An Attempt to Normalize the Hailstorm Variability for the Evaluation of Cloud Seeding

Abstract

Procedures have been developed from relationships between parameters of hailstone size distributions and storm thermodynamics to normalize the effects of storm thermodynamics of integral hail parameters observed at the ground. Hail parameters considered in this study are the number concentration, water content and kinetic energy. Results from statistical tests on these integral hail parameters obtained from various not-seeded storms showed that significant differences exist among these storms. The differences appear to be due to variations in storm intensity. However, once the normalization of the storm thermodynamic variability is applied to the data, no significant differences are found. The technique seems to reduce, if not eliminate, the variations due to the effects of storm thermodynamics on the integral hail parameters. Statistical analyses of the normalized surface integral hail parameters, obtained from two operationally seeded storms, are carried out to test the procedure for the evaluation of silver iodide seeding effects. A storm-by-storm approach is used to illustrate the anthropogenic variability such as differences in the execution of the seeding operation. Integral hail parameters obtained from a storm that is not seeded at the appropriate location as prescribed by the seeding hypothesis do not differ from those of the control storms. For the other storms seeded at the right location, a nominally significant difference is found in the hail total number concentration but not in the integral hail water content and kinetic energy. However, no conclusion as to the cause of the isolated significant difference can be drawn because the storms analyzed did not constitute a randomized experiment.