Laboratory study of the vertical concentration profile of water droplets near the ocean surface in a rain field

Secondary water droplets generated by the impact of raindrops on a deep-water pool are studied experimentally in an artificial rain facility. Artificial rain is produced by a rain generator that consists of a water-filled open-surface rectangular tank with an array of hypodermic needles attached to its bottom. Experiments are performed by mounting the rain generator above the water pool at a vertical distance of 2.2 m. With this distance, the impact velocities of the raindrops with a diameter of about 3.0 mm can reach 70% of their terminal velocity in natural conditions. Different rain rates are used. Droplets are measured at various heights above the pool's water surface by using a cinematic digital in-line holographic technique. Both diameters and motions of the secondary droplets and raindrops are extracted from the high-speed hologram movies. It is found that the diameters and concentrations of the secondary droplets in the rain field change drastically with the height above the pool's surface. The effects of rain rate and raindrop diameter on the vertical concentration profile of secondary droplets are investigated.