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Arlin B. Super

Abstract

A modified NCAR acoustical ice nucleus counter was used in an airborne mode to measure the characteristics of silver iodide plumes released in mountainous terrain. These measurements were a supplement to a winter randomized cloud seeding experiment conducted in the Bridger Range, Montana. It was found that the silver iodide was generally transported upslope from the seeding sites, over the Main Ridge of the Bridger Range some 1400 ft higher, and toward the intended target area. Plume widths were found to average 28° above the Main Ridge, while most of the seeding agent was confined to the lowest 1500 ft above the ridgeline. Ice nuclei concentrations were typically in the range of 100–1000 liter−1, effective at −20C. This is estimated to correspond to about 10–100 nuclei liter−1 at the warmer temperatures prevalent in the lowest 1500 ft above the Main Ridge during winter storms. The flux of ice nuclei was estimated on three occasions. Agreement was good, both from day to day and with the generator output as calibrated in a large isothermal cloud chamber.

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James A. Heimbach Jr. and Arlin B. Super

Abstract

Simulations of randomized winter orographic weather modification experiments were used to explore a possible cause of the many inconclusive results from previous statistical experiments. There is increasing evidence that the response to cloud treatment is highly variable due to differences in the availability of cloud liquid water, seeding agents, targeting effectiveness, and other factors. For this reason the simulations described in this paper focus on the sensitivity of previously applied statistical techniques to different responses to seeding. Data for the simulations came from two sources: the Bridger Range Experiment (BRE), conducted during the two winters from 1970 to 1972, and SNOTEL (snowpack telemetry) data from the Boise, Idaho, area during the winters of 1985–92. The principal focus is on the BRE data from which 6-h experimental units were extracted. This is because previous analyses of these data support the notion of a variable treatment response. Twenty-four-hour experimental units from the BRE and Idaho datasets were also incorporated into the simulations.

The simulations indicate a sensitivity to the size of the fraction of seeded units, which had a treatment response with the power of the test being significantly reduced as the fraction of seeded units showing a response decreased. It is suggested that past estimates of experimental duration, based on the simple model that assumed all seeded units have the same response, were overly optimistic. The results may partially explain the high frequency of inconclusive results from past statistical cloud seeding experiments. The implication of these results is described for past and future statistical weather modification experiments.

Monte Carlo techniques were applied in simulations that assumed a randomized target-control experiment. There were five models applied, which involved adding a percentage or constant responses to all or a fraction of the seeded units and capping the maximum increment. Experimental units were randomly selected from a pool of nonseeded cases. The selected units were randomly seeded or not seeded, and the seeded units were again randomly selected to have all or a fraction of them show a treatment effect, while keeping the net seasonal response approximately constant. For example, in the case of one out of three seeded units showing a treatment response, that unit would have triple the response of the simple model, which had each seeded unit showing a response. Not treatment responses were taken from the most successful partitions found for the BRE. Experimental units were added until a 0.05 one-tailed P level was achieved, where P is the probability of incorrectly concluding that there is a positive seeding effect when none exists (type I error). Each simulation was repeated 1000 times to estimate the number of experimental units needed to reach a specified power level (1 − β), where β is the probability of a type II error—the probability of not detecting a treatment response when one exists.

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Arlin B. Super and James A. Heimbach Jr.

Abstract

During January 1985 six aircraft sampling flights were made in cloud over the target area of an earlier randomized exploratory cloud seeding experiment in the Bridger Range, Montana. One of the two diver iodide (AgI) generator sites used in the earlier experiment was operated well up the wed (windward) slope of the north-south oriented Main Ridge, Crosswind aircraft sampling was done to within 300 m above the secondary ridge target about 17 km downwind of the AgI generator.

The AgI plume was detected over the target area on each of the six missions and was generally 5–8 km wide. Three of the missions detected supercooled liquid water (SLW) in the region of the AgI plume. The ice particle concentration (IPC) averaged about an order of magnitude higher in the seeded zone in these cases, and the estimated precipitation rate was greater, as compared with crosswind control zones. Most seeded ice particles were small hexagonal plates, appropriate for the prevailing temperatures and moisture conditions. The AgI generator was deliberately turned off in one of the experiments. and the seeding effects decreased with time beginning about one hour later.

The other three missions sampled negligible SLW in the seeded region over the target area. Observations did not indicate detectable changes in ice particle concentrations, sizes or habits.

The results of this series of physical experiments are in agreement with statistical suggestions from the earlier randomized experiment. It appears that seeding the stable orographic clouds over the Bridger Range sometimes caused marked increases in IPC, presumably leading to more surface snowfall. The physical observations indicate that enhanced IPC was largely dependent upon the availability of SLW when temperatures were cold enough for AgI nucleation.

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Arlin B. Super and James A. Heimbach Jr.

Abstract

A randomized exploratory single-area cloud seeding experiment was carried out in the Bridger Range of southwestern Montana during the winters of 1969–72. Seeding was accomplished using ground-based silver iodide (AgI) generators located more than midway up the west (windward) slope of the north–south Main Ridge, thereby avoiding trapping by lower stable layers. A secondary ridge from 5 to 20 km east of the Main Ridge was the expected target. An extensive airborne plume tracing program provided strong evidence of successful targeting of the AgI seeding material, with further evidence furnished by tracking of pibals and silver-in-snow analysis.

The experimental unit was 24 h beginning at local noon, a natural diurnal minimum in precipitation intensity. The response variable was daily precipitation amount as measured by a dense network of recording gages. Locally-launched rawinsondes and a thermograph atop the Main Ridge provided data for partitioning the experimental days.

A post hoc statistical analysis was conducted utilizing upwind and crosswind control gage data. Results from both the Wilcoxon rank-sum test and the recently developed multiresponse permutation procedure (MRPP) strongly suggest that increased target area snowfall resulted from seeding when AgI plume temperatures were colder than approximately − 9°C. Double ratios yielded estimates of ∼ 15% more seasonal target area precipitation than predicted by control gages on nonseeded days, while a target-control analysis of independent snow-course data strongly suggested seeding enhanced the seasonal snowpack by more than 15%.

Consideration of plume tracing findings and AgI generator calibration results suggest that the artificial ice nuclei concentration in the seeded volume would be quite limited at temperatures warmer than approximately − 9°C. This provides a plausible physical explanation for the results suggested by the statistical investigations.

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Arlin B. Super and Bruce A. Boe

Abstract

During March 1986, several airborne and ground-based silver iodide (AgI) seeding experiments were conducted over the Grand Mesa, Colorado, during a three-day period of northerly flow and shallow orographic cloud. While little natural snowfall was observed during these experiments, supercooled liquid water formed over the windward slopes and evaporated to the lee of the mesa of many hours. Seeding-induced microphysical changes coincident with the AgI plumes were found in all eight experiments, (including two that employed ground-based seeding) by aircraft sampling about 500 m above the mesa top. Precipitation rates estimated from ice particle images at light levels suggested increases within the seeded volumes in all but one experiment. Surface precipitation increases were observed in three aircraft seeding experiments and one ground-based seeding experiment that coincided with the passage of AgI plumes aloft. Surface observations were not possible during the other ground-based seeding experiment, but some increase in snowfall is thought probable. Three aircraft seeding experiments failed to show surface snowfall increases, and reasons for this are explored.

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James A. Heimbach Jr. and Arlin B. Super

Abstract

A convective complex weather modification experiment was simulated using Monte Carlo techniques. The purpose was to estimate the optimum raingage density for evaluation of a possible future experiment. The data base consisted of radar volume scans made within 150 km of Miles City, Montana, during May–July 1977. A total of 103 convective complexes were identified and tracked from radar data.

Raingage networks of various densities were simulated under the lowest-tilt radar scans to estimate total rainfall accumulation from each complex. Randomly chosen rainfall amounts were increased by given percentages to simulate assumed seeding treatments. A Monte Carlo scheme yielded estimates of the number of experimental units required for various combinations of α- and β-probability levels, treatment effects (percentage of increases) and raingage densities. Applying these results to the numbers of operationally available convective complexes as a function of area gave estimates of the optimal spacing and seasons required to detect a treatment. The results suggest that an unacceptably long field experiment would be necessary to detect treatment effects of 50% or less without some stratification of precipitation data.

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Arlin B. Super, Jack T. McPartland, and James A. Heimbach Jr.

Abstract

A field method of estimating the persistence of a commonly used silver iodide seeding agent is described. The method involved measurement of the AgI plume structure at two downwind distances from the ground generator(s). Distances between the nine available pairs of downwind measurement planes ranged from approximately 10 to 100 km. An NCAR acoustical ice nucleus counter in a light twin aircraft was used to sample the AgI plumes. A series of passes was made through the entire vertical and horizontal extent of the plume at each downwind distance. These measurements, together with pilot balloon observations, permitted calculation of the flux of AgI through each vertical cross-sectional plane. The difference in flux measurements yielded an estimate of the persistence of the seeding agent over the period of transport between the two vertical planes.

This method was applied at three separate locations, during different seasons, and with various degrees of cloudiness. Resulting estimates of deactivation rates of the ice nucleating ability of AgI ranged from no loss to 70% loss per hour. The implications for possible cloud seeding effects beyond the intended target area are discussed.

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Edmond W. Holroyd III, Jack T. McPartland, and Arlin B. Super

Abstract

A series of wintertime airborne tracing experiments was examined to determine some characteristics of the plumes of silver iodide smoke released either from the ground or from an aircraft over the Grand Mesa of Colorado. The plumes were identified in nearly every experiment by detecting the airborne AgI particles and often also by observing resulting ice particle plumes in essentially the same airspace. The lateral and vertical plume positions of Wound-released AgI from eight sites were determined for several wind, cloudiness and stability conditions. The instantaneous ground-released plume had a median spreading angle of 15° and meandered within a median angle of 38°. The median plume height above the crest exceeded 500 m. The lateral spreading rates of aircraft-released AgI were estimated at over 2 m s−1 for cloudy conditions and less in clear conditions. The implications for future cloud seeding strategies are discussed.

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Edmond W. Holroyd III, Arlin B. Super, and Bernard A. Silverman

Abstract

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James A. Heimbach Jr., Arlin B. Super, and John T. McPartland

Abstract

The problem of reducing data from continuously recording ice nucleus counters is addressed. These instruments characteristically have large response times which make their data difficult to interpret. A statistical theory of instrument response is developed which parameterizes the response of these instruments and reconstructs an estimate of the true count-versus-time profiles. Data from two NCAR ice nucleus counters used on separate field projects are summarized to provide example response characteristics. Field calibration techniques for these NCAR counters are also described. Although intended for use on ice nucleus counters, the interpretation techniques are applicable to other continuously recording instruments having significant response times.

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