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<br />I <br /> <br />I <br /> <br />The general sense of the results of cloud seeding <br />simulations for rain augmentation of warm season <br />convective clouds was that moderate sized clouds <br />tended to respond favorably (increase rainfall) to <br />treatment whereas large clouds do not. Some <br />redistribution of precipitation occurs in most cases. <br />Small clouds with cloud top temperatures in the range <br />00 to -50C may be completely unaffected. The effects of <br />silver iodide and dry ice were usually very similar, <br />especially when applied to weak to moderate clouds <br />(Kopp, 1988a; Orville and Kopp, 1990). <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Third generation seeding treatments have also been <br />applied to studies of the transport and dispersion of <br />inert tracers such as sulfur hexafloride (Huston et al., <br />1991; Kopp, 1988b r Recent work has also incorporated <br />third generation seeding treatments for AgI, CO2 and <br />SF6 in a three-dimensional cloud-scale framework <br />(Farley et al., 1994). This version of the Clark model <br />(Clark, 1977; 1979; Clark and Farley, 1984) has been <br />modified to include the microphysical scheme of Lin et <br />af. as described in Farley et al. (1992). This formulation <br />of the model should see increased applications in future <br />cloud seeding research efforts. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Most of the material presented above applies <br />strictly to extratropical convective clouds having a <br />highly continental microphysical character, and <br />relatively cool cloud base temperatures, i.e. typical <br />conditions for the northern Great Plains region. The <br />limited studies of cloud seeding applied to tropical <br />convective clouds (or those of a maritime <br />microphysical character with warm cloud base <br />temperatures) have tended to concentrate on the <br />dynamic effects of cloud seeding. Initial efforts in this <br />area addressed the seeding of hurricane rainband clouds <br />using first generation techniques as described in Chang <br />and Orville (1972). Heavy seeding of a Florida-type <br />cloud (using a second generation treatment) produced a <br />modest increase in updraft strength and enhanced <br />precipitation development early in the life cycle of the <br />cloud, but decreased precipitation in later stages (Chen, <br />1982). <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />Third generation cloud seeding simulations of two <br />clouds from the COHMEX project discussed in Orville <br />et af. (1989) gave results in good agreement with <br />effects noted for extratropical clouds in that the <br />moderate-sized cloud (8 km top) responded favorably <br />to seeding with a moderate increase in precipitation <br />whereas the larger cloud (12 km top) indicated a slight <br />decrease. The changes induced by ice-phase seeding of <br />these warm-base clouds were less dramatic than those <br />seen for cold-base clouds. In a related study of a warm- <br />base Illinois convective cloud, Orville et al. (1993) <br />showed that a strong ice-phase seeding signature was <br />produced only if the ice multiplication process was not <br />active. An active ice multiplication process tended to <br />overwhelm the effects of glaciogenic seeding for this <br />case. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />b. Stratiform/Cold Season <br /> <br />Numerical simulations of a cool season stratiform <br />cloud case from the Precipitation Enhancement Project <br />conducted in Spain under WMO auspices yielded some <br />surprising results as described in Orville et af. (1984). <br />Silver iodide seeding simulations on that case produced <br />strong dynamic responses in the model clouds, <br />triggering embedded convection within the stratiform <br />cloud deck. This occurred even though only small <br />amounts of supercooled cloud liquid were available. <br />The pronounced dynamic response to silver iodide <br />seeding in this case is illustrated in Fig. 7. <br /> <br />The strong dynamic effect was due to the heat <br />released by localized freezing of liquid and the <br />transformation from water to ice saturation. Additional <br />analyses reported in Orville et al. (1987) revealed that <br />less latent heat release than expected is produced in <br />high liquid water content cumulus clouds, and more <br />heating than expected is produced for low liquid water <br />content stratiform clouds. Simulations involving dry ice <br />seeding of the same case indicated only weak <br />microphysical effects due to the rapid fallout ofthe dry <br />ice and the limited time available for the seeding to take <br />effect. <br /> <br />The extended drought experienced in western <br />South Dakota during the late 1980s and early 1990s <br />triggered new interest in cloud seeding in the Black <br />Hills region, with particular emphasis being placed on <br />snowpack augmentation (Orville, 1990). This interest <br />was reflected in IAS modeling efforts as exemplified in <br />Farley et af. (1997) who applied the modified Clark <br />model described previously to a four-day spring storm <br />period in the Black Hills. In that study, only one of the <br />days (Day 3) responded positively to cloud seeding. <br />Figure 8 compares surface precipitation amounts (in <br />liquid equivalent) for unseeded and seeded runs of Day <br />3 at 360 min. A significant increase is evident for the <br />northern Black Hills. The effect was quite pronounced <br />over the first three hours of the simulation because the <br />natural clouds were not deep (cold) enough to launch <br />effective ice processes. <br /> <br />The first two days of the storm period failed to <br />respond favorably to seeding due to warm cloud top <br />temperatures for Day 1, and ineffective vertical <br />transport of the seeding agent released by ground-based <br />generators combined with relatively warm cloud top <br />temperatures for Day 2. Seeding was also ineffective <br />for Day 4, characterized by deep, naturally efficient <br />clouds. Results from this application of the Lin et af. <br />(1983) microphysical scheme indicated it had problems <br />in the representation of ice and snow forming processes <br />at low supercoolings. This had not been apparent in the <br />convective cases for which the scheme was designed. <br /> <br />21 <br />