On the collision of drops in turbulent clouds

TLDR

In turbulent clouds, collision rates of very small drops depend only on drop size, energy dissipation ε, and viscosity, and are estimated to lie between one and two due to spatial velocity variations. The study aims to develop a theory of collisions among small drops in turbulence, including equal and unequal drop interactions, and to assess turbulence’s role in initiating rainfall in atmospheric clouds. The authors derive a collision‑rate expression, numerically integrate it to model distribution evolution, estimate ε for typical cloud conditions, and compute initial collision rates, mean property changes, and large‑drop production. The study finds that turbulence has minimal impact in layer clouds but can broaden drop sizes in cumulus clouds, that heterogeneous clouds see increased collision rates dominated by drop inertia, and that turbulence‑induced unequal‑drop collisions rival gravity when ε ≈ 2000 cm² s⁻³.

Abstract

This paper proposes a theory of collisions between small drops in a turbulent fluid which takes into account collisions between equal drops. The drops considered are much smaller than the small eddies of the turbulence and so the collision rates depend only on the dimensions of the drops, the rate of energy dissipation ε and the kinematic viscosity v. Reasons are given for believing that the collision rate due to the spatial variations of turbulent velocity is shown to be between one and two. A numerical integration has been performed using this expression to show how an initially uniform distribution will change because of collisions. An approximate calculation is then made to take account also of collisions which occur between drops of different inertia because of the action of gravity and the turbulent accelerations.The results are applied to the case of small drops in atmospheric clouds to test the importance of turbulence in initiating rainfall. Estimates of ε are made for typical conditions and these are used to calculate the initial rates of collision, the change in mean properties and the rate of production of large drops. It is concluded that the effects of turbulence in clouds of the layer type should be small, but that moderate amounts of turbulence in cumulus clouds could be effective in broadening the drop size distribution in nearly uniform clouds where only the spatial variations of velocity are important. In heterogeneous clouds the collision rates are increased, and the effects due to the inertia of the drop soon become predominant. The effect of turbulence in causing collisions between unequal drops becomes comparable with that of gravity when ε is about 2000 cm2 sec−3.