GACP Projects
An Investigation of the Indirect Effect of Aerosols on Climate: Coupled Chemistry-Climate Modeling and Satellite Validation
Anthony D. Del Genio , PI
Dorothy Koch , Co-I
George Tselioudis , Co-I
Abstract: Previous estimates of the indirect radiative and microphysical effects of tropospheric aerosols on clouds are thought to be highly uncertain as a result of inadequacies in both the aerosol chemistry and cloud parameterization components of models used to predict these effects, as well as the assumptions made in coupling aerosol amount to cloud droplet number. Attempts to date to detect an indirect effect in observations have relied on simplistic land- ocean and/or hemispheric contrasts in cloud properties that ignore the ambiguity created by the varying temperature and dynamical setting of clouds in different regimes. We propose to reduce the uncertainty in the magnitude of the indirect effects using a combined modeling and observational approach. A newly developed sulfur chemistry model (including prognostic H2O2 to limit in-cloud sulfate formation), along with interactive source/transport models for carbonaceous aerosols, sea salt, and soil dust, will be coupled to the GISS GCM. The coupling philosophy will exploit the information in existing observed sulfate aerosol - cloud droplet number relationships about vertical velocity/turbulence levels and effects of non-sulfate aerosols. Autoconversion rates appropriate to GCM grid-scale liquid water contents will be derived from available cloud-scale parameterizations. Validation against a variety of relevant cloud parameter data sets will emphasize differences in cloud droplet size behavior among different cloud types and separation by dynamic regime as the most sensitive means of isolating aerosol effects from other meteorological influences. Estimates will be made of both the indirect radiative and microphysical effects, and the effect will be separated by aerosol type as well.
Our investigation uses as inputs existing cloud droplet - aerosol relations from various field programs, which differ from each other for reasons rarely investigated. It is hoped that despite these uncertainties, the satellite cloud data sets we use will provide some constraints that can allow us to narrow the range of estimates of the indirect aerosol effect. Serious investigation of the large-scale and cloud-scale meteorological conditions under which future aerosol data sets are acquired would greatly enhance the usefulness of the resulting data. We also require as inputs estimates of natural and anthropogenic sources of aerosols and their precursors. Since the indirect effect can only be estimated if the pre-industrial background is known, the lack of information on aerosol and cloud properties in relatively uncontaminated continental areas, to the extent that such areas exist, is a limiting factor in the interpretation of any indirect forcing estimate. Finally, since satellite droplet effective radius estimates are crucial to model validation yet sample only the first optical depth of the cloud, droplet size vertical profiles through the cloud should be part of any field program to measure aerosol effects on clouds.
