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- Author or Editor: F. Prodi x
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Abstract
The size of crystal grains in ice formed by accretion of supercooled droplets has been determined for a wide range of air (−11 to −25°C) and deposit (0 to −16°C) temperatures. At air temperatures of −14°C and below, the dependence of the crystal size on both air and deposit temperatures has been determined and found to be more marked at the lower temperatures.
The aspect ratio (as the average of the ratios of the length to maximum width of the crystals) has also been determined for the same accretions. This parameter, too, has been found to depend on air and deposit temperatures, in a way which could help in resolving the ambiguity deriving from the analysis of crystal size alone, when investigating the growth conditions of hailstones.
Abstract
The size of crystal grains in ice formed by accretion of supercooled droplets has been determined for a wide range of air (−11 to −25°C) and deposit (0 to −16°C) temperatures. At air temperatures of −14°C and below, the dependence of the crystal size on both air and deposit temperatures has been determined and found to be more marked at the lower temperatures.
The aspect ratio (as the average of the ratios of the length to maximum width of the crystals) has also been determined for the same accretions. This parameter, too, has been found to depend on air and deposit temperatures, in a way which could help in resolving the ambiguity deriving from the analysis of crystal size alone, when investigating the growth conditions of hailstones.
Abstract
The effect of annealing in accreted ice has been investigated for artificially grown ice deposits after ∼100 days of storage in a deep freeze unit. Cross sections of the cylindrical deposits have been cut and replicated soon after growth and after annealing to determine the average cross section σ¯, the maximum length l i, the maximum width w i, and the orientation of the c axis of the crystal grains.
Significant grain growth has been observed even at the relatively low temperature of −19°C. The average grain cross section increased for nearly all the deposits, and a decrease of the grain elongation was observed while the crystals tended to assume compact final shapes. The statistical distributions of the orientations of crystal grains was not modified substantially by annealing, thus indicating that information on hailstone growth may be contained more firmly in the orientation of crystals rather than in their size. The change of the crystal dimensions is interpreted in terms of a polycrystalline grain growth process.
The consequences of annealing on the crystal size analysis in hailstones are examined.
Abstract
The effect of annealing in accreted ice has been investigated for artificially grown ice deposits after ∼100 days of storage in a deep freeze unit. Cross sections of the cylindrical deposits have been cut and replicated soon after growth and after annealing to determine the average cross section σ¯, the maximum length l i, the maximum width w i, and the orientation of the c axis of the crystal grains.
Significant grain growth has been observed even at the relatively low temperature of −19°C. The average grain cross section increased for nearly all the deposits, and a decrease of the grain elongation was observed while the crystals tended to assume compact final shapes. The statistical distributions of the orientations of crystal grains was not modified substantially by annealing, thus indicating that information on hailstone growth may be contained more firmly in the orientation of crystals rather than in their size. The change of the crystal dimensions is interpreted in terms of a polycrystalline grain growth process.
The consequences of annealing on the crystal size analysis in hailstones are examined.
Abstract
The effect of size on ice nucleation has been experimentally investigated for aerosol particles in the aerodynamic range. Aerosol particles are separated according to their aerodynamic diameter while airborne and deposited on a membrane filter which is then developed in a static diffusion chamber. The effect of competition among nuclei and the effect of particle size, though acting together, have been studied separately by sampling various filters in such a way as to obtain a wide range of surface concentrations of particles in each interval of aerodynamic diameter. Moreover, when sampling atmospheric aerosols, the effect of competition among ice nuclei and cloud condensation nuclei is eliminated in practice, as the latter are mostly deposited, together with other small particles, at the end of the filter.
The effect of size has been investigated for homogeneous laboratory dispersed aerosols of clay (mont-morillonite, kaolinite and halloysite). Urban aerosols and the exhaust fumes from an oil-fired power station have also been studied.
The usefulness of the technique in investigating the fate of submicron ice nuclei which have become attached to coarser particles has been demonstrated.
Abstract
The effect of size on ice nucleation has been experimentally investigated for aerosol particles in the aerodynamic range. Aerosol particles are separated according to their aerodynamic diameter while airborne and deposited on a membrane filter which is then developed in a static diffusion chamber. The effect of competition among nuclei and the effect of particle size, though acting together, have been studied separately by sampling various filters in such a way as to obtain a wide range of surface concentrations of particles in each interval of aerodynamic diameter. Moreover, when sampling atmospheric aerosols, the effect of competition among ice nuclei and cloud condensation nuclei is eliminated in practice, as the latter are mostly deposited, together with other small particles, at the end of the filter.
The effect of size has been investigated for homogeneous laboratory dispersed aerosols of clay (mont-morillonite, kaolinite and halloysite). Urban aerosols and the exhaust fumes from an oil-fired power station have also been studied.
The usefulness of the technique in investigating the fate of submicron ice nuclei which have become attached to coarser particles has been demonstrated.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
The migration of water-insoluble particles during freezing was used to study the structure of ice grown by droplet accretion. Experiments were performed in a cold tunnel under different growth conditions using nickel powder as a tracer; the resulting deposit was examined using the x-ray microradiograph technique. Analysis of the types of tracer particle aggregates provided a description of the freezing processes. It was concluded that this method could be used as an additional tool in studies of the various ice growth conditions found in nature.
Abstract
The migration of water-insoluble particles during freezing was used to study the structure of ice grown by droplet accretion. Experiments were performed in a cold tunnel under different growth conditions using nickel powder as a tracer; the resulting deposit was examined using the x-ray microradiograph technique. Analysis of the types of tracer particle aggregates provided a description of the freezing processes. It was concluded that this method could be used as an additional tool in studies of the various ice growth conditions found in nature.
Abstract
Preliminary results of a chemical analysis of hailstone slice residues are reported. The radial pattern of Fe particles shows a marked outer ring of relatively high concentration. The possible causes are discussed and it seems reasonable to attribute it to a final freezing of contaminated liquid water contained in the hailstone outer layer.
Abstract
Preliminary results of a chemical analysis of hailstone slice residues are reported. The radial pattern of Fe particles shows a marked outer ring of relatively high concentration. The possible causes are discussed and it seems reasonable to attribute it to a final freezing of contaminated liquid water contained in the hailstone outer layer.
Abstract
The size and orientation of crystal grains in wet and spongy ice formed by accretion of supercooled droplets has been determined in a wide range of air temperatures (−8 to −25°C). Particular care has been taken in reducing the spurious ice crystals in the icing wind tunnel which according to former investigators could affect the crystal characteristics in these growth regimes. The c-axis orientation analysis of the grains has been used to check that spurious ice crystals have not been incorporated in a significant number into the structure.
As for the orientation of the c-axis it has been observed that different deposits grown at Ta ≥ −22°C show a rather similar behavior, especially in the F(θ) and F(ϕ) distributions; however, the probability that the distributions belong to the same populations is small. At Ta = −25°C there is a larger disorder in the c-axis orientation. Therefore the analysis of the crystal orientation should be primarily used to determine the growth regime, dry or wet-spongy, and not to determine Ta , which does not seem to be responsible for the observed differences in orientation, which are more probably due to variations of sponginess or surface roughness.
As for crystal size parameters, the results show that the average surface area ¯σ of the grains is lower in the wet-spongy accretions than in dry ones grown at the same air temperatures and at deposit temperatures just below 0°C. This observed difference could be correlated with the different freezing mechanism in the two regimes, which must be also responsible for the different behavior in the c-axis orientation. The mean maximum length l̄ has the same behavior as the mean surface area σ, while the mean maximum crystal width w̄, unlike in the dry growth regime, is almost independent of the air temperature Ta .
Therefore, considering the application to the analysis of natural hailstones, the mean surface area σ and the mean maximum length l̄ of the crystals seem to be the best parameters to follow in evaluating the air temperature Ta in wet and spongy regimes.
Abstract
The size and orientation of crystal grains in wet and spongy ice formed by accretion of supercooled droplets has been determined in a wide range of air temperatures (−8 to −25°C). Particular care has been taken in reducing the spurious ice crystals in the icing wind tunnel which according to former investigators could affect the crystal characteristics in these growth regimes. The c-axis orientation analysis of the grains has been used to check that spurious ice crystals have not been incorporated in a significant number into the structure.
As for the orientation of the c-axis it has been observed that different deposits grown at Ta ≥ −22°C show a rather similar behavior, especially in the F(θ) and F(ϕ) distributions; however, the probability that the distributions belong to the same populations is small. At Ta = −25°C there is a larger disorder in the c-axis orientation. Therefore the analysis of the crystal orientation should be primarily used to determine the growth regime, dry or wet-spongy, and not to determine Ta , which does not seem to be responsible for the observed differences in orientation, which are more probably due to variations of sponginess or surface roughness.
As for crystal size parameters, the results show that the average surface area ¯σ of the grains is lower in the wet-spongy accretions than in dry ones grown at the same air temperatures and at deposit temperatures just below 0°C. This observed difference could be correlated with the different freezing mechanism in the two regimes, which must be also responsible for the different behavior in the c-axis orientation. The mean maximum length l̄ has the same behavior as the mean surface area σ, while the mean maximum crystal width w̄, unlike in the dry growth regime, is almost independent of the air temperature Ta .
Therefore, considering the application to the analysis of natural hailstones, the mean surface area σ and the mean maximum length l̄ of the crystals seem to be the best parameters to follow in evaluating the air temperature Ta in wet and spongy regimes.
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.
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.
