Variability of the supersaturation in cumulus clouds

The quasi-steady supersaturations in cumulus clouds from the Cooperative Convective Precipitation Experiment of 1981 are calculated from measurements of vertical wind, cloud droplet size, temperature, and pressure. Mean values and spectral characteristics of the supersaturation are presented. The supersaturation values in 147 cloud regions averaged similar to 0%, with standard deviations in the (10-m average) supersaturation values that ranged from 0.1% in unmixed regions to similar to 0.4% in regions similar to 80% diluted by entrainment. The Eulerian variance spectra for vertical wind, integral radius, and supersaturation were determined, and a technique is described for estimating the Lagrangian spectra. The Lagrangian characteristic times for supersaturation were estimated to be similar to 50 sec for mixed regions, but substantially larger for some regions with strong updrafts. It is argued that the observed variability in supersaturation is comparable to that needed to explain typical dispersions in droplet size spectra.