Development and characterization of an advanced holographic instrument for atmospheric research

Abstract. In this study, we develop and characterize an advanced holographic instrument — HOLODROPS (HOLOgraphic DROPlet Sensor) — along with a complementary event-based Particle Image Velocimetry (ePIV) system. HOLODROPS operates in two configurations: (1) HOLODROPS, designed for long-term, high-precision measurements of cloud droplet size distributions, and (2) HOLODROPS-TRACK, designed for short-interval particle tracking to study turbulence and collision-coalescence processes. Unlike traditional optical scattering or imaging probes, both configurations can simultaneously capture the size, shape, and three-dimensional position of multiple droplets and ice crystals larger than 8.8 µm within a sample volume, eliminating the need for multiple instruments and yielding more accurate and cohesive data. However, the detailed three-dimensional particle tracking provided by HOLODROPS-TRACK requires significant computational resources for hologram reconstruction and data processing, limiting its use over extended measurement periods. To address this challenge, we developed the ePIV system, which leverages an event-based camera to assess key cloud motion features in real time with substantially lower data volume and processing. By rapidly characterizing the flow conditions, ePIV identifies optimal measurement windows and informs targeted deployment of HOLODROPS-TRACK, paving the road for efficient, high-resolution particle tracking when conditions are most scientifically valuable. All three systems have been successfully characterized and validated in the laboratory. Additionally, we tested HOLODROPS successfully in an outdoor setting with a drizzle event.