Constraining Small-Scale Cloud Microphysics with High-Resolution Scattering: The Novel Hyper-Angular Cloud Polarimeter and Synergistic Applications

Microscale properties of cloud particles govern processes that influence weather and climate from local to global scales, yet accurate detection and quantification of these properties remains a fundamental challenge. Addressing this, recently launched and planned satellite missions have prioritized instrument development based on multi-angle polarimetric imaging of particle light scattering. However, obtaining quantitative microscale properties from these measurements relies on retrieval algorithms that require robust validation against high-fidelity, in-situ data.

To address this critical need, we introduce the Hyper-Angular Cloud Polarimeter - Prototype Version (HACP-PV), a novel in-situ instrument designed and manufactured by schnaiTEC. The instrument's design foundation is shared with the optical principles of satellite-platform multi-angle polarimetry. Independent modulation of the instrument’s emitted and received light polarization states enables direct measurements of the unique elements needed to fully constrain the scattering matrix of the sample volume. We resolve angular scattering functions from approximately 101.5° to 168.5° at a resolution finer than 0.1°, tightly constraining measurements of the Particle Size Distribution (PSD). Crucially, the HACP-PV features an open-path sampling design, eliminating inlet-based sampling artifacts that typically introduce measurement uncertainty. We leverage these complete, artifact-free, scattering signatures to quantify key cloud microphysical quantities, including liquid water content and effective droplet diameter. In addition, the polarimetric measurements offer high sensitivity for discriminating ice from liquid particles, which is crucial for understanding phase-partitioning in mixed-phase clouds.

We report on the progress in bringing this prototype version of the HACP from concept to reality. Our presentation overviews its novel design and summarizes calibration metrics and performance benchmarks from laboratory characterization. We highlight results from an initial deployment in a controlled, cloud-simulating wind tunnel environment, demonstrating the first retrievals of high-fidelity, complete scattering matrix measurements for liquid droplet clouds. Subsequent evaluation of these measurements against scattering calculations based on Mie theory validates the instrument's measurement principle and demonstrates its readiness for synergistic cloud physics research across laboratory and field domains.

This prototype establishes a pathway towards a new benchmark for satellite validation or even serving as a reference standard at operational monitoring networks. Its direct, high-resolution measurements enable rigorous validation of remote sensing algorithms and assumptions. Ultimately, this work contributes to more accurate polarimetric scattering measurements of clouds, facilitating improved constraints on quantitative microphysical estimates, and an advanced understanding of small-scale cloud physics.