The Microstructure of Cumulus Cloud. Part II. The Effect on Droplet Size Distribution of the Cloud Nucleus Spectrum and Updraft Velocity

Abstract

The concentration of cloud droplets in natural cloud has been found to be fully consistent with existing theory and with observations of cloud nuclei and updraft speed. In general, turbulence in the updraft does not broaden the droplet size distribution markedly beyond that produced by condensation in a steady updraft. However, some updraft structures, particularly those involving a general acceleration, are capable of broadening the spectrum by the repeated activation of fresh nuclei and of producing a persistent, strongly bimodal, size distribution. In order to account for the almost invariable observations of numbers of droplets <10 µm diameter at heights of 100 m or so above cloud base, it appears necessary for the accommodation coefficient to be not greatly in excess of 0.05; and for droplets to grow to greater than this size in a reasonable period the accommodation coefficient cannot be markedly less than 0.02. Within these limitations it has been found possible to account fairly closely for the droplet size distributions actually observed.