Responses of Warm-Based, Midwestern Cumulus Congestus to Dynamic Seeding Trials

Abstract

Findings are reported from an analysis of AgI seeding effects on individual cumulus congestus clouds in the 1989 Illinois Exploratory Cloud Seeding Experiment. The experiment was designed around a dynamic seeding hypothesis. Randomized treatments of individual clouds were based on ?floating? experimental units, initially cantered on the first treated cloud. The analysis was based on 12 experimental units having a total of 67 treated echo core?32 treated with sand and 35 with AgI. Prior to any analysis for seeding effects, a check of many of the physical conditions at the time of treatment that would govern future cloud growth showed a bias for the sand-treated clouds to be expected to ultimately grow larger than the AgI-treated clouds. Thus, even though randomization produced numerical balance, direct comparison between the posttreatment behavior of the entire sample of sand- and AgI-treated echoes could not be expected to provide a true impression of possible seeding effects. In an attempt to overcome the bias, an empirically defined seedability index composed of criteria consistent with the Illinois dynamic seeding hypothesis was developed and applied as a filter to reduce the sample bias, and thereby reveal possible seeding effects. Results of two representative applications of the seedability index are reported: one for a subgroup of clouds with higher index values, and the other for a subgroup with lower index values. The primary impression from the ability index analysis was that AgI treatment did not have a pronounced initial effect on the behavior of individual echo cores, and that if seeding had any effect at all it may have been negative on maximum cloud-top height. This finding was not consistent with that expected from the Illinois dynamic seeding hypothesis.