How bias correction goes wrong: measurement of X_(CO_2) affected by erroneous surface pressure estimates

Creators: Kiel, Matthäus; O'Dell, Christopher W.; Fisher, Brendan; Eldering, Annmarie; Nassar, Ray; MacDonald, Cameron G.; Wennberg, Paul O.

Abstract

All measurements of X_(CO_2) from space have systematic errors. To reduce a large fraction of these errors, a bias correction is applied to X_(CO_2) 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-corrected X_(CO_2). For OCO-2, small errors in the knowledge of the pointing of the observatory (up to ∼130 arcsec) introduce a bias in X_(CO_2) 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 X_(CO_2) data.

Additional Information

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 05 Oct 2018 – Discussion started: 19 Nov 2018 – Revised: 21 Feb 2019 – Accepted: 21 Feb 2019 – Published: 12 Apr 2019. We thank David Crisp for helpful discussions on the viewing geometry of the observatory. We thank Callum McCracken for contributing to Fig. 4. This work was financially supported by NASA's OCO-2 project (grant no. NNN12AA01C) and NASA's Carbon Cycle and Ecosystems research program (grant no. NNX17AE15G). Data availability. All of the OCO-2 data products are publicly available through the NASA Goddard Earth Science Data and Information Services Center (GES DISC) for distribution and archiving (http://disc.sci.gsfc.nasa.gov/OCO-2; last access: 31 March 2019). TCCON data were obtained from the TCCON data archive hosted by CaltechDATA and are available from https://tccondata.org/ (last access: 31 March 2019). The authors declare that they have no conflict of interest. Edited by: Christof Janssen. Reviewed by: François-Marie Bréon and one anonymous referee.

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