On the Heat Transfer to Ice Spheres and the Freezing of Spongy Hail

Abstract

The rate of heat transfer during melting of solid ice spheres and freezing of spongy ice spheres has been measured over a range of experimental conditions. The results agree quite well with other published heat transfer data, and show that heat transfer to a melting hailstone, whose surface is covered by a film of water, is described by the same expression as that for a freezing hailstone with a dry surface. Using the measured heat transfer rates in a numerical model of the freezing of spongy hailstones in an updraft, it is shown that there is probably time for smaller spongy hailstones to freeze before reaching the ground, but that freezing of spongy hailstones 3 cm in diameter or larger is improbable. This finding, in conjunction with field measurements of liquid water contents of natural hailstones, casts doubt upon hailstorm models which imply growth of large spongy hailstones in an ?accumulation zone? of high supercooled liquid water content aloft.