A comparison of the transport of long-lived atmospheric trace gas species from two advection schemes incorporated into an atmospheric general circulation model

Two tracer advection schemes have been compared using an atmospheric chemical transport model based on the U.K. Met Office’s Unified Model (UM). Two experiments were carried out. The first used an inert calibration tracer (SF6) whilst the second experiment modelled two methane (CH4) carbon isotopic species. In both experiments, tracer emissions were prescribed at the Earth’s surface. Results from both experiments suggest that the global distribution of long lived trace species in the troposphere is not strongly dependent on the choice of the advection scheme. This is attributed to the fact that turbulent boundary layer mixing and vertical transport by convection, in addition to advection play a role in determining the large-scale tracer distribution in the troposphere. For the SF6 experiment, results from the UM were comparable to results from other transport models using the same experimental protocol. The UM’s default, total variation diminishing (TVD) advection scheme produced low values of marine boundary layer mixing ratios compared with other models and measurements. This was attributed to the TVD scheme being too diffusive resulting in unrealistically fast vertical transport. Comparisons between measured and modelled CH4 profiles in the stratosphere clearly shows that the vertical transport in the TVD scheme is too fast.