How bias correction goes wrong: measurement of XCO2 affected by erroneous surface pressure estimates

All measurements of XCO2 from space have systematic errors. To reduce a large fraction of these errors, a bias correction is applied toXCO2 retrieved from GOSAT and OCO-2 spectra using the ACOS retrieval algorithm. The bias correction uses, among other parameters, the surface pressure difference bet...

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Bibliographic Details
Published inAtmospheric measurement techniques Vol. 12; no. 4; pp. 2241 - 2259
Main Authors Kiel, Matthäus, O'Dell, Christopher W, Fisher, Brendan, Eldering, Annmarie, Nassar, Ray, MacDonald, Cameron G, Wennberg, Paul O
Format Journal Article
LanguageEnglish
Published Katlenburg-Lindau Copernicus GmbH 12.04.2019
Copernicus Publications
Subjects
Algorithms
Bias
Errors
Estimates
Pressure
Retrieval
Surface pressure
Systematic errors
Topography (geology)
New Zealand
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Summary:All measurements of XCO2 from space have systematic errors. To reduce a large fraction of these errors, a bias correction is applied toXCO2 retrieved from GOSAT and OCO-2 spectra using the ACOS retrieval algorithm. The bias correction uses, among other parameters, the surface pressure difference between the retrieval and the meteorological reanalysis. Relative errors in the surface pressure estimates, however, propagate nearly 1:1 into relative errors in bias-correctedXCO2. For OCO-2, small errors in the knowledge of the pointing of the observatory (up to ∼130 arcsec) introduce a bias inXCO2 in regions with rough topography. Erroneous surface pressure estimates are also caused by a coding error in ACOS version 8, sampling meteorological analyses at wrong times (up to 3 h after the overpass time). Here, we derive new geolocations for OCO-2's eight footprints and show how using improved knowledge of surface pressure estimates in the bias correction reduces errors in OCO-2's v9 XCO2 data.
ISSN:1867-1381
1867-8548
DOI:10.5194/amt-12-2241-2019