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Arthur L. Rangno

Abstract

Images of frozen drops with pieces missing were collected on two days of airborne sampling in shallow supercooled stratiform frontal clouds in the coastal waters of Washington State. In those limited regions where ice appeared to be newly formed, ice fragments with rounded portions accounted for about 5% of the total ice particle concentrations. These results are in rough agreement with the body of literature on laboratory experiments concerning the freezing of drops in free fall that have suggested a modest, though not insignificant, role for the fragmentation of freezing drops on total ice particle concentrations when larger supercooled drops are present.

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Arthur L. Rangno

Abstract

A six-season, randomized-by-season cloud seeding experiment consisting of three seeded seasons and three non-seeded seasons was conducted by Colorado State University (CSU) during the middle and late 1960's in the Wolf Creek Pass region of the San Juan Mountains of southwest Colorado. The results of the seeding have been reported in a series of papers as having produced statistically significant increases in precipitation at Wolf Creek Summit when the 500 mb temperature was ≥−23°C. Furthermore, it has been reported that increases in precipitation produced statistically significant increases in the runoffs from three target watersheds when compared to the runoffs from three control watersheds.

In this paper the results of the Wolf Creek Pass Experiment (WCPE) are reexamined. It is shown that the three non-seeded seasons occurred during meteorological conditions which brought “warm aloft” (500 mb temperatures ≥ −23°C) storm days with unusually light precipitation over a wide region of Colorado, northern New Mexico, southern Utah and northern Arizona. This bias produced high values of seed/no-seed precipitation ratios at Wolf Creek Summit which led to the misperception of large increases in precipitation due to cloud seeding.

It is also shown that nearly all central and southwest Colorado watersheds with similar exposures to the target watersheds for the WCPE had high runoffs during the three seeded seasons compared to the three control watersheds chosen. Hence, the increases in runoff reported from the three target watersheds were part of a large-scale pattern due to natural causes rather than to cloud seeding.

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Arthur L. Rangno

Abstract

Some of the complexities of clouds and precipitation that have been encountered in field projects are reviewed. These complexities highlight areas of cloud microstructure and precipitation development that need to be better understood before adequate conceptual or numerical models of orographic cloud seeding can be developed. Some concerns about cloud sampling with regard to the evolutionary behavior of supercooled clouds from water to ice are also discussed.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Several aspects of the study by Elliott et al. (1978) of the Colorado River Basin Cloud Seeding Pilot Project are discussed. Issues addressed include the design of the project, the diffusion of the seeding agent, anomalous ice nucleus concentrations in the target area, and statistical analysis of the randomized experiment.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

No abstract available.

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Arthur L. Rangno and Peter V. Hobbs

Abstract

Evidence is presented for the production of high concentrations of rather uniformly-sized ice crystals in a supercooled stratus cloud by a commercial, turbine, propeller-driven aircraft.

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Peter V. Hobbs and Arthur L. Rangno

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Peter V. Hobbs and Arthur L. Rangno

Abstract

In a previous analysis by Hastay and Gladwell (1969) of the Skagit Cloud Seeding Project, the actual runoffs of the Skagit River during the two seeded years were compared to the runoffs predicted by a principal component (or covariate) analysis technique. It was concluded that seeding from ground generators with silver iodide increased the annual runoff of the Skagit River by at least 15% in the second year of the Skagit Project (the 1964 water year) and that this result was significant at the 0.005 (or higher) level. In this paper it is shown that this conclusion cannot be substantiated due to the inclusion in their analysis of a control river which behaved anomalously during the 1964 water year and on which the statistical significance of Hastay and Gladwell's result rests. Comparisons of the runoff of the Skagit river during the 1964 water year with the runoffs of two similarly situated rivers, with which the Skagit is well correlated historically, show no significant effects due to seeding.

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Peter V. Hobbs and Arthur L. Rangno

Abstract

No abstract available.

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