EXPLORATORY ANALYSIS OF SEEDING EFFECTS ON RAINFALL - ILLINOIS 1989

Radar-indicated rainfall characteristics from six experimental units r andomly selected for AgI treatment were compared with those from six e xperimental units treated with sand during the summer of 1989 in an ex ploratory analysis. No differences were found between AgI and sand cel l 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 am ounts were determined for the units and for the extended areas around them. AgI units had higher median rainfall values by the end of the tr eatment and thereafter for 90 min than did the sand units, whereas ext ended area rainfall was less in AgI than sand cases. The unit rainfall s, adjusted to account for the unit-extended area relationships, showe d a systematic difference with AgI rain higher at the end of treatment (and statistically significant) and for 90 min thereafter. Comparison s based on 20 relevant meteorological;variables showed that seeding ap peared to produce an effect on days with high net buoyancy and low ini tial 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 summe r rain during some frontal cases when moderate to heavy rains occur. P retreatment conditions on the two AgI cases, found notably different f rom the sand cases, were compared with conditions on the other AgI cas es, showing that the echoes were younger on the two potentially effect ed days. This study found some weak evidence of augmented rainfall due to seeding but the sample size is too small to draw definitive conclu sions of an effect. As in many other seeding experiments, results sugg est that if a seeding effect occurred, it was present during only cert ain atmospheric conditions (some frontal situations producing heavy ra ins) and only with certain clouds (rapidly growing congestus treated i n the early stages of development).