Advancement of Techniques for Modeling the Effects of Atmospheric Gravity-Wave-Induced Inhomogeneities on Infrasound Propagation.

摘要

Propagation studies of a growing number of infrasound observations indicate that fine-scale atmospheric inhomogeneities contribute to infrasonic arrivals that are not predicted by standard modeling techniques. In particular, gravity waves, or buoyancy waves, are believed to contribute to the multipath nature of infrasound propagation and to cause penetration of infrasound into the classical shadow zones that are predicted by conventional modeling techniques. Propagation modeling studies using basic parameterizations of gravity wave spectra suggest that gravity waves represent the primary component of fine-scale atmospheric inhomogeneity that affects infrasonic arrivals at regional ranges. The influence of atmospheric gravity waves on the upper and middle atmosphere provides a significant source of uncertainty in atmospheric specifications. A large fraction of the gravity wave spectrum in operational numerical weather prediction models is either filtered out during the data assimilation process or else not resolved by the models. Prior approaches to modeling infrasound through gravity waves have relied on one-dimensional vertical wavenumber spectral models of gravity waves; this simplified model approach captures the vertical spatial scales in gravity waves as a function of height. Our recent research has developed improved resolution of these wave fields through more sophisticated computational techniques to achieve more complete spectral parameterization. Atmospheric specification techniques have been developed that incorporate realistic models of gravity waves that are self-consistent with the background flow field and that include effects of altitude, range-dependence, and time-dependence over relevant scales. One shortcoming of prior gravity wave field models recently used with infrasound propagation models is that they ignore the typically strong refraction of the background gravity waves by variations in the mean winds and mean stratification above 15 km. These limitations are addressed in this research by including the refraction effects of the gravity wave field by the background atmosphere as defined by the Naval Research Laboratory ground-to-space (NRL-G25) semi-empirical specification. A local atmospheric gravity wave field is represented using the summation of vertical eigenfunctions approximated by a computationally efficient Fourier-space ray-tracing technique. Using this methodology, wave perturbation fields (horizontal wind components, pressure, density, and temperature) are calculated that are self-consistent with the background atmospheric flow. Broadband, full-wave infrasound predictions computed using the parabolic equation (PE) method are used to model atmospheric infrasound from explosions at ranges of 100s to 1000s of km. Model predictions using the PE method are conducted using range-dependent specifications of the atmospheric wind and temperature from NRL-G125 that are modified by the addition of perturbation terms representing the gravity wave field, in order to characterize the fine-scale atmospheric structure not resolved by numerical weather prediction models, Synthesized infrasound waveforms, obtained using the Fourier-synthesis time-domain PE method, are compared with observed infrasound signals from ground-truth events in order to evaluate the modeling capabilities.