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| Titel |
Affect of a clear-sky bias on inversions of XCO2 |
| VerfasserIn |
Julia Marshall, Martin Jung, Christian Rödenbeck, Martin Heimann |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250133709
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| Publikation (Nr.) |
 EGU/EGU2016-14350.pdf |
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| Zusammenfassung |
| Since GOSAT XCO2 data have become available, several studies have illustrated the
systematic differences seen in flux patterns based on inversions of surface measurements vs.
those based on satellite measurements. Across retrievals, transport models, and inversion
setups, a clear shift towards larger sources from the tropics and a stronger sinks from
the extratropics is seen, often so marked that the two results are not statistically
consistent with one another within uncertainty bounds. Some studies have attributed
this redistribution of fluxes to biases in the measurements, perhaps the result of
inappropriately defined prior covariance assumptions in the satellite retrieval, while
others have argued that the result is robust. One aspect that has not been explored
thoroughly is the possibility that this difference might be the result of a clear-sky
bias in the measurements. By measuring only when there are gaps in the cloud,
the satellite measurements are skewed toward sampling in conditions of higher
uptake. Analysis using flux tower data shows the effect of this to be regionally
dependent with a strong seasonal signal, resulting in an overestimation in uptake in the
northern hemisphere extratropics. We extend the analysis using upscaled products to
assess the impact on the cumulative NEE fluxes at the time of satellite overpass,
comparing the all-sky values to those with clear-sky conditions. This difference in
cumulative fluxes is translated into an offset in XCO2, and the impact on inverted fluxes
is shown. Potential strategies to overcome such a systematic bias are discussed. |
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