Vorticity dynamics in a breaking internal gravity wave. Part 1. Initial instability evolution

A three-dimensional simulation of a breaking internal gravity wave in a stratified, compressible, sheared fluid is used to examine the vorticity dynamics accompanying the transition from laminar to turbulent flow. Our results show that baroclinic sources contribute preferentially to eddy vorticity generation during the initial convective instability of the wave field; the resulting counter-rotating vortices are aligned with the external shear flow. These vortices enhance the spanwise vorticity of the shear flow via stretching and distort the spanwise vorticity via advective tilting. The resulting vortex sheets undergo a dynamical (Kelvin-Helmholtz) instability which rolls the vortex sheets into tubes. These vortex tubes link with the original streamwise convective rolls to produce a collection of intertwined vortex loops. A companion paper (Part 2) describes the subsequent interactions among and the perturbations to these vortices that drive the evolution toward turbulence and smaller scales of motion.