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<br />53 <br /> <br />c. The unstable stage <br /> <br />The unstable stage of the storm that occurs during the last half of the <br /> <br />storm sequence meets most of the seedability criteria. The convective zone <br /> <br />ahead of the mountain provides a region of high liquid water content in a <br /> <br />seedable location, and under natural conditions much of this liquid water <br /> <br />is swept high over the mountain and is lost to the precipitation. The convec- <br /> <br />tion and the associated surface convergence zone provide a mechanism for <br /> <br />transport of the AgI into the clouds in an appropriate location ahead of <br /> <br />the target area. Also, the convective elements frequently contain the <br /> <br />lowest ice crystal concentrations of any portion of these clouds. The <br /> <br />only criterion which is questionable is that of residence time. <br /> <br />To investigate the time available, we have constructed some sample <br /> <br />trajectories for the airflow of 29 December 1974. These are shown in Fig. <br /> <br />3.8. Trajectories were constructed by considering the growth and fall of <br /> <br />unrimed PIa crystals in the airflow as inferred from our aircraft measure- <br /> <br />ments. The eTIlpirical formulation of Davis (1974) was used for the crystal <br /> <br />growth rates and terminal velocities. These sample trajectories do not <br /> <br />properly take into account the convective zone ahead of the mountain, which <br /> <br />may provide additional vertical transport either upvJ8rd or dO'<lnward, and <br /> <br />they do not take into account riming in the liquid ,<later region. Neverthe- <br /> <br />less, the region in Fig. 3.8 to the left of the starred line roughly in- <br /> <br />dicates the portion of the cloud where seeding could produce precipitation <br /> <br />that could fall to the target area. It is clear from this that the convective <br /> <br />region ahead of the mountain is the only realistic seeding target in this <br /> <br />storm. The liquid water region over the mountain may produce precipitation <br /> <br />on the eastern slope of the mountain or in the downwind valley, which may <br /> <br />be desirable in another context, but this region cannot be converted to <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 />