Forecast Improvements
Other Agency – Cloud Radiative Impacts of Giant CCN
Funding Agency: Office of Naval Research
Objectives
Demonstrate the radiative impact of drizzle using the new CIMMS parameterization
of giant cloud condensation nuclei (CCN).
Accomplishments
The addition of giant CCN (GCCN) to stratocumulus can have pronounced impacts,
not only on precipitation development, but also on cloud radiative properties
important for the large scale energy balance. The effect of adding giant
CCN to background clean and polluted environments on radiative properties
is evaluated in a large eddy simulation framework using the new CIMMS
GCCN parameterization. Adding GCCN to a clean CCN background has little
effect on cloud optical depth, largely because the clouds are already
drizzling. Additional GCCN, however, tend to make them drizzle even more,
and owing to the reduced optical depth and further depleted droplet concentration,
GCCN leads to a modest reduction in albedo. Adding GCCN to the polluted
background CCN results in a more noticeable reduction in optical depth,
though a similar decrease in albedo.
Aerosol indirect effects, one of the largest uncertainties in global climate change scenarios, are frequently formulated in terms of a sensitivity (called “susceptibility”) of albedo to a change in droplet (or aerosol) number. Absolute susceptibility varies little with GCCN concentration but is smaller in the polluted background CCN environment. Equivalent changes in droplet number produce more albedo response in the clean case than in the polluted case, yet cloud properties in the polluted environment are considered sensitive to the addition of GCCN. For this reason, susceptibility relative to a baseline (control with no GCCN) more aptly illustrates the sensitivity of albedo to change in droplet number. As expected, the relative susceptibility of the polluted case is much greater than that of the clean case and increases with increasing GCCN.
This project is completed.
Publications
Mechem, D., and Y. Kogan, 2007: A bulk parameterization of giant CCN. J.
Atmos. Sci., accepted.
Mechem, D. B., and Y. L. Kogan, 2006: Improving the representation of aerosol-cloud-precipitation interactions in numerical models. Proc., Sixteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting, U.S. Department of Energy.
Mechem, D. B., and Y. L. Kogan, 2006: Bulk parameterization of giant CCN. 12th Conference on Cloud Physics, Madison, WI, Amer. Meteor. Soc., CD-ROM P1.3.
Radiative quantities as a function of GCCN concentration for polluted and clean background CCN conditions. (a) optical depth; (b) albedo; (c) susceptibility; (d) susceptibility relative to the control simulations without GCCN.