Coalescence in a turbulent cloud

The results of two experiments show that the collision efficiency of small droplets is significantly increased when the interacting drops fall through a laminar shear flow of about 10 s −1 . The structure of microturbulence in clouds is considered and it is shown that shears of this magnitude are likely to occur only within a narrow spectral band. This peak in the shear spectrum occurs at a scale that is much larger than the interaction distance of cloud droplets having radii of about 20 γm. Hence it is a good approximation to describe droplet collisions in a turbulent cloud as though they occurred in a steady laminar shear flow. It is argued that this enhanced droplet coalescence in turbulent clouds may account for the observed development of large cloud droplets (radii < 30γm) in cumulus clouds at rates greater than can be explained by condensation and the efficiency of still‐air coalescence.