Physical Response of Winter Orographic Clouds over the Sierra Nevada to Airborne Seeding Using Dry Ice or Silver Iodide

Abstract

Cloud seeding experiments devoted to physical measurements of the effects of seeding shallow stable winter orographic clouds have been conducted in the central Sierra Nevada of California from 1984 to 1986. Seeding was done by aircraft using either dry ice or silver iodide at temperatures between ?6° and ?14°C. Aircraft, radar, and surface instruments were used to measure the effects. A trajectory model was used to target seeded precipitation to the ground where the surface instruments were deployed. Results from these experiments are presented in two case studies and a summary analysis of all 36 experiments. Observations from the various measurement platforms conformed with results expected from seeding in 35 percent of the seedlines sampled with a research aircraft, 4 percent of those observed with radar, and 17 percent of these which passed over the surface instrumentation; however, the complete seeding chain was believed to be documented in only 2 of 36 experiments. The failures result from difficult technical and logistical problems, and from the variability of even simple cloud systems, particularly in the spatial and temporal distributions of liquid water and in the natural fluctuations in ice crystal concentrations. Based on the difficulty of these experiments and the magnitude of seeding effects observed, a statistical experiment would be a formidable undertaking. During the two experiments when seeding effects were detected by all measurement platforms the following effects were observed. A high concentration, 50?100 L?1, of small compact ice crystals formed quickly along the seedline. Although aggregation was seldom observed, riming often began 5?10 min after seeding. The seeded ice crystals dispersed at 1 m s?1 and cloud liquid-water evaporated in regions corresponding to the seedlines. Seeding in a non-echoing region occasionally produced echoes of 3?10 dBZ in portions of the seedlines. At the surface seeding effects arrived 35 to 60 min after seeding, 20?30 km downwind. Snow crystal concentrations increased, snow crystal habits changed to small rimed particles, and precipitation rates increased by 0.1?1.0 mm h ?1. The duration of these effects was short, <10 min per seedline. Changes in ice particle development induced by seeding were similar when seeding with either dry ice or silver iodide. This was found to be the case even at temperatures as warm as ?6°C using AgI NH4I NH4ClO4 burned in an acetone solution.