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J. Rosinski and T. C. Kerrigan

Abstract

Variable concentrations of different sized water-insoluble particles were found in bulk rainwater and single raindrops (liquid phase) and in hailstones (solid phase) collected from severe thunderstorms in the Colorado-Nebraska region. It was possible to draw the following conclusions from studies of aerosol particles transferred into the liquid phase of the storm: 1) aerosol particles constitute an intrinsic part of a convective cloud in a severe thunderstorm; 2) the size distribution of cloud droplets is extended to larger sizes by thepresence of giant aerosol particles (> 75 n diameter); 3) giant aerosol particles begin to accrete cloud droplets as soon as they enter the cloud; and 4) the size distribution of raindrops falling at the leading edge of a severe storm depends on the concentration of giant aerosol particles ingested by the storm.

The presence of different sized particles in the solid phase led to the following conclusions: 1) transparenthailstones are formed from the frozen Equid phase of a cloud (cloud droplets and raindrops) around giantaerosol particles, some of the giant aerosol particles acting as ice-forming nuclei at temperatures as warmas - 6C; 2) milky hailstones originate in the presence of ice crystals which have grown through the liquid-vapor-solid phase transition; 3) the majority of ice-forming nuclei in milky hailstones are particles between6 and 12 y. diameter, this size range being the result of particle separation by size in an ascending cloud; and4) detailed analysis of particulate matter present in different forms of ice in mixed hailstones provides information on the environmental conditions during their formation and growth.

A numerical model shows possible conditions for the formation of raindrops around giant aerosol particles. The models calculations also predict strong separation of different sized aerosol particles ingested by the storm. The latter was verified in samples from a storm in which rainfall contained no 50-75 μ diameterparticles, while hailstones contained up to 2300 particles in the same size range, per gram of ice.

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J. Rosinski, C. T. Nagamoto, T. C. Kerrigan, and G. Langer

Abstract

The majority of hydrosol particles in the submicron range in precipitation are produced by separation from the surfaces of larger hydrosolized aerosol particles. Sail particles may produce freezing nuclei active between –10 and –7C in concentrations up to 100 cm−3 per soil particle, and freezing nuclei active at temperatures warmer than –20C in concentrations greater than 1000 cm−3 per soil particle. It is concluded that neither the role of freezing nuclei in the development of precipitation nor even the distribution of freezing nuclei among cloud particles, raindrops and hailstones can be deduced from the determination of freezing nucleus populations in bulk precipitation samples.

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J. Rosinski, C. T. Nagamoto, T. C. Kerrigan, and G. Langer

Abstract

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J. Rosinski, G. Langer, C. T. Nagamoto, T. C. Kerrigan, and F. Prodi

Abstract

There are two populations of aerosol particles in severe storms: normal background aerosol and aerosolized soil particles. Concentration of the latter, which depends on local wind speed and soil conditions, may be orders of magnitude higher than that of the former. Condensation nuclei are derived principally from the first source. Concentration of ice-forming nuclei, which derive from the soil particles, increases during storms up to 100 times the pre-storm (background) value. This concentration increase is less than that of the aerosol population, indicating that only a fraction of soil particles exhibit ice-nucleating properties. The fraction of soil particles active as ice-forming nuclei in a given particle size range increases with particle size; however, the concentration of ice-forming nuclei in air is counteracted by a decrease in the concentration of aerosol particles with size. Supercooled water drops are nucleated by hydrosol soil particles at temperatures as high as −5.3C.

The quantity of water vapor released during the freezing of supercooled water drops was determined theoretically and experimentally. This value depends primarily on the size of water drops and, to a lesser degree, on the temperature of supercooling. The released water vapor, equal to 0.03 to 3.5 mg per 1–5 mm diameter drops, produces high supersaturation with respect to water at the temperature of the environment in a volume of 300 to 105 cm3, respectively. The water vapor recondenses on cloud droplets and aerosol particles acting as condensation nuclei at higher supersaturation. Some of the aerosol particles acting as ice-forming nuclei will form ice crystals in the water vapor recondensation zone, and these particles will propagate the ice phase within an updraft. Giant aerosol particles, after becoming hydrosol particles, are the most effective freezing nuclei derived from the soil and should be responsible for the appearance of ice at the lowest altitude (warmest temperature).

The freezing temperature spectrum of different hydrosols made of various ices separated from natural hailstones revealed that the warmest freezing temperature was not necessarily associated with hailstone embryos. This indicates that many hailstone embryos form at higher altitudes (lower temperature zones) rather than forming at a freezing level corresponding to the temperature at which the warmest ice-forming nuclei are active.

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