Settling and Clustering of Snow Particles in Atmospheric Turbulence

Abstract

The effect of turbulence on snow precipitation is not incorporated into present weather forecasting models. Here we show evidence that turbulence is in fact a key influence on both fall speed and spatial distribution of settling snow. We consider three snowfall events under vastly different levels of atmospheric turbulence. We characterize the size and morphology of the snow particles, and we simultaneously image their velocity, acceleration, and relative concentration over vertical planes about 30 m2 in area. We find that turbulence-driven settling enhancement explains otherwise contradictory trends between the particle size and velocity. The estimates of the Stokes number and the correlation between vertical velocity and local concentration indicate that the enhanced settling is rooted in the preferential sweeping mechanism. When the snow vertical velocity is large compared to the characteristic turbulence velocity, the crossing trajectories effect results in strong accelerations. When the conditions of preferential sweeping are met, the concentration field is highly non-uniform and clustering appears over a wide range of scales. These clusters, identified for the first time in a naturally occurring flow, display the signature features seen in canonical settings: power-law size distribution, fractal-like shape, vertical elongation, and large fall speed that increases with the cluster size. These findings demonstrate that the fundamental phenomenology of particle-laden turbulence can be leveraged towards a better predictive understanding of snow precipitation and ground snow accumulation. They also demonstrate how environmental flows can be used to investigate dispersed multiphase flows at Reynolds numbers not accessible in laboratory experiments or numerical simulations.

arXiv comment22 pages, 13 figures

References in corpus (18)

• Fluid Particle Accelerations in Fully Developed Turbulence
• Measurement of Particle Accelerations in Fully Developed Turbulence
• Size-selective concentration of chondrules and other small particles in protoplanetary nebula turbulence
• Experimental Lagrangian Acceleration Probability Density Function Measurement
• Acceleration statistics of heavy particles in turbulence
• Statistical models for spatial patterns of heavy particles in turbulence
• The effect of Reynolds number on inertial particle dynamics in isotropic turbulence. Part I: Simulations without gravitational effects
• Experimental study of inertial particles clustering and settling in homogeneous turbulence
• The effect of Reynolds number on inertial particle dynamics in isotropic turbulence. Part II: Simulations with gravitational effects
• Dimensionality and morphology of particle and bubble clusters in turbulent flow
• Acceleration of heavy and light particles in turbulence: comparison between experiments and direct numerical simulations
• Preferential concentration of inertial sub-kolmogorov particles. The roles of mass loading of particles, Stokes and Reynolds numbers
• Observation of the sling effect
• Microbubbles and microparticles are not faithful tracers of turbulent acceleration
• Multiscale preferential sweeping of particles settling in turbulence
• The fall speeds of sub-100 micron ice crystals
• Small-scale dynamics of settling, bidisperse particles in turbulence
• Local analysis of the clustering, velocities and accelerations of particles settling in turbulence

Cited by in corpus (3)

• Rising and Sinking in Resonance: Probing the critical role of rotational dynamics for buoyancy driven spheres
• Imaging-based 3D Particle Tracking System for Field Characterization of Particle Dynamics in Atmospheric Flows
• Investigating the parametric dependence of the impact of two-way coupling on inertial particle settling in turbulence