Seeding Optimization for Hail Suppresion with Ground Generators

Abstract

An evaluation method of hail prevention by silver iodide ground seeding generators is developed. The method is based on correlations between the point hailfall intensities measured with hailpads and the silver iodide released prior to the hailfalls in the "feeding areas" where the hail cells developed. The time unit for the correlations is the day, but days can be aggregated together after data normalization. Former evaluations have shown that the number of hailstones larger than 0.7 cm is mainly responsive to the amount of seeding material released in a 13-km radius area centered on the place where the storm was developing 80 min before the hailfall, and that the seeding effect can be detected only for the days with at least 15 hailed pads.  In this paper, the method is applied to 24 major hail days with seeding recorded in the past 20 years in a hailed region north of the Pyrenees. For each day, 15 to 42 point hailfalls have been recorded, and they are used to compute the best negative daily correlation between hailfall intensity and seeding amount by moving the feeding area around its first approximate position. With this seeding area optimization, all the daily correlations are negative (more seeding, less hail), but they are weak, with a correlation coefficient reaching about r = -0.3 in only half of the cases. For the whole hailfall sample (561 hailfalls), the correlation computed with the ideal feeding areas determined as indicated above is r = -0.22, significant at the 0.01 level, subject to the data independency hypothesis. In average, the distance from the middle of the feeding area to the hailfall corresponds to a storm travel time of 66 min, but a numerical simulation of the seeding particle trajectories with the Meso-NH model suggests that the generators must be started at least 45 min before the storm travels above them.