Simpler Aerosol Representation Captures Essence of Their Influences on Climate
Aerosols affect the energy balance by scattering and absorbing sunlight and through their influence on cloud droplet and ice particle number concentrations. Climate simulations must account for all important radiative forcing mechanisms, including those from human-caused aerosols. DOE scientists at Pacific Northwest National Laboratory (PNNL) developed a detailed aerosol microphysical scheme, including seven distinct aerosol modes, each with its own size distribution and chemical mixing properties. However, it is computationally too expensive to represent this complexity in multi-century climate simulations. To address this challenge, the team developed a simpler three-mode aerosol scheme and compared simulations using the minimal representation of the aerosol to a more complex benchmark, showing that the minimal representation is both accurate enough for climate change simulations and sufficiently inexpensive to enable multi-century simulations. For either scheme, direct aerosol scattering and absorption effects nearly cancel one another. However, the aerosol indirect effect on clouds has a substantial cooling effect from enhanced low-level clouds in spite of a 25% offset from enhanced high-altitude clouds. The simpler, more efficient representation is being used in the Community Earth System Model to simulate future climate change for the Intergovernmental Panel on Climate Change.
Ghan, S. J., X. Liu, R. C. Easter, R. Zaveri, P. J. Rasch, J. H. Yoon, and B. Eaton. 2012. “Toward a Minimal Representation of Aerosols in Climate Models: Comparative Decomposition of Aerosol Direct, Semi-Direct, and Indirect Radiative Forcing,” Journal of Climate, DOI: 10.1175/JCLI-D-11-00650.1, in press.