Crystal Size and Orientation in Ice Grown by Droplet Accretion 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(?) 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 l? has the same behavior as the mean surface area σ, while the mean maximum crystal width w?, 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 l? of the crystals seem to be the best parameters to follow in evaluating the air temperature Ta in wet and spongy regimes.