| Summary: | The TOMCAT 3-D chemical transport model (CTM) has been used to investigate the cause of recent variations in global atmospheric methane (CH4), focusing on examining changes in the balance of sources and sinks of the species. The chemical loss, transport and emissions of methane have been studied and a new 4D-Var inverse version of TOMCAT has been created. The accuracy of the TOMCAT model transport was investigated by simulating the distribution of the long-lived species SF6. A range of model grid resolutions, boundary layer schemes and advection schemes were tested. New retrievals from the Atmospheric Chemistry Experiment (ACE) satellite instrument were used to test the model in the upper troposphere and lower stratosphere. The standard CTM simulated the observed distribution and growth of SF6 well. However, based on comparison with ground-based data, the interhemispheric transport in the TOMCAT model was found to be approximately 20% too slow, with too little temporal variation in southern hemisphere transport. On the whole, however, tracer transport in the CTM using its standard set-up was accurate. As a basis for the inverse model simpler advection and boundary layer (BL) schemes were tested. The advection scheme which conserved only up to first-order moments (rather than secondorder moments) did not significantly reduce the accuracy of the model transport. However, use of a local boundary layer mixing scheme rather than a non-local scheme did degrade the quality of the transport by reducing the speed of vertical mixing out of the BL. A number of currently used CH4 emission inventories were used with the forward TOMCAT model in order to examine the effect they have on the global CH4 budget, and two different estimates of the OH sink were also tested. A published OH field derived from global CH3CCl3 and a chemical box model was found to be more consistent with OH observations than the field from the full chemistry TOMCAT model. Although both OH fields produced global CH4 lifetimes consistent with published estimates, the TOMCAT OH field yielded model CH4 which was up to 100 ppb higher than observations at the surface. Data assimilation was used to improve the estimate of the stratospheric sink of CH4. Although this sink is small overall, it needs to be represented realistically in order to accurately reproduce global CH4 to within 10 ppbv. A new adjoint version of the TOMCAT model was produced by explicit coding, and was thoroughly tested. This was incorporated into a new 4D-Var inverse model which can be used to produce updated CH4 surface flux estimates which are constrained to agree with atmospheric observations. The inverse model was used to investigate emissions in the Arctic where the forward TOMCAT model and standard emissions revealed a seasonal cycle out of phase with surface CH4 observations. It was found that northern hemisphere summertime wetland emissions were overestimated in the GISS inventory by up to 100% for the period 2000-2006, and that this was likely due to the estimates of emission rates and thaw period used when producing that inventory. It was also found that increased Asian emissions suggested in the EDGAR V4.0 inventory are not consistent with observations unless mitigated by a corresponding drop in emissions elsewhere.
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