independence between the prior (respectively inverted) NEE and the flux measurements. The theoretical uncertainty reduction for the monthly NEE at the measurement sites is 53% while the inversion actually decreases the standard deviation of the misfits by as much as 38%. These results build confidence in the NEE estimates at the European/monthly scales and in their theoretical uncertainty from the regional inverse modeling system. However, the uncertainties at the monthly (respectively annual) scale remain larger than the amplitude of the inter-annual variability of monthly (respectively annual) fluxes, so that there is a low confidence in the inter-annual variations. The uncertainties at the monthly scale are significantly smaller than the seasonal variations. The seasonal cycle of the inverted fluxes is thus reliable. In particular, the CO2 sink period over the European continent likely ends later than represented by the ecosystem model.

Citation: Broquet, G., Chevallier, F., Bréon, F.-M., Alemanno, M., Apadula, F., Hammer, S., Haszpra, L., Meinhardt, F., Necki, J., Piacentino, S., Ramonet, M., Schmidt, M., Thompson, R. L., Vermeulen, A. T., Yver, C., and Ciais, P.: Regional inversion of CO2 ecosystem fluxes from atmospheric measurements: reliability of the uncertainty estimates, Atmos. Chem. Phys. Discuss., 13, 5769-5804, doi:10.5194/acpd-13-5769-2013, 2013.
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