Novel Possibilities of In-Situ Cloud Observation with Drone-Mountable Holographic Imager

Abstract. We present a lightweight holographic imaging instrument, the UAV-CPS (Unmanned Aerial Vehicle Cloud Particle Sensor), designed for high-resolution in-cloud particle measurements at a rate of 10 holograms per second. When mounted on a UAV, the instrument samples a well-defined volume of ~3 cm³ within a single hologram, providing sub-meter spatial resolution within targeted cloud regions such as stratiform cloud tops and cumuliform cloud edges. Compared to conventional platforms, such as research aircraft or tethered balloons, the UAV-based approach offers a cost-effective and versatile alternative, capable of accessing confined or remote areas. This enables improved validation of remote sensing products and provides observational constraints for cloud process rates in numerical cloud models. During a 16-day campaign comprising 14 flights at Pallas, Finland, UAV-CPS measurements were compared against reference instruments installed at the hilltop Sammaltunturi station. On average, UAV-CPS showed lower droplet concentrations in the 5–13 µm diameter range, while larger particles were captured with high accuracy. As with other holographic sensors, detecting and sizing the smallest droplets in a large measurement volume per hologram remains challenging. However, these limitations are quantified and can be mitigated through calibration, complementary measurements, and ongoing instrument refinement, such as optimizing hologram reconstruction procedures and employing a shorter-wavelength light source. In addition to introducing UAV-CPS, we present a case study that highlights the potential of such instrumentation for high-resolution cloud observations.