Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part IV: Analysis of Radar Data for Seeding Effect and Correlation with Hailfall

Abstract

Radar data collected during the seeding experiment of the National Hail Research Experiment are used in a search for possible effects of seeding. Two types of variables, denoted by P and Q, are defined as daily integrals of reflectivity and areas of reflectivity above a given threshold. These and other radar variables are examined for correlation with hailfall at the ground and for seeding effect. Though several variables are closely associated with the occurrence of hail in the network, according to the present sample, none is highly correlated with the amount of hail. A method for measuring hailfall by radar recently used in Switzerland with apparently good results was not successful when applied to the Colorado area. Ten radar variables were tested for seeding effect by comparing their values on seed and control days. Both the Student's t-test and the Wilcoxon-Mann-Whitney test were employed and gave comparable results. No variables tested showed a difference between seed and control days that was significant at the 10% level. An examination of regressions developed between two adjacent areas (one of which was expected to be much more strongly affected by seeding than the other) also failed to detect a statistically significant difference between seed and control days.