Droplet accretion during rime growth and the formation of secondary ice crystals

Abstract

Laboratory experiments have been performed to study rime growth and its relation to secondary ice crystal production (the Hallett-Mossop process). The rime was grown in a wind tunnel on a fixed vertical fibre 0.5 mm in diameter. The range of conditions extended from −1.0°C to −13°C in temperature, from 20cms−1 to 200cms−1 in air speed, and liquid water content up to 1.5 gm−3. The droplet size distributions were 5 to 25 μm and 8 to 50 μm, peaking at diameters of 9 and 22 μm respectively. Under conditions favourable for secondary ice production, the accreting droplets spread out on the ice surface; individual frozen droplets formed only at the lower temperatures. A separate experiment showed that supercooled water droplets froze as cones on a flat surface above −10.°C. At temperatures between −1. and −4.°C the spreading ratio of supercooled droplets on the basal surface of ice was less than on other faces, with lower molecular packing density. It is hypothesized that spreading takes place in a quasi-liquid layer which is thinner on the basal surface. Splinter formation by pressure build-up inside individual frozen droplets is therefore unlikely to be responsible for shatter; a new model is suggested in which thermal gradients give stresses leading to an ice crack at about − 5°C.