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5 Conclusions ant! Recommendations Although 40 years have passed since the first WAS report (NRC, 1964) on weather modification, this Committee finds itself very much in concurrence with the findings of that assessment (see Chapter 13. We conclude that the initiation of large-scale operational weather modification programs would be premature. Many fundamental problems must be answered first. It is unlikely that these problems will be solved by the expansion of present efforts, which emphasize the a posterior) evaluation of largely uncontrolled experiments. We believe that the patient investigation of atmospheric processes coupled with art exploration of the technological applications may eventually lead to useful weather modification, but we emphasize that the time-scale required for success may be measured in decades. CONCLUSIONS Below is a summary of the Committee's principal conclusions! presented in response to the tasks that the Committee was asked to address. Task 1. Rc~;iew the current state of the sciences elf Weather ~~/if~cation and the role of mate prediction curs it applies to weather modification, paying particular attention to the technological arid nZethadological developments of the last `;lecade. Principal conclusion. Over the past 30 years? there has teen significant advancement in observational and computational capabilities, providing new opportunities to address many of the outstanding questions underlying attempts to modify weather. It is the principal conclusion of this Committee that the field of atmospheric science is now in a position to mount a concerted and sustained effort to delineate the scope and expectations of future weather modification research. Such an effort must be directed at answering fundamental scientific questions that will yield results that go well beyond application to intentional modification. The emphasis must be on understanding processes and not on modification. Once understanding is achieved, 67

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68 CRITiCA L I~SSlJES IN TEL-A TlIER AlODIFI(-~A TION RESE-ARCi-f the focus can turn to application of this understanding, not only to intentional weather modification but also to inadvertent modification and other related fields? such as cloud n~ode]ing and weather forecasting. Status of weather modification research. Weather modification research has been in ~ state of decline in the United States for snore than two decades. Else reasons are many and include the lack of scientifica] ly demonstrable success in modification experi~ne~ts, extravagant claims, attendant unrealistic expectations (e.g., pressure from agencies to meet short-term operational needs rather than to achieve long-term scientific understanding), growing environmental concerns, and economic and legal factors. Within this context it became difficult to distinguish legitimate and important research from some cloud-seeding programs claiming success Title little or rho substantiation. This led many scientists to abandon the field and federal agencies to reduce funding for weather modification research dramatically. Status of weather ~nodif~cation operations. Despite the decline in research in the United States' weather modification remains a topic of substantial worldwide interest, with programs currently active in more than 24 countries. In the United States in 2001 there were at least 66 operational programs (supported by private and state entities) aimed at enhancing rain, enhancing snowpack, or suppressing hail. Evaluation methodologies vary but in general do not provide convincing scientific evidence for either success of Failure. Although there is Physical evidence that seeding affects cloud ~ . , ~ ,, . . . ~ , , , ,- . . `,, . , ~ , ,, ~ processes, ettectlve methods for slgnl~lcantty modl-rylng tile weather generally have not been demonstrated. Scientific evidence of seeding effects. The Committee concurs with the conclusion from Silverman (2001) that: `~Based upon a rigorous examination of the accumulated results of the numerous experimental tests of the static-mode and dynan~ic- 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." This statement was made specifically in reference to glaciogenic seeding of convective clouds. With the possible exception of winter orographic clouds, it applies to virtually all efforts aimed at precipitation er~haneeme~t or hail suppression. This does not challenge the scientific basis of cloud-seeding concepts; rather, it is recognition of the lack of credible evidence that applying these concepts will lead to predictable, detectable, and verifiable results. Recent experiments have renewed interest in the possibility of increasing rainfall from warm season convective clouds by cloud-base release of hydroscopic particles. These particles have just the right characteristics to promote the formation of drizzle, which grows by coalescence into rain There have been promising experiments conducted in South Africa and in Mexico, where measurements using new observing systems have demonstrated responses in clouds to treatment ilk accordance with understanding of the chain of physical reactions leading to precipitation. This appears to be a fruitful area for further research. Hazard mitigation. In the arena of hazard mitigation there are at least two examples of success. The suppression of cold fogs is clearly established and is used

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CO,N'CL ~ JSIO.VS A ND RE(-'O,tI7tIE.ND,4 TIO.NS 69 effectively at airports and other select locations. The use of lightning rods is an exceptionally effective method for protecting property, but no scientifically acceptable evidence exists that lightning can be suppressed or redirected through deliberate interventions in atmospheric processes. The inadvertent effect of air pollution on the fiequency and polarity of cloud-to-ground lightning strikes is an important new finding supported by some observations. There is no scientifically credible evidence that hail can be suppressed. Lack of knowledge and ability to observe the details of a large hailstorm limits our ability to target observations or to design experiments that can detect induced changes. Insurance data showing reduced Crop damage in areas of had] suppressions activity may serve to motivate the operational programs, belt they do not constitute scientific proof that hail fall can be r educed Almost no world has been conducted aimed at tornado mitigation. All work on modifying hurricanes, including numerical model simulations' ceased in 1980 Past hurricane modification studies contributed substantially to the knowledge of the structure arid inner workings of hurricanes, which led to improvements in forecasting hurricane motion and intensity. However a detailed understanding of the dynamics, the~nodynan~ics, and cloud physics of hurricanes must be attained before any actual modification experiments are considered. Atmospheric modeling and weather forecasting. Numerical simulation and prediction models are key components of a national weather modification program for use in planning and justification, operations, arid post-operation analysis. Simplified simulation models may be useful for learning about the sensitivity of a cloud system to various kinds of modification. while a prediction model must be able to conform to real initial and boundary conditions. The success of any weather modification program can best be tested by comparison with a prediction of what would have happened without the modification. However, this places an enormous burden on prediction since many of the uncertainties limiting quantitative precipitation forecasting in weather forecast models and cloud parameterizations are the same as those that limit understanding of the physics arid dynamics of seeded clouds. Thus, further advancement of numerical modeling capabilities is necessary, but weather modification-related research should not await an ability to make quantitative precipitation forecasting predictions. Improving modeling and quantitative precipitation forecasting are long-term, iterative processes that will continue to evolve for decades to come. In the meantime, there is a tremendous amount that can be learned by addressing other relevant research questions (e.g., precipitation formation mechanisms, cloud/storm dynamics). In fact, developments in these basic physical processes and in precipitation forecasting would benefit if done commensurately. Operational arid mesoscale predictions, supplemented by a program of numerical modeling and prediction aimed at resolving the much smaller scales of clouds (finer than I km) and incorporating the detailed physics of precipitation processes and evolution, would be useful for developing a resea~ch-quality weather modification program. The quality and validity of such cloud models have also improved substantially in the last two decades due to great increases in computer power and improved mathematical and

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7() CRITI(-~,4L [.SSlJES IN THEA TlIER AlODIFICA TIO.V RESEARClI numerical methods, including those for data assimilation. The models have not yet, however, demonstrated the ability to accurately represent and predict precipitation processes under all important natural conditions. Bin microphysics, which is believed to be the best current method for simulating cloud nucleus and hydrometeor evolution, is computationally demanding. It is currently on the borderline of: practical utility for simulating large convective clouds but Nay be more fully usable for simulating winter orographic clouds. Full testing of such models remains difficult because of the inadequacy of direct measurement of cloud water arid nucleus properties. thou satellite 1 ~ ~ ~ and other remote-sensing obs-e~vation methods help fill in the details. Evidence flom the best simulation models indicates bleat precipitation-forming processes may be strongly dependent oat the size spectrum of existing condensation and freezing nuclei, and that artificial modifications of nuclei concentrations may produce predictable results. Observational technologies. There have beets nanny advances in observational technologies in the past two decades. New remote and ill situ approaches have dramatically improved the ability to examine flee structure and hydrometeor content of clouds. Polarization-diversity radars can estimate in-cJoud particle shapes and sizes, allow tracking of the dispersion of seeding aerosols, and allow Close accurate estimates of precipitation. Millimeter-wave cloud radar can describe non-precipitating clouds. The national Doppler radar network (NEXRAD) provides opportunities for examining the evolution of radar signatures in all regions of the country and for applying cell tracking capabilities in field experiments designed to test hypotheses ~ elevant to cloud microphysical processes. Satellites provide observations of background aerosol and cloud mice ostructure as well as seeding signatures to be obtained. Applying these new observational technologies coherently can greatly advance our understanding of many key processes relevant to weather modification. Task2. Identify tile critical ~~nce'Atainties limiting advances in breathe' n~oclificatio'?.science and operation. Scientific and methodological uncertainties. The science underlying weather modification is replete with uncertainties and knowledge gaps. These include fundamental microphysics' the effectiveness of seeding methodologies, and the verifiability of modification procedures. At the most basic level important questions remain regarding liquid and ice nuclei numbers and nucleation processes; the presence concentration? and location of supercooled water in clouds; droplet and hydrometeor evolution processes; and the natural variability of all these factors. Methodological uncertainties are related to the effectiveness of particular seeding materials' the dispersion of seeding materials in clouds, interactions between clouds and cells within the same cloud system, effects outside of seeded areas' separation of the seeding effects from natural effects, and the use of surrogate measurements such as radar reflectivity factors to observe cloud and precipitation changes. The uncertainties of greatest interest to users of weather modification technologies relate to evaluation of the seeding effects, Densely, the determination of whether any significant effect on such things as rainfall or hail fall actually occurred. Improved statistical evaluation techniques could be beneficial in addressing this problem.

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CO~\7CLl.JSI0.7\rS AND RECO.LIiLlE.7\:DA TIOATS 7 1 / By recognizing these uncertainties orate can more readily identity the crucial gaps in understanding that impede progress in weather modification. Such issues are equally important to fundamental research in cloud physics and radiation, weather forecasting, and anthropogenic climate change. Opportunities abound for- collaboration among these various fields of interest. Task3. Identify future directions in weather modification research and operations for improving able manage~nent of floater resources and axle reductions in severe weather hazards. Opportunities for future progress. Given the lack of scientific evidence and the critical uncertainties, weather modification ~netl~odologies do not guarantee desired results. Therefore, fiom a scientific perspective these technologies do not appear ready for immediate application in water resource management or hazard mitigation strategies. Nevertheless, there aloe many advances in observing, computing, modeling, and statistics, all of which offer a means to establish hypotheses and evaluation criteria and to address many of the uncertainties that limit our confidence in weather modification approaches for operational use. Until this is done operational cloud-seeding programs likely will continue to make Blair decisions based on probabilistic cost-ve~sus-benefit analyses subj ect to considerable speculation. Use of existing resources. Existing national facilities such as the NEXRAD network, NCAR, NOAA/ETL, and the ARM/CART site could be used for fundamental cloud studies and as pilot program test-beds. Advanced computing capabilities enable high-resolution modeling, and community models at NCAR and models at several universities are available for researchers to use at their home institutions. These new tools form the basis of a coordinated program (WMO, 2000) in weather modification research. In addition, existing operational weather modification programs offer opportunities for focused research, and such collaborations are likely to be welcomed by the operational groups. It is, of course, important to ensure that such research be evaluated independently to provide a more r obust assessment of the results. Task4. Snuggest actions to idenfi,fy the potential infracts of localized Ether mollification on large-scale weather and climate patterns. Effects outside of seeded areas. There still is no convincing scientific evidence of the efficacy of intentional weather modification efforts' and there is even less evidence that weather modification efforts affect weatl~e~ outside of the seeded regions. Questions about whether cloud seeding in one location can reduce precipitation in other areas can only be addressed through carefully crafted hypotheses and carefully designed physical and statistical experiments. Since the direct effects of seeding may be small and difficult to detect, measuring effects outside of the seeded areas as well as regional or global effects is likely to be even more difficult. Numerical modeling simulations - -validated by observations whenever possiblemay prove to be a useful means for testing larger-scale effects, and it offers the best approach for examining the potential for inadvertent modification occurring as a consequence of intentional seeding. In addition, new satellite- remote-sensing capabilities arid the NEXRAD network may allow the identification of

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72 CRITICAL [~5SlJES IN [VEA TITER A10DIFI(-A TION REtSF-ARCI! some chancres in cloud structure and precipitation which may lead to substantive improvements in our current understanding of effects outside of the seeded areas. Inadvertent weather modification. There is ample evidence that inadvertent weather and global climate modification (e.g. go eenhouse gases affecting global temperatures arid anthropogenic aerosols affecting cloud properties) is a reality. The role of natural and anthtopogenic aerosols in influencing cloud drop size, precipitation, and lightning on regional scales has been increasingly observed and studied. Documentation of anthropogenic effects on the weather strengthens the physical basis for deliberate attempts to alter flee weather. In addition, the changing levels of background aerosols associated with inadvertent weather modification can influence the potential for deliberate weather modification. Therefore, cross-over studies of advertent and inadvertent modification will contribute to the understanding of both kinds of weather modification. RECOMMENDATIONS Recommendation: Because weather modification could potentially contribute to alleviating water resource stresses and severe weather hazards, because weather modification is being attempted r egarclless of scientific proof supporting or refuting its efficacy, because inadvertent atmospheric changes are a reality, and because an entire suite of resew 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 have implications well beyond issues of weather modification. Sustainable use of atmospl~e~;c water resources and mitigation of the risks posed by hazardous weather are important goals that deserve to be addressed through a sustained research effort. Recommendation: The Committee recommends 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 2.2. This program should not focus on nea~-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 the 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? What is the variability of cloud and cell properties (includirlg structure, intensity, evolution, and lifetimes within larger clusters, and how do clouds and cells interact with

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CO1~CL{JSIO.NiS AND RECO,VIIVIE.NDA TIO`VS 73 larger-scale systems? What ale the effects of localized seeding on the larger systems in which the 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'? ~ . . ~ . . . ~ several- government departments and T he tasks involved in weatl~e' 'codification resent-ch fall within tl~e mission agencies' and careful responsibilities of O -I 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 ~ variety of cloud and storm systems (e.g. Doppler lidars and airborne radars' microwave radiometers, mi]limetet-wave and polarimetric cloud radars, GPS and cell-stacking software, the Cloud Particle Images the Gerber Particle Volume Monitor, the Cloud Droplet Spectrometer). Initial field studies should concentrate or 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 whet e substantial further progress may occur (not listed in any priority) include ~ Hygro~scr~pic seeding to enhance rainfall. The small-scale experiments and larger-scale coordinated field efforts proposed by the Mazatlan workshop on hydroscopic seeding (WMO, 2000) could form a starting point for such efforts. A randomized seeding program with concurrent physical measurements (conducted over a period as short as three years) could help scientists to either confirm or 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 precipitations. ~ Steadies c)f-specific seeding effects. This may include studies such as those of the initial droplet broadening and subsequent formation of drizzle and rain associated with l~ygroscopic seeding, or of the role of large (~1 ~m) particles (e.g., sea spray) in reducing droplet concentrations in polluted regions where precipitation is suppressed due to excess concentrations of small CCN. . Improving cloud model treatment of cloud and precipitation physics. Special focus is needed on modeling cloud condensation nuclei, ice nuclei processes, and the growth, collision, breakup, and coalescence of water drops and ice particles. Such studies

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74 CRI TICK L I5~51JES IN IDEA TI -JER AlODIFICA TI ON REtSEA RCll must be based on cloud physics laboratory measurements, tested and tuned in model studies, and validated by ill situ and ground observations. . Improving and using cu' rent computational and data assimilation capabilities. Advances are needed to allow rapid processing of large quantities of data frown new 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 recodification 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 Intel ference to existing programs. A particularly promising opportunity for such a partnership is the DOE ARM/CART site in the southern Great Plains (Oklahoma/Kansas) augmented by the NASA Global Precipitation Mission. This site provides a concentration of the most advanced observing systems and an infrastructural base for sustained basic research. The NCAR and NOAA/ETL also could serve as important focal points for weather modification research. In pursuing reseal ch 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. Closing Thoughts 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 Or fresh water, the increasing levels of damage and loss of life resulting from severe weather, the undertaking of operational activities without the 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