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<br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Trajectory lowering might also be derived from updraft loading resulting from <br />rapid hydrometeor development within treated flanking line turrets. The additional total <br />water mass could slow the updraft, diminishing the storm's capacity for producing hail. <br /> <br />Promotion of coalescence of cloud droplets is accomplished by seeding the <br />flanking cells with hygroscopic materials near cloud base. Such treatment may cause <br />early rainout and/or trajectory lowering. It may also lead to the production of additional <br />hail embryos because of the freezing of large raindrops, which could in turn enhance <br />beneficial competition, Hygroscopic seeding promotes coalescence and is thus thought to <br />affect hail production. <br /> <br />The earlier release oflatent heat (see early rainout, above) would help release <br />convective instability within the smaller turrets, collectively over a larger area than in the <br />central mature cell. This could change storm dynamics, and as with the other concepts <br />previously stated, would be well suited to numerical modeling and simulations. <br /> <br />In addition, whenever precipitation falls out of clouds, downdrafts and outflows <br />are formed in the subcloud layer, further changing storm dynamics. <br /> <br />Another concept used in the past was complete glaciation. The aim of hail <br />suppression by glaciation is to introduce so many ice crystals via seeding that the ice <br />crystals consume all the available supercooled liquid water as they grow by vapor <br />deposition and riming of cloud droplets. To be effective this technique requires the <br />insertion of very large amounts of seeding materials in the storm updrafts. Modeling <br />studies (Weickmann, 1964; Dennis and Musil, 1973; English, 1973; Young, 1977) have <br />suggested that unless very large amounts of seeding material are used, the strongest <br />updrafts remain all liquid and hail growth is not substantially affected. Therefore, the <br />glaciation concept is generally thought not to be a feasible approach to hail suppression. <br />The glaciation concept is also not popular because many scientists think that it may result <br />in a reduction in rainfall along with hail. Since most hail-prone areas are semi-arid, the <br />loss of rainfall can have a greater adverse impact on agriculture than economic gains <br />from hail suppression, <br /> <br />Figure I depicts these concepts of seeding in a multicell thunderstorm. <br />Developing flanking line cells with weaker updrafts are shown on the left of the figure <br />and the mature cell with strong updrafts on the right. In multicellular storms, the <br />developing cells of the flanking line each in turn mature, becoming the dominant cell, <br />which eventually weakens and rains out. To better understand the figure, it is helpful to <br />consider the horizontal axis to represent time with zero on the left and the time of the <br />dissipating cells on the far right. <br /> <br />Important things to note from this discussion are that the concepts dictate that <br />developing cloud turrets are treated, invariably cumulus congestus, rather than the main <br />cell cumulonimbus. This means treatment of young clouds with modest updrafts, not the <br />mature cells with strong updrafts. Also, note that precipitation development is <br />accelerated. Promotion of coalescence is directed at liquid-phase processes primarily; the <br />other methods are based largely on glaciogenic seeding effects, Dynamic effects result <br /> <br />II <br />