Droplet Growth by Condensation and Coalescence in a Strong Updraft

Abstract

The growth of cloud droplets by condensation and coalescence in a strong updraft is investigated for different cloud conditions and different initial droplet distributions. Growth by coalescence is determined by solving the stochastic collection equation, and growth by condensation is calculated from the diffusion equation. The results indicate that under suitable conditions, starting from an initial droplet distribution centered at about 8 µm radius with a dispersion of 0.2, an appreciable number of cloud droplets will grow to precipitation sizes in times of the order of 10?15 min. The results are sensitive to the initial droplet concentration and moisture content of the cloud. The importance of combining coalescence and condensation raises the question of the validity of calculations of rain formation by coalescence only.