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Lawrence Cheng and Marianne English

Abstract

Hailstone size distributions have been determined from 41 time-resolved hailstone samples collected at the ground from seven storms that occurred in Alberta in the summer of 1980. Most size distributions were found to quite closely fit an exponential function of the form n(D) = n 0 e −λD. In studying variations in n 0 and λ, it was found that a relationship exists between the two. In particular, correlation coefficients of ∼−0.9 were found when least-square linear regressions were fitted to the values of logn 0 versus logλ. For Alberta storms, therefore, n 0 can be expressed in terms of λ as n 0 = 115λ3.63, and hail size distributions can be expressed in terms of the single parameter λ as n(D) = 115λ3.63 e −λD. From an examination of hail size distributions from one storm that occurred in Switzerland, it appears likely that similar relationships can be determined for hailstorms from other regions.

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Marianne English and John D. Marwitz

Abstract

Three convective clouds extending above a stratocumulus layer were identified as being seedable on one day and were then seeded in a random sequence with CO2 pellets, a placebo and droppable AgI flares. The radar and microphysical seeding effects were observed with the Alberta Hail Project S-band radar and with the University of Wyoming Queen Air aircraft. Distinct seeding effects were observed in both seeded clouds by both data systems. The CO2 seeded cloud developed a single curtain of precipitation particles 18 min after seeding which reached the ground 20 min after seeding and ceased precipitating 10 min later. The placebo cloud failed to develop any precipitation-sized particles or radar echo and dissipated after ∼30 min. The AgI seeded cloud developed its first echo 8 min after seeding near the threshold temperature for AgI (−7°C), produced precipitation at the ground 20 min after seeding, and continued to develop a new echo near the −7°C level and precipitate for ∼1 h. A natural echoing storm which occurred nearby was examined by radar and found to develop and evolve in a manner quite unlike the seeded clouds. It is plausible that the AgI continued to generate ice crystals in such a manner as to first initiate and then prolong the lifetime of precipitation while the curtain of CO2 pellets failed to initiate more than a single precipitation curtain.

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Raymond K. W. Wong, Norman Chidambaram, Lawrence Cheng, and Marianne English

Abstract

The use of a shifted gamma size distribution for hailstone samples is proposed. This is shown to provide a better fit than the usual exponential form, using time-resolved Alberta data. It is also concluded that there is a dependence of the shape of hailstone size distributions on the duration of sampling time. Such shape variations are associated with the sampling efficiency of the smaller size categories. The importance of the smaller sizes to the common hail integral estimates is also investigated. The minimum sizes required for sampling accuracy of these integral estimates are also obtained.

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