Measurement of snow settling dynamics using 3D particle tracking velocimetry*

Understanding the dynamics of snow settling is important for improving our ability to predict ground snow accumulation, snow water equivalent, and surface thermal properties in a number of applications. Following our previous work using planar particle tracking velocimetry (PTV) to quantify snow settling dynamics in atmospheric surface layer (Nemes et al. JFM, 2017; Li et al., JFM, 2021), we present a further investigation of the problem with large-scale field 3D PTV system composed of four WiFi-synchronized cameras using drone-based calibration approach. The field deployment was conducted on April 17^th, 2022 at the field research station to capture 3D snow settling motions in a sampling volume of 4 m x 4 m x 6 m with 6.4 mm and 200 Hz spatial and temporal resolution, respectively. Our study provides a detailed geometric characterization of snow settling trajectories and reveals the highly variable and meandering trajectories of snow particles even under a low level of turbulence. The acceleration of individual snowflake yields a strong correlation with the curvature of particle trajectory and its statistics shows a strong anisotropic behavior under the influence of turbulence and snow particle morphology. Our Voronoi analysis shows evidence of 3D clustering of snow particles.