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<br />. .. <br /> <br />generated AgI IN. The representativeness of these calibrations for winter orographic clouds is open to <br />question. Nevertheless, in the absence ofIPC measurements, cloud simulation AgI generator'calibrations <br />have been used 'in designing several seeding experiments and operational programs. <br /> <br />The ratios of AgI IN estimates by the two methods indicated that the more quantitative SF 6 observations <br />provided on average, about 4-5 times higher values than the NCAR IN counter measurements. The <br />differences of AgI IN estimates may be partially due to AgI losses by nucleation and scavenging and <br />partially to instrumentation limitations. Cloud characteristics and residence times are quite different <br />between winter orographic clouds and NCAR IN counter (and CSU) cloud chambers. The NCAR IN <br />counter uses a dense cloud in an attempt to compensate for the short residence time of IN. <br /> <br />The SF 6 gas measurement method provided first-approximation estimates of measured IPCs with all but <br />one of the estimates within a factor of six. Most estimates were higher than observed IPCs. First- <br />approximation estimates were also provided by the NCAR IN counter method, although values were lower <br />than provided by the SF 6-based method. <br /> <br />The single experiment reported suggests that seeding-caused IPCs can be estimated by tracer gas or <br />NCAR IN counter observations to within about one order of magnitude for the particular AgI generator <br />and aerosol used, and the sampled cloud conditions. While this results is encouraging, further <br />observations would be needed to test whether similar results can be obtained with other AgI generators, <br />AgI solutions and cloud conditions. In view of differences between orographic and simulated clouds, and <br />of known instrumentation limitations, the apparent good agreement from the single experiment may be <br />somewhat fortuitous. <br /> <br />Because of the uncertainties involved in AgI IN estimation, and in ice nucleation processes, it is clearly <br />. preferred to directly measure seeding-caused IPCs within winter orographic clouds. More direct IPC <br />measurements must be made if the field of winter orographic cloud seeding is to advance in scientific <br />understanding and credibility. However, such observations are difficult and expensive to obtain, and may <br />be impractical for many programs. Further testing of indirect methods may provide an alternative <br />. approach to direct observation ofIPCs. It is recommended that any further tests of indirect methods use a <br />current AgI generator calibration from the CSU or similar facility. Use of an improved IN counter, based <br />on current technology, would also be very desirable. <br /> <br />1995 articles and papers: <br /> <br />8.13. DeMott, P. J., A. B. Super, G. Langer, D. C Rogers, and J. T. McPartland, 1995: Comparative <br />characterizations of the ice nucleus ability of AgI aerosols by three methods. J. Weather ModifICation, <br />27,1-16. <br /> <br />ABSTRACT <br /> <br />Three methods were used to assess the ice nucleating ability of different AgI-based aerosols produced <br />using two solution combustion generators. The standard method employed was an isothermal cloud <br />chamber which has been historically used for "calibrations" of ice nucleus aerosol generators. <br />Comparisons with historical data showed consistency over a 20 year period for one generator, but not for <br />another. Data on rates of ice crystal formation were used to infer operative ice formation mechanisms, <br />and make inferences to the relative utility of the different aerosols in the atmosphere. Comparative <br />measurements of ice nucleus effectiveness at -20 oC were'made using two NCAR counters that have been <br /> <br />55 <br />