A Wind Tunnel Investigation of Interactions between Supercooled Precipitation-Size Water Drops

Abstract

Laboratory experiments were performed to investigate interactions between small (400?900 ?m) precipitation-size drops at temperatures colder than 0°C. The investigation was accomplished by creating a light shower of supercooled drizzle drops in a vertical wind tunnel. The experiments were performed to test the hypothesis that ice may nucleate by mechanical shock when supercooled precipitation-size drops collide. Experimental results did not strongly support collision?freezing as a viable mechanism that would fully explain the large discrepancy between observed concentrations of ?first? ice and ice nuclei in clouds characterized by an active coalescence mechanism. Also reported is the result of one trial in which a subject drop prematurely froze and subsequently interacted with a supercooled drop in the tunnel rain shower to form a dumbbell-shaped ice particle. Two-dimensional optical array probe data obtained at the ?10°C level in warm-based Midwestern rain clouds are presented that also show dumbbell-shaped images that were probably produced by the same mechanism observed during the wind tunnel trials. This conclusion was reached because 1) the number of dumbbell-shaped images found in the 2D dataset exceeded that expected from particle coincidence and 2) the number of liquid?frozen interactions between precipitation-size drops can be sufficient to account for observed concentrations of dumbbell-shaped ice particles in clouds. These findings suggest that dumbbell-shaped images in the image records of 2D optical array probe data should not be classified as artifacts.