MULTISCALE STRETCHED-GRIDS FOR FINE RESOLUTION SIMULATIONS OF ATMOSPHERIC COMPOSITION IN A MASSIVELY PARALLEL CHEMICAL TRANSPORT MODEL

Abstract

Modeling atmospheric composition at fine resolution globally is computationally expensive; the capability to focus on specific geographic regions using a multiscale grid is desirable. This work develops, validates, and demonstrates stretched-grids in the massively parallel version of the GEOS-Chem (GCHP) model of atmospheric composition. These multiscale grids are specified at runtime by four parameters that offer modelers nimble control of the region that is refined and the resolution of the refinement. The stretched-grid simulation is validated against global cubed-sphere simulations. The operation and flexibility of stretched-grid simulations is demonstrated with two case studies that compare simulated tropospheric NO2 column densities from stretched-grid and cubed-sphere simulations to retrieved column densities from the TROPOspheric Monitoring Instrument (TROPOMI). The first case study uses a stretched-grid with a broad refinement covering the contiguous US to produce simulated columns that perform similarly to a C180 cubed-sphere simulation at less than 1/9th the computational expense. The second case study experiments with a large stretch-factor for a global stretched-grid simulation with a highly localized refinement with approximately 10 km resolution for California. The refinement improves spatial agreement with TROPOMI columns compared to a C90 cubed-sphere simulation of comparable computational demands, despite the simulation being conducted at finer resolution than parent meteorological fields. Overall this work finds that stretched-grids are a practical tool for regional fine-resolution simulations of atmospheric composition, and enables this capability for a broad scientific community. Stretched-grids are publicly available in GEOS-Chem version 13.0.0.