HoloTrack: In-Situ Holographic Particle Tracking of Cloud Droplets

Abstract. We present HoloTrack, a novel, fully autonomous measurement system designed to capture three-dimensional cloud droplet data and provide detailed insights into droplet dynamics, their spatial distribution and velocity. The HoloTrack system integrates a high-accuracy holographic imaging system with environmental sensors, including pitot tubes for airflow measurements, and a motion-tracking system. Designed for deployment on platforms like the CloudKite and hence compact and autonomous design, HoloTrack is also ideally suited for deployment in laboratory or ground-based environmental research. The system records up to 25 hologram pairs per second, each of which provides two independent measurements of droplet position, size, and shape and measures individual droplet velocities in longitudinal and vertical direction. The holographic system reliably detects particles down to 10 µm, within a sample volume of 17 cm³ of each hologram, which results in 21.5 cm³ sampled particle position and size and 12.3 cm³ sampled velocity for a mean displacement of 0.5 cm within hologram pairs. Reliable sub-volumes for measuring droplets at different yaw angles, to account for the influence of the instrument body are defined. The droplet velocity is measured with errors of less than 1.5 % for mean velocities of 8–10 m/s, but the flexible timing allows adjustment for larger mean displacements which increases accuracy if desired. A series of ground tests and a maiden flight tests validated the system’s capabilities, confirming detection, robustness, automation and its ability to accurately measure droplet dynamics. HoloTrack’s unique combination of holographic particle measurements including capturing their velocities makes it a powerful tool for advancing our understanding of cloud microphysics, including droplet spatial distribution, coalescence, entrainment, and turbulent mixing processes.