Mixing of cloud and clear air in centimeter scales observed in laboratory by means of Particle Image Velocimetry

Cloudy air, containing small (∅ 14 μm) water droplets, undergoes mixing with the unsaturated environment inside the cloud chamber in the process resembling smallest scales of entrainment and mixing in real clouds. Particle Image Velocimetry (PIV) applied to images from the chamber interior is used to investigate dynamics of the process in scales from 1.2 mm to few centimeters. A special algorithm, allowing for investigation of droplets motion, is developed, tested and adapted to the experimental data. Two velocity components retrieved in the vertical cross-section through the chamber interior indicate anisotropy of small-scale turbulent motions, with the preferred vertical direction. This result confirms earlier numerical studies, indicating that evaporation of cloud droplets at the cloud–clear air interface may substantially influence the small-scale turbulence in clouds.