Turbulence Measurements in Shallow Water from a Subsurface Moored Moving Platform

Abstract

Preliminary results are presented from a moored, autonomous instrument measuring the upper ocean turbulent dissipa- tion rate together with the surface gravity wave field. Observations are made in the area approximately 30km southwest of Bergen, Norway between 28 and 30 November 2012, at about 8 m from the surface in 20 m deep water. The platform is the top element of a bottom-anchored mooring line, and moves in response to currents and waves. Shear probes mounted on the nose of the platform allow measurements of dissipation rate of turbulent kinetic energy, ɛ, in the frequency range between 1 to 20Hz in which shear probes are less contaminated by the surface gravity wave disturbances and platform motions. A high-resolution pressure sensor and an acoustic Doppler velocimeter allow estimates of surface bulk wave parameters and measurements of mean currents at a single depth. For the present deployment, in the shallow water within the wave-affected surface layer, at a depth of about 4 significant wave heights, good quality measurements of ɛ were limited by the instrument angle of attack and wave effects. Observations agree well with the scaling of dissipation due to wave breaking. Our preliminary results confirm that successful moored shear probes measurements are possible in the wave-affected layer of the upper ocean.