Freezing of Freely Suspended, Supercooled Water Drops in a Large Vertical Wind Tunnel

Abstract

The design and operation of a large vertical wind tunnel (1.8 m diameter updraft) are described, together with the associated special photographic equipment and techniques required for studies of the interactions and freezing of freely suspended water drops. During the first two winters of operation several new and important observations have been made while freezing freely suspended, large supercooled water drops. The terminal velocity of the frozen pellet was found to be very different than that of the liquid drop. If individual drops freeze at ?6C and colder they often exhibit a marked decrease (up to 4 m sec?1) in terminal velocity. Coalescence of a frozen pellet and a liquid drop produces an elongated ice pellet (8?15 mm horizontal axis) with a terminal velocity of 9 m sec?1. When an ice pellet becomes unstable and spins about a horizontal axis, it can obtain a rapid horizontal velocity. Two ice pellets frozen together display the same erratic tumbling. These observations indicate that some ice pellets have greatly increased distances and residence times to grow in the supercooled region of a cloud.