title: Challenges in constraining anthropogenic aerosol effects on cloud radiative forcing using present-day spatiotemporal variability.
abstract: A large number of processes are involved in the chain from emissions of aerosol precursor gases and primary particles to impacts on cloud radiative forcing. Those processes are manifest in a number of relationships that can be expressed as factors dlnX/dlnY driving aerosol effects on cloud radiative forcing. These factors include the relationships between cloud condensation nuclei (CCN) concentration and emissions, droplet number and CCN concentration, cloud fraction and droplet number, cloud optical depth and droplet number, and cloud radiative forcing and cloud optical depth. The relationship between cloud optical depth and droplet number can be further decomposed into the sum of two terms involving the relationship of droplet effective radius and cloud liquid water path with droplet number. These relationships can be constrained using observations of recent spatial and temporal variability of these quantities. However, we are most interested in the radiative forcing since the preindustrial era. Because few relevant measurements are available from that era, relationships from recent variability have been assumed to be applicable to the preindustrial to present-day change. Our analysis of Aerosol Comparisons between Observations and Models (AeroCom) model simulations suggests that estimates of relationships from recent variability are poor constraints on relationships from anthropogenic change for some terms, with even the sign of some relationships differing in many regions. Proxies connecting recent spatial/temporal variability to anthropogenic change, or sustained measurements in regions where emissions have changed, are needed to constrain estimates of anthropogenic aerosol impacts on cloud radiative forcing.
authors.affiliation: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352; Steve.Ghan@pnnl.gov.
authors.forename: Steven
authors.initials: S
authors.lastname: Ghan
authors.affiliation: Institute for Climate and Global Change Research, Nanjing University, 210023 Nanjing, China; School of Atmospheric Sciences, Nanjing University, 210023 Nanjing, China; Collaborative Innovation Center of Climate Change, 210023 Nanjing, China; Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352;
authors.forename: Minghuai
authors.initials: M
authors.lastname: Wang
authors.affiliation: Institute for Climate and Global Change Research, Nanjing University, 210023 Nanjing, China; School of Atmospheric Sciences, Nanjing University, 210023 Nanjing, China; Collaborative Innovation Center of Climate Change, 210023 Nanjing, China;
authors.forename: Shipeng
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland;
authors.forename: Sylvaine
authors.initials: S
authors.lastname: Ferrachat
authors.affiliation: National Center for Atmospheric Research, Boulder, CO 80305;
authors.forename: Andrew
authors.initials: A
authors.lastname: Gettelman
authors.affiliation: Information Technology Division, Norwegian Meteorological Institute, 0313 Oslo, Norway;
authors.forename: Jan
authors.initials: J
authors.lastname: Griesfeller
authors.affiliation: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford OX13PU, United Kingdom;
authors.forename: Zak
authors.initials: Z
authors.lastname: Kipling
authors.affiliation: Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland;
authors.forename: Ulrike
authors.initials: U
authors.lastname: Lohmann
authors.affiliation: National Center for Atmospheric Research, Boulder, CO 80305;
authors.forename: Hugh
authors.initials: H
authors.lastname: Morrison
authors.affiliation: Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland;
authors.forename: David
authors.initials: D
authors.lastname: Neubauer
authors.affiliation: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford OX13PU, United Kingdom; Department of Environmental Science and Analytical Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden; Bert Bolin Centre for Climate Research, Stockholm University, SE-106 91 Stockholm, Sweden;
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Partridge
authors.affiliation: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Oxford OX13PU, United Kingdom;
authors.forename: Philip
authors.initials: P
authors.lastname: Stier
authors.affiliation: Research Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan.
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Takemura
authors.affiliation: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352;
authors.forename: Hailong
authors.initials: H
authors.lastname: Wang
authors.affiliation: Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352;
authors.forename: Kai
authors.initials: K
authors.lastname: Zhang
keywords: aerosol radiative forcing; cloud−aerosol interactions; constraints; factors