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Bernard A. Silverman

Abstract

The original statistical evaluation of the South African hygroscopic flare seeding experiment described by Mather et al. focused on the amount of radar-estimated rain produced by randomly seeded and unseeded convective cloud complexes in 10-min time periods from −10 to +60 min with respect to their time of selection. Statistically significant differences in the quartile values in some of the 10-min time periods were reported, with indications that the response time to seeding increased with increasing cloud system size. It was found also that the rain mass of the seeded cloud complexes in the 10-min period prior to selection was greater than that of their unseeded counterparts; the statistical evaluation did not take this apparent “inadvertent bias” into account, however. The evaluation included all 5 yr of the experiment, although the design was changed after the third year to allow for seeding larger cloud systems with a larger number of flares per convective cloud system.

An independent statistical reevaluation of the South African cloud seeding experiment using hygroscopic flares is reported on here. Several interesting aspects of the results are found that require a physical explanation and, perhaps, a revision to Mather et al.’s seeding hypothesis; nothing was found that contradicts the claim that there is statistical evidence that hygroscopic flare seeding increased the rain mass from the South African convective cloud systems in the experiment, however.

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Bernard A. Silverman

Abstract

The needs of weather modification are examined from the vantage point of a manager, from the Federal sector, of applied research and development in precipitation management. Several problems in the perspective with which weather modification is viewed in the scientist, user and political communities are discussed. This image of weather modification has given rise to a credibility gap which hinders its technological development. Several courses of action are suggested to improve this image and move the field of weather modification forward both scientifically and socially.

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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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Bernard A. Silverman

During the past decade, statistically positive results have been reported for four major, randomized hygroscopic seeding experiments, each in a different part of the world. Experiments on cold convective clouds using hygroscopic flares were carried out in South Africa and Mexico. Experiments on warm convective clouds using hygroscopic particles were carried out in Thailand and India. The scientific evidence for enhancing rainfall from convective clouds by hygroscopic seeding from these four randomized experiments is examined and critically assessed. The assessment uses, as a measure of proof of concept, the criteria for success of any cloud seeding activity that were recommended in the Scientific Background for the 1998 AMS Policy Statement on Planned and Inadvertent Weather Modifications, criteria that required both statistical and physical evidence.

Based on a critical examination of the results of these four major, randomized hygroscopic seeding experiments, it has been concluded that they have not yet provided either the statistical or physical evidence required to establish that the effectiveness of hygroscopic seeding of convective clouds to increase precipitation is scientifically proven. The impressive statistical results from these experiments must be viewed with caution because, according to the proof-of-concept criteria, credibility of the results depends on the physical plausibility of the seeding conceptual model that forms the basis for anticipating seeding-induced increases in rainfall. The credibility of the hygroscopic seeding for microphysical effects hypothesis has been seriously undermined because it cannot explain the magnitude and timing of the statistically significant increases in precipitation that were observed. Theories suggesting that the microphysical effects of seeding-enhanced downdraft circulations to produce longer-lived clouds have been advanced; however, in the absence of any supporting physical or model evidence, they must be considered to be in the realm of speculation.

These results do not alter this author's basic position; cloud seeding is advocated in situations where it is scientifically and operationally appropriate, and it is strongly recommended that an independent evaluation accompany each research or operational project in order that the science of weather modification benefit from the experience.

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Bernard A. Silverman

Abstract

The effect of instrument baseline averaging on the estimated spectrum density of a homogeneous random field of a scalar variable in turbulent flow is investigated. The spectrum transfer function of the spatial filter is evaluated for all orientations of the baseline to the vector mean wind. It is shown that the influence of baseline averaging on the spectrum increases as the angle between the baseline and the vector mean wind decreases and as the ratio of the baseline length to the wavelength of the fluctuations increases. These results can be used to correct the computed spectra of spatially averaged components of the velocity field in turbulent flow and of spatially averaged conservative passive additives in a turbulent flow.

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Bernard A. Silverman

The scientific evidence for enhancing rainfall from convective clouds by static-mode and dynamic-mode seeding with glaciogenic agents is examined and critically assessed. The assessment uses, as a measure of proof of concept, the criteria for success of any cloud seeding activity that was recommended in the Scientific Background for the 1998 AMS Policy Statement on Planned and Inadvertent Weather Modification, criteria that require both statistical and physical evidence. Based on a rigorous examination of the accumulated results of the numerous experimental tests of the staticmode and dynamic-mode seeding concepts conducted over the past four decades, it has been found that they have not yet provided either the statistical or physical evidence required to establish their scientific validity. Thus, the conclusion of several high-level reviews of weather modification conducted by the Advisory Committee on Weather Control, the National Academy of Sciences, and the Weather Modification Advisory Board during the period from 1957 to 1978 that cloud seeding was promising, unproven, and worth pursuing is still valid today.

The research and experiments related to the static-mode and dynamic-mode seeding concepts, especially those conducted since 1978, provided physical insights about some important cold–cloud precipitation development mechanisms and the possible effect of glaciogenic seeding on them. Exploratory, post hoc analyses of some of the experiments have suggested positive effects of seeding under restricted meteorological conditions, at extended times after seeding and, in general, for reasons not contemplated in the guiding conceptual seeding models; however, these exploratory results have never been confirmed through subsequent experimentation. New experiments are needed to resolve the uncertainties, inconsistencies, and deficiencies in the statistical and physical evidence in support of static-mode and dynamic-mode seeding of convective clouds obtained thus far. Considering the statistically positive result of hygroscopic flare seeding of cold convective clouds in South Africa and its replication in Mexico, and of hygroscopic particle seeding of warm convective clouds in Thailand, efforts to obtain the physical evidence required to place the hygroscopic seeding concept on a secure scientific foundation is, perhaps, a more immediate and higher-priority investment.

Future experiments on glaciogenic seeding of convective clouds, indeed any cloud seeding technique, should feature well-defined physical–statistical tests of the seeding concepts, in accordance with the proof-of-concept criteria, in order to establish their scientific validity. People with water interests at stake who are investing in operational glaciogenic cloud seeding projects for precipitation enhancement should be aware of the inherent risks of applying an unproven cloud seeding technology and provide a means of evaluation that allows for an assessment of the scientific integrity and cost effectiveness of the operational seeding projects. Those who are contemplating investing in operational hygroscopic seeding projects for precipitation enhancement based on the statistically positive experimental results in South Africa, Thailand, and Mexico should be aware that, in the absence of physical evidence required by the proof-of-concept criteria, this cloud seeding technology is also unproven.

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Bernard A. Silverman
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Bernard A. Silverman and Morton Glass

Abstract

A time-dependent, one-dimensional model of the life cycle of an isolated warm cumulus cloud is presented that combines the vertical equation of motion, the equation of mass continuity, the first law of thermodynamics, and the equations of continuity of water vapor and liquid hydrometeors. The dynamic interaction between the cloud and its environment is modeled by two entrainment terms: turbulent entrainment representing lateral mixing at the side boundaries of the cloud, and dynamic entrainment representing the systematic inflow or outflow of air required to satisfy mass continuity. The formation and growth of drops by condensation, stochastic coalescence, and droplet breakup are modeled in detail for 67 logarithmically-spaced Eulerian size classes covering a range of particle sizes from 2 to 4040 μm in radius. The numerical method of simulating microphysical processes was investigated 1) by systematically reducing the number of mass classes used to represent the hydrometeor size spectrum and 2) by replacing the rigorous formulation of the microphysical processes by a set of parameterized expressions.

Calculations with the model gave results which were in good agreement with observations of the dynamical and microphysical properties of warm maritime cumuli. The formation and development of the tropical rain shower was particularly well simulated. Reasonable model predictions were obtained when as few as 45 logarithmically-spaced mass classes were used to characterize the hydrometeor size spectrum. When fewer mass classes or parameterized microphysical techniques were used, the model results were significantly different.

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Bernard A. Silverman and Alan Hecht
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Bernard A. Silverman and Wathana Sukarnjanaset

Abstract

A randomized, warm-rain enhancement experiment was carried out during 1995–98 in the Bhumibol catchment area in northwestern Thailand. The experiment was conducted in accordance with a randomized, floating single–target design. The seeding targets were semi-isolated, warm convective clouds, contained within a well-defined experimental unit, that, upon qualification, were selected for seeding or not seeding with calcium chloride particles in a random manner. The seeding was done by dispensing the calcium chloride particles at an average rate of 21 kg km−1 per seeding pass into the updrafts of growing warm convective clouds (about 1–2 km above cloud base) that have not yet developed or, at most, have just started to develop a precipitation radar echo. The experiment was carried out by the Bureau of Royal Rainmaking and Agricultural Aviation (BRRAA) of the Ministry of Agriculture and Cooperatives as part of its Applied Atmospheric Resources Research Program, Phase 2.

During the 4 yr of the experiment, a total of 67 experimental units (34 seeded and 33 nonseeded units) were qualified in accordance with the experimental design. Volume-scan data from a 10-cm Doppler radar at 5-min intervals were used to track each experimental unit, from which various radar-estimated properties of the experimental units were obtained. The statistical evaluation of the experiment was based on a rerandomization analysis of the single ratio of seeded to unseeded experimental unit lifetime properties. In 1997, the BRRAA acquired two sophisticated King Air 350 cloud-physics aircraft, providing the opportunity to obtain physical measurements of the aerosol characteristics of the environment in which the warm clouds grow, of the hydrometeor characteristics of seeded and unseeded clouds, and of the calcium chloride seeding plume dimensions and particle size distribution—information directly related to the effectiveness of the seeding conceptual model that was not directly available up to then.

The evaluation of the Thailand warm-rain enhancement experiment has provided statistically significant evidence and supporting physical evidence that the seeding of warm convective clouds with calcium chloride particles produced more rain than was produced by their unseeded counterparts. An exploratory analysis of the time evolution of the seeding effects resulted in a significant revision to the seeding conceptual model.

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