Toward the Operational Application of Hygroscopic Flares for Rainfall Enhancement in South Africa

A major challenge of any operational cloud seeding project is the evaluation of the results. This paper describes the development of verification techniques based on data collected during the first South African operational rainfall enhancement project in which hygroscopic flares were used to seed the bases of convective storms. Radar storm properties as well as historical rainfall records were used in exploratory studies. The storm-scale analyses are viewed as extremely important, because individual storms are the units that are seeded. Their response to seeding has to be consistent with that of the seeded group in a randomized experiment using the same seeding technology before a positive effect on area rainfall can be expected. Sixty storms were selected for seeding, mostly early in their lifetimes. This permits a time-of-origin analysis in which the group of seeded storms can be compared to a “control” group of unseeded storms from the time they were first identified as 30-dBZ radar storm volumes. One such control group was obtained by selecting unseeded storms by using certain threshold criteria obtained from the seeded storms. Another control group was obtained by simply selecting the 60 largest storms from the set of unseeded storms meeting the threshold criteria. Yet another control group was obtained by matching the seeded storms, in the first 20 min of their lifetimes, before seeding effects can be expected, with a corresponding set of unseeded storms. Comparisons with the National Precipitation Research Programme’s randomized hygroscopic flare seeding experiment database show consistency in the way seeded storms reacted toward producing more rainfall. The analyses on historic rainfall suggest trends in the same direction, but it is shown that one has to be careful in interpreting these trends. The importance of quantitatively linking storm-scale seeding effects to apparent area effects is highlighted.