A Review of Cloud Seeding Experiments to Enhance Precipitation and Some New Prospects

A Review of Cloud Seeding Experiments to Enhance Precipitation and Some New Prospects Roelof T. Bruintjes National Center for Atmospheric Research,* Boulder, Colorado ABSTRACT Water is one of the most basic commodities on earth sustaining human life. In many regions of the world, traditional sources and supplies of ground water, rivers and reservoirs, are either inadequate or under threat from ever-increasing demands on water from changes in land use and growing populations. This has prompted scientists and engineers to explore the possibility of augmenting water supplies by means of cloud seeding. This paper provides an overview of the current scientific status of weather modification activities to enhance precipitation for both glaciogenic and hygroscopic seeding experiments. It is important to emphasize that although funding for scientific studies has decreased substantially during the past decade, operational programs have actually increased. During the last 10 years there has been a thorough scrutiny of past experiments involving experiments using glaciogenic seeding. Although there still exist indications that seeding can increase precipitation, a number of recent studies have questioned many of the positive results, weakening the scientific credibility. As a result, considerable skep- ticism exists as to whether these methods provides a cost-effective means for increasing precipitation for water resources. Recent results from hygroscopic seeding experiments provided for some renewed optimism in the field of precipitation enhancement. Although promising results have been obtained to date, some fundamental questions remain that need to be answered in order to provide a sound scientific basis for this technology. 1. Introduction world where ground water is one of the primary sources of freshwater. This is particularly evident in Water is one of the most basic commodities on the southwest United States and Mexico. To help al- earth sustaining human life. In many regions of the leviate some of these stresses, cloud seeding for pre- world, however, traditional sources and supplies of cipitation enhancement has been used as a tool to help ground water, rivers and reservoirs, are either inad- mitigate dwindling water resources. equate or under threat from ever-increasing demands While many countries conducting weather modi- on water from changes in land use and growing popu- fication activities are located in semiarid regions lations. In many countries water supplies frequently of the world, several countries in the Tropics such come under stress from droughts and increased pollu- as Indonesia, Malaysia, India, and Thailand are also tion in rivers, resulting in shortages and an increase involved in weather modification activities. Although in the cost of potable water. Ground water tables have these countries receive a relatively large amount of been steadily decreasing in many areas around the rainfall, a 5% below normal rainfall year translates into a drought for them due to their infrastructure and agricultural practices that are more water intensive *The National Center of Atmospheric Research is sponsored by than in other parts of the world. Weather modification the National Science Foundation. activities to enhance water supplies have been con- Corresponding author address: Roelof T. Bruintjes, National ducted for a wide variety of users including water re- Center for Atmospheric Research, 3450 Mitchell Lane, Building source managers, hydroelectric power companies, and 2, Boulder, CO 80301. E-mail: [email protected] agriculture. In final form 4 January 1999. Only a small part of the available moisture in ©1999 American Meteorological Society clouds is transformed into precipitation that reaches Bulletin of the American Meteorological Society 805 the surface (National Academy of Sciences 1966a,b, ment program. This question may be answered differ- 1973; Weather Modification Advisory Board 1978). ently by scientists, water managers, or economists, and This fact has prompted scientists and engineers to ex- will depend on different factors depending on who plore the possibility of augmenting water supplies by answers the question. This difference is illustrated by means of cloud seeding. If more water could be trans- the fact that although scientific cloud seeding experi- formed into precipitation, the potential benefits appear ments have shown mixed results based on the level of very attractive. The ability to influence and modify proof required by the scientific community, many cloud microstructure in certain simple cloud systems operational cloud seeding programs are still ongoing. such as fog, thin layer clouds, simple orographic However, it also emphasizes the fact that the poten- clouds, and small cumulus clouds, has been demon- tial technology of precipitation enhancement is very strated and verified in laboratory, modeling, and ob- closely linked to water resources management. It is servational studies (American Meteorological Society thus important that the users of this potential technol- 1992). Although past experiments suggest that precipi- ogy are integrated into programs at a very early stage tation from single-cell and multicell convective clouds in order to establish the requirements and economic may be increased, decreased, and/or redistributed, the viability of any program (Ryan and King 1997). In ad- response variability is not fully understood. It appears dition, the continued need for additional water and the to be linked to variations in targeting, cloud selection fact that most programs currently ongoing in the criteria, and assessment methods. The complexity of United States and the rest of the world are operational atmospheric processes and specifically cloud and pre- programs emphasizes the need for continued and more cipitation development has prevented significant intensive scientific studies to further develop the sci- progress in developing a cloud seeding technology that entific basis for this technology. can be tested and verified in a repeatable manner with In the past, weather modification activities were the level of proof required by the scientific community. often initiated in times of a drought when desperate In the annual register of National Weather Modi- water needs exist. In many cases, the programs were fication Projects, compiled and published by the World discontinued when the drought was over. Apart from Meteorological Organization (WMO) since 1975, 27 the question of whether these programs are success- countries provided information on 88 ongoing weather ful or not, the more relevant question is whether cloud modification activities in 1994. It should be empha- seeding should be initiated during drought conditions sized that these data only pertain to countries that re- at all due to the limited number of clouds available port such data. Many countries do not report their for seeding. A better approach that has been adopted activities, making the above estimate conservative. In by some operational programs is to view the technol- the United States alone, the National Oceanic and At- ogy as a longer-term water resources management mospheric Administration (NOAA) reported informa- tool. It may be better to continue seeding during non- tion about 36 ongoing weather modification activities drought years in order to build up water supplies for in 12 states in 1994. The federal government spon- the future. sored only one of these projects. Twenty-nine projects In the scientific community weather modification were for precipitation enhancement. These activities is still viewed as a somewhat controversial topic. are operational programs that operate on the basis of Changnon and Lambright (1990) identified several past scientific results. problems and difficulties that have arisen during the The fact that many operational programs have been conduct of weather modification experiments. on going and have increased in number in the past 10 According to Changnon and Lambright, based on their years indicates the ever-increasing need for additional analyses of the National Hail Research Experiment water resources in many parts of the world, including and the Sierra Cooperative Pilot Program (SCPP), the United States. It also suggests that the level of proof the major scientific controversies were a result of six needed by users, water managers, engineers, and op- factors. These factors were 1) proceeding with an in- erators for the application of this technology is gener- adequate scientific knowledge base, 2) a flawed ally lower than what is expected in the scientific project-planning process, 3) differing views between community. The decision of whether to implement or funding agencies and project scientists, 4) lack of con- continue an operational program becomes a matter of tinuing commitment by the principal agency conduct- cost/benefit risk management and raises the question ing the experiment, 5) changes in project directors, and of what constitutes a successful precipitation enhance- 6) poor performance by project scientists. Because of 806 Vol. 80, No. 5, May 1999 the complex nature of precipitation enhancement experiments, it is extremely important to funding agencies, water managers, and scientists that current experiments are critically reviewed in terms of these six factors in order to avoid repeating the mistakes listed above. 2. Precipitation processes Weather modification research re- quires the involvement of a large range of expertise due to both the multifaceted

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