Results of the 1989 Exploratory Cloud Seeding Experiment in Illinois Based on Synoptic Weather Conditions

N E Westcott, S A Changnon, Jr., R R Czys, R W Scott, M S Petersen


The 1989 Illinois summertime cloud seeding results have been examined in the context of synoptic weather conditions. It was found that six of the 12 experimental units occurred under cold-front conditions, and four units under air-mass conditions when kinematic trigger was obvious. When cold fronts were present, the tallest daily echo tops, the highest first echoes, the strongest updrafts, and hail were observed. While deep convection occurred on both types of days, a larger amount of potential buoyancy was available and stronger vertical shear was present on cold-front days. Sixty echo cores treated near cloud top with silver iodide (Agl) or sand flares were examined before, at, and after treatment. It was expected that the cold-front cores would be larger than the air-mass cores. Instead, the air-mass cores in the mean were older, larger, more reflective, and contained more liquid water and a larger fraction of ice than the cold-front cores. The cold-front cores had a broad distribution in vertical motion as estimated by both aircraft and radar. This same broad distribution also was observed in the maximum values of height, area, and reflectivity. There were many small cold-front cores and a few large ones. The cold-front cores with the largest and strongest updrafts grew the tallest. This relationship was much weaker for the air-mass cores. It was postulated that the smaller cores were more subject to the detrimental effects of entrainment on cold-front days. On the air-mass days, all tracked cores were already joined with another echo by the time ot treatment. In this protected environment, less affected by vertical shear, even the smallest of the air-mass cores grew. Agl/sand differences also were examined. For the cold-front cores, it was found that at treatment time, there was a disproportionate number of small Agl-treated cores, and the largest cores were treated with sand flares. The sand cores also had the largest pre-treatment and treatment time growth rates. No seeding effect could be deduced from the cold front sample because of this bias towards smaller Agl-treated cores. No such bias was observed for the air-mass cores. However, the air-mass sand-treated cores grew taller than the Agl-treated cores. Since cloud seeding was expected to lead to an increase in vertical growth, if seeding affected the growth of the individually treated cores, it was in a negative way for this small sub-sample of air-mass cores. However, in terms of core duration, maximum horizontal area and maximum reflectivity, the sand- and Agl-treated cores were similar. These data failed to confirm the earliest steps of the dynamic seeding hypothesis suggesting that silver iodide treatment invigorates individual cloud growth.

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