Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

TL;DR

This study uses direct numerical simulations to investigate how supersaturation fluctuations influence cloud droplet growth, revealing that droplet size distributions broaden over time and are affected by Reynolds number but not by mean energy dissipation rate.

Contribution

It provides the first explicit demonstration that droplet size distribution evolution depends on Reynolds number and includes thermodynamic feedbacks in the simulation.

Findings

Droplet size distribution width increases with time.

Standard deviation of droplet surface area scales as t^{1/2}.

Size distribution evolution is sensitive to Reynolds number, not to mean energy dissipation rate.

Abstract

Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations with droplets being modeled as Lagrangian particles. The supersaturation field is calculated directly by simulating the temperature and water vapor fields instead of being treated as a passive scalar. Thermodynamic feedbacks to the fields due to condensation are also included for completeness. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as $t^{1/2}$. Also, for the first time, we explicitly demonstrate that the time evolution of the size distribution is sensitive to the Reynolds number, but insensitive to the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increases with increasing Reynolds number, therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the mean updraft velocity is almost zero.