Relationship between Cloud Microphysics and CCN in ACE-1

S.S. Yum, Y. Xie, and J.G. Hudson (All at : Desert Research Institute, P.O. Box 60220, Reno, NV 89506-0220; 702-677-3119, Fax: 702-677-3157, e-mail;

Cloud droplet concentrations, mean droplet diameters (MD), and cloud droplet standard deviations (sigma) are compared with drizzle drop concentrations and cloud condensation nuclei (CCN) spectra. Effective supersaturations (Seff) for ACE-1 clouds are determined and compared with Seff in other maritime clouds in both the same area (Southern Ocean Cloud Experiment--SOCEX) during the same and opposite seasons and in the northern Hemisphere.

The relationship between sigma and MD is examined. In several maritime cloud systems sigma has often been constant up to MD = 15 um, at least in adiabatic clouds. It increases at higher MD and this is usually closely related to drizzle. The apparent MD and sigma thresholds for drizzle (Hudson and Svensson 1995) are compared with these thresholds in other marine cloud systems.

Both the concentration of CCN and the slope of the CCN spectrum have been found to affect sigma. Therefore the relationship between CCN spectra and sigmais compared to that found in other marine cloud projects. This analysis will help determine if variations in CCN spectra affect autoconversion, which is the process that initiates drizzle in warm clouds. Some recent work has indicated that the concentration of large CCN (those with critical supersaturations below Seff) affect sigma and drizzle (Hudson and Yum 1996). These results and determinations of Seff will define which of the particles are the most important for cloud radiation and cloud stability.

Hudson, J.G. and G. Svensson, 1995: Cloud microphysical relationships in California marine stratus. J. Appl. Meteorol., 34, 2655-2666.
Hudson, J.G. and S.S. Yum, 1996: Droplet spectral broadening in marine stratus. Submitted J. Atmos. Sci. March.