Submitted as: development and technical paper
29 Sep 2023
Submitted as: development and technical paper |  | 29 Sep 2023
Status: this preprint is currently under review for the journal GMD.

Development of the tangent linear and adjoint models of the global online chemical transport model MPAS-CO2 v7.3

Tao Zheng, Sha Feng, Jeffrey Steward, Xiaoxu Tian, David Baker, and Martin Baxter

Abstract. We describe the development of the tangent linear (TL) and adjoint models of the MPAS-CO2 transport model, which is a global online chemical transport model developed upon the non-hydrostatic Model for Prediction Across Scales-Atmosphere (MPAS-A). The primary goal is to make the model system a valuable research tool for investigating atmospheric carbon transport and inverse modeling. First, we develop the TL code, encompassing all CO2 transport processes within the MPAS-CO2 forward model. Then, we construct the adjoint model using a combined strategy involving re-calculation and storage of the essential meteorological variables needed for CO2 transport. This strategy allows the adjoint model to undertake long-period integration with moderate memory demands. To ensure accuracy, the TL and adjoint models undergo vigorous verifications through a series of standard tests. The adjoint model, through backward-in-time integration, calculates the sensitivity of atmospheric CO2 observations to surface CO2 fluxes and the initial atmospheric CO2 conditions. To demonstrate the utility of the adjoint model, we conduct simulations for two types of atmospheric CO2 observations: tower-based in situ CO2 mixing ratio and satellite-derived column-averaged (XCO2). A comparison between the sensitivity to surface flux calculated by the MPAS-CO2 adjoint model with its counterpart from Carbon Tracker-Lagrange (CT-L) reveals spatial agreement but notable magnitude differences. These differences, particularly evident for XCO2, likely arise from differences in vertical mixing between the two systems. Moreover, this comparison highlights the substantial loss of information in the atmospheric CO2 observations due to CT-L’s simulation length and spatial domain limitations. Furthermore, the adjoint sensitivity analysis demonstrates that the sensitivities to both surface flux and initial CO2 conditions spread out throughout the entire northern hemisphere within a month. MPAS-CO2 forward, TL, and adjoint models stand out for their calculation efficiency and variable-resolution capability, making them competitive in computational cost. In conclusion, the successful development of the MPAS-CO2 TL and adjoint models, and their integration into the MPAS-CO2 system, establish the possibility of using MPAS’s unique features for investigating atmospheric CO2 transport sensitivity studies and for conducting inverse modeling with advanced methods such as variational data assimilation.

Tao Zheng et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2023-169', Peter Bosman, 14 Nov 2023
  • RC2: 'Comment on gmd-2023-169', Anonymous Referee #2, 28 Nov 2023

Tao Zheng et al.

Model code and software

The forward, tangent linear, and adjoint models of MPAS-CO2 V7.3 global online chemical transport model system Tao Zheng

Tao Zheng et al.


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Short summary
The tangent linear and adjoint models have been successfully implemented in the MPAS-CO2 system, a milestone that has undergone rigorous accuracy testing. This development lays the groundwork for a global carbon flux data assimilation system, which offers the flexibility of high-resolution focus on specific areas while maintaining a coarser resolution elsewhere. This approach significantly reduces computational costs, thus perfectly suited for future CO2 geo-stationery and imager satellites.