  We present a field study of snow settling dynamics based on simultaneous
measurements of the atmospheric flow field and snow particle trajectories.
Specifically, a super-large-scale particle image velocimetry (SLPIV) system
using natural snow particles as tracers is deployed to quantify the velocity
field and identify vortex structures in a 22 m $\times$ 39 m field of view
centered 18 m above the ground. Simultaneously, we track individual snow
particles in a 3 m $\times$ 5 m sample area within the SLPIV using particle
tracking velocimetry (PTV). The results reveal the direct linkage among vortex
structures in atmospheric turbulence, the spatial distribution of snow particle
concentration, and their settling dynamics. In particular, with snow turbulence
interaction at near-critical Stokes number, the settling velocity enhancement
of snow particles is multifold, and larger than what has been observed in
previous field studies. SLPIV measurements show higher concentration of snow
particles preferentially located on the downward side of the vortices
identified in the atmospheric flow field. PTV, performed on high resolution
images around the reconstructed vortices, confirms the latter trend and
provides statistical evidence of the acceleration of snow particles, as they
move toward the downward side of vortices. Overall, the simultaneous
multi-scale particle imaging presented here enables us to directly quantify the
salient features of preferential sweeping, supporting it as an underlying
mechanism of snow settling enhancement in the atmospheric surface layer.
