Generalized Criteria for Seeding Winter Orographic Clouds

Abstract

An a posteriori analysis was conducted utilizing precipitation, rawinsonde and seeding generator data from seven randomized winter cloud-seeding research projects conducted in orographic settings in the Rocky Mountain West and on the Pacific Coast of the United States. Variables were developed and investigated to establish generalized seedability criteria that are applicable to a variety of meteorological and topographic conditions. The variables were divided into four general categories: time available, water available, nuclei available and mixing available. This approach established stratifications under which positive (increases) or negative (decreases) seeding effects occurred. The study showed that positive seeding effects occurred at the crest under stable or unstable conditions when a “crest” trajectory was present, moderate-to-high cloud moisture was present and the cloud-top temperature was between −10 and −30°C. Decreases occurred at the crest for unstable clouds with a “blow-over” trajectory, with low cloud moisture and cloud-top temperature colder than −30°C. The precipitation for upwind and downwind regions of a barrier was also increased or decreased depending on stability, trajectory, cloud moisture and cloud-top temperature. Other stratifications are discussed in the paper.