Laserfiche WebLink
<br />Bulletin American Meteorological Society 1297 <br /> <br />TABLE 2. Direct observations of seeding effects on precipitation rate. <br /> <br />Seeding <br />Mode <br /> <br />Location <br /> <br />Reference <br /> <br />Aircraft <br />Aircraft <br />Aircraft <br />Aircraft <br />Aircraft <br />Ground** <br />Ground** <br /> <br />Russia <br />Colorado <br />California <br />Washington <br />Nevada (Fog) <br />Montana <br />Colorado <br /> <br />Leskov 1974 <br />Super & Boe 1988 <br />Deshler et at 1988 <br />Hobbs 1975 <br />Deshler et at 1987 <br />Super & Heimbach 988 <br />Super & Boe 1988 <br /> <br />CO2 <br />AgI <br />CO2-AgI <br />CO2-AgI <br />CO2 <br />AgI <br />AgI <br /> <br />* Concentration changes were 3-10 L-I at ground. <br />** Sampled at aircraft altitude. <br /> <br />min per seed line (Super and Boe [1988] show effects from <br />individual seedlines lasting 30-40 min over the Grand Mesa). <br />Given variations in LWC and restrictions in treating the entire <br />volume of air during a normal 3-hr seeding operation, seeding <br />effects at the ground may only be occurring 50 percent of the <br />time effects are expected. This might lead to an additional 0.6 <br />mm total precipitation in 3 hours. It is interesting to note that <br />in the Israeli 1 experiment increased precipitation related to <br />cloud seeding was only 1.9 mm per experimental day (Wurtele <br />1971). <br />If one looks at reported seeding increases from seeding pro- <br />grams utilizing ground-release methods for the experimental <br />unit length used, augmented precipitation rates can be calcu- <br />lated (table 3). These appear to be consistent between project <br />sites and consistent if not slightly smaller than the directly <br />observed effects. A summary of these seeding results combined <br />with physical observations and theoretical calculations per- <br />formed with realistic liquid-water distributions yields the fol- <br />lowing. <br /> <br />I. In the presence of water saturation or ice supersaturation, <br />glaciogenic seeding will produce increases in ice-crystal <br />concentrations. The crystals produced are generally small <br />and compact over the temperature range of - 20 to - 150C <br />due to local overseeding. Additional liquid water will be <br />necessary to produce accretional growth, or substantial <br />aggregation will be necessary (Prasad et al. 1989, pers. <br />comm.) to bring the particles to the ground usually 30- <br />50 min after treatment. <br />2. Observations show that in wintertime, widespread, shal- <br />low orographic clouds, liquid-water contents are low, the <br />temperature regime in which the liquid resides is rela- <br />tively warm (00 to -100C), and the precipitation-rate <br />increases expected from seeding are necessarily small. <br />Actual observations show precipitation-rate increases due <br />to aerial seeding of 0.1 to 0.6 mm . hr-I. <br />3. The area of treatment effect is larger for AgI than CO2 <br />because AgI continually nucleates until depleted, given <br />the presence of liquid water or ice supersaturations, whereas <br />CO2 is only effective at producing ice crystals at the point <br />of release. For a 35-km long seedline and the rate in- <br />creases described per seedline, CO2 would yield =20 <br />acre/ft (25 x 103 x m3) of additional water while AgI <br /> <br />Precip Rate Increases <br />From Seeding <br />mm' h-I <br /> <br />Seeding <br />Material <br /> <br />0.4 <br />0.10-0.40 <br />*0.3 -LO <br />0.15-0.9 <br />0.1 -0.6 <br />0.05-0.2 <br />0.1 <br /> <br />TABLE 3. Statistical inference of seeding effects on <br />precipitation rate. * <br /> <br /> Precipitation <br /> Rate <br /> Increases <br />Seeding By Seeding Seeding <br />Mode Location Reference mm. h-I Method <br />Ground California Mooney & Lunn 0.1 AgI <br /> (PG&E) 1969 <br />Ground Colorado Chappel et at 1971 0.1-0.7 AgI <br /> (Climax) <br />Ground Montana Super 1986 0.3 AgI <br /> (Bridger) <br /> <br />* These results have been questioned in a series of articles (Rangno <br />and Hobbs 1987). <br /> <br />would yield =40 acre/ft (50 x 103 x m)). Using AgI- <br />acetone burners on an aircraft is the least expensive seed- <br />ing method and provides the highest yield of nuclei. <br />Therefore, it is recommended that AgI-acetone burners <br />be utilized rather than CO2, even at temperatures near <br />- SoC. At higher temperatures CO2 or propane would be <br />the appropriate seeding agents. <br />4. For ground delivery, it has been observed that plume <br />lateral spread is approximately 150 and vertical transport <br />is approximately 1000 m above the generator height. As- <br />suming the generators are properly located, and the ma- <br />terial reaches to at least the - 50C level, precipitation <br />increases of approximately 0.1 mm hel might be ex-. <br />pected. Ground-delivery induced precipitation increase is <br />lower than that from aerial delivery because the residence <br />time of particles in cloud is less for ground delivery, <br />producing smaller particles. <br /> <br />5. Modeling efforts <br /> <br />Modeling studies are an important part of a cloud-seeding pro- <br />gram. These models can include conceptual models (Flueck <br />1986) as mentioned in this report, diagnostic numerical tar- <br />geting models, microphysical growth models, two-dimensional <br />time dependent models (Oroville et at 1984), and sophisticated <br />three-dimensional nested grid models. Each has a specific role <br />to play in the conduct and evaluation of a seeding program. <br /> <br /> <br />""---- <br />'- <br /> <br />