1Department of Physics, University of Oxford, Parks Road, OX1 3PU, Oxford, UK
2Hadley Centre, Met Office, Exeter, UK
3Joint Center for Earth Systems Technology, University of Maryland at Baltimore County, Baltimore, USA
4NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
5Laboratoire de Météorologie Dynamique, IPSL, CNRS/UPMC, Paris, France
6Pacific Northwest National Laboratory, Richland, USA
7Max Planck Institute for Meteorology, Hamburg, Germany
8Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Michigan, USA
9Atmospheric Sciences Research Center, State University of New York, Albany, USA
10Center for International Climate and Environmental Research Oslo (CICERO), Oslo, Norway
11GESTAR/Morgan State University, Baltimore, Maryland, USA
12Norwegian Meteorological Institute, Oslo, Norway
13Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan
14Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA
*now at: Department of Meteorology, University of Reading, Reading, UK
Abstract. Simulated multi-model "diversity" in aerosol direct radiative forcing estimates is often perceived as a measure of aerosol uncertainty. However, current models used for aerosol radiative forcing calculations vary considerably in model components relevant for forcing calculations and the associated "host-model uncertainties" are generally convoluted with the actual aerosol uncertainty. In this AeroCom Prescribed intercomparison study we systematically isolate and quantify host model uncertainties on aerosol forcing experiments through prescription of identical aerosol radiative properties in twelve participating models.
Even with prescribed aerosol radiative properties, simulated clear-sky and all-sky aerosol radiative forcings show significant diversity. For a purely scattering case with globally constant optical depth of 0.2, the global-mean all-sky top-of-atmosphere radiative forcing is −4.47 Wm−2 and the inter-model standard deviation is 0.55 Wm−2, corresponding to a relative standard deviation of 12%. For a case with partially absorbing aerosol with an aerosol optical depth of 0.2 and single scattering albedo of 0.8, the forcing changes to 1.04 Wm−2, and the standard deviation increases to 1.01 W−2, corresponding to a significant relative standard deviation of 97%. However, the top-of-atmosphere forcing variability owing to absorption (subtracting the scattering case from the case with scattering and absorption) is low, with absolute (relative) standard deviations of 0.45 Wm−2 (8%) clear-sky and 0.62 Wm−2 (11%) all-sky.
Scaling the forcing standard deviation for a purely scattering case to match the sulfate radiative forcing in the AeroCom Direct Effect experiment demonstrates that host model uncertainties could explain about 36% of the overall sulfate forcing diversity of 0.11 Wm−2 in the AeroCom Direct Radiative Effect experiment.
Host model errors in aerosol radiative forcing are largest in regions of uncertain host model components, such as stratocumulus cloud decks or areas with poorly constrained surface albedos, such as sea ice. Our results demonstrate that host model uncertainties are an important component of aerosol forcing uncertainty that require further attention.