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.
FTS measurements of column CO<sub>2</sub> at Sodankylä