Results of the South African cloud-seeding experiments using hygroscopic flares

Abstract

A new method of seeding convective clouds for the purpose of augmenting rainfall is being developed in South Africa. Flares that produce small salt particles (0.5-μm mean diameter) are attached to the trailing 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 hygroscopic materials, difficulties that made seeding with ice nuclei (AgI) a more attractive option. The research that has led to the development of this new technique was prompted by an encounter with a storm with dramatically altered microphysics that was growing over a Kraft paper mill in the research area. Hygroscopic seeding flares were subsequently developed, and seeding trials began in October 1990. Successful seeding trials quickly led to the design and execution of a randomized convective cloud-seeding experiment, the results of which show convincing evidence of increases in the radar-measured rain mass from seeded storms when compared to the control or unseeded storms. Heightened reflectivities aloft seen by the real-time storm-tracking software and observed in the exploratory analysis raises the possibility of developing a radar-measured seeding algorithm that can recognize in almost real time a successful convective seeding event. The implications of such a development would have far-reaching effects on the conduct of future convective cloud-seeding experiments and operations. The authors' seeding hypothesis postulates that the hygroscopic seeding at cloud base accelerates the growth of large hydrometeors in the treated clouds, which harvest more of the available supercooled water before it is expelled into the anvils by the strong updrafts that are a characteristic of the local storms, thereby increasing the efficiency of the rainfall process. The validity of this hypothesis is supported by microphysical measurements made from an instrumented Learjet and the results of the randomized experiment, both of which are supported by numerical condensation-coalescence calculations. There are also indications that the hygroscopic seeding may have an impact upon the dynamics of the treated storms, lengthening their lifetimes by strengthening the coupling of the updraft-downdraft storm propagation mechanism. The apparent sensitivity of rainfall in convective clouds to the aerosol concentration, size, and chemical content may have climatic implications. Higher concentrations 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 annual rainfall.