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Robert I. Sax, Jack Thomas, Marilyn Bonebrake, and John Hallett

Abstract

From in-cloud microphysical data collected during the Florida Area Cumulus Experiment (FACE), evidence is presented documenting the presence of significantly greater quantities of crystalline ice near the −10°C sampling level in convective towers previously seeded with AgI pyrotechnics compared to those growing naturally under similar environmental conditions. This finding helps to verify the first link in the dynamical seeding hypothesis—the conversion of supercooled water to ice. Evidence is also presented to show the development of significantly greater concentrations of crystalline ice in clouds seeded during 1976 compared to clouds seeded during 1975. It is suggested that, although some changes in experimental resources and sampling procedures took place between the two years, the observed differences in the evolution of ice crystal concentration were, to a large extent, related to a switch in the type of flare used in seeding. It is contended that a physical basis therefore exists for partitioning of the FACE rainfall results as a function of flare type useage.

The evolution of cloud water and ice in the form of graupel is also discussed in the framework of delineating differences between groupings of data based on a four-way (1975 seed, 1975 no-seed, 1976 seed and 1976 no-seed) partitioning scheme.

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William L. Woodley, John A. Flueck, Ronald Biondini, Robert I. Sax, Joanne Simpson, and Abe Gagin

The Florida Area Cumulus Experiment (FACE) is a long-term program to determine the potential of dynamic seeding for increasing convective rainfall over a fixed target area. The first phase of FACE (FACE-1) provided strong indications for increased, seeding induced rainfall. The second phase, FACE-2 (beginning in June 1978 and ending in August 1980), was conducted in an attempt to confirm these indications of a positive seeding effect. The criteria for confirmation in FACE-2 were published in a NOAA Technical Report prior to program commencement. A clarification and sharpening of these confirmatory criteria are discussed in this paper. In addition, a minority position of what is to constitute confirmation in FACE-2 involving the use of linear predictor models also is discussed. This paper was written and accepted for publication before the treatment decisions of FACE-2 were known.

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