Abstract
The design and Operation of neutrally buoyant floats that attempt to track the three-dimensional motion of water parcels in highly turbulent regions of the ocean, such as the upper mixed layer, are described. These floats differ from previous floats by combining high drag, a compressibility that nearly matches that of seawater, rapid (1 Hz) sampling, and short-range, high-precision acoustic tracking. Examples of float data are shown with the twin goals of demonstrating the utility of the floats and estimating the accuracy to which they are “Lagrangian.”
The analysis indicates that these floats follow the motion of the surrounding water to better than 0.01 m s−1 under most circumstances. Both the net buoyancy of the float and its finite size contribute to the error. The float's buoyancy is controlled by making its compressibility very close to that of seawater, by making its drag large, by reducing air pockets and bubbles on the float, and by carefully controlling variations in the float's mass and volume between deployments. The float accurately follows that part of the velocity field with Scales much larger than its own size (1 m) but does not follow components with scales smaller than itself. A model of this dependence is presented for turbulent flows.
Several unique measurements are possible with these floats. They measure vertical displacement using pressure and therefore accurately filter out the vertical velocity of surface waves, since linear surface waves have no pressure fluctuations along Lagrangian trajectories. Accurate measurements of vertical velocity in the oceanic mixed layer are therefore possible. This, combined with temperature, can be used to measure vertical heat flux. A compass measures the spin rate of the float and thus the vertical vorticity. In fully turbulent flows with outer scales much larger than the float size, the spectra of both vertical velocity and vorticity scale with ε (the turbulent kinetic energy dissipation) over a wide range of ε values, thus allowing ε to be measured. The floats con, in principle, therefore measure many important properties of turbulent flows even in the presence of surface waves.
