Turbulence and cloud microphysics

The effects of in-cloud turbulence on the growth of cloud droplets by condensation and coalescence in low level water clouds are reviewed. It is shown that while the entrainment of dry air into clouds may explain the observed variability in the droplet spectra, and particularly in droplet concentration, doubt still exists as to whether the entrainment of subsaturated air can give rise to enhanced droplet growth by condensation, that is growth in excess of that expected in an adiabatic parcel. It is also shown that the increase in the length of the path of some of the droplets in stratocumulus due to turbulent updraughts can significantly increase the rate of production of drizzle sized droplets by coalescence. The effects of turbulence are generally believed to increase the collection kernels of cloud droplets, although the magnitude of the increase is in doubt. The impact of such changes on the growth of a population of cloud droplets by coalescence is sensitive to their magnitude. In particular, such changes may be capable of partially overcoming the barrier to growth which occurs between the condensation and coalescence processes in still air at a droplet radius of around 20 μm, although further detailed studies are required.