ICE PARTICLE CONCENTRATIONS AND PRECIPITATION DEVELOPMENT IN SMALL POLAR MARITIME CUMULIFORM CLOUDS

Increases in the concentrations of ice particles, as well as precipitation development, can proceed very rapidly in even quite small maritime cumuliform clouds. In the upper portions of these clouds, ice particle concentrations can increase from < 1 per litre to > 100 per litre in approximately 10 minutes, even when cloud tops are warmer than -12-degrees-C. Just prior to the onset of the high ice particle concentrations in the upper regions of these clouds, a few liquid and frozen drizzle drops appear; these are followed almost immediately by high concentrations of largely vapour-grown ice crystals. Subsequently, the ice particle concentrations gradually decline as the ice particles grow, aggregate and fall out. Crystal habits and sizes indicate that nearly all of the ice crystals encountered in the upper regions of the clouds form close to those regions. Prior to the formation of high ice particle concentrations, the clouds generally contained, near cloud top, droplets > 30-mu-m diameter in concentrations > 1 cm-3, and often drizzle drops (100-400-mu-m diameter) in concentrations > 1 per litre. For maritime and continental cumuliform clouds with widths D greater-than-or-similar-to 3 km, the breadth of the droplet spectrum near the cloud top is a better predictor of the maximum ice particle concentrations that will develop in this region than is cloud-top temperature. Exceptions to the general picture described above are the very narrow (D < 3 km), 'chimney-type', maritime cumuliform clouds with tops that quickly subside after reaching their maximum altitude. For cloud-top temperatures between -7 and -13-degrees-C these clouds produce fewer ice particles (approximately 1-20 per litre), even though droplets with diameters > 30-mu-m are present in concentrations > 1 cm-3 and drizzle drops are often present in concentrations > 1 per litre. Several proposed mechanisms for the formation of high ice particle concentrations in clouds are discussed in the light of these field observations. In a detailed case-study ice particle concentrations produced by the ejection of ice splinters during riming were calculated to be about an order of magnitude less than the ice particle concentrations measured in the upper regions of the wider maritime clouds. The freezing of evaporating drops by contact nucleation could be responsible for the frozen drops that precede the high concentrations of vapour-grown ice crystals. The latter could be produced in localized regions of unexpectedly high supersaturation that may be produced when drops begin to grow by collisions. This explanation is consistent with several of our findings concerning the conditions under which high concentrations of vapour-grown crystals appear almost spontaneously in maritime clouds. Similarities between these natural crystals, and the conditions under which they appear, and those produced artificially by dry-ice seeding and by aircraft flying through supercooled clouds are discussed.