Sound propagation through and scattering by internal gravity waves in a stably stratified atmosphere

Abstract

A stably stratified atmosphere supports propagation of internal gravity waves ͑IGW͒. These waves result in highly anisotropic fluctuations in temperature and wind velocity that are stretched in a horizontal direction. As a result, IGW can significantly affect propagation of sound waves in nighttime boundary layers and infrasound waves in the stratosphere. In this paper, a theory of sound propagation through, and scattering by, IGW is developed. First, 3D spectra of temperature and wind velocity fluctuations due to IGW, which were recently derived in the literature for the case of large wave numbers, are generalized to account for small wave numbers. The generalized 3D spectra are then used to calculate the sound scattering cross section in an atmosphere with IGW. The dependencies of the obtained scattering cross section on the sound frequency, scattering angle, and other parameters of the problem are qualitatively different from those for the case of sound scattering by isotropic turbulence with the von Kármán spectra of temperature and wind velocity fluctuations. Furthermore, the generalized 3D spectra are used to calculate the mean sound field and the transverse coherence function of a plane sound wave propagating through IGW. The results obtained also significantly differ from those for the case of sound propagation through isotropic turbulence.

The Temporal Morphology of Infrasound Propagation

“…One possibility as already demonstrated by similar research is the example of the analysis of the multiyear time series of volcanic observations at I22FR (LE PICHON et al, 2005). Lastly for robust event location and screening of automated event bulletins by human analysts (BROWN et al, 2002a), we recommend that detailed propagation modeling techniques that account for range dependence (GOSSARD and HOOKE, 1975), topography (ARROWSMITH et al, 2007), and other effects such as interval gravity waves (OSTASHEV et al, 2005;CHUNCHUZOV, 2004) be utilized. This paper has presented time series of infrasound propagation characteristics.…”

“…These fast modes can be seen near the transition from the stratospheric to the thermospheric ducts for look directions to the west. Several limitations resulting from the various approximations in the sp method such as the shortcomings of linear ray-tracing theory (MILLET et al, 2007), the lack of explicit range dependence (DROB et al, 2003), and the influence of internal scattering by internal gravity waves (CHUNCHUZOV, 2004;OSTASHEV et al, 2005) are noteworthy but beyond the scope of the present discussion. With the doubling of processing capacities every few years following Moore's Law, more complex calculations that account for range-dependent variations in the background atmosphere should be investigated and eventually implemented in automated DLC algorithms.…”

Modeling propagation of infrasound signals observed by a dense seismic network

“…The corresponding spectrum of the vertical gradient of such fluctuations shows much slower power law decay ($k À1 z ) and contains the fluctuations with vertical scales of a few hundreds of meters. The latter are comparable to the wavelengths of infrasound waves in the frequency range 0.8-3 Hz, and therefore cause wave effects such as scattering of the infrasound field (Chunchuzov et al, 2011;Ostashev et al, 2005). Although ray-trace calculations through a perturbed atmosphere predict most of the observed acoustic-seismic coupled arrivals in the shadow zones the waveforms of these arrivals may be explained by taking into account wave effects that become essential in the presence of nonhomogeneities with scales comparable to the wavelengths of infrasound waves.…”

On the nonlinear shaping mechanism for gravity wave spectrum in the atmosphere

“…59-60) shows the existence of highly anisotropic spatial inhomoigeneities in the displacement and temperature fields, whose anisotropy depends on the value of e 0 1. This spectrum was used by Chunchuzov (2003, 2005) for explaining the spectra of stellar scintillations observed from space, and by Ostashev et al (2005) for modelling of a scattering of the acoustic waves from anisotropic wind speed and temperature inhomogeneities caused by internal wave field in the atmosphere.…”

“…Beside this, the knowledge of the space-time spectrum of the wave-induced temperature and wind speed fluctuations is needed for the prediction of the statistics of the amplitude and phase fluctuations of low-frequency acoustic waves (Chunchuzov et al, 2006;Ostashev et al, 2005) and electromagnetic waves Chunchuzov, 2003, 2005), propagating through a realistic atmosphere.…”