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. I t is shown that while the entrainment of dry air into clouds may expla in the observed variability in the droplet spectra, and particularly i n droplet concentration, doubt still exists as to whether the entrainm ent of subsaturated air can give rise to enhanced droplet growth by co ndensation, 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 o f some of the droplets in stratocumulus due to turbulent updraughts ca n significantly increase the rate of production of drizzle sized dropl ets by coalescence. The effects of turbulence are generally believed t o increase the collection kernels of cloud droplets, although the magn itude of the increase is in doubt. The impact of such changes on the g rowth of a population of cloud droplets by coalescence is sensitive to their magnitude. In particular, such changes may be capable of partia lly overcoming the barrier to growth which occurs between the condensa tion and coalescence processes in still air at a droplet radius of aro und 20 mu m, although further detailed studies are required.