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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: model evaluation paper 15 Nov 2019

Submitted as: model evaluation paper | 15 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

Characterizing model errors in chemical transport modeling of methane: Impact of model resolution in versions v9-02 of GEOS-Chem and v35j of its adjoint model

Ilya Stanevich1, Dylan B. A. Jones1, Kimberly Strong1, Robert J. Parker2,3, Hartmut Boesch2,3, Debra Wunch1, Justus Notholt4, Christof Petri4, Thorsten Warneke4, Ralf Sussmann5, Matthias Schneider6, Frank Hase6, Rigel Kivi7, Nicholas M. Deutscher8, Voltaire A. Velazco8, Kaley A. Walker1, and Feng Deng1 Ilya Stanevich et al.
  • 1Department of Physics, University of Toronto, Toronto, Canada
  • 2Earth Observation Science, Department of Physics and Astronomy, University of Leicester, Leicester, UK
  • 3National Centre for Earth Observation (NCEO) University of Leicester, Leicester, UK
  • 4Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 5Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-IFU), Garmisch-Partenkirchen, Germany
  • 6Karlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research (IMK-ASF), Karlsruhe, Germany
  • 7Finnish Meteorological Institute, Sodankyla, Finland
  • 8Centre for Atmospheric Chemistry, School of Chemistry, University of Wollongong, Wollongong, NSW, Australia

Abstract. The GEOS-Chem simulation of atmospheric CH4 was evaluated against observations from the Thermal And Near infrared Sensor for carbon Observations Fourier Transform Spectrometer (TANSO-FTS) on the Greenhouse gases Observing SATellite (GOSAT), the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and the Total Carbon Column Observing Network (TCCON). We focused on the model simulations at the 4° × 5° and 2° × 2.5° horizontal resolutions for the period of February–May 2010. Compared to the GOSAT, TCCON, and ACE-FTS data, we found that the 2° × 2.5° model produced a better simulation of CH4, with smaller biases and a higher correlation to the independent data. We found large resolution-dependent differences such as a latitude-dependent XCH4 bias, with higher columns abundances of CH4 at high latitudes and lower abundances at low latitudes at the 4° × 5° resolution than at 2° × 2.5°. We also found large differences in CH4 column abundances between the two resolutions over major source regions such as China. These differences resulted in up to 30 % differences in inferred regional CH4 emission estimates from the two model resolutions. We performed several experiments using 222Rn, 7Be and CH4 to determine the origins of the resolution-dependent errors. The results suggested that the major source of the latitude-dependent errors is excessive mixing in the upper troposphere and lower stratosphere, including mixing at the edge of the polar vortex, that is pronounced at the 4° × 5° resolution. At the coarser resolution, there is weakened vertical transport in the troposphere at mid- to high latitudes due to the loss of sub-grid tracer eddy mass flux in the storm track regions. We also identified reduced vertical transport at the coarser resolution. The vertical air mass fluxes are calculated in the model from the degraded coarse-resolution wind fields and the model does not conserve the air mass flux between model resolutions; as a result, the low resolution does not fully capture the vertical transport. This produces significant localized discrepancies, such as much greater CH4 abundances in the lower troposphere over China at 4° × 5° than at 2° × 2.5°. Although we found that the CH4 simulation is significantly better at 2° × 2.5° than at 4° × 5°, biases may still be present at 2° × 2.5° resolution. Their importance, particularly, in regards to inverse modeling of CH4 emissions, should be evaluated in future studies using on-line transport in the native general circulation model as a benchmark simulation.

Ilya Stanevich et al.
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Ilya Stanevich et al.
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Short summary
Systematic errors in atmospheric models pose a challenge for inverse modeling studies of methane (CH4) emissions. We evaluated the CH4 simulation in the GEOS-Chem model at the horizontal resolutions of 4° × 5° and 2° × 2.5°. Our analysis identified resolution-dependent biases in the model, which we attributed to discrepancies between the two model resolutions in vertical transport in the troposphere and in stratosphere-troposphere exchange.
Systematic errors in atmospheric models pose a challenge for inverse modeling studies of methane...