Microscopic Observations of Riming on an Ice Surface Using High Speed Video

Microscopic droplet riming events on an ice surface have been observed using high speed video. Observations included greater propensity for droplet spreading at temperatures higher than -15°C on flatter ice surfaces, and subsequently, the formation of growing rime spires into the flow, allowing glancing droplet collisions and more spherical freezing of smaller droplets. Insight into differences between laboratory observations of the Hallett-Mossop process is offered, relating to the nature of droplet spreading associated with the structure of the rimer surface prior to impact. Observations of a difference between air speed and resulting droplet impact speed on an ice surface may affect interpretations of riming laboratory studies, and may explain recent observations of a high secondary ice production rate in supercooled layer clouds.