Development of a grid-independent GEOS-chem chemical transport model as an atmospheric chemistry module for Earth System Models

6 The GEOS-Chem global chemical transport model (CTM), used by a large atmospheric chemistry 7 research community, has been re-engineered to also serve as an atmospheric chemistry module for Earth 8 System Models (ESMs). This was done using an Earth System Modeling Framework (ESMF) interface 9 that operates independently of the GEOS-Chem scientific code, permitting the exact same GEOS-Chem 10 code to be used as an ESM module or as a stand-alone CTM. In this manner, the continual stream of 11 updates contributed by the CTM user community is automatically passed on to the ESM module, which 12 remains state-of-science and referenced to the latest version of the standard GEOS-Chem CTM. A major 13 step in this re-engineering was to make GEOS-Chem grid-independent, i.e., capable of using any 14 geophysical grid specified at run time. GEOS-Chem data “sockets” were also created for communication 15 between modules and with external ESM code via the ESMF. The grid-independent, ESMF-compatible 16 GEOS-Chem is now the standard version of the GEOS-Chem CTM. It has been implemented as an 17 atmospheric chemistry module into the NASA GEOS-5 ESM. I The coupled GEOS-5/GEOS-Chem 18 system was tested for scalability and performance with a tropospheric oxidant-aerosol simulation (120 19 coupled species, 66 transported tracers) using 48-240 cores and MPI parallelization. Numerical 20 experiments demonstrate that the GEOS-Chem chemistry module scales efficiently for the number of 21 processors tested. Although inclusion of atmospheric chemistry in ESMs is computationally expensive, 22 the excellent scalability of the chemistry module means that the relative cost goes down with increasing 23 number of MPI processes. 24 25