18 November 2013

Estimating Aerosol Effects on Cloud Radiative Forcing


The Intergovernmental Panel on Climate Change Fifth Assessment Report recommends that anthropogenic aerosol effects on the planetary energy balance be expressed as an effective radiative forcing that allows clouds to respond to the aerosol while surface temperature is prescribed. The anthropogenic aerosol influence on clouds is often approximated as the anthropogenic aerosol-induced change in the cloud radiative forcing (the impact of clouds on the Earth’s energy balance). In a recent publication, a DOE climate scientist from Pacific Northwest National Laboratory suggests a more representative method by using the change in cloud radiative forcing calculated as a diagnostic with aerosol scattering and absorption neglected. The proposed method also yields an aerosol radiative forcing decomposition that includes a term quantifying the impact of changes in surface albedo. The method requires only two additional diagnostic calculations: the whole-sky Fclean and clear-sky Fclear,clean, top-of-atmosphere radiative flux with aerosol scattering and absorption neglected. All future simulations performed to quantify aerosol radiative forcing should save F, Fclean, and Fclear,clean in the simulation history. All climate models already have the ability to calculate Fclear as a diagnostic for estimating cloud radiative forcing, so these additional diagnostics should not introduce an excessive burden on simulations.

S J Ghan
Pacific Northwest National Laboratory (PNNL)
Ghan, SJ.  2013.  "Technical Note: Estimating Aerosol Effects on Cloud Radiative Forcing."  Atmospheric Chemistry and Physics 9971-9974, doi:10.5194/acp-13-9971-2013.