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. We 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

Variational data assimilation of satellite remote sensing observations for improving methane simulations in chemical transport models

Atmospheric concentrations of methane (CH4), the second most important anthropogenic greenhouse gas, have been rapidly rising since 1850, however, the rate of increase has varied in recent decades. In order to attribute these trends, significant effort has been put into characterizing CH4 surface emissions using inverse modelling ("top-down") approaches, which rely on the ability of chemical transport models (CTMs) to simulate atmospheric CH4 fields. However, systematic errors in models can significantly reduce the quality of the CH4 simulation and result in biased CH4 emission estimates. Until now, errors in models have been poorly characterized. The objective of this thesis was to characterize and investigate the origin of the CH4 model errors in the GEOS-Chem CTM and quantify their impact on inferred CH4 emission estimates. The weak constraint four-dimensional variational data assimilation scheme in GEOS-Chem, together with CH4 data from the Greenhouse gases Observing SATellite (GOSAT), were used to characterize model errors in GEOS-Chem at the horizontal resolutions of 4x5 and 2x2.5. Large biases in CH4 were found in the stratosphere and in vertical transport in the troposphere at mid-latitudes. The identified errors were significantly larger at 4x5 than at 2x2.5. It was determined that a major cause of the biases at 4x5 is excessive mixing due to increased numerical diffusion manifested in enhanced stratosphere-troposphere exchange, and stronger quasi-isentropic mixing through the edges of the "tropical pipe" and the polar vortex in the stratosphere. Coarsening of the model grid also weakened vertical transport in the troposphere due to the loss of advective air mass fluxes and sub-grid tracer eddy mass fluxes. A key outcome of this work is the recommendation that the 4x5 version of GEOS-Chem should not be used for inverse modeling of CH4 emissions. The thesis also investigated the sensitivity of North American CH4 emission estimates in the nested version of GEOS-Chem (at the 0.5x0.67) to biases in boundary conditions from the coarse global resolution model. It was shown that biases not fully mitigated in the global CH4 simulation could result in biases as large as 30-35% in monthly mean surface emission estimates on local to regional scales.Ph.D

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 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 column 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, 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 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 online transport in the native general circulation model as a benchmark simulation