Laserfiche WebLink
<br />." <br /> <br />(1975a), Gagin et al. (1986) found no <br />difference in the relationships between <br />cell height and other variables, <br />including rainfall volume, for unseeded <br />and silver iodide seeded cells. <br />Therefore, they interpreted all of the <br />apparent FACE seeding effect in terms of <br />changes in cell height. <br /> <br />On the other hand, Rosenfeld and <br />Woodley (1989) found for a seeding <br />experiment in Texas a "large indicated <br />effect of treatment on cell rain volume, <br />despite the small indicated effect on <br />maximum cell heights." It is <br />interesting to note that the Texas <br />result, which is only tentative due to <br />the small number of cases analyzed to <br />date, is in line with the Cloud Catcher <br />seeding hypothesis, which stressed a <br />combination of microphysical and dynamic <br />effects, rather than the hypothesis in <br />the Texas project design, which was <br />patterned after FACE and considered <br />increases in cell height as the <br />principal, if not the sole, source of <br />additional rainfall (Rosenfeld and <br />Woodley, 1989). <br /> <br />Any future experiments near the <br />Black Hills should be designed to shed <br />light on these questions, as well as <br />interactions between the clouds and the <br />Black Hills themselves. Seeding clouds <br />over the Black Hills to maximize <br />precipitation requires consideration of <br />wind, temperature, and moisture fields <br />over the entire area. Kuo and Orville <br />(1973) showed how the interactions <br />between the Black Hills and the air <br />streams passing over them set up <br />preferred areas for cloud and shower <br />formation. Because seeding influences <br />cloud dynamics, one should not assume <br />that seeding over one part of the Black <br />Hills does not influence subsequent <br />cloud developments elsewhere. <br /> <br />A unifying concept, which seems to <br />bring together various pieces of <br />evidence, is that rainfall increases are <br />produced when seeding helps convective <br />clouds merge into clusters, rather than <br />continuing as single-cell showers <br />(Dennis et al., 1976). Such <br />organization promotes the precipitation <br />efficiency of clouds. It also appears <br />to increase the total amount of water <br />vapor processed, which accounts for the <br />great attention paid to cloud mergers in <br />cloud seeding experiments in Florida and <br />Texas (e.g., simpson and Dennis, 1974; <br />Rosenfeld and Woodley, 1989). <br /> <br />The apparent success of seeding on <br />Rapid Project shower days has already <br />been noted, as well as the lack of <br />overall success on storm days. On Rapid <br /> <br />Prc)j ect storm days with southwesterly <br />flow, there were indications of rainfall <br />decreases at 1:he edge of the Black <br />Hills, but of substantial inc::reases some <br />30 to 40 km northeastward, still within <br />the target areas (Dennis et al., 1976). <br />It appears that, for that particular <br />situation, additional organi:~ation or <br />clustering induced by seeding was still <br />beneficial over the open prairie, but <br />could not be produced (or was not <br />beneficial) over the Black Hills <br />the!msel ves . <br /> <br />These inferences are only tentative,' <br />but: suggest a possible stratE!gy to be <br />followed in any future field <br />experiments. On shower days, only <br />clouds supported by low level <br />convergence induced by the Black Hills <br />themselves have any chance of producing <br />significant showers. Seedinsr would be <br />directed at the most promising growing <br />clouds, wherever they occurre!d. On <br />storm days, as evidenced by positive <br />vorticity advection (synoptic-scale <br />forcing), systems over the Black Hills <br />would organize with no help, and seeding <br />there likely would have no effect. <br />However, clouds forming in the less <br />disturbed regions might be of a size <br />(COP around 4 to 6 km) that could be <br />stimulated to further growth and <br />organization. Evaluation should be on <br />both a cell or cluster basis and an <br />area-wide basis to study the larger <br />scale effects. <br /> <br />continued work with powerful <br />numerical models, such as the new three- <br />dimensional models developed by Clark <br />and others (e. g., SmolarJdewicz et al., <br />1988), is required to sort out the <br />various possible effects. In time, it <br />should be possible to integrate detailed <br />parameterizations of microphysical <br />processes in individual clouds into <br />three-dimensional mesoscale models <br />capable of simulating air motions over <br />and around the Black Hills. The output <br />of such models would suggest additional <br />seeding hypotheses, and might even <br />provide statis.tical controls for seeding <br />experiments. This would be an extremely <br />important development, as it 'would <br />provide to single-area experiments a <br />degree of sensitivity now obtainable <br />only with target-control or randomized <br />crossover experiments. As we have seen, <br />the interpretation of results from <br />target-control or randomized crossover <br />experiments is ambiguous when strong <br />dynamic effects are present or <br />suspected. <br /> <br />9 <br />