The trapped mountain waves produced when the incident wind near the surface is small compared with its value aloft are analyzed with a theory adapted from Long (1953) and compared with fully nonlinear simulations performed with the Weather Research and Forecasting model (WRF). Although small near‐surface incident winds occur naturally in fronts via a combination of the thermal wind balance and the boundary layer, they pose at least two problems in mountain meteorology: zero surface incident winds produce no wave in the fully linear case; they also correspond to places where mountain waves have a critical level. Despite these problems, theory and WRF show that, for small mountains, trapped lee waves (a) can occur and (b) are favored when the surface Richardson number J = N 2/(∂u/∂z)2 is small. This last result is related to the theoretical fact that the surface absorption of stationary gravity waves increases when J increases. The relation with flow stability is corroborated further by the fact that the trapped lee waves resemble the Kelvin–Helmholtz (KH) modes of instability that exist when J < 0.25. For medium mountains, some aspects of the theory still hold but need to be adapted, the more intense winds and foehn that occur along the lee side of the mountain having a tendency to increase the surface flow stability. For “initially” small J, this can limit the onset of trapped lee waves, again consistent with the fact that mountain‐wave surface absorption increases with surface flow stability. For large J, the dynamics produces wave breaking on the lee side, destabilizing the flow in the wake of the mountain. In the region where the Richardson number is small, trapped waves develop despite the fact that the surface Richardson number can be quite large, suggesting that the trapped lee waves now result from an absolute instability of the wake.

Atmospheric mountain flows, produced when the incoming wind is small near the surface and continuously increases with altitude, are evaluated with models of increasing complexity. All models confirm that foehn can be produced by a mountain gravity wave critical level mechanism, where the critical level is located below the surface. This mechanism does not involve humidity, upper-level wave breaking, upstream blocking, downward wave reflections or hydraulic control, as often suggested by popular theories. The first model used is a theoretical one which combines linear gravity wave dynamics with a nonlinear boundary condition: in this model the wave breaking does not feed back onto the dynamics by construction. Partial linear wave reflections are also minimized by using smooth profiles of the incident wind and a uniform stratification N 2 = constant, and can even be suppressed when the incident wind shear is also constant, U z = constant. The second model is a numerical mesoscale model (Weather Research and Forecasting), and we show that it predicts mountain wave fields which can be reproduced by the theoretical model, provided that we specify an adequate boundary-layer depth in the theoretical model.

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.

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.