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<br />~ <br /> <br />temperatures and higher condensation rates. In such cases snow may not <br />develop or the precipitation process may be inefficient. <br />If artificial ice nuclei can be activated in the saturated oro- <br />graphic stream far enough upwind of the mountain barrier, a more <br />efficient conversion of cloud water to ice crystals should result in <br />increased snowfall. Otherwise, the unconverted cloudwater evaporates <br />to the lee of the mountain barrier. The modification potential assoc- <br />iated with these microphysical processes has been designated as "static <br />modification potential" by Chappell (Ref. 7). A "dynamic modification <br />potential" may also exist when seeding alters buoyancy effects within <br />the cloud system by changing the latent heat release in ascending air <br />parcels. This may result in warming of the cloud system, increase cloud <br />tops or alter the vertical motion field over the orographic barrier. <br />The overall result could be to change the rate of condensation or cloud <br />geometry during seeded conditions (Ref. 7). <br />Grant et al., (Ref. 17) have developed a simple model for showing <br />the variation of optimum ice-nuclei concentration as a functian of <br />cloud system temperatures. The optimum ice-nuclei concentration was <br />defined as that which enabled the cloud system to grow ice by diffusion <br />at a given condensation rate. Chappell (Ref. 7) has derived a physical <br />model of the cold orographic cloud system in a climatological mode <br />suitable for testing with results from cloud seeding experiments at <br />Climax and San Juan mountain areas in Colorado. A parameterized <br />numerical model for simple two-dimensional representation of orographic <br />precipitation has been derived by Willis (Ref. 35). The latter two <br />models consider the physical effects of artificial seeding but not <br />the transport and dispersion aspect. <br />The early literature on the delivery of seeding material to oro- <br />graphic cloud systems has been summarized by Orgill et al., (Ref. 30). <br />Current research efforts on this subject are under investigation in <br />Wyoming (Ref. 2), Colorado (Refs. 30 and 31), Montana (Ref. 32) and <br />other western states. <br /> <br />2 <br /> <br />Purpose, Goals and Method <br />The overall purpose of this research is to develop laboratory <br />physical models as a tool for modeling the atmospheric planetary <br />boundary-layer over mountainous terrain and the transport-dispersion of <br />a passive tracer material simulating the silver-iodide seeding material. <br />A second phase of the research involves obtaining limited field data <br />that will assist in enlarging our understanding of the transport-diffusion <br />process in the field and also providing relevant data to check on the <br />laboratory physical models. The more specific objectives for the <br />research were as follows: <br />1) Investigate and review the mathematical aspects of similarity <br />for atmospheric transport and dispersion of particulate material, <br />such as silver-iodide, over complex terrain. <br />2) Determine the full capability for laboratory simulation of <br />airflow over complex roughness features. <br />3) Evaluate the use of laboratory simulation of airflow and trans- <br />port for various types of orographic terrain as related to weather <br />modification operations. <br />4) Obtain field information on the relative dispersion and trans- <br />port characteristics of tracers with particle sizes ranging from <br />meter to molecular sizes. <br /> <br />~ <br /> <br />13 <br />