THE RAPID GLACIATION OF SLIGHTLY SUPERCOOLED CUMULUS CLOUDS

It is demonstrated that, contrary to current opinion, the rapid glacia tion of shallow supercooled cumulus clouds with tops no colder than -8 degrees C can be accounted for by the growth of a very low concentrat ion (<10 m(-3)) of graupel pellets in a cloud of modest liquid-water c ontent of similar to 1 g m(-3) and a modest updraught of -3 m s(-1). I n an illustrative calculation, a graupel pellet, in ascending from the -3 degrees C to the -7 degrees C level and returning to the -5 degree s C level, attains a radius of 3.6 mm and sheds > 10(4) ice splinters by the Hallett-Mossop mechanism in 750 s, one-third of them in only 15 0 s. Ice splinters shed in the early stages of graupel growth develop into small graupel pellets of diameter similar to 1 mm and account for these being observed in concentrations of similar to 1 per litre. The whole process, from activation of the ice nuclei at the -8 degrees C level to the final stage of explosive glaciation, takes about one hour and therefore occurs only in aged cumulus with weak updraughts and no t in rapidly ascending towers.