ASSESSING THE POTENTIAL FOR RAIN AUGMENTATION - THE NELSPRUIT RANDOMIZED CONVECTIVE CLOUD SEEDING EXPERIMENT

The experimental design, analyses, and results of the first Nelspruit randomized cloud seeding experiment are described. The experiment ran for three years, commencing in October 1984, and involved the on-top s eeding of new cloud turrets growing on the hanks of isolated multicell ular storms using dry ice delivered from a Learjet at around the heigh t of the -10 degrees C isotherm. All storms were tracked by a radar op erating in computer-controlled volume scan mode. A total of 169 storms were examined, of which 94 passed the selection criteria. The most im portant criterion was based upon a microphysical classification scheme obtained from measurements made by the instrumented Learjet. This sch eme, based upon a ratio of cloud-base temperature to potential buoyanc y at 500 mb, rejected those storms in which the production of precipit ation via coalescence was unlikely. A key element of the experiment wa s the ability to objectively track the storms using an automatic storm tracking algorithm. Storms were analyzed in terms of their track prop erties, some of the more important of which were storm volume, area, a nd rain flux. Analyses of these track properties in 10-min time interv als either side of decision time (the time the seed/no-seed decision w as made) proved to be the most revealing in terms of observed changes and rates of changes in convective cloud processes. This analysis show ed an almost fourfold percentage increase in radar-measured rain flux and storm area when the seeded and control storms were compared. A con firmatory experiment was conducted in the third season. Storm track pr operties that showed an apparent response to seeding in each of the fi rst two seasons were selected prior the commencement of the third seas on. All but one of these track properties either stayed the same or sh owed increases in the third season, confirming the hypothesis that the re were radar-detected differences between the seeded and control stor ms.