Toward the operational application of hygroscopic flares for rainfall enhancement in South Africa

A major challenge of any operational cloud seeding project is the evaluatio n of the results. This paper describes the development of verification tech niques based on data collected during the first South African operational r ainfall enhancement project in which hygroscopic hares were used to seed th e 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 techn ology before a positive effect on area rainfall can be expected. Sixty stor ms 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 compa red to a "control" group of unseeded storms from the time they were first i dentified as 30-dBZ radar storm volumes. One such control group was obtaine d by selecting unseeded storms by using certain threshold criteria obtained from the seeded storms. Another control group was obtained by simply selec ting the 60 largest storms from the set of unseeded storms meeting the thre shold criteria. Yet another control group was obtained by matching the seed ed storms, in the first 20 min of their lifetimes, before seeding effects c an be expected, with a corresponding set of unseeded storms. Comparisons wi th the National Precipitation Research Programme's randomized hygroscopic f lare seeding experiment database show consistency in the way seeded storms reacted toward producing more rainfall. The analyses on historic rainfall s uggest trends in the same direction, but it is shown that one has to be car eful in interpreting these trends. The importance of quantitatively linking storm-scale seeding effects to apparent area effects is highlighted.