Physical Response of Convective Clouds over the Sierra Nevada to Seeding with Dry Ice

Abstract

The effect of seeding convective clouds with dry ice was studied using simultaneous aircraft and radar observations. Clouds that were initially ice-free with supercooled liquid water contents of 0.5 g m?3 when the tops reached the ?10°C level had similar responses to seeding, although significant natural variability existed. Aircraft particle probes detected sharp increases of small crystals (<100 ?m) in 3?6 min followed by > 1 mm aggregates about 10 min after seeding. Observations supported the expectation that riming growth should not be important at these liquid water contents. Initial radar echoes formed in 7 win with distinctive time-height profiles of reflectivity. Most radar echoes forming downwind of the seeding line were small and relatively weak compared with the natural echoes forming further downwind over the mountains. The impact of the seeding was shown to be observable but relatively small. It was found that unseeded clouds formed radar echoes later, and produced reflectivity time-height profiles that were significantly different from the seeded ones. The difference are considered in part to be due to variability in the initial cloud properties as well as the obvious and well-documented effects of injection of the seeding material early in the cloud lifetime. While the meteorological impact was small, documentation of the evolution of the seeding effect from cloud to ground is a prerequisite to further experimentation.