An investigation of infrasound propagation over mountain ranges

Abstract

Linear theory is used to analyze trapping of infrasound within the lower tropospheric waveguide during propagation above a mountain range. Atmospheric flow produced by the mountains is predicted by a nonlinear mountain gravity wave model. For the infrasound component, this paper solves the wave equation under the effective sound speed approximation using both a finite difference method and a Wentzel-Kramers-Brillouin approach. It is shown that in realistic configurations, the mountain waves can deeply perturb the low-level waveguide, which leads to significant acoustic dispersion. To interpret these results, each acoustic mode is tracked separately as the horizontal distance increases. It is shown that during statically stable situations, situations that are common during night over land in winter, the mountain waves induce a strong Foehn effect downstream, which shrinks the waveguide significantly. This yields a new form of infrasound absorption that can largely outweigh the direct effect the mountain induces on the low-level waveguide. For the opposite case, when the low-level flow is less statically stable (situations that are more common during day in summer), mountain wave dynamics do not produce dramatic responses downstream. It may even favor the passage of infrasound and mitigate the direct effect of the obstacle.

Direct numerical simulation of stratified Ekman layers over a periodic rough surface

“…These characteristics not only change the atmospheric dispersion of tracers and pollutants, as has been well documented in the past, but also, as discussed in recent work, strongly influence acoustic propagation in the nocturnal boundary layer (Talmadge et al. 2008) with this influence being modified by hilly terrain (Damiens, Millet & Lott 2018).…”

“…The simulation results are related to meteorological characteristics that are distinctive features of the stable ABL: low-level jets (Smedman, Tjernström & Högström 1993;Cuxart et al 2000;Banta et al 2002), collapse of surface-layer turbulence (Banta et al 2007), local turbulence peaks at locations above the surface layer (Mahrt 1985;Smedman et al 1993;Banta et al 2007) and unsteadiness of turbulence statistics (Banta 2008;Pichugina et al 2008;Sun et al 2012). These characteristics not only change the atmospheric dispersion of tracers and pollutants, as has been well documented in the past, but also, as discussed in recent work, strongly influence acoustic propagation in the nocturnal boundary layer (Talmadge et al 2008) with this influence being modified by hilly terrain (Damiens, Millet & Lott 2018).…”

A New Crater Near InSight: Implications for Seismic Impact Detectability on Mars

“…Though an impulsive pressure signal could be expected from a close impact event, the long duration HF pressure wave packet we observed is similar to scattered pressure signals related to explosions seen in infrasound records on Earth (Green et al, 2011). Such scatterings of acoustic energy can occur when small‐scale gravity waves perturb the lower atmosphere wave guide (e.g., Damiens et al, 2017; Green et al, 2011). Nonetheless, other facts concur to put aside the impact hypothesis as a source of the observed pressure fluctuations: (1) High‐frequency pressure fluctuations are recorded by InSight almost every sol in the evening, (Banfield et al, 2020); (2) acoustic propagation models (Garcia et al, 2017; Spiga et al, 2018) show that the InSight lander is in a shadow zone for infrasound waves generated at the impact location (Figure S11); and (3) owing to the noise levels of the respective instruments, if infrasound signals were seen in InSight pressure data, they would also be seen in the seismic data.…”

Mountain-Associated Waves and their relation to Orographic Gravity Waves

“…Concerning feedback mechanisms within turbulent flows, the impact of OGWs on the acoustic wave field is of great interest. Damiens et al (2018) have addressed this topic by modeling the effect of tropospheric winds, OGWs, and low-altitude critical levels on the sound propagation in mountainous regions. Sabatini et al (2019) have recently investigated the infrasound propagation through turbulent layers caused by breaking OGWs.…”

“…Moreover, it is noted that single detections could result from small-scale fluctuations; for instance, upward-propagating GWs could temporarily establish a ground-to-stratosphere waveguide if such perturbations of the wind speed sufficiently increase the effective sound speed ratio in the upper stratosphere. Note that, for the troposphere, Damiens et al (2018) also modeled an impact of OGWs and tropospheric winds on the acoustic wave field in mountainous regions. However, the high number of signals in winter would be more reasonable if the explanation can be found in the source generation mechanism.…”