Theoretical predictions for the growth of a population of cloud droplets by vapor diffusion in a rising, undiluted cloud parcel gives a narrow, monodisperse cloud droplet spectra. Observations, however, show actual cloud droplet spectra in clouds to be broad and complex. The cause for this discrepancy has been the subject of a large number of papers in past years. Two basic areas of study have been defined. The first involves studies of factors effecting the condensation process itself, and the second the effects of dry air entrainment on the cloud droplet spectra. The principal recent contribution to the first area is that of Fukuta [1992]. He presented analytical equations describing the competitive growth of cloud droplets in a rising cloud parcel under both Maxwellian (normal vapor diffusion) and diffusion kinetic conditions and showed that competitive kinetics effects could be described by a factor that is a function of the ratio of the cloud droplet radius and the distance between cloud droplet center and the boundary with the surrounding droplets. Application of this theory to real cloud conditions, however, showed negligible differences due to competitive kinetics for typical cloud conditions.
In a later study, Fukuta [1993] showed theoretically that water vapor supersaturations (% above saturation) as high as 10% near cloud base may occur during diffusional growth of cloud droplets due to parcel lifting in convective clouds as a result of gas kinetic effects related to the condensation and thermal accommodation coefficients. Due to the slower diffusional growth when gas kinetic effects were included, supersaturations were able to reach much higher values than previously thought.
A number of studies showed continued evidence for broadening of the cloud droplet spectra during dry air entrainment. Politovich [1993] showed that the variations in vertical velocity associated with the entrainment process may lead to large variations in supersaturation, which in turn may lead to cloud droplet spectra broadening through correlations of supersaturation with selected cloud droplet trajectories. She based her analysis on University of Wyoming data collected during the Cooperative Convective Precipitation Experiment (CCOPE). Measurements were obtained in growing cumulus clouds and droplet sizes were small, in most cases less than 20 microns in diameter. Baker and Latham [1992] showed with a semi-analytical model that the liquid water content, effective radius and optical depth in ice free entraining cumulus clouds growing in surroundings typical of Hawaii and Montana were all sensitive to the entrainment scale.