Simulations of internal gravity waves approaching a critical level

Internal gravity waves are ubiquitous throughout the atmosphere, because it is a stably-stratified fluid. These waves propagate naturally throughout the atmosphere in all directions interacting with various flows, such as changing winds, turbulence, and other gravity waves. One common interaction is when the relative frequency of the gravity wave tends to zero: a critical level. These critical levels represent areas where internal gravity waves tend to become unstable, resulting in wave steepening, overturning, or losing energy to other flows. Although critical levels due to steady shear have been well studied, when the background flow is unsteady different results ensue. In particular, if a larger scale, inertial wave is present a smaller-scale wave may begin to approach a critical level while propagating through, but it may not actually lose any energy permanently. Fully nonlinear, two-dimensional numerical simulations allow for simple methods to determine which parameters, such as wave numbers and amplitude, will affect the outcome of these wave interactions. Such outcomes involve transmitting energy from one flow to another, wave breaking and the turbulence that follows, or interactions where the waves remain virtually unchanged.