Ivan Calic

Powder snow avalanches (PSAs) radiate infrasound energy, yet the source mechanism remains unclear, limiting hazard monitoring and mitigation with infrasound-based technologies. Here, we analyze a unique data set from a large PSA to improve the understanding of the source mechanism. Through comparison of cluster activity within the airborne layers of the PSA and the recorded infrasound signal in the frequency domain, we demonstrate that infrasound is mainly generated from particle clusters suspended by turbulent eddies or ejected from the denser basal layer. Further correlating infrasound amplitudes with radar-derived spatial distributions of these clusters, we reveal a distributed source extending hundreds of meters behind the avalanche front. Additionally, we establish a relationship between infrasound and kinetic energy of suspended particles. These findings deepen our understanding of the complex dynamics of infrasound generation, offering valuable insights for avalanche detection and early warning strategies, and fundamental comprehension of PSA dynamics.

Powder snow avalanches (PSAs) pose significant threats to human lives and infrastructure in mountain regions. Measuring inside these natural flows is challenging because of their destructive power, which limits our understanding of these events. Previous research revealed a complex layering, characterized by a superposition of three distinct layers. The dense basal layer, where particle-particle interactions are  ominant, is situated below two aerial layers: the suspension and transition layer, both of which are particle laden turbulent flows. In contrast to the suspension layer, the transition layer hosts a higher amount of suspended mass and intense clustering processes, resulting in high local density fluctuations. Up to now technological limitations have prevented measurements within the dilute flows, especially the inability to capture turbulence at large scales. This lack of direct observations of dilute layers complicates model development and inhibits accurate predictions of PSAs’ destructive impacts.
To address this gap, we developed a non-invasive measurement technique consisting of a high-speed camera array. We installed this array on a vertical structure at the Vallée de la Sionne test site in Switzerland,  robing the transition and suspension layers of fully developed PSAs. The new array consists of three high-speed cameras and captures images at mm resolution from 5 m up to 11 m above ground.
On December 2nd, 2023, we measured the first PSA using the new setup. The collected images show individual suspended snow particles inside the transition and suspension layers, revealing complex air-particle dynamics. With image processing algorithms, including particle image velocimetry, we extracted essential flow characteristics such as the vertical velocity profiles.
This research paves the way for understanding the destructive potential of PSAs, but also lays the basis for improving prediction models and mitigation strategies. Additionally, it offers intriguing images from the inside of a PSA.

We investigate the response of a Ramsey-type experiment on an additional oscillating magnetic field. This superimposed field is oriented in the same direction as the static main magnetic field and causes a modulation of the original Larmor spin precession frequency. The observable magnitude of this modulation decreases at higher frequencies of the oscillating field. It disappears completely if the interaction time of the particles matches the oscillation period, which we call resonant cancellation. We present an analytical approach that describes the effect and compare it to a measurement using a monochromatic cold neutron beam.