Wind and Turbulence Effects on Raindrop Fall Speed

Wind and turbulence effects on raindrop fall speeds were elucidated using field observations over a 2-yr time pe-riod. Motivations for this study include the recent observations of raindrop fall speed deviations from the terminal fall speed pre-dictions (Vt) based upon laboratory studies and the utilizations of these predictions in various important meteorological and hydrological applications. Fall speed (Vf) and other characteristics of raindrops were observed using a high-speed optical dis-drometer (HOD), and various rainfall and wind characteristics were observed using a 3D ultrasonic anemometer, a laser-type disdrometer, and rain gauges. A total of 26 951 raindrops were observed during 17 different rainfall events, and of these ob-served raindrops, 18.5% had a subterminal fall speed (i.e., 0.85Vt $ Vf) and 9.5% had a superterminal fall speed (i.e., 1.15Vt # Vf). Our observations showed that distributions of sub-and superterminal raindrops in the raindrop size spectrum are distinct, and different physical processes are responsible for the occurrence of each. Vertical wind speed, wind shear, and turbu-lence were identified as the important factors, the latter two being the dominant ones, for the observed fall speed deviations. Turbulence and wind shear had competing effects on raindrop fall. Raindrops of different sizes showed different responses to turbulence, indicating multiscale interactions between raindrop fall and turbulence. With increasing turbulence levels, while the raindrops in the smaller end of the size spectrum showed fall speed enhancements, those in the larger end of the size spectrum showed fall speed reductions. The effect of wind shear was to enhance the raindrop fall speed toward a superterminal fall.