Laboratory studies of the effects of mixing on the evolution of cloud droplet spectra

Abstract

Experiments were performed in which a cloud of droplets formed by condensation was drawn slowly down a cylindrical tube of length 5 m. the effect on the spectrum of mixing-in humidified but undersaturated air was studied. The most striking observation was that the mixing process was highly inhomogeneous. Some droplets from all size categories were completely evaporated, while others were not significantly affected by the admixture of undersaturated air. the dispersion of the observable spectrum remained essentially constant. If this spectrum, with substantially reduced water content, was then allowed to grow by producing a supersaturation the measured dispersion increased appreciably. Calculations showed that the spectral evolution observed in a selected experiment was inexplicable in terms of homogeneous mixing, as envisaged by other workers. However, it could be explained by a rudimentary model of inhomogeneous mixing, which was found to offer a qualitative explanation for features of cloud droplet evolution observed by Warner, surrounding which there has been considerable recent discussion. This mixing process can be considered in terms of fluctuations in supersaturation not tied to fluctuations in vertical air velocity.