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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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Arlin B. Super, Bruce A. Boe, Edmond W. Holroyd III, and James A. Heimbach Jr.

Abstract

A series of winter orographic cloud seeding experiments is described in which the seeding agent and associated changes in cloud microphysics are monitored to within 300 m of the target areas (Montana and Colorado), and at the surface (Colorado only). This, the first paper in a three-part series, discusses the underlying physical hypothesis and experimental approach, and describes in detail the instrumentation used. The results of the physical evaluations, presented in Parts II and III, show that marked microphysical changes were caused by both ground-based and aircraft seeding with silver iodide.

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Paul L. Smith, L. Ronald Johnson, David L. Priegnitz, Bruce A. Boe, and Paul W. Mielke Jr.

Abstract

The basis for the cloud seeding operations of the North Dakota Cloud Modification Project (NDCMP) is first outlined. Then the multiresponse permutation procedures are applied in an analysis of crop hail insurance data for the NDCMP target area and for an upwind control area in eastern Montana. A historical analysis of the annual hail insurance loss ratios for the target area indicates lower hail-loss experience during the NDCMP operational years 1976–88. A corresponding analysis for the control area shows no indication of a difference during those years, suggesting the absence of any significant climatological variation. Analysis of a target–control scatterplot of the loss ratios also indicates that the target area experienced relatively smaller hail losses during the NDCMP period. An inference that the difference can be attributed to the NDCMP seeding operations appears to be justified, and the reduction in hail insurance loss ratios in the target area during the NDCMP years is estimated to be about 45%.

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Bruce A. Boe, James A. Heimbach Jr., Terrence W. Krauss, Lulin Xue, Xia Chu, and John T. McPartland

Abstract

Part I of this paper presents the results from a series of plume-tracing flights over the Medicine Bow and Sierra Madre Ranges in south-central Wyoming. These flights, conducted during February and early March of 2011, were part of the Wyoming Weather Modification Pilot Project. Effective targeting of ground-based silver iodide plumes to supercooled clouds has long been a problem for winter orographic cloud-seeding projects. Surface-based ice nucleus (IN) measurements made at a fixed location near the Medicine Bow Range target area had confirmed the effective transport of IN plumes in many cases, but not all. Airborne plume tracing, undertaken to further illuminate the processes involved, provided additional insight into the plume behavior while providing physical measurements that were later compared with large-eddy-simulation modeling (Part II). It was found that the plumes were most often encountered along the flight paths set out in the experimental designs and, in the absence of convection, appear to be mostly confined to the lowest 600 m above the highest terrain. All passes above 600 m above ground level revealed IN concentrations greater than background levels, however. An estimate of IN flux measured over the Medicine Bow Range was approximately 85% of that produced by the five ground-based IN generators active at the time.

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Bruce A. Boe, Jeffrey L. Stith, Paul L. Smith, John H. Hirsch, John H. Helsdon Jr., Andrew G. Detwiler, Harold D. Orville, Brooks E. Mariner, Roger F. Reinking, Rebecca J. Meitín, and Rodger A. Brown

The North Dakota Thunderstorm Project was conducted in the Bismarck, North Dakota, area from 12 June through 22 July 1989. The project deployed Doppler radars, cloud physics aircraft, and supporting instrumentation to study a variety of aspects of convective clouds. These included transport and dispersion; entrainment; cloud-ice initiation and evolution; storm structure, dynamics, and kinematics; atmospheric chemistry; and electrification.

Of primary interest were tracer experiments that identified and tracked specific regions within evolving clouds as a means of investigating the transport, dispersion, and activation of ice-nucleating agents as well as studying basic transport and entrainment processes. Tracers included sulfur hexafluoride (SF6), carbon monoxide, ozone, radar chaff, and silver iodide.

Doppler radars were used to perform studies of all scales of convection, from first-echo cases to a mesoscale convective system. An especially interesting dual-Doppler study of two splitting thunderstorms has resulted.

The objectives of the various project experiments and the specific facilities employed are described. Project highlights and some preliminary results are also presented.

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