Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-4341-2006 http://www.atmos-chem-phys-discuss.net/6/4341/2006/ http://www.atmos-chem-phys-discuss.net/6/4341/2006/acpd-6-4341-2006.html http://www.atmos-chem-phys-discuss.net/6/4341/2006/acpd-6-4341-2006.pdf 4341 4373 2006-05-31 Impact of cloud-borne aerosol representation on aerosol direct and indirect effects S. J. Ghan R. C. Easter Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington, USA Aerosol particles attached to cloud droplets are much more likely to be removed from the atmosphere and are much less efficient at scattering sunlight than if unattached. Models used to estimate direct and indirect effects of aerosols employ a variety of representations of such cloud-borne particles. Here we use a global aerosol model with a relatively complete treatment of cloud-borne particles to estimate the sensitivity of simulated aerosol, cloud and radiation fields to various approximations to the representation of cloud-borne particles. We find that neglecting transport of cloud-borne particles introduces little error, but that diagnosing cloud-borne particles produces global mean biases of 20% and local errors of up to 40% for many variables of interest. A treatment that predicts the total mass concentration of cloud-borne particles for each mode yields smaller errors and runs 20% faster than the complete treatment.