Tropospheric Transport Differences Between Models Using the Same Large-Scale Meteorological Fields

The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. A common approach is to transport gases using the winds from meteorological analyses, either using them directly in a chemical transport model or by constraining the flow in a general circulation model. Here we compare the transport of idealized tracers between different models developed at NASA and at NCAR that use the same meteorological fields taken from MERRA. We show that, even though the models have the same large-scale flow, there are substantial differences in their global-scale tropospheric transport properties due to large differences in parameterized convection between the simulations, especially over the subtropical and tropical oceans in the Northern Hemisphere. Furthermore, we find that the transport differences between constrained versions of the models is larger than the differences between free-running simulations of the models, which have differing large-scale flow but much more similar convective mass fluxes. Our results indicate that to better understand large-scale tropospheric transport more attention needs to focus on convective parameterizations in models and the methods by which they are applied in simulations using analyzed winds.