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Broadening of cloud droplet size spectra by stochastic condensation: effects of mean updraft velocity and CCN activation.

Broadening of cloud droplet size spectra by stochastic condensation: effects of mean updraft... AbstractWe study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a Large Eddy Simulation (LES) approach. We investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed to a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending our previous results regarding stochastic condensation (Sardina et al. 2015), we introduce a new theoretical estimation of the droplet size spectrum broadening which accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Broadening of cloud droplet size spectra by stochastic condensation: effects of mean updraft velocity and CCN activation.

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References (54)

Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0469
DOI
10.1175/JAS-D-17-0241.1
Publisher site
See Article on Publisher Site

Abstract

AbstractWe study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a Large Eddy Simulation (LES) approach. We investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed to a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending our previous results regarding stochastic condensation (Sardina et al. 2015), we introduce a new theoretical estimation of the droplet size spectrum broadening which accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Nov 27, 2017

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