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Properties of ice accreted in two‐stage growth
Author(s) -
Prodi F.,
Santachiara G.,
Franzini A.
Publication year - 1986
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49711247408
Subject(s) - materials science , porosity , penetration (warfare) , seeding , ice crystals , bulk density , penetration depth , stage (stratigraphy) , mineralogy , composite material , geology , meteorology , optics , soil science , paleontology , physics , operations research , engineering , soil water , aerospace engineering
The hypothesis that hailstones may grow in two stages by the soaking and refreezing of a previously formed porous structure, has been explored by obtaining artificial accretions on rotating cylinders in a wind tunnel, in controlled laboratory conditions. Once the first stage of growth was obtained, the inner plastic embryo was extracted and slices were cut for a series of analyses; the remaining pieces of the deposit were rejoined and a new layer was grown onto it or, alternatively, the deposit was partially melted in a flow of warm air, and other slices were sawn for another set of analyses. Changes in accreted ice characteristics due to soaking have been determined for local density, crystallographic parameters and morphology. While deposits obtained at high updraught velocity (26 ms −1 ) were already at a high density in the first stage, the lower density of deposits grown at lower impact velocities (10 ms −1 or less) was increased, due to water penetration, to values similar to those observed in natural hailstones. The local density measurements were obtained by X‐ray contact micrography. This technique enabled us to record even minor density variations crucial for correct interpretation and analysis. All crystallographic parameters: the average grain size, σ, the mean maximum crystal length, l , and the mean maximum crystal width, w , showed an increase after penetration due to both annealing during the time spent at 0°C and the filling of voids. In deposits grown at impact speeds ⩽10 ms −1 the appearance and shape of the first stage deposits are considerably changed as a result of soaking, and the changes introduced are more important the lower the density of the first stage. The dry growth lobes are smoothed and the ice made translucent, radial lines of bubbles are formed within the channels between lobes, and sometimes features are propagated to the external layer. Features similar to those found in natural hailstones have been observed. The lobe structure defined in the first stage, though modified by soaking, retains its general characteristics. Evidence of two‐stage growth in natural hailstones is given and the origin of features such as cuspidated lobes, radial lines of bubbles and rounded air bubbles is explained. The evidence that two‐stage growth may be a frequent occurrence in hailstones is changing the criteria for natural hailstone interpretation by introducing new criteria and suggesting modifications to those currently adopted.