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Title:
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Regional inversion of CO2 ecosystem fluxes from atmospheric measurements: reliability of the uncertainty estimates
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Author(s):
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Broquet, G.; Chevallier, F.; Bréon, F.-M.; Kadygrov, N.; Alemanno, M.; Apadula, F.; Hammer, S.; Haszpra, L.; Meinhardt, F.; Morguí, J.A.; Necki, J.; Piacentino, S.; Ramonet, M.; Schmidt, M.; Thompson, R.; Vermeulen, A.T.; Yver, C.; Ciais, P.
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Published by:
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Publication date:
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ECN
Environment & Energy Engineering
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17-9-2013
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ECN report number:
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Document type:
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ECN-W--13-045
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Article (scientific)
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Number of pages:
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18
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Published in: Atmospheric Chemistry and Physics Discussions (Copernicus Publications), , 2013, Vol.13, p.9039-9056.
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 where50
eddy covariance sites are operated. These inversions are conducted
for the 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. Averaged over monthly periods and
over the whole domain, the misfits are in good agreement
with the theoretical uncertainties for prior and inverted NEE,
and pass the chi-square test for the variance at the 30% and
5% significance levels respectively, 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 decreases the standard
deviation of the misfits by 38 %. These results build
confidence in the NEE estimates at the European/monthly
scales and in their theoretical uncertainty from the regional
inverse modelling 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 this study does not engender
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
prior ecosystem model
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