Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part VII: Seeding Logistics and Post Hoc Stratification by Seeding Coverage

Abstract

An analysis of the seeding operations during the National Hail Research Experiment 1972?74 randomized seeding program is carried out for the purpose of critiquing the seeding procedures and establishing the actual rates at which seeding material was dispensed as opposed to the prescribed rates. The seeding coverage, a parameter defined in the paper, is found to be only about 50% on the average. The reasons for the low seeding coverage are discussed in terms of seeding logistics and storm evolution, and three case studies are presented to illustrate the problems that can arise. Some results on the rate at which storm cells can develop and on the duration of convective activity over a fixed target area are presented. It is concluded that seeding convective clouds using aircraft flying near cloud base is more difficult than is widely acknowledged. Since the seeding operations were more thorough on some days than on others, one might reasonably expect that seeding effects, if they exist, would be more marked on the days with the higher coverage. Post hoc analyses that stratify the surface hail and rain data according to seeding coverage are presented. The results do not allow one to reject the hypothesis that seeding had no effect on surface precipitation.