The relationship between cloud microphysics and CCN spectra in ASTEX

Extensive airborne measurements of cloud condensation nuclei (CCN) spectra throughout ASTEX are compared with cloud droplet spectra. CCN concentrations ranged from 10 to 1500 cm^-3. Cloud droplet concentrations displayed similar variability that was related to CCN concentrations. Moreover, droplet size and cloud drop (those larger than 50 um diameter) concentrations were often inversely related to droplet concentrations. These observations indicate that cloud albedo, precipitation efficiency, and cloud scavenging processes are influenced by the CCN.

Vertical distributions usually showed lower CCN concentrations in the cloudy boundary layer; this is similar to the Pacific (Hudson and Frisbie, 1991; Hudson, 1993 where coalescence scavenging appeared to be responsible for the reduced concentrations. On many occasions the vertical gradient in the Atlantic was reversed when outbreaks of continental aerosol from Europe elevated concentrations in the marine boundary layer.

Comparisons between CCN spectra and cloud droplet spectra revealed the effective supersaturations, S_eff. Since S_eff seemed to be a function of the CCN concentration, the subset of the aerosol that became involved in cloud nucleation changed. This indicates a feedback in which cloud scavenging and the inhibition of cloud scavenging (by high CCN concentrations) affect CCN concentrations, which in turn influence cloud microphysics that in turn determines cloud scavenging.

References

Hudson, J.G. and P.R. Frisbie, Coud condensation nuclei near marine stratus, J.G.R. 96, D11, 20, 795-20-808, 1991
Hudson, J.G., Cloud condensation nuclei near marine cumulus, J.G.R. in press, 1993.