Abstract

The generation of inertia-gravity waves in complex flows is examined using a geometric-optics approach. This approach considers the dynamics of a small-scale wavepacket in prescribed time-dependent, balanced flow. The wavepacket is assumed to be in the so called wave-capture regime, where the wave intrinsic frequency is negligible compared with the Doppler shift. The dynamics is reduced to a number of ordinary differential equations describing the evolution of the wavepacket position, of the wavevector, and of three scalar fields describing the wavepacket amplitude and polarisation. The approach clearly identifies two classes of wave-generation processes: unbalanced instabilities, associated with linear interactions between inertia-gravity waves, and spontaneous generation, associated with a conversion between vortical and inertia-gravity modes. Applications to idealised and realistic flows are discussed.