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Robert D. Elliott

Abstract

This review provides a sketchy background of orographic weather modification activities prior to the 1960s, followed by a more critical review of major orographic projects carried out and reported in the scientific literature during the past 25 years. In the earlier of these major projects, evaluation of results had been based largely upon comparisons of seeded and nonseeded precipitation experimental units stratified by various sounding-derived parameters in an attempt to amplify the physical significance of the seeding effects within various sub-types of orographic clouds.

The later major projects are still underway with no final evaluations having been presented. However, a wealth of significant data analyses have been reported that provide important insights into the various natural and seeding precipitation mechanisms. Much of this is attributable to the new observational tools in use, which include airborne and ground microphysical sensors, doppler radar, and microwave radiometers.

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Geoffrey E. Hill

Abstract

This article is a review of work on the subject of seedability of winter orographic clouds for increasing precipitation. Various aspects of seedability are examined in the review, including definitions, distribution of supercooled liquid water, related meteorological factors, relationship of supercooled liquid water to storm stage, factors governing seedability, and the use of seeding criteria.

Of particular interest is the conclusion that seedability is greatest when supercooled liquid water concentrations are large and at the same time precipitation rates are small. Such a combination of conditions is favored if the cloud-top temperature is warmer than a limiting value and as the cross-barrier wind speed at mountaintop levels increases.

It is also suggested that cloud seeding is best initiated in accordance with direct measurements of supercooled liquid water, precipitation, and cross-barrier wind speed. However, in forecasting these conditions or in continuation of seeding previously initiated, the cloud-top temperature and cross-barrier wind speed are the most useful quantities.

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Bernard A. Silverman

Abstract

A review of the state of knowledge of the physics of the static mode seeding hypothesis for convective clouds is presented. The central thesis of the review is that the results of past experimental work are diverse but valid and that credibility of the science depends on understanding the physical reasons for the diverse results. Areas of uncertainty and conflicts in evidence associated with the statement of physical hypothesis, the concept of seedability, the seeding operation, and the chain of physical events following seeding are highlighted to identify what issues need to be resolved to further progress in precipitation enhancement research and application.

It is concluded that the only aspect of static seeding that meets scientific standards of cause-and-effect relationships and repeatability is that glaciogenic seeding agents can produce distinct “seeding signatures” in clouds. However, the reviewer argues that a body of inferential physical evidence has been amassed that provides a better understanding of which clouds are seedable (susceptible to precipitation enhancement by artificial seeding) and which are not, even though the tools for recognizing and properly treating them are imperfect. In particular, the inferred evidence appears to support the claims of physical plausibility for the positive statistical results of the Israeli experiments.

It is suggested that future work continue to be designed for physical understanding and evaluation through comprehensive field studies and numerical modeling. Duplicating the Israeli experiments in another location should receive high priority but, in general, future experiments should move upscale from cumulus congestus to convective complexes. In doing so, a new, more complex physical hypothesis that accounts for cloud–environment and microphysical–dynamical interactions and their response to seeding will have to be developed.

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George A. Isaac

Abstract

Any observing program studying summer cumulus clouds should attempt to measure cloud lifetime. This parameter is important for determining whether a cloud will last long enough for precipitation to form by either natural or artificially stimulated mechanisms. When reporting cloud lifetime, the definition used and the method of calculation should be clearly specified. In North America, after a summer cumulus cloud has been identified and selected, lifetimes, at temperatures below –5°C, of approximately 10 to 12 min are being reported. This lifetime must be considered marginal for static mode seeding to produce precipitation by artificial ice nucleants.

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William R. Cotton

Abstract

In this paper, testing, implementation, and evolution of both static and dynamic seeding concepts are reviewed. A brief review of both waterspray and hygroscopic seeding is first presented. This is followed by reviews of static seeding of stable orographic clouds and supercooled cumuli. We conclude with a review of dynamic seeding concepts with particular focus on the Florida studies.

It is concluded that it is encouraging that our testing procedures have evolved from single-response-variable “blackbox” experiments to randomized experiments that attempt to test a number of components in the hypothesized chain of physical responses to seeding. It is cautioned, however, that changes in the seeding strategy to optimize detection of a physical response (in any of the intermediate links in the hypothesized chain of responses) can have an adverse effect upon rainfall on the ground.

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Ronald M. Welch, Stephen K. Cox, and John M. Davis

Abstract

No Abstract available.

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