Exploratory Analysis of Seeding Effects on Rainfall: Illinois 1989

Abstract

Radar-indicated rainfall characteristics from six experimental units randomly selected for AgI treatment were compared with those from six experimental units treated with sand during the summer of 1989 in an exploratory analysis. No differences were found between AgI and sand cell frequencies before and after treatment. The areal extent of rain in all AgI-treated units grew during treatment, whereas four of the sand units decreased, but the differences were not significant. Rainfall amounts were determined for the units and for the extended areas around them. AgI units had higher median rainfall values by the end of the treatment and thereafter for 90 min than did the sand units, whereas extended area rainfall was less in AgI than sand cases. The unit rainfalls, adjusted to account for the unit-extended area relationships, showed a systematic difference with AgI rain higher at the end of treatment (and statistically significant) and for 90 min thereafter. Comparisons based on 20 relevant meteorological variables showed that seeding appeared to produce an effect on days with high net buoyancy and low initial echo heights. Much of the AgI-sand difference was due to two AgI-treated units with heavier rain conditions across the area. Both were cold-frontal cases, and in general, the results resembled those found at Chicago and St. Louis, where urban influences act to increase summer rain during some frontal cases when moderate to heavy rains occur. Pretreatment conditions on the two AgI cases, found notably different from the sand cases, were compared with conditions on the other AgI cases, showing that the echoes were younger on the two potentially effected days. This study found some weak evidence of augmented rainfall due to seeding but the sample size is too small to draw definitive conclusions of an effect. As in many other seeding experiments, results suggest that if a seeding effect occurred, it was present during only certain atmospheric conditions (some frontal situations producing heavy rains) and only with certain clouds (rapidly growing congestus treated in the early stages of development).