Constraining cloud droplet number concentration in GCMs suppresses the aerosol indirect effect

• Published in: Geophysical research letters Vol. 36; no. 12; pp. L12807 - n/a
• Main Authors: Hoose, C., Kristjánsson, J. E., Iversen, T., Kirkevåg, A., Seland, Ø., Gettelman, A.
• Format: Journal Article
• Language: English
• Published: Washington, DC American Geophysical Union 01.06.2009; Blackwell Publishing Ltd; John Wiley & Sons, Inc
• Subjects: aerosol indirect effects; Aerosols; Atmospheric Composition and Structure; Atmospheric Processes; Atmospheric sciences; Biogeochemical cycles, processes, and modeling; Climate change; Climate models; Climate science; Cloud/radiation interaction; Clouds; Clouds and aerosols; Earth; Earth sciences; Earth, ocean, space; Exact sciences and technology; Global Change; Global climate models; Marine; Oceans; Radiation; global climate models; aerosol indirect effects; Lower bound; General circulation models; aerosols; droplets; concentration; Climate models; global; digital simulation; clouds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2009GL038568

Global aerosol‐climate models with prognostic treatment of cloud droplet number concentration (CDNC) often prescribe lower bounds for CDNC or aerosol concentrations. Here we demonstrate that this possibly unphysical constraint reduces the simulated aerosol indirect effect by up to 80%, caused by extensively uniform CDNCs. In present‐day conditions, the impact of the prescribed lower bound for CDNC is mainly visible over oceans, while with preindustrial emissions, large parts of both land and ocean areas are influenced. We furthermore show that imposing the same constraints on aerosol instead of on CDNC reduces the aerosol indirect effect to a lesser extent.