Ice particles in maritime clouds near Tasmania

Abstract An instrumented aircraft was used to sample particles in the upper regions of small supercooled cumulus clouds in maritime air off the Tasmanian coast in May 1968. 114 clouds were studied on nine days, and it was found on average that clouds had a 50 per cent chance of containing ice particles when a summit temperature of about − 8°C was reached. Not only cloud temperature but also cloud width had a profound effect on whether ice particles were found. The cloud summit temperature at which ice particles first reached detectable concentrations in narrow clouds on a particular day was related to the ice nucleus concentration as measured in a cloud chamber, being, in general, highest on days of high ice nucleus concentration. However, in some glaciated clouds ice crystal concentrations of the order of 100 1−1 were found. These were greater by a factor of ∼ 104 than the estimated number of active ice nuclei. The process of multiplication of ice crystals may be associated with the riming of ice particles. Riming was assisted by the presence of large droplets, up to drizzle size, in these clouds. Multiplication of the number of ice crystals by a factor of 10 took place in about 8 minutes. The various stages in the glaciation process are described.