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. This work examines the causes of this dissipation and how it can be cured. GEOS-5 meteorological data at 0.25°×0.3125° horizontal resolution and ~0.6 km vertical resolution in the free troposphere are used to drive a worldwide ensemble of GEOS-Chem CTM plumes at resolutions from 0.25°×0.3125° 10 to 4°×5°, in both 2-D (horizontal) and 3-D. 2-D 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 plume until GEOS-Chem’s high-order advection scheme cannot resolve gradients and fast numerical diffusion ensues. This divergence, measured by the Lyapunov exponent (λ), is much stronger at extratropical latitudes than in the tropics, resulting in accelerated decay. The plume decay constant (α) is linearly related to λ, and increasing grid resolution provides only modest benefits toward plume 15 preservation. 3-D 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. Increases in vertical grid resolution in the free troposphere should be prioritized over horizontal resolution if intercontinental transport in global Eulerian models is to be resolved.