American Geophysical Union Fall Meeting, San Francisco, CA, December 6-10, 1998

A Study of the Impact of Atmospheric Aerosol on Cloud Microphysics

C. Andronache, L. J. Donner, V. Ramaswamy, C. J. Seman, and R. S. Hemler
Princeton University, GFDL NOAA, Princeton, New Jersey, 08542

Abstract. Tropospheric aerosols influence the microphysical and radiative characteristics of clouds and it has been proposed that an increase of anthropogenic aerosols can have an indirect effect on climate. During the development of cloud, the characteristics of the cloud drop size distribution are initially determined by the size distribution and chemical properties of the aerosol particles that form cloud condensation nuclei and by the local updraft velocity. It is common in current studies that address the indirect effect of aerosols to use parameterizations that link the cloud droplet concentration to atmospheric sulfate concentration. Such empirical relationships based on available measurements in continental and maritime clouds do not account for the local updraft velocities and saturation conditions predicted by the model and might cause uncertainties.

In an attempt to link the tropospheric aerosol properties to cloud microphysics and to understand some of the uncertainties in the current estimations of the indirect radiative forcing, we use a three dimensional dynamic cloud resolving model developed at the Geophysical Fluid Dynamics Laboratory. The model has an explicit scheme for cloud microphysics that includes processes such as droplet activation from sea salt and sulfate aerosol, growth by condensation and collection, conversion of cloud droplets in ice, snow, rain, and removal by precipitation. The relationship between aerosol loading and cloud droplet concentration is illustrated for cases of convective activity over maritime regions based on field experiments in western and tropical Pacific.

Copyright 1998 by the American Geophysical Union.