Abstract
Measurements of the droplet size distribution collected in a field of small nonprecipitating cumuli are analyzed. It appears that despite the high variability of the measured droplet concentration, the droplet spectral shape at given levels is almost invariable. An original method, based on the coincidence effects in the droplet counter, is used to show that most of the low concentration samples are in fact heterogeneous and that the actual droplet concentration in the cloudy parts of these samples is comparable to the concentration in regions of the cloud which are uniform.
Observed spectra are much broader than provided by adiabatic lifting. The frequency distribution of the spectral shapes is compared to the results of a one-dimensional cloud model for various values of the entrainment rate. The limitations of the previous models are discussed and the inhomogeneous mixing formulation of Baker et al. is then modified to allow nucleus activation in the entrained air before its fine scale mixing with the cloudy droplets. Finally the proposed scheme is simplified to derive a parameterization of the broad spectra. The range of spectral shapes reproduced at any level by the parameterization is in agreement with their observed frequency distribution.
