Precipitation from Stratocumulus Clouds Affected by Fallstreaks and Artificial Seeding

Abstract

Cloud and precipitation processes in a stratocumulus cloud layer (?1 km thick) were investigated by means of airborne, radar and ground observations for three situations: 1) the stratocumulus alone, 2) fallstreaks from altocumulus falling into the stratocumulus, 3) regions of stratocumulus not appreciably affected by fallstreaks but strongly affected by artificially seeded dry ice. In case 1) the cloud was composed primarily of supercooled droplets. In cast 2) dendritic ice crystals in the fallstreaks increased their mass by riming as they passed through the stratocumulus; derived precipitation rates for this case were ?0.02?0.08 mm h?1. In addition, it appeared that the dendrites provided a source for high concentrations of needle crystals in the stratocumulus; these crystals were estimated to give a precipitation rate of ?0.01?0.03 mm h?1. In case 3) high concentrations of needle crystals were produced by the dry ice seeding and it was deduced that these also produced precipitation rates ?0.01?0.03 mm h?1. Some implications of the results for areal artificial seeding experiments are discussed.