Holographic Particle Tracking Velocimetry: Resolving cloud droplet dynamics

We present the newly developed holographic particle tracking velocimeter (HPTV) designed for use on the Max Planck CloudKite helikites. With airborne in-line holography we can retrieve information about cloud droplets in a large three-dimensional sample volume from a single image. We can resolve individual cloud particles, their cross-section and 3D position in space. The low true airspeed of the CloudKite Helikite, combined with a high sample volume of 0.1 l per image and a high sampling rate in our latest holographic imaging system HPTV, allows a detailed view into each cloud section. Most importantly, the low true airspeed allowed us to develop a holographic particle tracking instrument: The HPTV can capture up to 35 image pairs per second. The time between the images of each pair is short enough that we can track the particles from image to image. This allows us to measure not only the position and cross-sectional size, but also the speed of the cloud particles with just one image system. With this instrument, we will therefore gain more insight into the spatial and temporal properties of cloud droplets on small scales and deepen our understanding of clustering, inhomogeneities and mixing processes in clouds. HPTV can also be used to study the shape and orientation of non-spherical objects such as ice crystals in clouds, volcanic and forest fire ash, and the dispersion of microplastics in the atmosphere.