Snowfall from a Heavily Seeded Cloud

Abstract

Few documented cases exist to demonstrate that highly convective supercooled clouds can be completely glaciated or overseeded. By ?overseeding? we imply a sufficient concentration of ice nuclei to accommodate all the water generated in the updraft and to consume rapidly the existing cloud liquid water. One such case is herein presented that describes the ground variations in snow crystal type, size and concentration as a seeded cloud passed by. During this period, snow crystal concentrations increased by approximately two orders of magnitude, and, within the limits of accuracy of the experiment, showed a one-to-one correspondence with the concentration of silver iodide released. Snowflake aggregates were dominant and individual crystals comprising the aggregates averaged only 200 ?, in general agreement with model predictions. Riming of crystals was significantly reduced, with thick plates and solid columns indicative of a ?dry? environment replacing the original rimed dendrites. It was evident that heavy seeding, while limiting the riming and size of individual crystals, amplified the snowflake aggregation mechanism.