Unraveling snow settling dynamics: a field study on the effects of snow morphology and atmospheric turbulence

Abstract

Accurate modeling of ground snow accumulation is important for applications such as snow hazard warnings, hydrology, and traffic regulation during snow events. However, current predictions often fall short due to an incomplete understanding of how snow particle morphology, density, and atmospheric turbulence influence snow settling dynamics. While laboratory experiments and numerical simulations have explored the impact of turbulence on particles with simple geometries, few studies have addressed the complexities of real-world conditions, where a wide range of turbulence scales and intricate snow particle shapes come into play. To bridge this gap, field investigations are essential for enhancing the understanding of snow settling behavior in the atmosphere and improving predictive models for ground snow accumulation. This study investigates the influence of snow morphology and atmospheric turbulence on snow particle settling behavior, including settling velocity and spatial distribution. A snow particle analyzer, incorporating a digital inline holography system and a high-precision scale, is used to simultaneously measure snow particle morphology (i.e., size, shape, type) and density, which are critical for predicting the aerodynamic properties and terminal velocity of snow particles. Additionally, planar and three-dimensional imaging techniques, such as super-large-scale particle image velocimetry (SLPIV), large-scale particle tracking velocimetry (LSPTV), and 3D particle tracking velocimetry (3D PTV), are employed to visualize snow settling dynamics and capture atmospheric turbulence in the field. These methods were deployed during selected snow events at the EOLOS field research station in Rosemount, MN, alongside measurements of atmospheric turbulence by a meteorological tower. The field experiments yield significant insights into how morphology and turbulence affect snow settling dynamics. Specifically, measurements from the snow particle analyzer have contributed to the understanding of the relationship between snow particle morphology and density, which is later used for the estimation of snow aerodynamic properties and terminal velocity. In addition, simultaneous planar measurement techniques (SLPIV and LSPTV) have provided direct evidence of preferential sweeping of snow particles by atmospheric turbulence, with higher concentration and enhanced settling velocity observed on the downward side of both prograde and retrograde vortices. Furthermore, the 3D PTV study under weak turbulence has demonstrated a significant impact of snow particle morphology on their meandering behavior (magnitude and frequency), which in turn influences their settling velocity. Last but not least, 3D PTV measurements under moderate and high turbulence have offered deeper insights into how turbulence affects the settling velocity of snow particles with varying morphologies. These findings contribute valuable information for enhancing the accuracy of snow settling velocity models in winter weather forecasting.