Possible source mechanisms for a frequently occurring infrasonic signal

Abstract

Commonly occurring infrasonic waves recorded during the winter months are believed to originate by means of aerodynamic source mechanisms. Acoustic power spectra of these commonly observed signals are compared to the power spectra one would observe from theoretical source mechanisms. From this comparison, the theoretical source mechanism believed responsible for production of the observed signals is thought to be isotropic turbulence in the lee of mountain peaks. Experimentally recorded signals and their power spectra are shown. Source regions of three signals are identified.

Mountain-Associated Waves and their relation to Orographic Gravity Waves

“…Bedard (1978) combined infrasound observations using sensors in the Rocky Mountains (USA) and aircraft observations. The latter were supposed to support the theory of air turbulence being a source of MAW excitation, which was also proposed by Thomas et al (1974) before. However, Rockway et al (1974) remarked that the effect of atmospheric conditions on the propagation and detection of MAWs might have been underestimated in previous theories.…”

“…As a result of thermodynamic instabilities in the mesosphere causing the GWs to break, these produced upstream-and downstreampropagating acoustic waves. Previously, Thomas et al (1974) had rejected the theory of breaking lee waves being involved in the MAW production, which relied upon the evaluation of power spectra slopes of selected MAW events. Following the findings of Lund et al (2018), the valve layers over New Zealand or the Tibetan Plateau could also be altitude layers where MAWs are excited as a result of breaking stationary OGWs.…”

“…According to Campbell and Young (1963), auroral activity was known to produce sound in this frequency range (see also Wilson et al, 2010), but Cook (1969) found, as a result of triangulation, that his observations traced back to mountainous regions (Larson et al, 1971). Therefore, these acoustic waves have been referred to as mountain-associated sound (Chimonas, 1977) or, more commonly, as MAWs (Larson et al, 1971;Rockway et al, 1974;Thomas et al, 1974;Greene and Howard, 1975;Bedard, 1978). Larson et al (1971) used data of three sites in the USA -in Alaska, Colorado, and Idaho -and measured amplitudes of 0.05 Pa to 0.7 Pa.…”