Articles | Volume 17, issue 4
Research article
20 Feb 2017
Research article |  | 20 Feb 2017

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

Sebastian D. Eastham and Daniel J. Jacob

Abstract. 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°  ×  0.3125° horizontal resolution and  ∼  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°  ×  0.3125° to 4°  ×  5°, 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 (λ) 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 (α) is linearly related to λ, 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 (∼  1°) is sufficient.

Short summary
Intercontinental atmospheric transport can disrupt local chemistry and cause air quality issues thousands of kilometers from the source, complicating correct attribution of air quality exceedances. This transport occurs in long, thin plumes which current-generation models consistently fail to reproduce. Our study investigates the cause of this failure, finding that greater vertical resolution than is currently available is required to reliably resolve the plumes and their effects.
Final-revised paper