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Peter W. Summers

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Paul J. Denison and Peter W. Summers
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Peter W. Summers, Graeme K. Mather, and Dennis S. Treddenick

Abstract

Many of the severe and persistent hailstorms in Alberta propagate by means of new cloud development on the southern or western flank. A concept is proposed whereby these cumulus towers are seeded with freezing nuclei early in their development. In order to accomplish this a droppable pyrotechnic flare system was developed and tested.

A T-33 jet aircraft was used as the seeding platform. It was equipped with a Ku-band weather radar, a flare rack and firing control panel, a 14 channel recorder and a 3-cm transponder. Seven inch pyrotechnic flares were manufactured to the following specifications: delay burn 50 sec, silver iodide burn time 30 sec, and flare output 24 gm of silver iodide producing a total of 2.4 × 1014 freezing nuclei active at −10C. A unique feature was the incorporation of 10-cm radar chaff which was released at the flare burn-out and used as a position marker. Flare performance was evaluated using radar, visual and photographic tracking. Total fall distance as a function of release height was determined. For typical drop altitudes used to seed storms the flares fell 9500 ft with silver iodide being released during the last 2700 ft.

In the summers of 1970 and 1971 the seeding system was used on sixteen occasions in experiments which emphasized physical understanding rather than statistical inference. On two occasions turbulence measurements were made in cumulus towers with a second T-33 aircraft. The calculated dissipation rates indicate that there is sufficient diffusion to produce silver iodide nuclei concentrations in excess of 100 liter−1 active at −10C through cloud volumes of several cubic kilometers within a few minutes after seeding.

The operational logistics of this seeding system are quite straightforward and the system appears to be a practical one for applying the direct injection seeding technique to multicell hailstorms. By means of radio communication between the project control room and the seeding aircraft, it was always possible to unambiguously identify and seed the selected target storm. The radar chaff was limited usefulness as a marker of seeding location.

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Charles A. Knight, G. Brant Foote, and Peter W. Summers

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The three-year, randomized hail suppression test of the National Hail Research Experiment (NHRE) used seeding methods patterned after the Soviet hail suppression activities, applied over a specified target area on a randomly selected portion of days identified as having a high potential for hailfall by a radar criterion. Statistical tests for seeding effects were performed on data from ground networks of hail measuring instruments within the target area using total hail mass within the target area as the primary response variable. The statistical results indicate a range of possible mean seeding effects from a reduction of 60% to an increase of as much as 500% in total hail mass, depending on the test used, within the 90% confidence limits. The range is so wide that no conclusion about a seeding effect can be drawn. Analysis of hail size, embryo type and various radar parameters, and stratification either in terms of estimates of “hail potential” or operational seeding efficiency have also not revealed a convincing seeding effect.

These results can be rationalized on the basis that the seeding method was to put extra nuclei into the strong updraft portions of the storms, as the Soviet workers do in order to seed “accumulation zones” of supercooled water. Direct and indirect evidence from the research component of NHRE has shown that accumulation zones, if indeed they exist in northeast Colorado, rarely participate in the hailstone growth. Virtually all radar echo from above the freezing level is from ice, and hail embryos are predominantly graupel. In these circumstances, seeding in the strong updraft is unlikely to produce beneficial competition and a hail suppression effect.

In order to devise a seeding technique that has a good chance to be effective in these types of storms, and to strengthen the statistics to the point where a seeding test has a reasonable chance of detecting seeding effects, research must be done on several fundamental aspects of hailstorms. Paramount among these are the location and time of the natural ice nucleation events that lead to the formation of hail embryos, the trajectories followed by the growing hailstones, and the role of depletion of cloud water in determining hail size.

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G. Brant Foote, Charles G. Wade, James C. Fankhauser, Peter W. Summers, Edwin L. Crow, and Mark E. Solak

Abstract

An analysis of the seeding operations during the National Hail Research Experiment 1972–74 randomized seeding program is carried out for the purpose of critiquing the seeding procedures and establishing the actual rates at which seeding material was dispensed as opposed to the prescribed rates. The seeding coverage, a parameter defined in the paper, is found to be only about 50% on the average. The reasons for the low seeding coverage are discussed in terms of seeding logistics and storm evolution, and three case studies are presented to illustrate the problems that can arise. Some results on the rate at which storm cells can develop and on the duration of convective activity over a fixed target area are presented. It is concluded that seeding convective clouds using aircraft flying near cloud base is more difficult than is widely acknowledged.

Since the seeding operations were more thorough on some days than on others, one might reasonably expect that seeding effects, if they exist, would be more marked on the days with the higher coverage. Post hoc analyses that stratify the surface hail and rain data according to seeding coverage are presented. The results do not allow one to reject the hypothesis that seeding had no effect on surface precipitation.

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