The Effects of Small-Scale Turbulent Motions on the Growth of a Cloud Droplet Spectrum

Fausto Carlos de Almeida National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

The effects of small-scale cloud turbulent motions on the growth of a droplet spectrum are investigated by numerically solving the stochastic collection growth equation. Four different initial distributions of cloud droplets characterized by different mean mass radii R0 are analyzed. The growth of these initial droplet distributions using the turbulent collision efficiency function of Almeida and the non-turbulent (still-air) collision efficiency functions of Davis and Sartor and of Almeida are compared. Increased growth rates are obtained for the turbulent collision efficiency functions presented by Almeida.

For initial droplet distributions with R0 ≤ 8 µm, light to moderate growth rates are observed for the turbulent case, while no growth is observed for the non-turbulent cases. For a droplet distribution of R0 = 10 µm (typical of a condensation grown cloud droplet distribution), the still-air collection cases still do not product any growth, but the turbulent collection case does produce precipitation-size drops in less than 20 min. For larger mean mass radius distributions, R0 ≥ 12 µm, both growth cases produce precipitation-size drops, although for the same time interval turbulent growth produces larger concentrations.

The results of these droplet growth calculations are analyzed in connection with understanding warm rain initiation and its consequences in cloud modification experiments.

Abstract

The effects of small-scale cloud turbulent motions on the growth of a droplet spectrum are investigated by numerically solving the stochastic collection growth equation. Four different initial distributions of cloud droplets characterized by different mean mass radii R0 are analyzed. The growth of these initial droplet distributions using the turbulent collision efficiency function of Almeida and the non-turbulent (still-air) collision efficiency functions of Davis and Sartor and of Almeida are compared. Increased growth rates are obtained for the turbulent collision efficiency functions presented by Almeida.

For initial droplet distributions with R0 ≤ 8 µm, light to moderate growth rates are observed for the turbulent case, while no growth is observed for the non-turbulent cases. For a droplet distribution of R0 = 10 µm (typical of a condensation grown cloud droplet distribution), the still-air collection cases still do not product any growth, but the turbulent collection case does produce precipitation-size drops in less than 20 min. For larger mean mass radius distributions, R0 ≥ 12 µm, both growth cases produce precipitation-size drops, although for the same time interval turbulent growth produces larger concentrations.

The results of these droplet growth calculations are analyzed in connection with understanding warm rain initiation and its consequences in cloud modification experiments.

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