Infrasound propagation across mountain ranges

The aim of this thesis is to explain the discrepancies between the calculated acoustic signals and the observations at the infrasound stations of the International Monitoring System near mountain ranges. In chapter 2, we present the theoretical bases of the infrasound wave models (normal mode methods) and gravity models used in this thesis work. Chapter 3 is devoted to the development of a linear model to compute the atmospheric perturbations generated by the presence of a mountain subjected to boundary layer flow. Systematic comparisons with the American meteorological model WRF are also presented. The results of the linear model, validated by the simulations carried out with WRF, provide a new perspective on the dynamics of the Foehn effect. In Chapter 4, the impact of mountain waves and the Foehn effect on infrasound propagation will be detailed by analysing infrasound propagation mode by mode. We will show how mountain flow can promote infrasound propagation, whereas the approach using simple masks and neglecting the waveguide fails. We will also show how in other circumstances, the atmospheric disturbance produced by the mountain can completely destroy a waveguide. Chapter 5 concludes this manuscript and generalizes the concepts developed in chapters 3 and 4, and in particular the modal approach, to precisely characterize the effect of a relief on the propagation of infrasound from a combined use of the WRF code and the finite element method to solve the wave equation. The originality of this approach is to take into account the random dimension of the problem, by integrating into the problem the transient effects and strong non-linearities that occur when a flow meets a relief. (author)