SUMMARYAircraft observations of cloud and precipitation particles just below the tops of Florida cumuli at temperatures near -4 and -9°C show that fresh, vigorously growing towers (updraughts of order IOms-I) tend to contain low concentrations ( < O l I-') of graupel, whereas older towers with lower updraught speeds contain much higher concentrations (> 101-I). The ice phase, mostly as graupel, was often found to appear within a period of five minutes. Cloud drops of diameter > 25 pm were always found to be present in concentrations of 10 to lOOcm-' between -4 and -6°C. a condition, according to criteria established by laboratory studies, necessary for secondary ice crystal production by the riming of graupel particles. Vapourgrown columns in concentrations as high as 50 I-' were sometimes observed at the colder level. an observation consistent with the concept of a copious production zone below. Supercooled raindrops in concentrations > 1 I-' occur in many regions of the clouds, often concurrently with graupel and crystalline ice. It is shown by means of a simple analytical model that aerodynamic capture of secondary ice particles by the supercooled raindrops and the subsequent transformation of the raindrops to new splinter-producing graupel may be the main mechanism responsible for the rapid increase in observed graupel concentrations. The coalescence processes are therefore seen to be instrumental in the evolution of the ice phase. This process is favoured in lower updraught regions where the particles can be retained in the -4 to -6°C production zone long enough (" 100s) for the entire feedback process of splinter production, capture, drop freezing and renewed splinter production to take place. 1, INTRODUCTIONAlthough the evolution of the ice phase has been thought to be the dominating influence on precipitation formation in most clouds extending significantly above the freezing level, evidence is mounting that the effective glaciation of such clouds is itself dependent upon the prior evolution of the liquid phase by the coalescence precipitation mechanism. Aircraft observations (Koenig 1963) have shown that large precipitation-sized supercooled drops were associated with the rapid evolution of the ice phase in Missouri summer cumuli having top temperatures mostly above -10°C. Mossop et a/. (1968, 1970, 1972) have provided additional evidence for rapid and prolific ice formation in maritime clouds developing over Australia, the observed concentrations of ice particles exceeding an extrapolation of detected concentrations of ice nuclei by factors > lo4. They concluded that the riming process and the presence of large drops are of prime importance for rapid glaciation.More recently, the laboratory studies of Hallett and Mossop (1974) and Mossop and Hallett (1974) showed that the production of secondary ice particles during the growth of graupel between -4 and -6°C required the presence of a cloud drop spectrum with a tail extending beyond approximately 25 pm diameter, with one ice particle produced duri...

SUMMARYAitken-sized (0.01pm) particles of AgI, CuI, and 'contaminated' Ag,O were able to nucleate various sized water droplets in free fall (20-80@ni in radius) by a direct, diffusive (Brownian) contact mechanism. The AgI aerosol was found to be the most efficient nucleator, showing a probability of 100 per cent at -16"C, 50 per cent at about -14"c, and 1 per cent at -11°C. These nucleation characteristics for AgI can be modelled from heterogeneous nucleation theory if the value of the contact parameter is assumed to be 0.70. Ag,O, passing through carrier tubing previously contaminated with iodine, was found to be only a slightly less efficient nucleating agent than AgI. CuI was found to have a nucleating efficiency corresponding to that which would theoretically be expected from an aerosol with a contact parameter of 0.55. Aerosols of the 'pure' oxides of silver, platinum, and chromium were unable to cause nucleation (to within detectable limits) at temperatures as low as --1S"C. Auxiliary experiments indicated that the AgI aerosol had a small, but finite nucleating efficiency at -5'C. Aitken-sized particles of AgI did not lose their nucleating ability when captured at temperatures higher than 0°C.It is postukated that contact nucleation by the Brownian capture of Aitken particles may be an important enough process in the atmosphere to contribute significantly to the large amounts of ice which have been observed at relatively high (-10°C) temperatures in some clouds.

S U M M A R YThe latent heat thermodynamically available in the supercooled water of deep convective clouds is released at rates governed by the glaciation mechanism, presumed here to involve primary nucleation, capture nucleation of rain by small ice particles and secondary ice production (rime-splintering hypothesis). A simple microphysical model of the glaciation shows the importance of regenerative feed-back on the evolution of ice when the primary nucleation and regenerative processes proceed simultaneously, leading to insensitivity to the primary ice nucleus concentration. This allows rapid glaciation and heat release to occur naturally in clouds exhibiting a broad supercooled drop size spectrum.New observational data and model results of artificially induced glaciation are consistent with the idea that the primary microphysical role of seeding is the creation of many small ice particles which substitute for the secondary ice splinters of naturally induced glaciation. The aerodynamic capture of the splinters by the supercooled rain leads to the formation of new graupel particles and the rapid release of fusional heat, shown by calculation to dominate the heat release mechanisms. With a seeding agent acting in the contact mode the small (cloud drop) end of the spectrum is required, since Brownian scavenging of the nuclei by the few large rain drops is inefficient. The quantitative analysis of these glaciation concepts also demonstrates that realistic seeding under conditions conducive to ice multiplication could increase materially the rate of heat release and offer opportunities for artificially invigorating the dynamic structure of a cloud if glaciation is induced to occur within a relatively narrow 'time window' for seeding. Present affiliation: Joseph Oat Cow.. Camden. NJ 08104, USA.

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