Aerosol impacts on upper-tropospheric ice formation

Paul J. DeMott

Colorado State University

Most research concerning the role of aerosol particles as forcing agents on the cloud/climate system has focused on aerosol effects on warm clouds.

Nevertheless, the effects of aerosol particles on ice-phase cloud processes remain poorly understood, including the role of aerosols in affecting precipitation formation processes and the radiative properties of cold clouds, particularly cirrus clouds. Numerical model studies suggest that cirrus microphysics may be much less sensitive to aerosol particle increases than are warm clouds, but that this conclusion depends on assumptions regarding ice nucleation mechanisms (both heterogeneous and homogeneous). Which mechanisms dominate ice nucleation in cirrus is determined by the types of aerosols reaching cirrus altitudes, the activation conditions for different processes and cloud dynamics. Measurements we have made using laboratory surrogates for atmospheric particles and measurements directly in the atmosphere suggest:

1. We may understand practically, if not theoretically, the conditions under which homogeneous freezing usually ensues on background atmospheric aerosol particles;
2. Ice nuclei concentrations active at cirrus conditions are sometimes quite low, but are also quite variable;
3. Dust particles are a strong source for heterogeneous ice nuclei in the free troposphere and may even influence anvil cirrus properties in some regions;
4. Organic aerosol components are sometimes associated with impeding homogeneous freezing nucleation.