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<br />33 <br /> <br />c. Precipitation-forming mechanisms <br /> <br />It is of interest to consider the precipitation-forming mechanisms in <br /> <br />these storms, because the relative.importance of the various mechaniSlns must <br /> <br />influence the seeding technique. Three growth processes are possible for <br /> <br />ice crystals: diffusional vapor growth, accretional growth, and aggregation. <br /> <br />All three are significant in these storms. To assess their relative impor- <br /> <br />tance, we have examined the surface crystal collections taken across Holf <br /> <br />Creek Pass by Dr. Vardiman. Most of his crystals showed some rime, but the <br /> <br />majority of crystals were still recognizable. Diffusional growth accounted <br /> <br />for most of the mass in the precipitation. Riming was not as significant as <br /> <br />,.,e had expect,~d in these storms, except in the more unstable situations which <br /> <br />occasionally produced graupel sho,.,ers. However, rime is significant not only <br /> <br />for its contribution to the mass of the precipitation but because it increases <br /> <br />the fall speed of the ice crystals. This is especially important in the case <br /> <br />of the more unstable periods, which generally have higher average updraft <br /> <br />speeds against which the crystals must fall. <br /> <br />The extent to which riming is important is closely related to the droplet <br /> <br />spectrum. Droplet diameters of IS ~m or more are required for efficient riming <br /> <br />(Pitter and Pl'uppacher, 1974), and these were present only in low concentrations <br /> <br />in stable or neutral storm stages. This dependence was reflected in the sur- <br /> <br />face observations, where rimed crystals ,.,ere generally collected only during <br /> <br />unstable storm stages. <br /> <br />Aggregation of ice crystals was observed on many occasions in these <br /> <br />storms, both in our airborne observations and in Dr. Vardiman's surface <br /> <br />collections. Apparent requirements for significant aggregation to occur 'in <br /> <br />these storms were: concentrations of about SO/liter or more and sizes larger <br /> <br />than 400 pm. Below either of these limits, little aggregation was observed. <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 />I <br />