Model intercomparison of indirect aerosol effects J. E. Penner1, J. Quaas2,*, T. Storelvmo3, T. Takemura4, O. Boucher5,**, H. Guo1, A. Kirkevåg3, J. E. Kristjánsson3, and Ø. Seland3 1University of Michigan, Department of Atmospheric, Oceanic and Space Sciences, Ann Arbor, USA 2Laboratoire de Météorologie Dynamique, CNRS/Institut Pierre Simon Laplace, 4, place Jussieu, 75005 Paris, France 3University of Oslo, Department of Geosciences, Oslo, Norway 4Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan 5Laboratoire d'Optique Atmosphérique, CNRS/Universite de Lille I, 59655 Villeneuve d'Ascq Cedex, France *now at: Max Planck Institute for Meteorology, Bundesstraße 53, Hamburg, Germany **now at: Hadley Centre, Met Office, FitzRoy Road, Exeter EX1 3PB, UK
Abstract. Modeled differences in predicted effects are increasingly used to help
quantify the uncertainty of these effects. Here, we examine modeled
differences in the aerosol indirect effect in a series of experiments that
help to quantify how and why model-predicted aerosol indirect forcing varies
between models. The experiments start with an experiment in which aerosol
concentrations, the parameterization of droplet concentrations and the
autoconversion scheme are all specified and end with an experiment that
examines the predicted aerosol indirect forcing when only aerosol sources
are specified. Although there are large differences in the predicted liquid
water path among the models, the predicted aerosol first indirect effect
for the first experiment is rather similar, about −0.6 Wm−2 to −0.7 Wm−2.
Changes to the autoconversion scheme can lead to large changes in the liquid
water path of the models and to the response of the liquid water path to
changes in aerosols. Adding an autoconversion scheme that depends on the droplet
concentration caused a larger (negative) change in net outgoing shortwave
radiation compared to the 1st indirect effect, and the increase varied from
only 22% to more than a factor of three. The change in net shortwave
forcing in the models due to varying the autoconversion scheme depends on
the liquid water content of the clouds as well as their predicted droplet
concentrations, and both increases and decreases in the net shortwave
forcing can occur when autoconversion schemes are changed. The
parameterization of cloud fraction within models is not sensitive to the
aerosol concentration, and, therefore, the response of the modeled cloud
fraction within the present models appears to be smaller than that which
would be associated with model "noise". The prediction of aerosol
concentrations, given a fixed set of sources, leads to some of the largest
differences in the predicted aerosol indirect radiative forcing among the
models, with values of cloud forcing ranging from −0.3 Wm−2 to −1.4 Wm−2.
Thus, this aspect of modeling requires significant improvement in
order to improve the prediction of aerosol indirect effects.
Citation: Penner, J. E., Quaas, J., Storelvmo, T., Takemura, T., Boucher, O., Guo, H., Kirkevåg, A., Kristjánsson, J. E., and Seland, Ø.: Model intercomparison of indirect aerosol effects, Atmos. Chem. Phys., 6, 3391-3405, doi:10.5194/acp-6-3391-2006, 2006.