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Publication Abstract

Results from the 1989 Exploratory Cloud Seeding Experiment in Illinois Czys, Robert R., Stanley A. Changnon, Jr., K. Ruben Gabriel, Mary Schoen Petersen, Robert W. Scott, and Nancy E. Westcott, 1993  Illinois State Water Survey, Champaign, IL,  ISWS B-72    Full Text Available

A randomized exploratory cloud seeding experiment was conducted in Illinois from mid-May through July 1989. The experiment was designed around the dynamic seeding hypothesis with the intent to investigate whether silver iodide seeding near the tops of growing cumulus clouds increased individual cloud growth and/or longevity and enhanced cloud system rainfall. Treatmentrandomization was based on a "floating" experimental unit encompassing the cloud system to obtain a 50/50 split between clouds and cloud systems that received silver iodide treatment and those that received sand treatment as a placebo. Allocation and delivery of the seeding material was devised so that project scientists and other personnel were unaware of the type of treatment used.

During the course of the experiment, 82 clouds were treated, 36 with sand (in six experimental units) and 46 with silver iodide (also in six units). Most findings from the summer experiment are based on analysis of 67 clouds that produced trackable radar echo cores, 35 treated with silver iodide and 32 treated with sand. The analysis focused on 13 predictor variables that described the synoptic, in-cloud, and radar properties prior to and at treatment, and 11 response variables based on radar measurements. Comparison of predictor variable means and extremes revealed a statistically significant sample difference between silver iodide and control clouds, even though every effort was made to randomly select "similar" clouds according to a strict set of visual and in-cloud criteria Thus, the 1989 experiment resulted in a classic example of the "bad draw."

In spite of the sample bias, the experiment produced a number of important findings about cloud and rainfall modification in Illinois. These findings relate to effects on clouds that could be seen, effects measured by the radar properties of individual echo cores, and effects on radar-estimated rainfall. To test whether seeding effects could be observed in outward cloud appearance, the pilot and the flight meteorologist kept independent records of what type of effect they thought occurred in each experimental unit. This test revealed a very high level of skill for the cloud seeding pilot and marginal skill for the flight meteorologist in assessing seeding effects in outward cloud appearance. This finding implies that treatment dosages were large enough to enhance cloud glaciation, and thus alter cloud appearances.

Analyses used to eliminate the sample bias revealed that no conclusion could be drawn for seeding effects in clouds that developed under cold-front conditions. Analysis of clouds that developed during air-mass conditions revealed essentially no seeding effect. Further analysis and comparison among clouds with high suitability for dynamic seeding indicated that if seeding had any effect at all, it negatively affected echo-core height, area, and reflectivity. Curiously, clouds characterized by low seeding suitability showed weak evidence of positive seeding effects. These findings are generally not expected to occur from the dynamic seeding technique, and taken alone, they suggest that its use does not produce desired results in echo-core behavior.

An analysis using median radar-estimated rainfall suggested that precipitation enhancement may have occurred at the scale of the rain cloud system, in spite of possible negative effects on individual clouds. However, sample size was too small to draw firm conclusions.

When taken together, the results raise questions about the validity of using the dynamic seeding hypothesis in Illinois. The findings are sufficiendy negative to call for either a major revision in the modification hypothesis or rejection of the dynamic hypothesis in favor of an alternative seeding technique. They also suggest that the present technology needs advancement for the practical use of cloud seeding in Illinois to produce more than marginal benefits.

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