Microphysical relationships in warm clouds

Relationships among cloud microphysical parameters from six aircraft projects carried out over different parts of the globe in different seasons are presented. Comparisons of project averages showed that projects with higher droplet (2–50 μm diameter) concentrations generally had smaller droplets and less drizzle (diameter >50 μm). Hence, when all projects were combined there were negative correlations between droplet concentrations and sizes. However, this was not the case for individual cloud penetrations where only 24% had negative droplet concentration–size correlations. For the stratocumulus clouds, these negative correlations were more likely to be found in clouds with greater droplet spectral broadening and drizzle. The small cumulus clouds in this study were unique in that 95% of the droplet concentration–size correlations were positive regardless of spectral broadening or drizzle. Thus, for the majority of clouds, droplets were smaller in parcels with lower droplet concentrations. Entrainment in these clouds was then consistent with homogeneous mixing that occurs locally and intermittently. The dearth of observations of larger droplets in diluted cloud parcels suggested that mixing of entrained air did not promote droplet growth.