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| Titel |
Estimating surface CO2 fluxes from space-borne CO2 dry air mole fraction observations using an ensemble Kalman Filter |
| VerfasserIn |
L. Feng, P. I. Palmer, H. Bösch, S. Dance |
| 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 ; 9, no. 8 ; Nr. 9, no. 8 (2009-04-15), S.2619-2633 |
| Datensatznummer |
250007202
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| Publikation (Nr.) |
 copernicus.org/acp-9-2619-2009.pdf |
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| Zusammenfassung |
| We have developed an ensemble Kalman Filter (EnKF) to estimate 8-day regional
surface fluxes of CO2 from space-borne CO2 dry-air mole fraction
observations (XCO2) and evaluate the approach using a series of
synthetic experiments, in preparation for data from the NASA Orbiting
Carbon Observatory (OCO). The 32-day duty cycle of OCO alternates every 16 days
between nadir and glint measurements of backscattered solar radiation at
short-wave infrared wavelengths. The EnKF uses an ensemble of states to
represent the error covariances to estimate 8-day CO2 surface
fluxes over 144 geographical regions. We use a 12×8-day lag
window, recognising that XCO2 measurements include surface flux
information from prior time windows. The observation operator that
relates surface CO2 fluxes to atmospheric distributions of XCO2
includes: a) the GEOS-Chem transport model that relates surface fluxes
to global 3-D distributions of CO2 concentrations, which are sampled
at the time and location of OCO measurements that are cloud-free and
have aerosol optical depths <0.3; and b) scene-dependent averaging
kernels that relate the CO2 profiles to XCO2, accounting for
differences between nadir and glint measurements, and the associated
scene-dependent observation errors. We show that OCO XCO2
measurements significantly reduce the uncertainties of surface CO2
flux estimates. Glint measurements are generally better at constraining
ocean CO2 flux estimates. Nadir XCO2 measurements over the
terrestrial tropics are sparse throughout the year because of either
clouds or smoke. Glint measurements provide the most
effective constraint for estimating tropical terrestrial CO2 fluxes
by accurately sampling fresh continental outflow over neighbouring
oceans. We also present results from sensitivity experiments that
investigate how flux estimates change with 1) bias and unbiased errors,
2) alternative duty cycles, 3) measurement density and correlations, 4)
the spatial resolution of estimated flux estimates, and 5) reducing the
length of the lag window and the size of the ensemble.
At the revision stage of this manuscript, the OCO instrument failed to
reach its orbit after it was launched on 24 February 2009. The EnKF
formulation presented here is also applicable to GOSAT measurements of
CO2 and CH4. |
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