A Re-Evaluation of the Swiss Hail Suppression Experiment Using Permutation Techniques Shows Enhancement of Hail Energies When Seeding

Armin Auf Der Maur,Urs Germann

doi:10.3390/atmos12121623

Abstract

Grossversuch IV is a large and well documented experiment on hail suppression by silver iodide seeding. The original 1986 evaluation remained vague, although indicating a tendency to increase hail when seeding. The strategy to deal with distributions of hail energy far from normal was not optimal. The present re-evaluation sticks to the question asked and avoids both misleading transformations and unsatisfactory meteorological predictors. The raw data show an increase by about a factor of 3 for the hail energy when seeding. This is the opposite of what seeding is supposed to do. The probability to obtain such a result by chance is below 1%, calculated by permutation and bootstrap techniques applied on the raw data. Confidence intervals were approximated by bootstrapping as well as by a new method called “correlation imposed permutation” (CIP).

Highlights

Hail damage to crops, fruits, cars, buildings and even people is a disaster which leads especially farmers to seek for protection
They state in their conclusions “A supercell storm exhibits a kind of natural selection mechanism which tends to restrict the number of embryos, natural or artificial, entering the hail growth region
The present study is based on data found in the appendix of [7]: the hail energy on the ground ln(EGR + 1), reconverted to energy on ground (EGR), the seeding coverage sc, the beginning t0 and the end t f of the seeding criterion met within the experimental area

Summary

Introduction

Fruits, cars, buildings and even people is a disaster which leads especially farmers to seek for protection. On the other hand an artificial increase of the number of hail embryos could reduce the size of hailstones and the kinetic energy of hail by the competition for the available supercooled water. This idea, promoted mainly by Sulakvelidze [2], was the basis for many operational programs of hail suppression in the former USSR, in Eastern Europe and more than 20 other countries worldwide, see Figure 1 in [3]. A comprehensive review on hail suppression by different methods was done by Wieringa and Hollemann [6], and, more recently, by Rivera et al [3], who confirms the uncertainty about a possible benefit of 60 years of hail suppression in Mendoza (Argentina)

Results

Discussion

Conclusion