A Review of Hygroscopic Seeding Experiments to Enhance Rainfall
AbstractField experiments and computer modeling studies of the possibility to promote the coalescence process by hygroscopic seeding for rainfall enhancement are reviewed. Most previous experiments have focused on the use of water sprays or common salt particles, but the practical delivery of the massive bulk sources of these products has been a limiting factor. Although most past efforts have not provided convincing scientific evidence of seeding effects, they leave the impression that effects were generally consistent with the hygroscopic seeding hypothesis under investigation (i.e., broadening of the cloud droplet distribution, and triggering of coalescence which possibly alters echo morphology). Rainfall enhancement from hygroscopic seeding remains to be demonstrated. Seeding effects beyond those expected on the coalescence process may also be apparently possible. Effects on the initiation and evolution of ice may have been noted perhaps because of the enhanced presence of supercooled drizzle and rain drops, or because rime-splintering may have been enhanced by the presence of broader distributions of supercooled cloud droplet distributions in ice multiplication zones, or for perhaps both reasons. Indications of "dynamic" effects may have been found which possibly occurred either in conjunction with the latent heat of condensation, or from a more active conversion of supercooled water to ice, or for both reasons. Computer modeling has indicated that seeding at cloud base with appropriately sized cloud condensation nuclei to foster Langmuir-type precipitation growth trajectories may be a desirable seeding strategy, and that seeding does not necessary have to be limited to cold based clouds characterized by a marginal coalescence process. The use of "new" hygroscopic seeding flares at cloud base in deep warmer-based South African clouds has produced very encouraging results from a limited amount of experimentation, and the new seeding flares have apparently overcome earlier problems associated with transporting the seeding materials. Thus, the technique of hygroscopic seeding deserves reexamination.
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