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Critical issues in weather modification research (2003)

Principal conclusions abstract:

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: Review the current state of the sciences of weather modification and the role of weather prediction as it applies to weather modification, paying particular attention to the technological and methodological developments of the last decade.

Principal conclusion: Over the past 30 years there has been 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 to 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, the focus can turn to application of this understanding, not only to intentional modification but also to inadvertent modification and other related fields, such as cloud modeling and weather forecasting.

Status of weather modification research. Weather modification research has been in a state of decline in the United States of more than two decades. The reasons are many and include the lack of scientifically demonstrable success in modification experiments, 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 with little or no substantiation. This led many scientists to abandon the field and federal agencies to reduce funding for weather modification research dramatically.

Status of weather modification 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 snow pack, or suppressing hail. Evaluation methodologies vary but in general do not provide convincing scientific evidence for either success or failure. Although there is physical evidence that seeding affects cloud processes, effective methods for significantly modifying the 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 dynamic-mode seeding concepts conducted over the past 4our 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 enhancement 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 those 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 hygroscopic 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 the clouds to treatment in accordance with understanding of the chain of physical reactions leading to precipitation. This appears to be a fruitful area for further research.

Recommendations abstract:

Recommendation: Because weather modification could potentially contribute to alleviating water resource stresses and sever 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 have 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 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 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 the national program.

The complete study is available online to read or print here: National Academies Press

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