The interpretation of the crystalline and air bubble structures of hailstones

Abstract

AbstractThe factors which determine the crystalline and air bubble structures of accreted ice have recently been elucidated. It is now possible to ascertain the growth histories of hailstones of approximately spherical symmetry from studies of their internal structure. In principle this analysis can be extended to oblate hailstones once their aerodynamic behaviour is known. Six large hailstones 4 to 6.5cm maximum dimension, two from each of three severe storms, have been analysed. The orientation distributions and sizes of the crystals in the hailstone layers indicate that most of the growth of the hailstones took place at ambient temperatures between about −20 and −25°C, i.e. between 2.7 and 3.7km above the freezing level. The air bubble concentrations and size distributions show that all the hailstones grew with surface temperatures ranging from −1°C for the clear layers to about −11°C for opaque layers comprised of very small crystals. Heat balance considerations indicate that the effective liquid water concentrations in which the hailstones consequently grew were between 3 and 1g m−3. On the assumption that the median volume radius of the cloud droplet distribution was 10μm, the growth of most of the hailstones took place in liquid water concentrations of about the adiabatic values. However, fluctuations in the liquid water concentration of up to some 30% are required to explain the observed air bubble and crystal structures. The fact that the hailstones grew largely between the −20 and −25°C levels indicates that they were balanced in the updraught for most of their history. Since the hailstone fallspeed increases with radius, this meant that the updraught experienced by the hailstone increased steadily with time. There are two possible explanations for this: the strength of the updraught could have increased with time; alternatively, the hailstones could have moved around the main core of the updraught to encounter increasing updraught speeds, as suggested in the Browning and Foote (1975) three‐dimensional model of severe storms.