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Executive Summary
The weather 03~ planet Earth is ~ vital and sometimes fatal force in human affairs.
Efforts to control or reduce the harmful impacts of weather go back for in time. In recent
decades our ability to observe and predict various types of meteorological systems has
increased tremendously. Yet during this same period there has been a progressive decline
in weather modifications research. Extravagant claims, unrealistic expectations, and
failure to provide scientifically demonstrable success are among the factors responsible
for this decline. Significantly, every assessment of weather modification dating frotn the
first National Academies' ~ eport in 1 964 has found that scientific proof of the
effectiveness of cloud seeding was lacking (with a few notable exceptions' such as the
dispersion of cold fog). Each assessment also has called for a dedicated research effort
directed at removing or reducing basic scientific uncertainties before proceeding with the
application of weather modification methods. Yet, this type of intensive, committed effort
leas not been carried out.
In this, the latest National Academies' assessment of weather modification, the
Committee was charged to provide an updated assessment of the ability of curt ent and
proposed weather modification capabilities to provide beneficial impacts on water
resource management and weather hazard mitigation It alas asked to examine new
technologies, such as ground-based, in situ' and satellite detection systems, and fast
reacting seeding materials and dispensing methods. The Committee also was asked to
review advances in numerical remodeling on the cloud- and r~eso-scale and consider how
improvements in computer capabilities might be applied to weathers modification. This
study was not designed to address policy implications of weather modification; rather it
focused on the research and operational issues. Specifically, the Committee was asked to:
· review the current state of the science of weather modification and the rode of
weather prediction as it applies to weather modification' paying particular attention to the
technological and methodological developments oftl~e last decade;
. identify the critical uncertainties limiting advances in weather modification
science and operation;
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CRITIC. AL [~SSlJES IN [PEA TI-lER AlOD lric~A TION RESEARCI!
. identify future directions in weather modification research and operations for
improving tile management of water resources and the reduction in severe weather
hazards; and
. suggest actions to identify the potential impacts of localized weather
modification on large-scale weather and climate patterns.
ISSUES AND TRENDS IlS WEATHER MODIFICATION
Motivation
Increasing demands for water make the potential for enhancing the sources'
storage, and recycling of fresl~water a legitin~ate area of study. Destruction and loss of
life due to severe weather, which is increasing with population growth and changing
demographics, require float we examine ways to reduce these impacts. In addition, there is
ample evidence that human activities such as the emission of industrial air pollution, can
alter atmospheric processes on scales ranging from local precipitation patterns to global
climate. These inadvertent impacts on weather arid climate require a concerted research
effort, yet the scientific community has largely failed to take advantage of the fact that
many of the scientific underpinnings of intentional and unintentional weather
modification are the same.
Current Operational and Research Efforts
Operational weather modification programs, which primarily involve cloud-
seeding activities dinted at enhancing precipitation or mitigating hail fall, exist in more
than 24 countries and there were at least 66 operational programs being conducted in l 0
states across the United States in 2001. No federal funding currently is supporting any of
these operational activities in the United States. Despite the large number of operational
activities, less than a handful of weather modification research programs ale being
conducted worldwide. After reaching a peak of $20 million per year in the late 1970s,
support for weather modification research in the United States has dropped to less than
$500,000 per year.
The Paradox
Clearly, there is a paradox in these divergent trends: The federal government is
not willing to fund research to understand the efficacy of weather modification
technologies, but others are willing to spend funds to apply these unproven techniques.
Central to this paradox is the failure of past cloud-seeding experiments to provide an
adequate verification of attempts at rmodif~ing the weathers A catch-22 ensues in which
the inability to provide acceptable proof damages the credibility of the entire field,
resulting in diminished scientific effort to address problems whose solutions would
almost certainly lead to better evaluations.
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l
l
F,XECUTII7E SlJill,;~RY
3
Limitations and Problems
The dilemma in weather modification thus remains. We know that humans
activities can affect the weather, and we know that seeding will cause some changes to a
cloud. Flowever, we still are unable to translate these induced changes into verifiable
changes ifs rainfall, hail fall, and snowfall on the ground, or to employ methods that
produce credible, repeatable changes in precipitation. Among the Actor s that have
contributed to an almost unifor~n failure to verify seeding effects are such uncertainties as
the natural variability of precipitation, tl~e inability to measure these variables with the
required accuracy or resolution, the detection of a small induced effect under these
conditions, and the need to randomize and replicate experiments.
CONCLUSIONS
The Committee concludes that there still is no convincing scientific proof of flee
efficacy of intentional weather modification efforts In some instances there are strong
indications of induced changes, but this evidence has not been subjected to tests of
significance and reproducibility. This does not challenge the scientific basis of weather
modification concepts Rather it is the abser~ce of adequate understanding of critical
atmospheric processes that, in turn, lead to a failure in producing predictable' detectable,
and verifiable results. Questions such as the transferability of seeding techniques or
whether seeding in one location can reduce precipitations in other areas can only be
addressed through sustained research of the underlying science combined with carefully
crafted hypotheses and physical and statistical experiments.
Despite the lacl: of scientific proof the Committee concludes that scientific
understanding has progressed on many fronts since the last National Academies' report
and that there have been many promising developments and advances. For instance' there
have been substantial improvements in the ice-nucleating capabilities of new seeding
materials. Recent experiments using hygroscopie seeding particles in water and ice
(mixed-}ghase) clouds have shown encouraging results, with precipitation increases
attributed to increasing the lifetime of the rain-producing systems. There are strong
suggestions of positive seeding effects in winter orographic glaciogenic systems (i e.'
cloud systems occurring, over mountainous terrain). Satellite imagery has underlined the
role of high concentrations of aerosols in influencing clouds, rain, and lightning, thus
drawing the issues of intentional and inadvertent weather modifications closer together.
This and other recent work has highlighted critical questions about the microphysical
processes leading to precipitation, the transport and dispersion of seeding material in the
cloud volume, the effects of seeding on the dynamical growth of clouds, and the logistics
oftranslating storm-scale effects into an area-wide precipitation effect. By isolating these
cat itical questions, which curl entry hamper pi ogress in weather modification, fetus e
research efforts call be focused and optimized
Additional advances in observational, computational, and statistical technologies
have been made over the past two to three decades that could be applied to weather
modification. These include, respectively, the capabilities to (1) detect and quantify
relevant variables on temporal and spatial scales not previously possible; (2) acquire,
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CRITI(-',4L I.5SUES IN TT'EA TI-lER AlODJFICA TION RESE-ARCI-f
store, and process vast quantities of data; and (3) account for sources of uncertainty and
incorporate complex spatial and temporal relationships. Computer power has enabled the
development of models that range in scale from a single cloud to the global atmosphere.
Nun~erical modeling simulations validated by observations wherever possible are
useful for testing intentional weather modification and corresponding larger-scale effects.
Few of these tools, however, have been applied in any collective and concerted fashion to
resolve critical uncertainties in weather modification. These numerous methodological
advances thus leave not resulted in greater scientific unclerstar~ding of flee principles
underlying weather modification. This has not been due to flawed science but to the lack
of support for this particular field of the science over the past few decades. As a result
there still is no conclusive scientific proof of the efficacy of intentional weather
modification, although the probabilities for seeding-induced alterations are Leigh in some
instances. Despite this lack of scientific pi cot, operational weather modification
programs to increase ~ ain and snowfall and to suppress hail formation continue
worldwide based on cost versus probabilistic benefit analyses.
RECOMMENDATIONS
Recommendation: Because weather modification could potentially contribute to
alleviating water resource stresses and severe weather hazards, because weather
modification is being attempted regardless of scientific proof supporting or refuting
its efficacy, because inadvertent atmospheric changes are a reality, and because an
entire suite of new tools and techniques now exist that could be applied to this issue,
the Committee recommends that there be a renewed commitment to advancing our
knowledge of fundamental atmospheric processes that are central to the issues of
intentional and inadvertent weather modification. The lessons learned from such
research are likely to leave implications well beyond issues of weather modification.
Sustainable use of atmospheric water resources and mitigation of the risks posed by
hazardous weather are important goals that deserve to be addressed through a sustained
r esearch effort.
Recommendation: The Committee r ecommends that a coordinated national
program be developed to conduct a sustained research effort in the areas of cloud
and precipitation microphysics, cloud dynamics, cloud modeling, and cloud seeding;
it should be implemented using a balanced approach of modeling, laboratory
studies, and field measurements designed to reduce the key uncertainties listed in
Box ES.1. This program should not focus on near-term operational applications of
weather modification; rather it should address fundamental research questions from these
areas that currently impede progress and understanding of intentional and inadvertent
weather modification. Because a comprehensive set of specific research questions cannot
possibly be listed here, they should be defined by individual proposals funded by a
national program. Nevertheless, examples of such questions may include the following:
· What is the background aerosol concentration in various places, at different times
of the year, and during different meteorological conditions? To what extent would
weather modification operations be dependent on these background concentrations?
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EXECUTIVE SlJ,\~.~ARY
.
What is the variability of cloud and cell properties (including structure, intensity,
evolution, arid lifetime) within larger clusters, arid how do clouds and cells interact with
larger-scale systems? What are the effects of localized seeding on the larger systems in
which flee seeded clouds are embedded?
. How accurate are radar reflectivity measurements in measuring the differences
between accumulated rainfall in seeded and unseeded clouds? How does seeding affect
the drop-size distribution that determines the relationship between the measured radar
parameter and actual rainfall at the surface?
BOX ES.1
Summary of Key Uncertainties
The statements in boldface type are considered to have the highest priority.
Cloud/p'.ecipitc~tio'~ microphysics issues
. Background concentration, sizes, and chemical composition of
aerosols that participate in cloud processes
Nucleation processes as they relate to chemical composition, sizes, and
concentrations of hydroscopic aerosol particles
· Ice nucleation (primary arid secondary)
. Evolution of the droplet spectra in clouds and processes that contribute to
spectra broadening and the onset of coalescence
· Relative importance of drizzle in precipitation processes
C,louddy~7~amics assures
.
clouds
Cloud-to-cloud and mesoscale interactions as they relate to updraft
and downdraft structures and cloud evolution and lifetimes
~ Cloud and sub-cloud dynamical interactions as they relate to
precipitation amounts arid the size spectrum of hydrometeors
· Mictophysical, thermodynamical7 and dynamical interactions within
Cloud modeling issues
· Combination of the best cloud models with advanced observing
systems in carefully designed field tests and experiments
.
Extension of existing and development of new eloud-resolving models
explicitly applied to weather modification
. Application of short-te~-m predictive models including precipitation
forecasts and data assimilation and adjoins methodology in treated and untreated
situations
.
.
Evaluation of predictive models for severe weather events and
establishment of current predictive capabilities including probabilistic forecasts
Advancement of the capabilities in cloud models to simulate dispersion
tt ajectories of seeding material
· Use of cloud models to examine effects of cloud seeding outside of
seeded areas
.
seeding effects
Combination of cloud models with statistical analysis to establish
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CRITICAL [~SSlJES IN [VEA Tl--lER AJODiFICA T10V RESEARCI-f
.
Seeding-relc~ted issues
· Targeting of seeding agents, diffusion and transport of seeding
material, and spread of seeding effects throughout the cloud volume
Measurement capabilities and limitations of cell-tracking software,
radar' and technologies to observe seeding effects
.
~ - D - °
Analysis of recent observations with new instruments of high
concentrations of ice crystals
· Interactions between different hydrometeors ifs clouds and how to latest
mode] them
.
simulations
Modeling arid prediction of treated and untreated conditions for
Mechanisms of transferring the sto~n-scale effect into an area-wide
precipitation effect and tracking possible downwind changes at the single cells
cloud cluster, and floating target scales
The tasks involved in weather modification research T;~11 within the mission
responsibilities of several government departments and agencies, and careful
coordination of these tasks will be required.
Recommendation: The Committee recommends that this coordinated research
program include:
· Capitalizing on new remote and in situ observational tools to carry out
exploratory and confirmatory experiments in a variety of cloud and storm systems
(e.g., Doppler lidars and airborne radars, microwave radiometers, millimeter-wave and
polarimetrie cloud radars, global positioning system (GPS) and cell-tracking software, the
Cloud Particle Imager, the Gerber Particle Volume Monitor, the Cloud Droplet
Spectrometer). Initia] field studies should concentrate on areas that are amenable to
accurate numerical simulation and multiparameter, three-dimensional observations that
allow the testing of clearly formulated physical hypotheses. Some especially promising
possibilities where substantial further progress may occur (not listed in any priority)
include
~ H~'gro.scr~pic seeding to enhance '~c~inic~ll. The small-scale experiments and
larger-scale coordinated field efforts proposed by the Mazatlan workshop on
hydroscopic seeding (VIMO, 2000) could form a starting point for such efforts. A
randomized seeding program with concurrent physical measurert~ents (conducted
over a period as short as three years) could help scientists to either confirm of
discard the statistical results of recent experiments.
~ Orographic cloud seeding to enhance precipitation. Such a program could
build on existing operational activities in the mountainous western United States.
A randomized program that includes strong modeling and observational
components, employing advanced computational and observational tools, could
substantially enhance our understanding of seeding effects and winter orographic
precipitation.
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EXECUTIVE SlJ,Tl,~ARY
7
~ Studies of specific seeding Affects. This may include studies such as those of
the initial droplet broadening and subsequent; formation of drizzle and rain
associated with hydroscopic seeding, or of the role of large (>1 army particles
(e.g., sea spray) in reducing droplet concentrations in polluted regions where
precipitation is suppressed due to excess concentrations of small cloud
condensation nuclei (CCN).
. Improving cloud model treatment of cloud and precipitation physics. Special
focus is needed on modeling CON' ice nuclei processes, and the growth' collision'
breakup, and coalescence of water drops and ice particles. Such studies nest be based on
cloud physics laboratory measurements, tested and tuned in model studies, and validated
by in situ and ground observations.
. Improving and using current computational and data assimilation
capabilities. Advances are needed to allow rapid processing of large quantities of data
from resew observations and better simulation of moist cloud and precipitation processes.
These models could subsequently be used as planning and diagnostic tools in future
weather modification studies, and to develop techniques to assist in the evaluation of
seeding effects.
.
Capitalizing on existing field facilities and developing partnerships among
research groups and select operational programs. Research in weather modification
should take full advantage of opportunities offered by other field research programs and
by operational weather modification activities. Modest additional research efforts
directed at the types of research questions mentioned above can be added with minimal
interference to existing progran~s. A particularly promising opportunity for such a
partnership is the Department of Energy Atmospheric Radiation Measurement
program/Cloud and Radiation Test bed (DOE ARM/CART) site in the southern Great
Plains (Oklahoma/Kansas) augmented by the National Aeronautics and Space
Administration (NASA) Global Precipitation Mission. This site provides a concentration
of the most advanced observing systems and an infrastructural base for sustained basic
research. The National Center for Atmospheric Research (NCAR) and the National
Oceanic and Atmospheric Administration's Environmental Technology Laboratory
(NOAA/ETL) also could serve as important focal points for weather modification
research.
in pursuing research related to weather modification explicit, financial and
collegial support should be given to young aspiring scientists to enable them to contribute
to our fundamental store of knowledge about methods to enhance atmospheric resources
and reduce the impacts of hazardous weather. It must be acknowledged that issues related
to weather modification go well beyond the limits of physical science. Such issues
involve society as a whole, and scientific weather modification research should be
accompanied by parallel social, political, economic, environmental, and legal studies
The Committee emphasizes that weather modification should be viewed as a
fundamental and legitimate element of atmospheric and environmental science Owing to
the growing demand for fresh water, the increasing levels of damage and loss of life
resulting from severe weather, the undertaking of operational activities without the
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CRITIC,4 L [.SSZJELS IN [VE4 TILLER AJODIFICA TION RESEARCl-I
guidance of a careful scientific foundation, and the reality of inadvertent atmospheric
changes, the scientific community now has the opportunity, challenge, and responsibility
to assess the potential efficacy and value of intentional weather modification
technologies.
Representative terms from entire chapter:
modification research
