First Direct Observations of Internal Flow Structures in a Powder Snow Avalanche: Turbulence, Instability and Particle Distribution

Abstract

Powder snow avalanches are highly dynamic, multiphase gravity-driven flows typically composed of a dense basal layer overlain by airborne layers in which snow particles are suspended within a turbulent air phase. Despite extensive work on related systems such as pyroclastic density currents and turbidity currents, all gravity current communities face a fundamental limitation: the lack of direct, high-resolution particle-scale field data. Here, we present the first direct optical observations of individual particle motion inside the airborne layers of a natural powder snow avalanche using high-speed imaging. The flow is segmented into three regions: an initial short living surge, a highly dynamic suspension phase, and a final wake. Across these phases, we quantify flow velocity and turbulence characteristics, including integral length scales, and use image intensity as a proxy for particle concentration. We identify fluctuations exceeding the turbulent integral scale and use linear stability analysis to link them to Kelvin-Helmholtz-type shear instabilities. Observed changes in clustering behavior, stratification, and the decoupling of particles from the flow mark the transition from an unstable suspension layer characterized by a high level of turbulent activity to a stable one dominated by passive snow settling. Together, these findings provide the first empirical constraints on turbulence and instability dynamics in airborne avalanche layers, with direct implications for the refinement of numerical avalanche models and closure schemes in multiphase gravity current simulations.

Figures (13)

• Figure 1: a) Artificially released PSA at VdlS from winter 2015, shown for illustration only. The approximate release area of the studied avalanche # 20243024 is highlighted. b) A PSA reaches the pylon (Photo: PhotoTerre et Nature O. Born). c) Overview of the VdlS experimental site. The black line outlines the boundaries of the avalanche analyzed in this study. The red shaded area represents the region observed by the GEODAR system. Red dashed lines within this area indicate reference positions ($d_i$) used to relate GEODAR measurements to flow velocity and terrain location (see Fig. \ref{['fig:avaVelocity']}). The red dot marks the location of the bunker housing the GEODAR. The original map is obtained from the Swiss Geoportal (2025) in Swiss coordinate system CH1903 / LV03 (EPSG:21781). (d) A 20 m high steel pylon located in the runout zone (black rectangle in panel c) is equipped with three high-speed cameras (red boxes) mounted inside protective hatches. A LED panel on the pylon projects a light sheet (green area) perpendicular to the cameras' field of view. Optical sensors (yellow marks) at the base of the pylon measure flow velocity.
• Figure 2: Image processing workflow for velocity and particle size extraction. (a) Preprocessed image after background subtraction and contrast enhancement, used as input for further analysis. (b) Resulting velocity field obtained by PIV. Green arrows indicate particle displacements converted to velocities; the yellow box marks the vertical band used for subsequent high-resolution temporal analysis. (c) Detected particles with sizes estimated from fitted ellipses (green). The yellow rectangle shows a zoomed region with particle identification.
• Figure 3: (a) MTI plot from the GEODAR of avalanche #20243024. (b) Front velocities extracted from the MTI plot. Dashed red lines denote the positions in the avalanche path as shown in Fig. \ref{['fig:method']}. Colored lines distinguish the separate surges in both the MTI and front velocity plot.
• Figure 4: a) Zoomed section of the GEODAR derived MTI plot focused around the position of the pylon (horizontal black line). b) Corresponding velocities measured by the optical sensors on the pylon. The avalanche slid on an old snow cover up to 1.5 m thick.
• Figure 5: Space-time visualization of the avalanche flow recorded by the high-speed camera array. Each panel shows a stitched sequence of central vertical strips (1 m in height) from three cameras, creating a panoramic view from 5 to 11 m above ground. The upper image provides a zoomed-in view of the initial flow, highlighting the early surge. Black horizontal spacers indicate gaps between the cameras’ fields of view, corresponding to vertical regions not captured by the cameras. A high-resolution version of this figure is provided in the auxiliary material, allowing detailed inspection by zooming.