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<br />I. INTRODUCTION <br /> <br />Statement of Problem <br />Several weather modification field programs are now in progress <br />to augment water resources in the western states by artificially <br />seeding wintertime orographic cloud systems. Artificial ice nuclei <br />in the form of silver-iodide smoke are released from ground-based, <br />air-borne or rocket-borne generators in the natural airstream where <br />wind transport, mechanical turbulence and convection currents are <br />expected to carry and disperse the seeding material into supercooled <br />water clouds and thus, initiate precipitation by the Bergeron ice- <br />crystal process. <br />One of the greatest areas of uncertainty within a winter oro- <br />graphic cloud-seeding program is that of obtaining the optimal <br />distribution of seeding material within the cloud system. The complex- <br />ity and variability of airflow over mountains is well known. The <br />seeding material may be trapped, channeled and forced to flow over <br />as well as around mountain barriers. Plume width, depth and direction <br />may change as atmospheric stability, wind speed and direction change. <br />In addition, the number of effective nuclei may change as the result <br />qf temperature variations and depletion mechanisms. <br />Successful cloud seeding of orographic clouds depends upon the <br />introduction of sufficient artificial nuclei (e.g., silver iodide) into <br />supercooled clouds to obtain optimum crystal concentrations. If the <br />concentration of crystals in the cloud should be less than the optimum <br />concentration, then not all of the vapor provided by the orographic <br />updraft can be readily condensed upon the snow crystals. When the <br />concentration of crystals is above the optimum numbe~ overseeding may <br />occur and the resultant precipitation may be less than would have <br />occurred naturally. <br />The realization of the delivery of the optimal distribution of <br />seeding material to orographic cloud systems presents a complex theo- <br />retical and operational problem. In order to help solve this complex <br />problem several questions need to be answered in a quantitative manner. <br />Such questions are: <br />1) Under given storm conditions, will artificial freezing nuclei <br />reach the target area? <br />2) How much of the cloud volume will be covered (i.e., horizontal <br />and vertical dimensions of seeding plume), and in what concentration? <br />3) What are the effects of stability, wind shear, orographic fea- <br />tures and other natural factors in dispersion of the seeding material? <br /> <br />- <br /> <br />~ <br />~ <br /> <br />? <br /> <br />Background <br />The physical basis for modifying cold orographic clouds by artificial <br />seeding has been discussed by Bergeron (Ref. 3), Ludlam (Ref. 23), Grant <br />et al., (Ref. 18) and Chappell (Ref. 7). The orographic clouds which <br />fOrm-along and windward of the mountain ranges over the western United <br />States are frequently composed of supercooled liquid droplets. The <br />temperature activation spectrum of natural nuclei is such that the <br />number of effective natural ice-nuclei does not meet cloud requirements <br />for converting the cloud water to ice form at the warmer cloud <br /> <br />~ <br /> <br />E <br /> <br />12 <br />