Spectral broadening of infrasound tones in mountain wave fields

Abstract

Linear theory of acoustic propagation is used to analyze how infrasounds trapped within the lower tropospheric waveguide propagate across mountain waves. The atmospheric disturbances produced by the mountains are predicted by a semi-theoretical mountain gravity wave model. For the infrasounds, we solve the wave equation under the effective sound speed approximation both using a spectral collocation method and a WKB 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, we follow each acoustic mode separately and show which mode is impacted and how. We show that during statically stable situations, roughly representative of winter or night situations, the mountain waves induce a strong Foehn effect downstream which shrinks significantly the waveguide. This yield a new form of infrasound absorption, a form that can largely outweigh the direct effect of the mask the mountain induce on the low-level waveguide. On the opposite, when the low level flow is less statically stable (summer or day situations), the mountain wave dynamics does not produce dramatic responses downstream, it can even favor the passage of infrasound waves, somehow mitigating the direct effect of the obstacle.