A Sensitivity Test for Hail Prevention Assessment with Hailpad Measurements

Abstract

The evaluation of the French hail prevention project with silver iodide ground generators is based on daily correlations between the running time of the generators and the intensity of point hailfalls as indicated by hailstone number determined with hailpads. A normalization of these two parameters by their daily mean values allows the aggregation of hail days, and the setting-up of larger data samples for a statistical examination in which the random nature of hail becomes less important. In this paper, the evaluation is made from the 1948 point hailfalls recorded in an area of 16,000 km^2 of the Midi-Pyrénées region during 17 hail seasons. A cumulative method of correlation between the seeding and hailfall data shows that only the major hail days, with at least 15 point hailfalls measured in a hailpad network of 7 km mesh, may enable the detection of a seeding effect from a ground generator network of 10 km mesh. With this observation, the correlation between the seeding and hailfall data for438 hailfalls on 18 major hail days indicates a beneficial effect of the seeding on 15 days, with a hail decrease of 40% for the correctly seeded events. This ratio amounts to 50% when the hailfall kinetic energy is considered instead of the hailstone number.