Three-dimensional transport and concentration of SF6 - A model intercomparison study (TransCom 2)

Sulfur hexafluoride (SF6) is an excellent tracer of large-scale atmospheric transport, because it has slowly increasing sources mostly confined to nor thern midlatitudes, and has a lifetime of thousands of years. We have simul ated the emissions, transport, and concentration of SF, for a 5-year period , and compared the results with atmospheric observations. In addition, we h ave performed an intercomparison of interhemispheric transport among 11 mod els to investigate the reasons for the differences among the simulations. M ost of the models are reasonably successful at simulating the observed meri dional gradient of SF6 in the remote marine boundary layer, though there is less agreement at continental sites. Models that compare well to observati ons in the remote marine boundary layer tend to systematically overestimate SF6 at continental locations in source regions, suggesting that vertical t rapping rather than meridional transport may be a dominant control on the s imulated meridional gradient. The vertical structure of simulated SF6 in th e models supports this interpretation. Some of the models perform quite wel l in terms of the simulated seasonal cycle at remote locations, while other s do not. Interhemispheric exchange time varies by a factor of 2 when estim ated from 1-dimensional meridional profiles at the surface, as has been don e for observations. The agreement among models is better when the global su rface mean mole fraction is used, and better still when the full 3-dimensio nal mean mixing ratio is used. The ranking of the interhemispheric exchange time among the models is not sensitive to the change From station values t o surface means, but is very sensitive to the change from surface means to the full 3-dimensional tracer fields. This strengthens the argument that ve rtical redistribution dominates over interhemispheric transport in determin ing the meridional gradient at the surface. Vertically integrated meridiona l transport in the models is divided roughly equally into transport by the mean motion, the standing eddies, and the transient eddies. The vertically integrated mass flux is a good index of the degree to which resolved advect ion vs. parameterized diffusion accomplishes the meridional transport of SF 6. Observational programs could provide a much better constraint on simulat ed chemical tracer transport if they included regular sampling of vertical profiles of nonreactive trace gases over source regions and meridional prof iles in the middle to upper troposphere. Further analysis of the SF6 simula tions will focus on the subgrid-scale parameterized transports.