Impact of snow morphology and atmospheric turbulence on snow settling behavior*

Measuring and predicting snow settling velocity is essential for improving ground snow accumulation estimates in weather forecast models. Both variability in snowflake morphology (dendrites, plates, needles, graupels, aggregates) and atmospheric turbulence significantly influence the settling velocity. To elucidate these mechanisms, we conducted a comprehensive study under various field conditions deploying a three-dimensional particle tracking velocimetry system and a snow particle analyzer. These tools allowed us to measure particle settling trajectories, characterize snowflake size, shape, and density, and estimate their aerodynamic properties. Our previous work on snow settling in mild turbulence highlighted the significant impact of non-spherical snow shapes in modulating the settling velocity. We observed pronounced meandering motions in dendrites and aggregates, which affects their trajectories curvature. These meandering motions also lead to orientation changes of snow particles, causing reduced average projected area and enhanced settling velocity. Analyzing settling trajectories under more intense atmospheric turbulence, we observed converging acceleration statistics across various snowflake morphologies, and a more pronounced turbulence effect on the particle fall speed. Our findings offer key insights for modeling snow settling velocity, considering morphology and turbulence impacts determined by settling parameter (Sv_L) and Stokes number (St_η).