HoloTrack: in-situ holographic particle tracking velocimetry of cloud droplets

Abstract. We present HoloTrack, a novel, fully autonomous measurement system designed to capture three-dimensional (3D) 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 navigation system. Designed for deployment on airborne platforms like the CloudKite and hence having a 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 (50 holograms per seconds with the resolution of 65 megapixels), each of which provides two independent measurements of droplet position, size, and shape and measures individual droplet velocities. With laboratory tests we confirmed, that the holographic system reliably detects particles down to 10 µm, within a sample volume of 17 cm3 (1.84 cm × 1.84 cm × 5 cm) of each hologram. For a recorded hologram pair with mean displacement of 0.5 cm caused by e.g. an inter-frame time of 500 µs and a mean velocity of 10 m s−1, this results in 21.5 cm3 combined volume, where particle position and size is sampled and 12.3 cm3 overlapping volume where the two-frame particle velocimetry can be applied to resolve individual droplet velocities. Reliable sub-volumes for measuring droplets at different yaw angles, to account for the influence of the instrument body are further defined. The droplet velocity in longitudinal and vertical direction is measured with errors of less than 0.07 m s−1 for inter-frame times of 500 µs. The transverse velocity is less accurate with errors in the range of 0.1–0.5 m s−1, depending on the position of the particles in the sample volume. Nevertheless, the flexible timing allows the adjustment to different displacements to optimize the overlapping volume and 3D velocity uncertainties according to the needs of the experiment. A series of laboratory tests and a maiden flight tests validated the system's capabilities, characterizing the detection, robustness, automation and its ability to 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, collision-coalescence, entrainment, and turbulent mixing processes.