Diversity of Aerosol Simulations in the NASA GEOS-5 Model: Impacts of Spatial Resolution, Meteorology, and Model Physics

Abstract:

Uncertainties in aerosol distributions, properties, and processes allow for a wide diversity of aerosol representations in global models, which complicates the attribution of simulated climate change, for example, to specific forcing agents. A component of model diversity results from fundamental unknowns in critical aerosol parameters, such as optical properties. Other components of model diversity, however, arise from structural considerations of the models themselves, for instance, spatial resolution and basic model physics. We investigate these sources of diversity using different instances of the NASA Goddard Earth Observing System version 5 (GEOS-5) Earth system model. GEOS-5 runs the Goddard Chemistry, Aerosol, Radiation, and Transport (GOCART) aerosol module online and radiatively coupled within its atmospheric general circulation model. Three specific structural considerations are addressed: horizontal spatial resolution, meteorology, and model physics. For all cases the GOCART aerosol module is held fixed. Results of the different simulations are intercompared and evaluated in the context of aerosol remote sensing observations from MODIS, MISR, AERONET, and CALIOP. Spatial resolution is considered by running the system at grid resolutions varying from about 2º (typical of climate simulations) to 0.25º (typical of global weather forecasting systems). Meteorology is considered by driving the model with either internally generated winds or else by replaying from the MERRA meteorological reanalysis. Model physics is considered by testing two versions of the GEOS-5 system. For all cases we consider the resulting diversity among the simulated aerosol distributions and properties, in comparison with observations, as well as the impact on computed aerosol radiative fluxes.