Internal gravity waves generated by turbulent convection: spectral characteristics and non-linear interactions

We investigate flows in a stratified layer adjacent to a turbulent convective layer. Such configurations are found in planetary atmospheres, in stars and potentially in the Earth core. The dynamics of the stratified layer have long been neglected. However, it requires a systematic investigation: the convective layer generates internal gravity waves (IGW) which transport energy and momentum. Their non-linear interactions can drive large-scale flows (e.g. Quasi Biennial Oscillation in the Earth atmosphere). It is thus crucial to understand how much of the convection energy is transported by the waves, with which spectral signature and how large-scale flows appear.

To investigate this experimentally, we use a peculiar feature of water: its density maximum is at 4°C. By cooling down the bottom of a tank at 0°C and heating up the top at a given temperature, the requested two-layer configuration spontaneously appears, with the stratified layer situated above the convective layer. Here, I will introduce the first results of our ongoing study using PIV: convection and IGW spectra, and preliminary observations of mean flow reversals in the stratified layer. Comparisons with a 1D analytical model, extending the classical Lindzen & Holton (1968) model will also be discussed.