Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part IX: Overall Discussion and Summary in the Context of Physical Research

Abstract

The three-year, randomized hail suppression test of the National Hail Research Experiment (NHRE) used seeding methods patterned after the Soviet hail suppression activities, applied over a specified target area on a randomly selected portion of days identified as having a high potential for hailfall by a radar criterion. Statistical tests for seeding effects were performed on data from ground networks of hail measuring instruments within the target area using total hail mass within the target area as the primary response variable. The statistical results indicate a range of possible mean seeding effects from a reduction of 60% to an increase of as much as 500% in total hail mass, depending on the test used, within the 90% confidence limits. The range is so wide that no conclusion about a seeding effect can be drawn. Analysis of hail size, embryo type and various radar parameters, and stratification either in terms of estimates of ?hail potential? or operational seeding efficiency have also not revealed a convincing seeding effect. These results can be rationalized on the basis that the seeding method was to put extra nuclei into the strong updraft portions of the storms, as the Soviet workers do in order to seed ?accumulation zones? of supercooled water. Direct and indirect evidence from the research component of NHRE has shown that accumulation zones, if indeed they exist in northeast Colorado, rarely participate in the hailstone growth. Virtually all radar echo from above the freezing level is from ice, and hail embryos are predominantly graupel. In these circumstances, seeding in the strong updraft is unlikely to produce beneficial competition and a hail suppression effect. In order to devise a seeding technique that has a good chance to be effective in these types of storms, and to strengthen the statistics to the point where a seeding test has a reasonable chance of detecting seeding effects, research must be done on several fundamental aspects of hailstorms. Paramount among these are the location and time of the natural ice nucleation events that lead to the formation of hail embryos, the trajectories followed by the growing hailstones, and the role of depletion of cloud water in determining hail size.