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.
