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| Titel |
On the statistical optimality of CO2 atmospheric inversions assimilating CO2 column retrievals |
| VerfasserIn |
F. Chevallier |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 19 ; Nr. 15, no. 19 (2015-10-07), S.11133-11145 |
| Datensatznummer |
250120081
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| Publikation (Nr.) |
 copernicus.org/acp-15-11133-2015.pdf |
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| Zusammenfassung |
| The extending archive of the Greenhouse Gases Observing Satellite (GOSAT)
measurements (now covering about 6 years) allows increasingly robust
statistics to be computed, that document the performance of the
corresponding retrievals of the column-average dry air-mole fraction of
CO2 (XCO2). Here, we demonstrate that atmospheric inversions
cannot be rigorously optimal when assimilating current XCO2 retrievals,
even with averaging kernels, in particular because retrievals and inversions
use different assumption about prior uncertainty. We look for some practical
evidence of this sub-optimality from the view point of atmospheric inversion
by comparing a model simulation constrained by surface air-sample
measurements with one of the GOSAT retrieval products (NASA's ACOS). The
retrieval-minus-model differences result from various error sources, both in
the retrievals and in the simulation: we discuss the plausibility of the
origin of the major patterns. We find systematic retrieval errors over the
dark surfaces of high-latitude lands and over African savannahs. More
importantly, we also find a systematic over-fit of the GOSAT radiances by
the retrievals over land for the high-gain detector mode, which is the usual
observation mode. The over-fit is partially compensated by the retrieval
bias-correction. These issues are likely common to other retrieval products
and may explain some of the surprising and inconsistent CO2 atmospheric inversion results obtained with the existing GOSAT retrieval
products. We suggest that reducing the observation weight in the retrieval
schemes (for instance so that retrieval increments to the retrieval prior
values are halved for the studied retrieval product) would significantly
improve the retrieval quality and reduce the need for (or at least reduce
the complexity of) ad-hoc retrieval bias correction. |
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