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

Abstract

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.

On what scales can GOSAT flux inversions constrain anomalies in terrestrial ecosystems?

“…The spatial resolution is varied to examine whether changes in model transport significantly impact our results. It has previously been shown that there are significant differences in tracer transport as model resolution is decreased in GEOS-Chem (Yu et al, 2018;Stanevich, 2018). In particular, Stanevich (2018) show that resolution-induced biases of up to 30 % can be introduced on the scale of TransCom regions for 4 • × 5 • relative to 2 • × 2.5 • for atmospheric methane (CH 4 ) inversions.…”

“…The results of this study indicate that the spatial resolution of the model used in the inversion analysis (2 • × 2.5 • or 4 • × 5 • ) has a relatively minor impact on posterior NEE anomalies. This somewhat surprising since recent studies (Yu et al, 2018;Stanevich, 2018) have shown significant transport differences for different resolution versions of GEOS-Chem. Also, Deng et al (2015) showed that there are large biases in CO 2 in the upper troposphere and lower stratosphere in GEOS-Chem that impact inferred flux estimates.…”

Attribution of the accelerating increase in atmospheric methane during 2010–2018 by inverse analysis of GOSAT observations

“…This is thought to result from excessive meridional transport in the stratosphere, a common problem in global models (Patra et al, 2011). In particular, coarse-resolution global models have difficulty resolving polar vortex dynamics that control the distribution of stratospheric methane in the winter-spring hemisphere (Stanevich et al, 2019). GEOS-Chem model evaluation with stratospheric sub-columns derived from ground-based TCCON column measurements shows that the stratospheric bias varies seasonally (Saad et al, 2016).…”

On what scales can GOSAT flux inversions constrain anomalies in terrestrial ecosystems?

“…has a relatively minor impact on posterior NEE anomalies. This somewhat surprising since recent studies (Yu et al, 2018;Stanevich, 2018) have shown significant transport differences for different resolution versions of GEOS-Chem. Also, Deng et al (2015) showed that there are large biases in CO 2 in the upper troposphere and lower stratosphere in GEOS-Chem that impact inferred flux estimates.…”

“…The spatial resolution is varied to examine whether changes in model transport significantly impact our results. It has previously been shown that there are significant differences in tracer transport as model resolution is decreased in GEOS-Chem (Yu et al, 2018;Stanevich, 2018). In particular, Stanevich (2018) show that resolution-induced biases of up to 30% can be introduced on the scale of TransCom regions for 4°× 5°relative to 2°× 2.5°for atmospheric methane (CH 4 ) inversions.…”