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Mixing and the Evolution of Cloud Droplet Size Spectra in a Vigorous Continental Cumulus

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  • 1 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

Aircraft measurements in a vigorous, highly turbulent continental cumulus show predominantly bimodal and multiple peaked cloud droplet spectral shapes. The data are 100 m (1 s) averages. Three factors involved in the development of the cloud droplet size spectrum are discussed. First, evidence is given for local activation, far above cloud base, of significant concentration of new cloud droplets. This evidence is that strong peaks in the concentration of very small droplets are found in this data set and are in one-to-one correspondence with the simultaneous occurrence of a low concentration of the larger cloud droplets and a vigorous updraft. Second, a simple model of droplet sedimentation and evaporation at boundaries between clear, unsaturated air and cloud air, as occurs during entrainment, shows the development of broad and bimodal droplet spectra in boundary zones in periods of a few tens of seconds. Third, an analysis of the relation between the droplet size and concentration at the large-droplet peak of the spectrum shows nearly constant size at constant altitude regardless of major concentration variations. The only explanation of this fact that we have found is speculative and postulates the existence of major, small-scale nonuniformities in the droplet population. If true, this would in turn seem to imply that the turbulence is not fully developed to the smallest scales in the cloud studied.

Abstract

Aircraft measurements in a vigorous, highly turbulent continental cumulus show predominantly bimodal and multiple peaked cloud droplet spectral shapes. The data are 100 m (1 s) averages. Three factors involved in the development of the cloud droplet size spectrum are discussed. First, evidence is given for local activation, far above cloud base, of significant concentration of new cloud droplets. This evidence is that strong peaks in the concentration of very small droplets are found in this data set and are in one-to-one correspondence with the simultaneous occurrence of a low concentration of the larger cloud droplets and a vigorous updraft. Second, a simple model of droplet sedimentation and evaporation at boundaries between clear, unsaturated air and cloud air, as occurs during entrainment, shows the development of broad and bimodal droplet spectra in boundary zones in periods of a few tens of seconds. Third, an analysis of the relation between the droplet size and concentration at the large-droplet peak of the spectrum shows nearly constant size at constant altitude regardless of major concentration variations. The only explanation of this fact that we have found is speculative and postulates the existence of major, small-scale nonuniformities in the droplet population. If true, this would in turn seem to imply that the turbulence is not fully developed to the smallest scales in the cloud studied.

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