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Ij <br />1 <br />P <br />1 <br />approach would require significant electrical power for pumps, making it impractical for mountain <br />seeding sites. <br />Silver Iodide (AgI), either in rather pure form or with various additives, is by far the most commonly <br />used heterogeneous seeding agent. It has a threshold temperature near -6 °C, where a small fraction of <br />the total AgI particle population begins to nucleate ice particles. It has usually been impractical to <br />achieve significant ice crystal concentrations several kilometers downwind from an AgI generator at <br />temperatures warmer than -9 °C. The reason is the two to three or more orders of magnitude increase in <br />ice nucleating activity between -6 and -9 °C. A typical generator has an output of about 1014 ice crystals <br />per gram of AgI, effective at -9 °C. <br />Besides this strong temperature dependence, the effectiveness of AgI seeding depends on the specific <br />type of chemical and particular generator design used and the atmospheric conditions at the release point. <br />Many operational programs use relatively pure AgI released well below cloud. The resulting contact - <br />freezing nucleation process depends on cloud droplet concentration. Because winter orographic clouds <br />have relatively low droplet concentrations, the contact - freezing process is known to be quite slow. That <br />is, only a fraction of the AgI particles with the potential to nucleate ice will do so over a typical in -cloud <br />residence time of 20 to 30 min. <br />The effectiveness of various seeding solutions, loosely referred to simply as AgI but often much more <br />complex, is known to vary widely. The strong temperature dependence and less pronounced wind speed <br />dependence of the ice nucleation effectiveness of particular AgI solutions has been widely documented, <br />such as by DeMott et al. (1995). This same reference shows that some types of AgI with additives are <br />fairly fast acting because they operate by the condensation - freezing mechanism at water saturation which <br />does not depend on droplet concentration. Changing from the commonly used AgI aerosol, which <br />nucleates by contact - freezing, to the AgICI- 0. 125NaCl aerosol, which acts by a faster condensation - <br />freezing process, would provide about an order of magnitude increase in seeding effectiveness during a <br />20 min in -cloud transit time. Other types of AgI have also been shown to be faster - acting, and more <br />efficient in the important -6 to -9 °C range, than the relatively pure AgI aerosol which operates by <br />contact - freezing. However, some of these agents cause operational problems in the field such as nozzle <br />clogging. <br />A special case of condensation - freezing is the "forced" condensation - freezing mechanism discovered by <br />Finnegan and Pitter (1988). It is achieved if AgI is released within cloud where the consumption of the <br />AgI- acetone solution and propane add abundant local water vapor just above the generator stack. This <br />approach causes large transient supersaturat ions, resulting in the formation of vast numbers of ice crystals <br />immediately downwind from the generator stack if the cloud is colder than -6 °C. This mechanism has <br />been shown to work with all silver iodide- containing aerosols in rapidly and efficiently forming ice <br />crystals at the relatively warm temperature of -6 °C. <br />Li and Pitter (1997) reported on numerical simulations of ground -based AgI seeding for two ice crystal <br />formation mechanisms, contact - freezing and "forced" (very rapid) condensation - freezing. A relatively <br />simple orographic cloud model was used to investigate how the different mechanisms affect snowfall <br />patterns and intensities. A temperature sensitivity analysis showed large effects on snowfall production <br />by forced condensation- freezing because ice nucleation is a strong function of temperature. The authors <br />cite Feng and Finnegan (1989) who showed ice nucleation efficiency is enhanced by four orders of <br />magnitude from -6 to -10 °C. They considered the field results of Super and Heimbach (1983) at the <br />Bridger Range of Montana to be strong evidence for forced condensation- freezing because suggested <br />seeding effects were for ridge top temperatures of -9 °C and colder. That condition corresponds to high <br />altitude generator site temperatures colder than -6 °C. The generators were usually operated in- cloud. <br />7 <br />