Séminaire
Jet stream variations (location, strength) have strong impacts on the weather, for example in Europe, and the contribution of the Golf stream on the transport of heat and biomass is uncontested. But jet streams are also unstable and model their evolutions require representing those instabilities. The most unstable mode of a wind jet is a essentially ageostrophic (or unbalanced) instability, named inertial (symmetric) instability. This instability will be characterized, as why a non-hydrostatic framework, and including the full Coriolis force, is needed to do so.
Internal gravity waves are of great importance for atmosphere and oceanic sciences, for example for the thermohaline and the mesopause circulations. But those waves still need to be parameterized in present-day models. Wave-mean flow interactions can be effectively described by ``Wentzel-Kramers-Brillouin'' (WKB) theory, while a stable numerical implementation can be provided by a description of the wave propagation in location-wavenumber phase-space.
The vertical propagation of a localized gravity wave in a non-rotating, non-Boussinesq large-scale flow (time-altitude dependent) is well described by a ray-tracer, knowing that gravity wave propagation can strongly be modified by the interaction between the wave and the wave-induced mean flow.
The 3-D propagation of gravity waves from a very simple but continuous source is then studied, on a global model which the middle atmosphere. The non-zonal time-dependent large-scale flow considered here consist in solar tides and a climatology flow. It is shown that the horizontal propagation and refraction of gravity waves matters. The effect of gravity wave forcing on solar tides is studied.