Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part VI: Post Hoc Stratification by Storm Intensity and Type

Abstract

The surface hail and rain data collected during the randomized seeding experiment of the National Hail Research Experiment (NHRE) are stratified according to storm intensity in a search for seeding effects. The problems encountered in attempting to stratify the data according to whether the storm cells exhibited radar vaults are discussed, but no stratification is attempted on this basis. The stratification by storm intensity is accomplished using a one-dimensional steady-state cloud model in conjunction with measured radar echo top heights to estimate the maximum updraft speed of the storm. It is shown that the technique is quite successful in classifying the storm days according to hailfall intensity. Analysis of hail and rain data on seed and control days in the various strata do not show an effect of seeding as assessed by 90% confidence intervals. The confidence intervals are quite wide, however, and a variety of possible effects of seeding are also not inconsistent with the data. The frequency with which vaulted cells were observed during the experiment is documented, and they are shown to constitute only ?1% of the total. However, the hailfall from vaulted cells, which tend to be very large and long-lived, can be extreme. Three vaulted cells were seeded during the experiment. Analysis of the radar reflectivity structure of these cells did not reveal any obvious effects of seeding.