Laboratory measurement of secondary droplets generated by the impact of raindrops on water surface*

Secondary 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 360 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 2.6 mm can reach 72% of their terminal velocity in natural conditions. Secondary droplets are measured at various heights above the pool's water surface by using a cinematic digital in-line holographic technique.  A deep learning approach is employed to extract the diameters and 3D motions of the secondary droplets and raindrops from the high-speed hologram movies. It is found that the diameters and velocities of the secondary droplets in the rain field change drastically with the height above the pool's surface. The correlation between the energy of secondary droplets and raindrop energy is explored.