Assessment of Hail Prevention experiments and operations in Spain: comparison seeded and unseeded periods

Despite the difficulties in assessing the results of Hail Prevention Projects, some positive effects have been described. Studies such as Dessens (1998), Dessens et al. (1998) or Sánchez et al. (2005) have found that it is possible to assess hail suppression projects by using ground truth data. A strict control of the seeding activities is required, as well as an excellent hailpad network on the ground. From 2001 to 2004 an operational Hail Prevention Project was made in Lérida (Ebro Valley). The target area had an extension of 3000 km2. Most storms in the studied zone are multicellular storms. Seeding material was spread by means of 70 ground base generators. From 2005 to 2009 no seeding material was used. In order to have the ground truth, 173 hailpads were deployed in the target area. The database was built including the following data for each hail day in the study zone: a) Storm cells in the study zone (using radar imagery), b) Hailpads in the study zone, c) The amount of seeding material (2001-05) or placebo (2005-09) released close to the storm, whenever the storm had been seeded at least 30 minutes before the hail fall. d) For each hailstorm included in the data base, the seeding material released from ground base generators or the placebo plumes were simulated. Finally we obtain 118 for the seeding period and 119 for the unseeded period. A database was set up with information obtained from each hailpad with impacts of hailstones (number of hailstones with a diameter of more than 5 mm), and the seeding rate that would have had the chance of affecting the storm. For each point with evidence of ‘hail fall', two variables were defined: 1) ΔNi, i.e., the difference between the number of hailstones at that point and the average number of hailstones produced by that storm; 2) ΔSi, the seeding rate released at least 30 minutes before the storm reached that point, and the average rate seeded in the storm

In the seeded period, a significant correlation (r = 0.23, α=0.01) was found between the seeding rate and the number of hailstones on seeding days. It means the relationship between the AgI emission rate of a network of ground generators and the number of hailstones showed a statistically significant correlation that illustrates the positive effect of cloud seeding activities for hail suppression. In the unseeded period no significant correlation was found. It is to say no correlation was found using placebo and the same methodology. These results are indicating that a physical evidence of signature caused by AgI. 79.152.14.63 on 8-1-2010-->