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<br />\ i <br /> <br />against cloud physics aircraft data (Holroyd, <br />1984) and with a more accurate combination <br />receiver-radiometer'that utilizes absorption <br />of a signal from the COMSTAR satellite as <br />well as the absorption of microwave energy by <br />liquid water (Snider et a1., 1980). <br /> <br />Although equivalent intercomparisons between <br />the aircraft and radiometer were not possible <br />due to the sampling differences (aircraft <br />measured at one level), it was found that if <br />the radiometer detected liquid water, the <br />aircraft did also, and vice versa. If it was <br />assumed that the amount of liquid detected at <br />one level by the aircraft was evenly distri- <br />buted through the cloud depth, a very close <br />correspondence of vertically integrated <br />liquid water values ~as found to exist. <br /> <br />In comparisons between liquid water derived <br />from the dual-wavelength and satel1ite- <br />receiver radiometers~ the correlation coef- <br />ficient between the two sets of data points <br />was 0.95 with the v31ues from the dual wave- <br />length system being about 11% higher. <br /> <br />The Bureau of Reclamation is not the only <br />group who is using the radiometer in support <br />of weather modification studies. Researchers <br />at Utah State University (Hill, 1984), the <br />Desert Research Institute (Long and Walsh, <br />1984) and Colorado State University (Rauber <br />et al., 1984) have all reported that radiome- <br />ters are very useful detectors of supercooled <br />1 iquidwater. <br /> <br />In the near future the Bureau of Reclamation <br />plans to convene in a workshop with others <br />who operate radiometers in order to gain more <br />knowledge regarding such things as: <br /> <br />, 1. accuracy of vertically integrated 1 iq- <br />uid values <br />2. stability of measurements <br />3. intercomparabi1ity between radiometers <br />4. optimum calibration techniques <br />5. data gatheri ng procedures (vertical <br />pointing vs. scanning) <br />6. combining data from two or more radio- <br />meters to achieve reasonable estimates <br />of vertical profiles of liquid water <br /> <br />The workshop will consist of a field program <br />using several radiometers supported by a <br />cloud physics aircraft followed by indepen- <br />dent, but coordinated analyses to assemble a <br />comprehensive set of results and conclusions. <br /> <br />b. Ice nucleatihg characteristics of <br />silver iodide <br /> <br />ihe real effectiveness of silver iodide as a <br />seeding agent depends on the number of ice <br />crystals it produces at the cloud tem- <br />peratures specified by the seeding hypothe- <br />sis. The ice crystal yield of the seeding <br />material, as distinguished from its cloud <br />chamber calibration of effectivity (potential <br />effectiveness), is mainly determined by its <br /> <br />nucleation rate and residence time in the <br />portion of the cloud of interest. The <br />nucleation rate is, in turn, determined by <br />the particular silver iodide agent's mode of <br />nucleation and the cloud environment factors <br />which affect it. <br /> <br />Using chemical kinetic theory and experimen- <br />- tal methodology, sci ent i st s at the Co 1 o,ado <br /> <br />:~ :::3 :,8 , <br /> <br />~ <br />~{) <br /> <br />State University Cloud Simulatron and Aerosol <br />Laboratory have invest i gated the nuc 1 eat fon <br />~ode and nucleation time constants of a <br />variety of silver iodide seeding complexe). <br />DeMott et al., (1983) have shown that seeding <br />agent~ such as silver iodide-silver chloride <br />and silver iodide-ammonium iodide appear to <br />act primarily by the contact-freezing , <br />nucleation mode and have long time constants <br />of nucleation. Their nucleation rate is' <br />dependent on Brownian coagulation whi chis a <br />function of temperature and the size and con- <br />centration of both the silver iodide aerosols <br />and cloud droplets. They showed that at tem- <br />peratures ,warmer than -16 oC it takes from <br />about 15 min to 1 h for 90% of the silver <br />iodide-silver chloride aerosols to nucleate <br />ice crystals by the contact-freezing mode, <br />the exact times being dependent on the speci- <br />fic chemical compositon and cloud chamber <br />conditions used. At temperatures colder than <br />-16 oC, DeMott et ale concluded that the <br />deposition nucleation mode was dominant with <br />even longer nucleation time constants. These <br />findings are generally consistent with field <br />observations (Dye et al., 1976; Strapp et <br />al., 1979; English and Marwitz, 1981) which <br />report activation of ice crystals following <br />silver iodide seeding for 15 min and longer. <br /> <br />Seeding agents such as silver iodide-sodium <br />iodide and silver iodide-potassium iodide <br />whi ch appear to act by the condensat i on- , <br />freezing mode of nucleation have somewhat <br />shorter nucleation time constants. <br />Blumenstein et ale (1983) report that a <br />silver iodide-sodium iodide nucleant produces <br />90% of its ice crystals in about 14 to 20 min <br />depending on temperature, in a cloud cha~ber <br />at water saturation. However, when subjected <br />to transient supersaturations with respect to <br />water, both the nucleation rate and effec- <br />t ivity of the nucl eanti ncreased. In super- <br />saturated conditions this nucleant produced <br />90% of its ice crystals in about 4 min and <br />its effectivity increased by almost one order <br />of magnitUde. Rilling et al., (1984) found <br />that a silver iodide-potassium iodide <br />nucleant behaves similarly. Finnegan et ale <br />(1984) showed that the incorporation of a <br />hygroscopic salt, like sodium chloride, into <br />the silver iodide-silver chloride nucleus <br />composition changed its mode of nucleation <br />from contact to condensation-freezing, <br />thereby increasing both its effectivity and <br />nucleation rate, especially under transient <br />supersaturation ,conditions. <br /> <br />The combined effects of a silver iodide <br />seeding agent's nucleation rate and residence <br />time in a cloud is to make the nucleant's ice <br />crystal yield less than its effectivity at <br />all temperatures, the amount of reduction <br />depending on the mode of nucleation and the <br />cloud conditions where and when nucleation <br />actually occurs. In effect it lowers the <br />temperature threshold of activity for most <br />silver iodide seeding agents to about _90 to <br />-10 oC. These factors may help to explain <br />the diverse results of some of the past cloud <br />seeding experiments. <br /> <br />c. Physical evaluation of experiments <br /> <br />HIPLEX-l represents a significant advance in <br />our abil ity to design, carry out and evaluate <br />a randomized seeding experiment to improve <br /> <br />, ~,....1-91 <br />... "..;, l~ ~ <br />