The modeling of tropospheric methane: How well can point measurements be reproduced by a global model?

Global model simulations of tropospheric methane are presented, using state of the art representations of its terrestrial sources. Parameters critical for its tropospheric sink and transport have been evaluated using CH3CCl3 and SF6. We assess how well available methane measurements can be reproduce d by the model, and how model and measurements can most efficiently be comp ared. Using European Centre for Medium-Range Weather Forecasts reanalyzed m eteorological fields, direct comparisons between model results and flask or in situ measurements are presented, as opposed to comparing multiannual av eraged seasonal cycles and trends as was done in previous studies. When com paring monthly means derived from weekly flask sampling and the model, the agreement at stations as Bermuda East and Mace Head is improved if, instead of sampling the model at each model time step, samples are taken at the sa me times as the measurements were taken. A method is presented to estimate the potential influence of subgrid variability using a marked tracer that i s emitted in the vicinity of observational stations only. From the contribu tion of this tracer to the computed methane concentration at a particular s tation, the potential contribution of subgrid sources can be estimated. Rad on 222 is used to select baseline conditions in the model to improve the co mparability of model and measurements when a clean air sector is selected f or sampling. Comparisons of model results and measurements, screened for lo cal influences and artifacts of wind sector selection, indicate that the mo del has in particular difficulty reproducing seasonal cycles at higher lati tude stations of the Northern Hemisphere. Sensitivity simulations show that the simulated annual variation at these stations is sensitive to the param eterization of wetland emissions. Also at the South China Sea, model simula tions point to errors in the representation of methane sources. Marked trac er simulations indicate that this is most likely related to emissions from natural wetlands and rice paddies, in line with recent inverse modeling and up-scaling estimates.