Abstract
Abstract. Chemical plumes in the free troposphere can preserve their identity for more than a week as they are transported on intercontinental scales. Current global models cannot reproduce this transport. The plumes dilute far too rapidly due to numerical diffusion in sheared flow. We show how model accuracy can be limited by either horizontal resolution (Δx) or vertical resolution (Δz). Balancing horizontal and vertical numerical diffusion, and weighing computational cost, implies an optimal grid resolution ratio (Δx ∕ Δz)opt ∼ 1000 for simulating the plumes. This is considerably higher than current global models (Δx ∕ Δz ∼ 20) and explains the rapid plume dilution in the models as caused by insufficient vertical resolution. Plume simulations with the Geophysical Fluid Dynamics Laboratory Finite-Volume Cubed-Sphere Dynamical Core (GFDL-FV3) over a range of horizontal and vertical grid resolutions confirm this limiting behavior. Our highest-resolution simulation (Δx ≈ 25 km, Δz ≈ 80 m) preserves the maximum mixing ratio in the plume to within 35 % after 8 days in strongly sheared flow, a drastic improvement over current models. Adding free tropospheric vertical levels in global models is computationally inexpensive and would also improve the simulation of water vapor.
Highlights
Global transport of pollution mainly takes place in the free troposphere where winds are strong and pollutant lifetimes are long
Eastham and Jacob (2017) used Goddard Earth Observing System (GEOS)-FP meteorological data with 0.25◦ × 0.3125◦ horizontal resolution and 72 vertical levels to drive the offline GEOS-Chem chemical transport model (CTM) with horizontal resolutions ranging from 0.25◦ × 0.3125◦ to 4◦ × 5◦, all with 72 vertical levels and using conservative regridding of the native meteorological fields for the coarser simulations
They found that increasing the horizontal resolution is effective in preserving plumes in 2-D simulations but fails with 3-D plumes because the coarse vertical resolution of the native GEOS forward processing (GEOS-FP) data incurs large vertical numerical diffusion
Summary
Global transport of pollution mainly takes place in the free troposphere where winds are strong and pollutant lifetimes are long. Eastham and Jacob (2017) used GEOS-FP meteorological data with 0.25◦ × 0.3125◦ horizontal resolution and 72 vertical levels to drive the offline GEOS-Chem chemical transport model (CTM) with horizontal resolutions ranging from 0.25◦ × 0.3125◦ to 4◦ × 5◦, all with 72 vertical levels and using conservative regridding of the native meteorological fields for the coarser simulations They found that increasing the horizontal resolution is effective in preserving plumes in 2-D simulations (horizontal-only, no vertical dimension) but fails with 3-D plumes because the coarse vertical resolution of the native GEOS-FP data incurs large vertical numerical diffusion. Increasing free tropospheric vertical resolution in GCMs would have meteorological implications for the transport of water vapor, similar to chemical plumes (Tompkins and Emanuel, 2000; Pope et al, 2001). A realistic sheared/stretched atmospheric flow can be simulated in a dry dynamical core by triggering baroclinic instability (Jablonowski and Williamson, 2006)
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