Direct imaging of snow settling around turbulent eddies in the atmospheric surface layer

Turbulence-induced settling enhancement of inertial particles has been observed in the lab, simulations, and recent field measurements of snow particles (Nemes et al. JFM 2017; Li et al. JFM 2021). However, there is still no direct imaging of interaction between turbulent eddies and snow particles in the field. Here we present the first simultaneous field measurements of atmospheric flow using PIV (20 m wide x 40 m tall sample area) and snow particle trajectories using PTV (3 m x 5 m within the PIV domain). Such measurements show a preferential concentration of snow particles on the downward side of both retrograde and prograde vortices in the flow, with, in particular, the concentrated snow near the bottom of retrograde vortices. Moreover, snow particles are found to accelerate as they move toward the downward side of vortices and decelerate or even be lifted on the upward side. Additionally, we observe a stronger settling enhancement (i.e., a larger difference of settling velocity between downward and upward sides of vortices) by prograde vortices than that by retrograde vortices. Overall, our measurements provide direct field evidence and underlying physical processes of turbulence-induced snow settling enhancement in the atmospheric surface layer.