Abstract
Global simulations of atmospheric chemistry are commonly conducted with off-line chemical transport models (CTMs) driven by archived meteorological data from general circulation models (GCMs). The off-line approach has advantages of simplicity and expediency, but incurs errors due to temporal averaging in the meteorological archive and the inability to reproduce the GCM transport algorithms exactly. The CTM simulation is also often conducted at coarser grid resolution than the parent GCM. Here we investigate this cascade of CTM errors by using 222Rn-210Pb-7Be chemical tracer simulations offline in the GEOS-Chem CTM at rectilinear 0.25° ×0.3125° (≈25 km) and 2° ×2.5° (≈200 km) resolutions, and on-line in the parent GEOS-5 GCM at cubed-sphere c360 (≈25 km) and c48 (≈200 km) horizontal resolutions. The c360 GEOS-5 GCM meteorological archive, updated every 3 hours and remapped to 0.25° ×0.3125°, is the standard operational product generated by the NASA Global Modeling and Assimilation Office (GMAO) and used as input by GEOS-Chem. We find that the GEOS-Chem 222Rn simulation at native 0.25° ×0.3125° resolution is affected by vertical transport errors of up to 20% relative to the GEOS-5 c360 on-line simulation, in part due to loss of transient organized vertical motions in the GCM (resolved convection) that are temporally averaged out in the 3-hour meteorological archive. There is also significant error caused by operational remapping of the meteorological archive from cubed-sphere to rectilinear grid. Decreasing the GEOS-Chem resolution from 0.25°×0.3125° to 2°×2.5° induces further weakening of vertical transport as transient vertical motions are averaged out spatially as well as temporally. The resulting 222Rn concentrations simulated by the coarse-resolution GEOS-Chem are overestimated by up to 40% in surface air relative to the on-line c360 simulations, and underestimated by up to 40% in the upper troposphere, while the tropospheric lifetimes of 210Pb and 7Be against aerosol deposition are affected by 5-10%. The lost vertical transport in the coarse-resolution GEOS-Chem simulation can be partly restored by re-computing the convective mass fluxes at the appropriate resolution to replace the archived convective mass fluxes, and by correcting for bias 20 in spatial averaging of boundary layer mixing depths.
Highlights
Accurate simulation of transport is crucial for global models of atmospheric composition
We isolated the different sources of transport errors resulting from performing off-line chemical transport model (CTM) simulations with archived meteorological data from a general circulation model (GCM)
We used as a reference an online simulation of the 222Rn–210Pb–7Be chemical tracer suite in the Goddard Earth Observing System (GEOS-5) GCM at cubed-sphere c360 (≈ 25 km) resolution
Summary
Accurate simulation of transport is crucial for global models of atmospheric composition. GCMs compute grid-resolved winds, sub-grid turbulence, and convection properties that determine the transport of chemical species through the corresponding continuity equations (Brasseur and Jacob, 2017). We examine how the off-line archiving of GCM meteorological data, including temporal and spatial averaging, affects the simulation of transport in the GEOS-Chem CTM, and we recommend some corrections for these errors. We use for this purpose the 222Rn–210Pb–7Be tracer suite, which provides a standard basis for evaluating transport and aerosol scavenging in CTMs (Jacob et al, 1997; Liu et al, 2001; Considine et al, 2005; Zhang et al, 2017). Has recently been integrated within the GEOS GCM so that simulations with detailed chemistry can be conducted either online or off-line using the exact same module (Long et al, 2015)
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