Uncovering the effect of morphology on snow particle settling dynamics under mild wind conditions

Natural snowflakes, with crystal categories including dendrites, plates, needles, graupels, and aggregates, exhibit complex settling kinematics due to their intricate shapes and atmospheric turbulence. To elucidate the impact of snow crystal shapes and atmospheric turbulence on these dynamics, we conduct a comprehensive study under various field conditions using a three-dimensional particle tracking velocimetry (3D PTV) system and a digital inline holography (DIH) system with a high-precision scale. These tools allow us to measure particle settling trajectories and simultaneously characterize snow size, shape, and density, thereby estimating particle aerodynamic properties. The current work focuses on the 3D kinematics of snow settling under low turbulence, where snow-shape effects are prominent. Our findings reveal diverse settling behaviors across crystal categories. Notably, we observed periodic fluctuations of the spanwise acceleration, suggesting the meandering motion of measured snow particle trajectories. The oscillation frequencies and amplitudes cover a wide range of scales and vary across crystal categories, with dendrites exhibiting the smallest frequency and largest amplitude and graupels showing the opposite. These variations likely depend on the particles' aerodynamic properties (e.g., drag coefficient, particle response time) and turbulence characteristics. Based on shape alone, needles settle faster than aggregates, followed by graupels and dendrites.