Particle Image Velocimetry (piv) for Cloud Droplets – Laboratory Investigations

Interaction between small-scale turbulence and cloud particles is a key issue in warm rain formation mechanism, which is an important meteorological and climatological challenge [1],[2]. We investigate this interaction observing motion of cloud droplets in a glass walled chamber 1m deep, 1m wide and 1.8m high [3]. The second smaller chamber, placed above the main one is filled with water droplets of diameters in range from 7 to 25 micrometers. Cloudy air containing water droplets enters the main chamber forming the negatively buoyant, turbulent plume, mixing with unsaturated air in the main chamber and producing additional turbulence by evaporative cooling of droplets. The plume is illuminated with a 1.2mm thick sheet of laser light (Nd:YAG, λ=532nm), forming the vertical cross-section through the central part of the chamber. Images, formed due to Mie scattering of laser light by cloud droplets are recorded by a high-resolution CCD camera placed outside the chamber, with the optical axis perpendicular to the light sheet. Images are recorded in pairs in order to retrieve information on droplets' velocities. Application of standard PIV algorithms to these pairs resulted in a large amount of errors and artifacts in the retrieved velocity field due to chaotic nature of motion in a chamber. Therefore a special multi-scale algorithm was developed.