Collisioncoalescence

Collisioncoalescence is a key microphysical process in cloud physics in which individual cloud droplets collide and merge to form larger droplets. It is especially important for warm rain formation in clouds with temperatures above freezing, where condensation alone cannot quickly produce raindrops large enough to fall.

Collisions arise from differences in gravitational settling speeds and from turbulent motions in the air. Larger droplets settle faster than smaller ones, overtaking them, while turbulence creates random relative motions that enhance encounter rates. The probability that a collision results in coalescence is described by a coalescence efficiency, and the resulting droplet size increases according to the collision kernel that depends on droplet sizes, ambient humidity, and air properties. In models, the process is represented by methods such as the Smoluchowski equation or bin microphysics schemes that track droplets across a spectrum of sizes.

Collisioncoalescence is a principal mechanism by which clouds convert many tiny droplets into a few large drops capable of rain formation. Its effectiveness depends on having a broad initial droplet size distribution and sufficient residence time within the cloud. In clouds dominated by ice processes or with very weak turbulence, the mechanism is less important, and other pathways for precipitation may prevail.

Measurement and modeling involve radar observations, aircraft measurements, and high-resolution imagery, while numerical weather and climate models parameterize collisioncoalescence through kernels and efficiencies. Uncertainties arise from variations in droplet charges, subgrid-scale turbulence, and environmental conditions that affect collision rates and coalescence probabilities. The concept also appears in aerosol science and chemical engineering, where colliding particles can merge to form larger aggregates.