ASSESSMENT OF SEEDING EFFECTS IN SNOWPACK AUGMENTATION PROGRAMS - ICENUCLEATION AND SCAVENGING OF SEEDING AEROSOLS

Trace chemical analysis techniques have been used in a series of cloud -seeding experiments in the central Sierra Nevada with the ultimate pu rpose of distinguishing whether the submicron-sized aerosol particles used for seeding are removed by nucleation or by scavenging in snowfal l. The research programs used submicron-sized seeding aerosols with di fferent nucleating characteristics. When winter storms were seeded wit h silver iodide in the Lake Tahoe and Lake Almanor watersheds, positiv e correlations were observed between silver concentrations and precipi tation amounts in both catchment areas. This is considered to be evide nce that the AgI aerosols are not being removed in the snowfall entire ly by scavenging processes. When two separate aerosols of silver iodid e and indium sesquioxide were released simultaneously from the same gr ound locations during winter snowstorms in the Lake Almanor watershed, it was found that considerably more of the ice-nucleating aerosol par ticles (Agr) were removed by the snowfall than the non-ice-nucleating ones (In2O3). Under the experimental conditions employed, scavenging a lone of the two aerosols would lead to a chemical ratio of Ag:In in th e snowfall of 0.83:1. Ratios as high as 17.2:1 were observed, the mean ratio being 4:1. These results are considered to be evidence of the r emoval of substantial numbers of the AgI aerosol particles through dir ect nucleation of ice crystals.