A Statistical Reanalysis of the Replicated Climax I and II Wintertime Orographic Cloud Seeding Experiments

Abstract

A reanalysis of the Climax I and II experiments is described. The concern prompting this reanalysis is a suggestion arising from Colorado State University analyses of extended area effects. Those analyses suggested a regionwide pattern of precipitation that, by chance, may have favored the randomly selected seeding days for some of the important meteorological partitions used in earlier analyses. In order to address this concern, this reanalysis employs excellent covariate relationships developed before the initiation of Climax II and which account for over half of the target variability for most meteorological partitions of major interest (e.g., warm 500 mb temperatures and southwest 700 mb wind directions). The statistical evidence of seeding-induced increases for this reanalysis is generally much stronger than in the previous analysis, which did not utilize covariates. For example, the joint one-sided Wilcoxon test statistic P-value for testing the null hypothesis that seeding did not induce a precipitation increase during warm 500 mb temperatures of the Climax I and II experiments is now 0.0013, compared to 0.0550 in the previous analysis. However, the reanalysis also indicates that previous estimates of increases attributed to seeding based strictly on ratios of seeded to non-seeded precipitation amount means are apparently too large. For example, the estimated precipitation increase of the combined Climax I and II data for the warm 500 mb temperature partition is reduced from 41 to 25% when the full set of data is employed in this reanalysis.