Global simulation of tropospheric chemistry at 12 . 5 km resolution : performance and evaluation of the GEOS-Chem chemical module within the NASA GEOS Earth System Model

We present a full-year on-line global simulation of tropospheric chemistry (158 coupled species) at cubed-sphere c720 (~12.5× 12.5km) resolution in the NASA GEOS Earth System Model, Version 5 (GEOS-5 ESM) with GEOS-Chem as a chemical module (G5NR-chem). The GEOS-Chem module within GEOS uses the exact same code as the off-line GEOSChem chemical transport model (CTM) developed by a large atmospheric chemistry research community. In this way, continual updates to the GEOS-Chem CTM by that community can be seamlessly passed on to the GEOS chemical module, which 5 remains state-of-the-science and referenceable to the latest version of GEOS-Chem. The 1-year G5NR-chem simulation was conducted to serve as Nature Run for observing system simulation experiments (OSSEs) in support of the future geostationary satellite constellation for tropospheric chemistry. It required 31 walltime days on 4707 compute cores with only 24% of the time spent on the GEOS-Chem chemical module. Results from the GEOS-5 Nature Run with GEOS-Chem chemistry were shown to be consistent to the off-line GEOS-Chem CTM and were further compared to global and regional observations. The simulation 10 shows no significant global bias for tropospheric ozone relative to the Ozone Monitoring Instrument (OMI) satellite, and is highly correlated with observations spatially and seasonally. It successfully captures the ozone vertical distributions measured by ozonesondes over different regions of the world, as well as observations for ozone and its precursors from the Aug-Sep 2013 SEACRS aircraft campaign over the Southeast US. It systematically overestimates surface ozone concentrations by 10ppbv at sites in the US and Europe, a problem currently being addressed by the GEOS-Chem CTM community and from which the 15 GEOS ESM will benefit through the seamless update of the on-line code.