| 14 Aug 2026
Single-Particle Polarization Tomography Enables Highly Accurate Phase Identification of Cloud Particles
Abstract. The phase state of cloud particles and their transformation processes are fundamental to understanding cloud and precipitation formation, variations in radiative energy budgets, and the evolution of severe weather systems. At present, for small-scale cloud particles smaller than 50 μm, conventional identification methods based on morphological features are readily constrained by imaging resolution and feature overlap, making high-accuracy phase discrimination challenging. To address this issue, we propose a polarization tomographic imaging method that integrates digital holography and polarimetric imaging, enabling the simultaneous acquisition of morphological parameters of individual particles within a particle ensemble, including three-dimensional coordinates, particle size, area, perimeter, the major and minor axes of the minimum-area bounding rectangle, and circularity, together with polarization parameters. Through observation experiments in an ice cloud chamber, liquid droplet and ice crystal samples smaller than 50 μm were analyzed. By combining morphological and polarization parameters, accurate identification of liquid droplets and ice crystals in mixed-phase particles was achieved, with an accuracy of 98.33 %, representing an improvement of 4.16 percentage points over the conventional circularity-based identification method. In addition, a continuous 10 s observation window during the late stage of mixed-phase cloud glaciation in the ice cloud chamber experiment was selected to calculate liquid water content, the number concentrations of liquid droplets and ice crystals, and their particle-number fractions. The results show that the droplet number concentration and liquid water content decrease with time, whereas the relative fraction of ice crystals gradually increases, reflecting the microphysical process of liquid water conversion into ice in a mixed-phase environment. This method can not only effectively improve the accuracy of phase identification for small-scale cloud particles but also provide information on the microphysical evolution of cloud particles, offering important support for studies of cloud microphysical processes, improvement of model parameterizations, weather modification, and early warning of severe weather events.
Mixed phase clouds are important constituents of the atmosphere, for instance with respect to radiation effects and precipitation generation. Their microphysical characterization, which is crucial for modelling and forecasting, is nevertheless difficult, mostly due to the different phases of hydrometeors in the cloud. Since the interplay between particles of different phases of water, i.e. water droplets and ice particles, governs the most interesting microphysical processes, distinguishing between liquid and solid when measuring the particles is crucial. In the manuscript, Chen et al. present a digital holography method integrating polarization tomographic imaging that is applied for phase identification of small particles in a mixed phase cloud environment. The method is very promising, and the topic doubtlessly fits within the scope of Atmospheric Measurement Techniques.
The paper is in general well written, easy to follow but contains some repetitions. The figures are of good quality. I list some points that should be considered before publication:
Major points
If I understand correctly, the authors introduce their method and an instrument based on that method. As an example of applications, they show some results of measurements in a cloud chamber. First, the experiments for instrument characterization and testing should be described in more detail and in a distinguishable manner. I recommend moving the description of the cold chamber to the main part of the manuscript, to the Experimental System section. Second, it is mentioned that glass beads were used for size and polarimetric characterization. Do they work for water and ice? If yes, why? If not, which correction should have been applied? Third, concentration, size, LWC, etc. have been measured, but these were not compared to other measurement techniques. It would be important to present such an intercomparison; however, I understand that the authors might only want to show one application for which their method/instrument can be employed. If this latter is indeed the case, it should be emphasized in the paper.
Minor points