Errors and improvements in the use of archived meteorological data for chemical transport modeling

Global simulations of atmospheric chemistry are generally 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, in part, 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 222 Rn- 210 Pb- 7 Be chemical tracer simulations off-line in the GEOS-Chem CTM at rectilinear 0.25° x 0.3125° (a 25 km) and 2° x 2.5° (a 200 km) resolutions, and on-line in the parent GEOS-5 GCM at cubed-sphere c360 (a 25 km) and c48 (a 200 km) horizontal resolutions. The c360 GEOS-5 GCM meteorological archive, updated every 3 hours and remapped to 0.25° x 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 222 Rn simulation at native 0.25° x 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° x 0.3125° to 2° x 2.5° induces further weakening of vertical transport as transient vertical motions are averaged out spatially as well as temporally. The resulting 222 Rn 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 210 Pb and 7 Be 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 in spatial averaging of boundary layer mixing depths.