The Nature of Winter Clouds and Precipitation in the Cascade Mountains and their Modification by Artificial Seeding. Part III: Case Studies of the Effects of Seeding

Abstract

Three case studies are described in which detailed airborne and ground observations were made in order to evaluate the effects of artificial seeding from the air on cloud structure and snowfall on the ground in a small predetermined target area in the Cascade Mountains. On 31 January 1972, stratocumulus clouds just west of the Cascade crest were heavily seeded with silver iodide for 72 min. Airborne observations showed that the clouds were glaciated by the artificial seeding, new crystal types appeared, and there was an increase in convective activity. Particle trajectory analysis showed that snowfall in the target area should have been affected by the seeding. On the ground in the target area near the Cascade crest new snow crystal types appeared, riming decreased, freezing nuclei in the snowfall increased, and the snowfall rate decreased during the predicted period-of-effect (PPE) of seeding. Further east in the target area, new crystal types appeared, freezing nuclei in the snowfall increased, and the only precipitation of the day fell during the PPE. Stratocumulus clouds over the Cascades were heavily seeded with silver iodide and Dry Ice for 77 min on 19 January 1973. Clouds, glaciated by the seeding, were tracked over the target area from the aircraft. A 2.5-fold and 10-fold increase in precipitation occurred in the target area at Snoqualmie Pass and Kachess Dam, respectively, during the PPE. At the same time the amounts of silver in the snowfall increased. At Kachess Dam graupel particles were replaced by aggregates of dendrites and stellars and double crystals during the PPE. The increase in precipitation at Kachess Dam during the PPE was caused mainly by an increase in the number concentration of ice particles rather than an increase in their average mass. Twenty-six cumulus clouds over the Cascades were seeded with silver iodide on 27 March 1973. There were sharp increases in the ice content of these clouds and snow fell from them over the target area during the PPE causing snow showers at Snoqualmie Pass and Hyak. At Snoqualmie Pass graupel particles were replaced by aggregates of unrimed stellars and dendrites during the PPE. At Hyak, snow which fell during the PPE contained 100 times more silver than normal. The concentrations of freezing nuclei in the snow increased at both stations during the PPE. These detailed physical evaluations support predictions, based on observations of the natural clouds and precipitation and theoretical computations, that snowfall across the Cascade Mountains can be redistributed and increased by artificial seeding.