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

The GEOS-Chem simulation of atmospheric CH$_{4}$ 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 CH$_{4}$, with smaller biases and a higher correlation to the independent data. We found large resolution-dependent differences such as a latitude-dependent XCH$_{4}$ bias, with higher column abundances of CH$_{4}$ 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 CH$_{4}$ column abundances between the two resolutions over major source regions such as China. These differences resulted in up to 30 % differences in inferred regional CH$_{4}$ emission estimates from the two model resolutions. ... mehrWe performed several experiments using 222Rn, 7Be, and CH$_{4}$ 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, which is pronounced at the 4°×5° resolution. At the coarser resolution, there is weakened vertical transport in the troposphere at midlatitudes to high latitudes due to the loss of sub-grid tracer eddy mass flux in the storm track regions. 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 CH$_{4}$ abundances in the lower troposphere over China at 4°×5° than at 2°×2.5°. Although we found that the CH$_{4}$ 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 CH$_{4}$ emissions, should be evaluated in future studies using online transport in the native general circulation model as a benchmark simulation.