01 Sep 2023
 | 01 Sep 2023
Status: this preprint is currently under review for the journal AMT.

The importance of digital elevation model accuracy in XCO2 retrievals: improving the OCO-2 ACOS v11 product

Nicole Jacobs, Christopher W. O'Dell, Thomas E. Taylor, Thomas L. Logan, Brendan K. Byrne, Matthäus Kiel, Rigel Kivi, Pauli Heikkinen, Aronne Merrelli, Vivienne H. Payne, and Abhishek Chatterjee

Abstract. Knowledge of surface pressure is essential for calculating column average dry-air mole fractions of trace gases, such as CO2 (XCO2). In the NASA Orbiting Carbon Observatory 2 (OCO-2) Atmospheric Carbon Observations from Space (ACOS) retrieval algorithm, the retrieved surface pressures have been found to have unacceptable errors, warranting a parametric bias correction. This correction depends on the difference between retrieved and a priori surface pressures, which are derived from a meteorological model that is hypsometrically adjusted to the surface elevation using a digital elevation model (DEM). As a result, the effectiveness of the OCO-2 bias correction is contingent upon the accuracy of the referenced DEM. Here, we investigate several different DEM datasets for use in the OCO-2 ACOS retrieval algorithm: the OCODEM used in ACOS v10 and previous versions, the NASADEM+ used in ACOS v11, the Copernicus DEM, and two polar regional DEMs (ArcticDEM and REMA). We find that variations of 10 m in DEM elevations lead to variations in XCO2 of approximately 0.4 ppm. Given large-scale differences north of 60° N between the OCODEM and NASADEM+, we find that replacing the OCODEM with NASADEM+ yields a ∼ 100 TgC shift in inferred carbon uptake for the zones spanning 30–60° N and 60–90° N, which is on the order of 5–7 % of the estimated pan-Arctic land sink. Our analysis suggests that the Copernicus DEM has superior global continuity and accuracy compared to the other DEMs, motivating a post-processing update from OCO-2 v11 lite files (which used NASADEM+) to OCO-2 v11.1 by substituting the Copernicus DEM globally. We find that OCO-2 v11.1 improves accuracy and spatial continuity in the bias-corrected XCO2 product relative to both v10 and v11 in high latitude regions, while resulting in marginal or no change in most regions within ± 60° latitude. In addition, OCO-2 v11.1 provides increased data throughput after quality control filtering in most regions, partly due to the change in DEM, but mostly due to other corrections to quality control parameters.

Nicole Jacobs et al.

Status: open (until 06 Oct 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-151', Anonymous Referee #2, 21 Sep 2023 reply
  • RC2: 'Comment on amt-2023-151', Anonymous Referee #1, 21 Sep 2023 reply

Nicole Jacobs et al.

Nicole Jacobs et al.


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Short summary
The accuracy of the NASA Orbiting Carbon Observatory 2 (OCO-2) XCO2 product is sensitive to the referenced digital elevation model (DEM). This motivates our investigation of the quality of several global DEMs, used in current and past versions of the OCO-2 retrieval along with the Copernicus DEM. We also explore the impacts of changing the DEM on biases in OCO-2 retrieved XCO2 and inferred CO2 fluxes. Our findings lead to a version update to OCO-2 v11.1 that uses the Copernicus DEM globally.