Internal gravity waves generated by katabatic flows in a valley and induced mixing

In this talk we will present results of high-resolution large-eddy simulations of the generation of internal gravity waves by katabatic flows in a valley. These are motivated by previous experimental studies, which have reported oscillations in katabatic flows either on a simple slope or more rarely in a valley (e.g. Gryning et al., 1985; van Gorsel et al., 2004). Only few papers mention the presence of internal gravity waves, despite oscillations are often reported. Nonetheless, the valley atmosphere being stable, internal gravity waves must be generated by katabatic flows. The simulations reported in our study focus on an idealized valley with dimensions close to those of the Chamonix (France) or the Riviera (Switzerland) valleys, for which sparse nocturnal and/or wintertime data are available. We will present examples of internal gravity waves generated by gravity currents, as well as discuss the atmospheric stability conditions for which they may be clearly identified and characterized (for instance using Hovmoller diagrams). The waves propagate upward and thus carry energy at higher altitudes, where their breaking contributes to the vertical mixing in the atmosphere. We will discuss conditions for which waves are reflected back down toward the ground surface or are damped with height because of the stratification of the upper layers. Trapped waves between the ground surface and the layer aloft may then contribute to the mixing of the stable valley atmosphere. Finally, we will comment on the quantification of the induced mixing and on the interaction of the waves with katabatic flows. The presented results may help to improve parameterization of the effect of internal gravity waves on turbulence for larger-scale models.