Large-Scale Tropospheric Transport: Constraints from Models and Observations
The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. A natural way to quantify tropospheric transport is in terms of the distribution of transit times from the Northern Hemisphere (NH) midlatitude surface, where various greenhouse gases and ozone-depleting substances are emitted. Here, we show that the transit-time distribution (TTD), as simulated by the NASA Global Modeling Initiative Chemistry Transport Model is characterized by mean transit times or “mean ages” that are significantly larger than their corresponding modal transit times. Idealized loss and mean age tracers are then used to constrain different TTD timescales both from observations (e.g. sulfur hexafluoride and chlorofluorocarbons) and from a broad range of models participating in the Chemistry Climate Modeling Initiative. A comparison of simulations using the Goddard Earth Observing System Chemistry Climate Model and the Whole Atmosphere Community Climate Model, wherein the large-scale flow is constrained to MERRA meteorology, reveals large (30%) differences in interhemispheric transport that are related to large differences in convection over the oceans. Our results indicate that more attention needs to focus on convective parameterizations in models and the methods by which they are applied in simulations using analyzed winds.