Simulations of Convective Excitation of Internal Waves in Water

We will present a series of simulations of convective excitation of internal gravity waves (IGWs) in water. We mimic the experimental set-up of Perrard et al. (2013), where water is cooled to zero degrees at the bottom of a tank, and keep at approximately room temperature at the top of a tank. The density maximum of water at 4 degrees renders the fluid convectively unstable between zero and four degrees, and stably stratified above four degrees. Our 2D simulations of the experiment show qualitatively similar IGW excitation spectrum. We then investigate two commonly discussed excitation mechanisms: interface forcing, and deep excitation. We run simplified simulations testing these two excitation mechanisms using the data from the full simulation, and compare the wave fields. We find that the interface forcing simulations overestimate the excitation of high frequency waves because high frequency interface motions are associated with nonlinear convection and not linear IGWs. On the other hand, the deep excitation of IGWs by Reynolds stresses accurately reproduces excitation spectrum. The correlation between the full simulation wave field and the deep excitation wave field is ~0.95. This suggests that deep excitation is the dominant excitation mechanism for this system.