Experimental and field studies of precipitation particles formed by the freezing of supercooled water

Drops which freeze in isolation or by accretion on an ice particle are found to be single or polycrystalline depending on the drop supercooling and the particle temperature. Subsequent growth from the vapour depends on the number and orientation of these crystals. The airflow around the particle, characterized by the Reynolds Number, Re, is important in the initial accretion process. The particle falls steadily for small Re, but oscillates and eventually tumbles as Re approaches 500. The deformation of accreted drops depends both on the dendrite freezing velocity through the liquid compared with the impact velocity and on the drop impact kinetic energy compared with its surface energy. Viscous dissipation becomes important as the deformation, defined as the ratio of the base radius of the deformed drop to its height, exceeds 10. Deformations of about 10 have been measured for 40 μ radius supercooled drops impacting on an ice surface at 13.5 m sec −1 . The heat economy of a freezing accreted drop is dominated by a collecting particle which is large compared with the drop, when complete freezing takes place from the contact area outwards. Symmetrical freezing, from top and bottom of the drop, only occurs for surface temperature approaching 0°C, or when the drop accretes on a narrow spike, when there is a possibility of drop shatter and separation of electric charge. Drops accreting on particles growing spongily may interact first with a liquid layer whose thickness may be of the same order as the drop size and produce splash droplets even at large supercoolings for critical values of layer thickness. Opacity is related to bubble size. Opaque ice forms when the particle is growing spongily or dry, with transparent ice forming when the growth is just wet. Just spongy growth at low temperatures is associated with small crystals and opaque ice.