Turbulence measurements from a glider

To test the feasibility of measuring small-scale turbulence from an ocean glider, a self-contained package carrying velocity shear probes and FP07 thermistors is deployed in a small lake on a Slocum ocean glider. The package's turbulence sensors are augmented by a high-resolution pressure sensor and a set of orthogonally mounted accelerometers monitoring the glider's attitude and vibrations. The package is neutrally buoyant and does not limit the glider's manoeuvrability. It attaches to the top of the glider's fuselage with the turbulence sensors positioned just ahead of the glider nose. The package receives power from the glider and can record turbulence data independently for up to 35 days. Data from the a test in Ashumet Pond near Cape Cod show that vibration levels of the glider are generally small and do not interfere with the measurement of small-scale turbulence shear. The accelerometer spectra show vibration peaks at 28, 60, and 80 Hz, which are caused by vibrations of the glider's tail fin assembly. These vibrations are stimulated by the action of the glider's buoyancy pump, which acts at the top and bottom turn-around points, and by the action of the rudder, which is activated at regular intervals (~6 seconds) during the flight in order to control the heading of the glider. The vibration peaks have a small magnitude and narrow bandwidth and do not interfere with the shear probe spectrum. The shear probes resolved dissipation rates between 5 × 10
^-11
W/kg in the quiescent layer below the thermocline and 5 × 10
^-7
W/kg in the surface mixing layer. All measured shear spectra fit well with the Nasmyth Empirical Spectrum.