Abstract : 1A.2
Internal gravity wave breaking as a possible source of mixing in the stably-stratified atmosphere of a valley

Charles Chemel, Chantal Staquet, Matthieu Tavernier
Centre for Atmospheric and Instrumentation Research

The flow dynamics in an alpine valley under stable conditions (e.g. either at night or in winter) is mainly dominated by down-slope flows. Indeed, gravity currents are induced by the horizontal temperature gradients, which result from the radiative cooling of the slopes on the mountain sides. Internal gravity waves must be generated by these down-slope flows and propagate within the stably-stratified valley atmosphere. Several papers based on in situ measurements, either on a simple slope or within a valley (e.g. Gryning et al., 1985), reported the existence of `oscillations' and of internal gravity waves. A simple estimate of the oscillation period, assuming that the waves are trapped within the valley, was found to be in a rather good agreement with the observed period (van Gorsel et al., 2004). The possible occurrence of trapped waves implies that standing waves may form within the valley, and that breaking and fluid mixing are likely to result. To our knowledge, internal gravity wave breaking has not been studied previously as a possible source of turbulence under stable conditions in a narrow valley. Hence, we conducted a carefully controlled numerical study of the atmospheric dynamics in an alpine valley under stable conditions, in order to investigate the features of the waves, which should be produced by down-slope flows, and to quantify the mixing resulting from the breaking waves. High-resolution simulations were conducted for this purpose, using the ARPS code. We focus on the Chamonix valley, which has already been investigated with ARPS, and on an idealized valley with dimensions close to those of the Chamonix valley. Using different diagnostic methods, we found that the wave characteristics are controlled by those of the valley and that the waves are therefore trapped. Our talk will also report about the mixing properties of the waves, which we currently study both in the idealized and in the realistic Chamonix valley configurations.