Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data
Creators: name: Quaas, Johannes
name: Ming, Yi
name: Menon, Surabi
name: Takemura, Toshihiko
name: Wang, Minghuai
name: Penner, Joyce E.
name: Gettelman, Andrew
name: Lohmann, Ulrike
name: Bellouin, Nicolas
name: Boucher, Olivier
name: Sayer, Andrew M.
name: Thomas, Gareth E.
name: McComiskey, Allison
name: Feingold, Graham
name: Hoose, Corinna
name: Kristjansson, J. Egill
name: Liu, Xiaohong
name: Balkanski, Yves
name: Donner, Leo J.
name: Ginoux, Paul A.
name: Stier, Philip
name: Feichter, Johann
name: Sednev, Igor
name: Bauer, Susanne E.
name: Koch, Dorothy
name: Grainger, Roy G.
name: Kirkevåg, Alf
name: Iversen, Trond
name: Seland, Oyvind
name: Easter, Richard
name: Ghan, Steven J.
name: Rasch, Philip J.
name: Morrison, Hugh
name: Lamarque, Jean-François
name: Iacono, Michael J.
name: Kinne, Stefan
name: Schulz, Michael
Publisher: name: ETH Zurich
Year: 2009

description: Aerosol indirect effects continue to constitute one of the most important uncertaintiesfor anthropogenic climate perturbations. Within the international AEROCOM initiative,the representation of aerosol-cloud-radiation interactions in ten different general cir-culation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects,and none of the models explicitly parameterizes aerosol effects on convective clouds.We compute statistical relationships between aerosol optical depth (τa) and variouscloud and radiation quantities in a manner that is consistent between the models andthe satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at leastover the ocean. The relationship betweenτaand liquid water path is simulated muchtoo strongly by the models. It is shown that this is partly related to the representationof the second aerosol indirect effect in terms of autoconversion. A positive relationshipbetween total cloud fraction (fcld) andτaas found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them.In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strongfcld–τarelationship, our results indicate that none can be identified asunique explanation. Relationships similar to the ones found in satellite data betweenτaand cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR –τarelationship show a strongpositive correlation betweenτaandfcld. The short-wave total aerosol radiative forc-ing as simulated by the GCMs is strongly influenced by the simulated anthropogenicfraction ofτa, and parameterisation assumptions such as a lower bound onNd. Never-theless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred fromthe combination of these predictors for the modelled forcings with the satellite-derivedstatistical relationships yields a global annual mean value of−1.5±0.5 Wm−2. An alternative estimate obtained by scaling the simulated clear- and cloudy-sky forcingswith estimates of anthropogenicτaand satellite-retrievedNd–τaregression slopes,respectively, yields a global annual mean clear-sky (aerosol direct effect) estimate of−0.4±0.2 Wm−2and a cloudy-sky (aerosol indirect effect) estimate of−0.7±0.5 Wm−2, with a total estimate of−1.2±0.4 Wm−2.
description: Atmospheric Chemistry and Physics Discussions, 9 (3)
description: ISSN:1680-7375
description: ISSN:1680-7367