Applicability of numerical two-layer depth-averaged models for pyroclastic density currents to powder snow avalanches: Toward developing a unified model for powder snow avalanches and pyroclastic density currents

Abstract

Powder snow avalanches and pyroclastic density currents (PDCs) are particle-gas gravity currents with stratification of particle concentrations. This paper reviews an existing two-layer PDC model and discusses the applicability of two-layer PDC models to powder snow avalanches. In two-layer PDC models, the upper region of PDCs is modeled as a dilute turbulent suspension flow and the lower region is modeled as a dense fluidized granular flow. Considering the interactions between these flows (e.g., particle transfer from one flow to the other), the two-layer PDC models can evaluate the flowing and stopping of PDCs. In particular, to evaluate the run-out distance of the dilute current of PDCs (i.e., the process in which the hot dilute current becomes lighter than the ambient air and lifts off the ground), the effect of thermal expansion of the ambient air entrained into the current is taken into account. Many physical processes of PDCs are common to powder snow avalanches. Unlike for PDCs, however, the run-out distance of the dilute current of the snow avalanches can be explained by the physical process of the fall-out of all particles in the dilute current due to decreased turbulent velocities in the flow. Incorporating this physical process into the two-layer PDC models may allow us to construct a unified model for powder snow avalanches and PDCs.