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<br />however. would be required for supply- <br />ing crucial operational information and <br />servicing equipment. <br />c. General procedures (computer model) <br />for controlling operations <br />(1) Input data for the computer ::nodel <br />(a) Weather system description <br />[1] Macroscale <br />[2] Mesoscale <br />[3] Microscale <br />(b) Stored data programs <br /> <br />[1] Opportunity definition <br />[2] Seeding equipment <br />capabilities <br />[3] Delivery capabilities <br />[4] Watershed and streamflow <br />characteristic s <br />[5] Cost-benefit relationships <br />[6] Etc. <br />(c) Computer output as shown <br />under item 4 of the first por- <br />tion of this section of the <br />report. <br /> <br />V. STATUS REPORT ON VARIOUS PHASES OF THE OPERATIONAL <br />ADAPTATION FROGRAM FOR THE COLORADO RIVER BASIN <br /> <br />A. Physical basis for weather modification op'=rations <br />1. Introduction <br />Since Wegner (1911) first suggested the <br />rapid growth of ice crystals within supercooled water <br />clouds, the artificial stimulation of precipitation was <br />believed possible under certain conditions. After <br />Schaefer (1946) demonstrated the conversion of a <br />supercooled water cloud to ice in a cold chamber <br />utilizing dry ice, and V onnegut (1947) reported a <br />method for nucleating ice formations in the atmo- <br />sphere, attempts at precipitation augmentation began <br />in earnest. <br /> <br />The physical basis for treating cold <br />orographic clouds by seeding was presented by <br />Bergeron (1949), and discussed in more detail by <br />Ludlam (1955). The orographic induced clouds along <br />and windward of the mountain ranges over the western <br />United States are frequently composed of supereooled <br />liquid droplets. The temperature activation spectrum <br />of natura 1 nuclei is such th::lt tnf' number of effective <br />natural ice nuclei may not meet cloud requirements <br />for converting the cloud water to ice form at the <br />warmer cloud temperatures and higher condensation <br />rates. In such cases snow may not develop, or the <br />precipitation process may be inefficient. If artificial <br />ice nuclei can be activated in the saturated orographic <br />stream far enough upwind of thc mountain barrier, a <br />more efficient conversion of cloud water to ice <br />crystals should result in increased snowfall. Other- <br />wise, the unconverted cloud water evaporates to the <br />lee of the mountain barrier. Modification potential <br />(type 1) can exist from a difference in the supply rate <br />of condensate and the growth rate of ice in the cloud <br />system, and is related to the evaporation rate of the <br />cloud system. , <br /> <br />Other potential for modification (type II) <br />exists if, by treating the cloud system, an additional <br />latent heat release can affect a change in the conden- <br />sation rate itself by altering the vertical motion <br />field. It seems this type of modification potential <br />would mainly coexist with that of type 1. If the con- <br />version of cloud water to ice form is transpiring at <br />an optimum efficiency, little lasting change in the <br />latent heat release can be affected by adding addition- <br />al ice nuclei. Seeding under this condition should <br />;1 result in only small effects related to altering the <br /> <br />size of the snow crystals growing in the cloud <br />system. Through this paper "modification potential" <br />will connote the type I meaning described above. <br /> <br />A simple model presented by Grant <br />et al., (1968) showed roughly the variation of <br />optimum ice nuclei concentrations as a function of <br />cloud system temperatures. The optimum ice <br />nuclei concentration was defined as that which <br />enabled the cloud system to grow ice by diffusion at <br />a given condensation rate. Preliminary results of <br />the Climax, Colorado, weather modification experi- <br />ment for the years 1960-65 showed the distribution <br />of seeding effects with 500 mb temperature followed <br />the trend indicated by the model. <br /> <br />,Following a similar approach a more <br />refined and improved model is derived by Chappell <br />et al.(1969) that is tailored for existing cloud <br />conditions at Climax and Wolf Creek Pass, Colorado. <br />Ff'aturf'S of the model are appraised utilizing <br />natural snowfall observations from the two experi- <br />mental sites. Finally, the results from independent <br />data samples acquired during cloud seeding experi- <br />ments conducted at the Climax and Wolf Creek Pass <br />areas are compared for consistency and discussed <br />relative to the improved model. <br /> <br />2. A Model to Delineate Modification Potential <br />The major assumptions embodied in <br />the model to be derived are (1) the rate of <br />extraction of cloud water by growing ice crystals <br />is mainly by diffusional growth, (2) the supply rate <br />of cloud water is adequately defined by parcels <br />following a pseudoadiabatic process, and (3) the <br />cloud system is embedded in the 700 mb to 500 mb <br />layer with a vertical temperature distribution <br />equivalent to the moist adiabatic lapse rate. <br /> <br />Refinements in the model include the <br />incorporation of observations at Climax to evaluate <br />factors in the diffusional growth equation that were <br />not included originally. Also, all variables of <br />temperature have been expressed in terms of the <br />temperature at the 500 nib level. The incorporation <br />of diffusional growth only into the model is sub- <br />stantiated by snow crystal samples collected at <br />Climax. These snow crystal replicas rarely show <br /> <br />5 <br />