RESULTS OF THE SOUTH-AFRICAN CLOUD-SEEDING EXPERIMENTS USING HYGROSCOPIC FLARES

A new method of seeding convective clouds for the purpose of augmentin g rainfall is being developed in South Africa. Flares that produce sma ll salt particles (0.5-mu m mean diameter) are attached to the trailin g edge of the wings of seeding aircraft and ignited in updrafts below the cloud base of convective storms. This method of delivery overcomes most of the difficulties encountered in the handling and the use of h ygroscopic materials, difficulties that made seeding with ice nuclei ( AgI) a more attractive option. The research that has led to the develo pment of this new technique was prompted by an encounter with a storm with dramatically altered microphysics that was growing over a Kraft p aper mill in the research area. Hygroscopic seeding flares were subseq uently developed, and seeding trials began in October 1990. Successful seeding trials quickly led to the design and execution of a randomize d convective cloud-seeding experiment, the results of which show convi ncing evidence of increases in the radar-measured rain mass from seede d storms when compared to the control or unseeded storms. Heightened r eflectivities aloft seen by the real-time storm-tracking software and observed in the exploratory analysis raises the possibility of develop ing a radar-measured seeding algorithm that can recognize in almost re al time a successful convective seeding event. The implications of suc h a development would have far-reaching effects on the conduct of futu re convective cloud-seeding experiments and operations. The authors' s eeding hypothesis postulates that the hygroscopic seeding at cloud bas e accelerates the growth of large hydrometeors in the treated clouds, which harvest more of the available supercooled water before it is exp elled into the anvils by the strong updrafts that are a characteristic of the local storms, thereby increasing the efficiency of the rainfal l process. The validity of this hypothesis is supported by microphysic al measurements made from an instrumented Learjet and the results of t he randomized experiment, both of which are supported by numerical con densation-coalescence calculations. There are also indications that th e hygroscopic seeding may have an impact upon the dynamics of the trea ted storms, lengthening their lifetimes by strengthening the coupling of the updraft-downdraft storm propagation mechanism. The apparent sen sitivity of rainfall in convective clouds to the aerosol concentration , size, and chemical content may have climatic implications. Higher co ncentrations of small aerosols produced by pollution, biomass burning, etc., could adversely affect the efficiency of the rainfall process. The negative consequences of this effect would be magnified in regions that depend upon convective storms to provide the bulk of their annua l rainfall.