The Evolution of Droplet Spectra and the Rate of Production of Embryonic Raindrops in Small Cumulus Clouds

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  • 1 Civil Engineering Department, University of Washington, Seattle 98195
  • | 2 Department of Physics, UMIST, Manchester, England
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Abstract

Calculations have been made of the evolution of droplet spectra within small cumulus clouds which are entraining undersaturated environmental air. The mixing process is assumed to be highly inhomogeneous. In the extreme situation considered, environmental air is entrained in discrete blobs or parcels, causing some droplets of all sizes to be completely removed from the condensate spectrum, while others do not change in size. This model, which is based on laboratory experiments, corresponds to a situation in which the time constant for droplet evaporation is small relative to that for turbulent mixing; in the classical (homogeneous) model, which has been used by other workers, the reverse applies. The calculations produce spectral shapes which agree well with those observed in cumulus by Warner (1969), and they indicate that favored droplets may grow very much faster through the condensate spectrum than is predicted classically.

Abstract

Calculations have been made of the evolution of droplet spectra within small cumulus clouds which are entraining undersaturated environmental air. The mixing process is assumed to be highly inhomogeneous. In the extreme situation considered, environmental air is entrained in discrete blobs or parcels, causing some droplets of all sizes to be completely removed from the condensate spectrum, while others do not change in size. This model, which is based on laboratory experiments, corresponds to a situation in which the time constant for droplet evaporation is small relative to that for turbulent mixing; in the classical (homogeneous) model, which has been used by other workers, the reverse applies. The calculations produce spectral shapes which agree well with those observed in cumulus by Warner (1969), and they indicate that favored droplets may grow very much faster through the condensate spectrum than is predicted classically.

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