Exploratory analysis of the effect of hail suppression operations on precipitation in Alberta

An operational hail suppression program has been based in southern Alberta, Canada, since 1996. The program is designed to reduce hail damage to property in towns and cities. The analysis of the effect of the cloud seeding on the rainfall was motivated by concerns that hail suppression operations might reduce rainfall and thus offset any economic gain offered by a reduction of hail damage. An exploratory analysis of volume-scan, C-band radar data using sophisticated storm cell tracking software was used to calculate radar-derived rainfall characteristics from 160 seeded and 1167 non-seeded storms, on 82 days with seeding, during the summers of 2001 and 2002. The seeded storms (stratified according to maximum radar-derived cell top height) have greater mean durations (+50%), have greater mean precipitation rates (+29%) and have a greater mean total rain area–time integral (+54%). There is statistical evidence to reject the null hypothesis of no effect of cloud seeding on the total volume of rainfall. The data support the claim that seeding causes an increase in rainfall. The seeding effect is estimated to increase the mean rainfall volume (averaged for categories 7.5–11.5-km height storms) by a factor of 2.2, with an average 95% confidence interval of (1.4, 3.4).