Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation

Journal article, 2018

The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by
means of a large-eddy simulation (LES) approach. The authors 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 onto a turbulent field reduces the broadening of the
droplet size spectra induced by the turbulent fluctuations alone. Extending the authors’ previous results regarding
stochastic condensation, the authors introduce a new theoretical estimation of the droplet size
spectrum broadening that 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.