Evaluation of a 2-Month Cooperative Ground-Based Silver Iodide Seeding Program

Abstract

Field investigations to determine the effectiveness of ground-based AgI seeding generators to treat Sierra Nevada winter cloud systems were conducted by the Sierra Cooperative Pilot Project (SCPP) from November 3, 1986, to January 9, 1987. Of 18 randomized events, 12 were seeded and 6 were left unseeded. A postanalysis using the data collected by a variety of in situ devices and the results of a numerical targeting model identified periods which appeared to contain the best seeding potential and estimated the effectiveness of placing the effects within the desired target area. Criteria used in real time for declaring an experiment may have been too lenient. Nearly one-third of the seeding cases were conducted when the -5 °C level was at an elevation that would not be expected to be reached by the ground-released nucleants. A numerical targeting model, GUIDE (Rauber et al, 1988), adapted for ground release of seeding material, computed nucleation and fallout locations from each generator on a given day. Results indicated fallout downwind of the target, primarily due to a high -5 °C level and a strong southerly wind component. During southerly winds, nucleants released from low elevation sites were often predicted to travel parallel to the barrier, displacing fallout north of the target. Meteorological conditions with low freezing levels and light westerly winds were predicted to produce the most effective targeting allowing fallout a short distance downwind of the generator, especially for those above 2000 m. Aircraft plume tracing studies, using airborne ice nucleus counters under both visual flight rules (VFR) and instrument flight rules (IFR) conditions, detected the plumes downwind of the release sites. The observations indicated a 10- to 15-degree angle of spread and an approximate 0.3 m s^-1 rise rate for plumes over mountainous terrain. Although it was difficult to map the horizontal and vertical extent of the plumes, the GUIDE model appeared to provide reasonable estimates of plume transport and diffusion. In addition to the above studies, a chemistry analysis of snowpack samples showed very limited silver dispersion, with less than 15 percent of the samples indicating any silver above background. This rather poor result may indicate inadequate generator coverage and/or poor generator performance.