Model dynamics and vertical collapse in decaying strongly stratified flows

Recent laboratory experiments with decaying strongly stratified grid turbulence at moderate Reynolds numbers reveal remarkable behavior. These experiments document the evolution of an initial sea of columnar dipole vortex pairs with dominant vertical vorticity to stratified ‘‘pancake’’ vortex sheets with dominant horizontal vorticity, together with a concurrent dominance of vertical dissipation of kinetic energy as compared with horizontal dissipation. Here we build exact solutions of the equations for low Froude number limiting dynamics, which capture basic qualitative features observed in the above experiments. Unlike the actual turbulent experiments, these exact solutions are laminar and do not involve a cascade of many scales in the horizontal. The exact solutions of the limiting dynamics involve a periodic array of dipole vortices in a weakly vertically sheared horizontal flow. The effect of finite Rossby numbers on the collapse of these exact solutions is also described here. At moderately large Rossby numbers, the effect of rotation is to inhibit the vertical collapse process. © 1997 American Institute of Physics. @S1070-6631~97!01210-5#