New Modal Aerosol Module for Community Atmosphere Model


Accurately simulating climate change requires inclusion of full interactions between tiny aerosol particles, clouds, and climate. This, in turn, requires that aerosol size and mixing conditions be resolved and that multiple species be carried in the climate model. A new aerosol scheme that includes these features is now available for the Community Earth System Model (CESM1). DOE researchers at Pacific Northwest National Laboratory led the development of a Modal Aerosol Module (MAM) for the Community Atmospheric Model version 5 (CAM5), the atmospheric component of CESM1. MAM can simulate the aerosol size distribution and mixing states between different aerosol components, and can treat numerous aerosol physical and chemical processes. Two versions of MAM were developed: a complete version with seven aerosol modes serving as the benchmark and used for detailed aerosol studies, and a simplified version with three aerosol modes used for decade-to-century climate simulations. MAM does a good job of simulating the temporal and spatial distributions of aerosol mass, number, and size distribution, and aerosol optical depth compared to observations, although some biases, such as underestimation of black carbon in the Arctic and underestimation of aerosol loading near source regions, will require further development. MAM is being used in CESM1 for the Intergovernmental Panel on Climate Change Fifth Assessment Report. MAM has also been adopted by other major global and regional models (e.g., NASA GEOS-5 and the Weather Research Forecast Model). The complexities of aerosol properties and processes and limitations of computer resources have made it a challenge for global climate models (GCMs) to realistically represent aerosols. MAM’s ability to minimally represent aerosols in GCMs while capturing the essentials of aerosol forcing is a substantial achievement.


Liu, X., R. C. Easter, S. J. Ghan, R. Zaveri, P. Rasch, X. Shi, J.-F. Lamarque, A. Gettelman, H. Morrison, F. Vitt, A. Conley, S. Park, R. Neale, C. Hannay, A. M. Ekman, P. Hess, N. Mahowald, W. Collins, M. J. Iacono, C. S. Bretherton, M. G. Flanner, and D. Mitchell. 2012. “Toward a Minimal Representation of Aerosols in Climate Models: Description and Evaluation in the Community Atmosphere Model CAM5,” Geoscientific Model Development 5,709-39. DOI: 10.5194/gmd-5-709-2012.