Measurements of vertical overturns in a stably stratified turbulent flow

The vertical overturns occurring behind a biplane grid placed in a continuously stratified water channel were measured with a high-speed vertical density profiler. The resulting scale (LT)rms, known as the Thorpe scale is compared to the Ozmidov scale LR=(ε/N3)1/2 and the Eulerian turbulent scale l=2ρ′/(dρ̄/dz). For buoyancy-dominated or stratified turbulence, l and (LT)rms are linearly related, but when all turbulent overturning events have ceased, (LT)rms is shown to decay at a faster rate. Vertical displacements larger than the Ozmidov scale are observed implying that some previously turbulent scales have become fossil turbulence.