Evaluation of Echo Core Responses in the 1989 Illinois Exploratory Cloud Seeding Experiment using a Seedability Index

Robert R Czys, Stanley A Changnon, Nancy E Westcott, Mary Schoen Petersen, Robert W Scott


Evaluation of individual echo core responses to silver iodide seeding in the 1989 exploratory cloud seeding experiment in Illinois is the focus of this paper. The analysis was based on a seeding suitability index defined in terms of 20 predictor variables chosen to represent either in-cloud, echo core, or synoptic/meso-scale conditions that might be suspected of being desirable based on the dynamic seeding hypthosis that was developed for Illinois. Two applications of the seedability index are reported; one for a subgroup of clouds with a high level of seedability index, and the other for a subgroup of clouds with a low level of seedability index. Reduction in sample size by creation of these subgroups, along with other factors such as the analysis being based on data from a single summer, precludes arriving at statistically significant conclusions about seeding effects on individual echo cores. However, examination of the data suggests that if AgI seeding had any initial effect on echo behavior that it may have been negative for cores with high seedability index and positive for cores with lower seedability index. The possibility that the initial reaction of individual echo cores may have been contrary to that expected according to the dynamic seeding hypothesis suggests that the initial steps of the seeding hypothesis, at least as it applies to Illinois, needs reconsideration. An alternative rationale for the initial effect of AgI seeding on echo core behavior is offered which is based on the recognition that it may not be possible to perfectly target seeding material to updraft regions, as is required in dynamic seeding, when the echo cores are characterized by a multi-updraft structure. Caution is expressed about the extent to which these results reflect on those reported for other experiments designed around the dynamic seeding hypothesis because of certain differences between the operational procedures followed during the 1989 Illinois experiment and those traditionally followed.

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