Cloud droplets are formed by condensation of water vapor on cloud condensation nuclei (CCN). Consequently, the CCN spectra, i.e., the concentration and sizes of CCN, have a direct influence on the concentration and sizes of the cloud droplets. The radiative properties of clouds depend on cloud microphysics. Therefore, the CCN spectra indirectly affect the radiative properties of clouds. The association of CCN spectra with cloud microphysics has come to be known as the Twomey effect or indirect aerosol effect (Charlson et al., 1992), i.e., higher CCN concentrations cause higher concentrations of smaller sized droplets, and thus increase cloud albedo (Twomey, 1977). Furthermore, smaller cloud droplets caused by higher CCN concentrations are less likely to initiate coalescence precipitation, and this would cause further radiative cooling by extending cloud lifetime. This constitutes a secondary indirect effect or the second Twomey effect.
In this study, in-situ measurements of CCN spectra are compared with cloud droplet spectra in Arctic stratus measured in SHEBA both in May and July, 1998. Correlations between CCN spectra, and cloud droplet concentration, mean droplet diameter, and drizzle amount determine the influence of different CCN spectra on cloud microphysics and precipitation. CCN spectra both below and above cloud will be used for the correlations. Preliminary results indicate that cloud droplet concentrations often exceeded boundary layer CCN and even condensation nuclei (CN) concentrations. This suggests that the higher concentrations of CCN above cloud may be influencing droplet concentrations. The results obtained in the Arctic stratus will be compared with the analyses made for other stratus clouds in other locations (e.g. Yum et al.,1998; Hudson and Svensson, 1995).
The comparisons between CCN and cloud droplet spectra also result in determinations of effective supersaturation (Seff), which indicates the CCN that become cloud droplets (Hudson, 1984). Nuclei with critical supersaturation (Sc) less than Seff become cloud droplets and those with Sc greater than Seff remain as haze droplets. Comparisons will be made of Seff between spring and summer and with other stratus clouds mentioned above.
References
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Hudson, J. G., and G. Svensson, 1995: Cloud microphysical relationships in California marine stratus. J. Appl. Meteor., 34, 2655-2666.
Twomey, S., 1977: The influence of pollution on the shortwave albedo of clouds. J. Atmos. Sci., 34, 1149-1152.
Yum, S. S., J. G. Hudson and Y. Xie, 1998: Comparisons of cloud microphysics with cloud condensation nuclei spectra over the summertime Southern Ocean. J. Geophys. Res., in press.