Limits on the ability of global Eulerian models to resolve intercontinental transport of chemical plumes

Quasi-horizontal chemical plumes in the free troposphere can preserve their concentrated structure for over a week, enabling transport on intercontinental scales with important environmental impacts. Global Eulerian chemical transport models (CTMs) fail to preserve these plumes due to fast numerical dissipation. We examine the causes of this dissipation and how it can be cured. Goddard Earth Observing System (GEOS-5) meteorological data at 0.25 degrees x 0.3125 degrees horizontal resolution and similar to 0.5 km vertical resolution in the free troposphere are used to drive a worldwide ensemble of GEOS-Chem CTM plumes at resolutions from 0.25 degrees x 0.3125 degrees to 4 degrees x 5 degrees, in both 2-D (horizontal) and 3-D. Two-dimensional simulations enable examination of the sensitivity of numerical dissipation to grid resolution. We show that plume decay is driven by flow divergence and shear, filamenting the plumes until GEOS-Chem's high-order advection scheme cannot resolve gradients and fast numerical diffusion ensues. This divergence can be measured by the Lyapunov exponent (lambda) of the flow. Dissipation of plumes is much faster at extratropical latitudes than in the tropics and this can be explained by stronger divergence. The plume decay constant (alpha) is linearly related to lambda, and increasing grid resolution provides only modest benefits toward plume preservation. Three-dimensional simulations show near-complete dissipation of plumes within a few days, independent of horizontal grid resolution and even in the tropics. This is because vertical grid resolution is inadequate in all cases to properly resolve plume gradients. We suggest that finer vertical grid resolution in the free troposphere is essential for models to resolve intercontinental plumes, while current horizontal resolution in these models (similar to 1 degrees) is sufficient.