Effects of Growth Temperatures and Surface Roughness on Crystal Orientation of Ice Accreted in a Dry Regime

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  • 1 Istituto FISBAT-CNR, Reparto Nubi e Precipitazioni, Bologna,Italy
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Abstract

The dependence of crystal orientation on air and deposit temperatures in ice accreted in the dry growth regime has been re-examined by producing accretions over a wide range of these temperatures and critically examining the structural aspects of the deposits, such as surface roughness. It is shown that the variation in the preferred c-axis orientation with the air and deposit temperatures during growth is an intrinsic effect of she freezing mode of ice, rather than an effect of surface roughness.

The Fϕ distributions of the angle ϕ between the accretion radius and the crystal c-axis, and those of its component angles F(η) and F(), have been obtained. It is shown that for cloud temperature Ta > −18°C, the F(ϕ) distributions have a main maximum at small angles. For −18°C ≥ Ta ≥ −23°C, the location of the maximum depends strongly on the deposit temperature Td reaching values near 45° for Td < −10°C. For Ta < −23°C the distributions show a large disorder at Td ≥ −5°C, but a maximum about 45° when Td is below this value.

Sharp secondary peaks frequently appear at the side of or superposed to, the main maxima. The application of the χ2-test to the distributions shows that secondary maxima may not be used to establish a more precise definition of Ta and Td. They may on the other hand be a consequence of the shape of the lobes, which may determine the splitting of the main maxima.

The application of the results to natural hailstone analysis is discussed.

Abstract

The dependence of crystal orientation on air and deposit temperatures in ice accreted in the dry growth regime has been re-examined by producing accretions over a wide range of these temperatures and critically examining the structural aspects of the deposits, such as surface roughness. It is shown that the variation in the preferred c-axis orientation with the air and deposit temperatures during growth is an intrinsic effect of she freezing mode of ice, rather than an effect of surface roughness.

The Fϕ distributions of the angle ϕ between the accretion radius and the crystal c-axis, and those of its component angles F(η) and F(), have been obtained. It is shown that for cloud temperature Ta > −18°C, the F(ϕ) distributions have a main maximum at small angles. For −18°C ≥ Ta ≥ −23°C, the location of the maximum depends strongly on the deposit temperature Td reaching values near 45° for Td < −10°C. For Ta < −23°C the distributions show a large disorder at Td ≥ −5°C, but a maximum about 45° when Td is below this value.

Sharp secondary peaks frequently appear at the side of or superposed to, the main maxima. The application of the χ2-test to the distributions shows that secondary maxima may not be used to establish a more precise definition of Ta and Td. They may on the other hand be a consequence of the shape of the lobes, which may determine the splitting of the main maxima.

The application of the results to natural hailstone analysis is discussed.

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