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<br />51 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />diffusion data obtained on 10, 14 and 17 March, one would expect that the <br /> <br />seeding material would not rise more than 1 to 1.5 km above the general <br /> <br />terrain because the transport time of the seeding material from the genera- <br /> <br />tors to the mountain crest was 1 to 3 hours. <br /> <br />D. Seeding Potential of San Juan Storms <br /> <br />1. Seedability of different storm stages <br /> <br />The typical storm progresses from a stable initial stage through a <br /> <br />neutral stage to an unstable stage, and finally to a dissipation stage. The <br /> <br />different stages of the storm sequence appear to have different seeding <br /> <br />potentials. The stable storm stage at the beginning of the sequence has <br /> <br />low seeding potential, and the unstable storm at the end has the highest <br /> <br />potential. <br /> <br />a. The stable stage <br /> <br />In the stable storm stage, little liquid water is present and high <br /> <br />crystal concentrations are common because of the high cloud tops and long <br /> <br />upwind extent of the cloud. Generaily, the cloud is completely glaciated <br /> <br />except where gravity waves provide short-lived regions of low liquid <br /> <br />~vater content. A blocking flow pattern is often present at the surface, <br /> <br />resulting in a reduced effective height for the mountain barrier and re- <br /> <br />duced vertical displacement of the flow. Under these conditions, the <br /> <br />primary precipitation mechanism is diffusional growth of the ice crystals; <br /> <br />riming is insignificant. <br /> <br />The vertical diffusion of the seeding material to cloud level is <br /> <br />seriously inhibited by the stable stratification, and the AgI simply "pools" <br /> <br />in the blocked flow ahead of the mountains. This failure of the AgI to <br /> <br />reach cloud level in stable situations is now documented by our airborne <br /> <br />and ground based ice nucleus measurements as well as by the earlier and <br />