Evidence of preferential sweeping during snow settling in atmospheric turbulence

Abstract

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 centred 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. 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. Super-large-scale particle image velocimetry measurements show a higher concentration of snow particles preferentially located on the downward side of the vortices identified in the atmospheric flow field. Particle tracking velocimetry, 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.

How does two-way coupling modify particle settling and the role of multiscale preferential sweeping?

“…This preferential sweeping effect was observed and confirmed using DNS in Wang & Maxey (1993). Several subsequent numerical and experimental studies provided confirmation of settling enhancement due to the preferential sweeping effect (Bec, Homann & Ray 2014;Ireland et al 2016b;Rosa et al 2016;Petersen et al 2019;Li et al 2021b). In the original study of Maxey (1987), the preferential sweeping mechanism was described theoretically for particles with weak inertia, i.e.…”

“…It is also worth mentioning that exciting new field experiments have provided evidence for settling enhancement due to turbulence in diverse naturally occurring flows (Nemes et al 2017;Li et al 2021a). Moreover, Li et al (2021b) provided direct evidence that this enhancement is, in fact, associated with the preferential sweeping mechanism. Since these field experiments could be impacted by 2WC, the results may suggest that the preferential sweeping mechanism still applies in that regime.…”

“…2017; Li et al. 2021 a , b ), both of which inherently have 2WC effects, have also observed an enhancement of settling velocity compared with the Stokes settling velocity. Hence, there is a need to explore the physical mechanism(s) responsible for generating and whether there are different mechanisms at play for the 1WC and 2WC systems.…”

“…Moreover, Li et al. (2021 b ) provided direct evidence that this enhancement is, in fact, associated with the preferential sweeping mechanism. Since these field experiments could be impacted by 2WC, the results may suggest that the preferential sweeping mechanism still applies in that regime.…”

“…2019; Li et al. 2021 b ). In the original study of Maxey (1987), the preferential sweeping mechanism was described theoretically for particles with weak inertia, i.e.…”