Gravity Wave–Fine Structure Interactions. Part I: Influences of Fine Structure Form and Orientation on Flow Evolution and Instability

Four idealized direct numerical simulations are performed to examine the dynamics arising from the superposition of a monochromatic gravity wave (GW) and sinusoidal linear and rotary fine structure in the velocity field. These simulations are motivated by the ubiquity of such multiscale superpositions throughout the atmosphere. Three simulations explore the effects of linear fine structure alignment along, orthogonal to, and at 45° to the plane of GW propagation. These reveal that fine structure alignment with the GW enables strong wave–wave interactions, strong deformations of the initial flow components, and rapid transitions to local instabilities and turbulence. Increasing departures of fine structure alignment from the GW yield increasingly less efficient wave–wave interactions and weaker or absent local instabilities. The simulation having rotary fine structure velocities yields wave–wave interactions that agree closely with the aligned linear fine structure case. Differences in the aligned GW fields are only seen following the onset of local instabilities, which are delayed by about 1–2 buoyancy periods for rotary fine structure compared to aligned, linear fine structure. In all cases, local instabilities and turbulence primarily accompany strong superposed shears or fluid “intrusions” within the rising, and least statically stable, phase of the GW. For rotary fine structure, local instabilities having preferred streamwise or spanwise orientations often arise independently, depending on the character of the larger-scale flow. Wave–wave interactions play the greatest role in reducing the initial GW amplitude whereas fine structure shears and intrusions are the major source of instability and turbulence energies.