Cloud droplet spectral width relationship to CCN spectra and vertical velocity

Cloud droplet spectral width (standard deviation; sigma; sigma) was inversely related to cloud condensation nuclei (CCN) concentrations (NCCN) in the Rain in Cumulus over the Ocean (RICO) project. This was determined from thorough comparisons between flight-averaged NCCN and microphysics of the lowest altitude cloud passes of 17 RICO flights. Adiabatic model predictions of droplet spectra based on complete below cloud CCN spectra and within-cloud vertical velocity (W) showed good agreement with measured droplet concentrations and mean diameter (MD) but sigma predictions were only weakly correlated with measured sigma. Significantly better sigma predictions were obtained for mixtures of droplet spectra for distributions of W that were measured in each flight. Adiabatic model predictions for various W applied uniformly to all RICO flights displayed a trend of correlation coefficients (R) for sigma-NCCN plotted against W that changed from positive to negative with increasing W. The sigma-NCCN positive R range at low W corresponds to previous results in stratus clouds where it has been suggested that albedo calculations that include sigma reduce the indirect aerosol effect (IAE). The sigma-NCCN negative R range of higher W corresponds to more convective clouds such as RICO where albedo calculations that include sigma thus might seem to augment IAE.