Infrasound propagation in the Arctic

Abstract

This report summarizes results of the basic research project “Infrasound Propagation in the Arctic.” The scientific objective of this project was to provide a baseline understanding of the characteristic horizontal propagation distances, frequency dependencies, and conditions leading to enhanced propagation of infrasound in the Arctic region. The approach emphasized theory and numerical modeling as an initial step toward improving understanding of the basic phenomenology, and thus lay the foundation for productive experiments in the future. The modeling approach combined mesoscale numerical weather forecasts from the Polar Weather Research and Forecasting model with advanced acoustic propagation calculations. The project produced significant advances with regard to parabolic equation modeling of sound propagation in a windy atmosphere. For the polar low, interesting interactions with the stratosphere were found, which could possibly be used to provide early warning of strong stratospheric warming events (i.e., the polar vortex). The katabatic wind resulted in a very strong low-level duct, which, when combined with a highly reflective icy ground surface, leads to efficient long-distance propagation. This information is useful in devising strategies for positioning sensors to monitor environmental phenomena and human activities.

Equations for finite-difference, time-domain simulation of sound propagation in moving media with arbitrary Mach numbers

“…Starting with the linearized equations of fluid dynamics, a new set of two coupled equations for the sound pressure and acoustic particle velocity was derived, Eqs. ( 20) and (21). These equations have a wide range of applicability: they are valid for arbitrary Mach numbers, in the high-frequency approximation, and for arbitrary frequencies, for low Mach numbers.…”

“…( 20) and ( 21) can be done using the staggered-in-space and collocated-in-time numerical grids, with a minimal modification of the existing algorithms. These equations can be used in FDTD calculations and other methods for sound propagation in the near-ground atmosphere, aeroacoustics, and infrasound propagation in the upper atmosphere (the high-frequency approximation is routinely used in the infrasound research [20,21,22]).…”

“…( 6) and ( 17), ∇P and ∇ • v can be omitted and these equations reduce to Eqs. ( 20) and (21). Thus, Eqs.…”

“…Equations (20) and (21) are derived in the high-frequency approximation and are valid for arbitrary Mach numbers M = v/c, where v = |v|. Interestingly, these equations are also valid for arbitrary frequencies, for low Mach numbers (M 1).…”

“…Despite these similarities, it is important to emphasize that a new set of Eqs. ( 20) and (21) for p and w is valid for arbitrary Mach numbers M (in the high-frequency approximation), while the old set of Eqs. ( 21) and ( 23) is valid only for M 1.…”