# Fluid Mechanics and Acoustics Laboratory - UMR 5509

LMFA - UMR 5509
Laboratoire de Mécanique des Fluides et d’Acoustique
Lyon
France

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Article in J. Mar. Res. (2015)

## Distinguishing turbulent overturns in high-sampling-rate moored thermistor string observations

Hans van Haren & Louis Gostiaux

Turbulent overturns are distinguished from salinity-compensated intrusions in high-resolution moored thermistor string observations. The buoyancy frequency N is used to make the time dimensionless, "$t^\star$." This results in a primary, visual means to easily compare the duration of overturns with N, the natural frequency that separates internal waves from turbulent overturns. As a secondary means, the shapes of overturns are investigated. Above various sloping topography between $500$ and $1,000\, \mathrm{m}$ water depth where the buoyancy period varies between $\sim1,300$ and $2,600\,\mathrm{s}$, vertical overturns of $\sim40\,\mathrm{m}$ last $\Delta t^\star=0.2–0.4$. This corresponds with the timescale of growth of model-stratified turbulence in the wake of a grid. Smaller-scale, weaker-turbulent, shear-induced Kelvin-Helmholtz overturns of $\sim5\,\mathrm{m}$ are observed to last approximately $\Delta t^\star=0.03$, whereas the passage of their train of multiple consecutive overturns lasts up to approximately $\Delta t^\star=0.95$. Although the shape of overturns can distinguish salinity-compensated intrusions from turbulent overturns, the present observations from internal wave breaking above sloping topography show complex results of mixed features.