Trajectories of Cloud Droplets around a Rain Drop Observed in Vertical Wind Tunnel

We use digital in-line holography to investigate collisions and near collisions of cloud droplets with a rain drop. The measurements were performed in the Mainz vertical wind tunnel in a laminar air flow. A single collector drop of 700 µm diameter is injected into the flow, where the flow velocity is matched to the terminal velocity of the drop. This causes stationary levitation of the drop in the measurement section of the wind tunnel. Smaller cloud droplets of diameters between 10 and 30 µm are injected upstream into the flow below the levitating raindrop. This drop and droplets are illuminated from one side by a laser beam and imaged by a high speed camera located on the other side of the measurement section. The scattered light from the particles and the background laser light superimpose creating interference patterns on the camera's CCD chip which forms the digital hologram. Numerical reconstruction of the holographic images allows determination of the positions and the diameters of both small droplets and the collecting drop. Successive images allow for reconstruction of the small droplet trajectories around the large drop. We present details of the experimental setup, the data processing procedure, as well as selected droplet trajectories and first results.