A LAGRANGIAN FLOAT

The design and operation of neutrally buoyant Boats that attempt to tr ack the three-dimensional motion of water parcels in highly turbulent regions of the ocean, such as the upper mixed layer, are described. Th ese floats differ from previous floats by combining high drag, a compr essibility 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 th e Boats and estimating the accuracy to which they are ''Lagrangian.'' The analysis indicates that these Boats follow the motion of the surro unding water to better than 0.01 m s(-1) under most circumstances. Bot h the mt buoyancy of the Boat and its finite size contribute to the er ror. The Boat's buoyancy is controlled by making its compressibility v ery close to that of seawater, by making its drag large, by reducing a ir pockets and bubbles on the Boat, and by carefully controlling varia tions in the float's mass and volume between deployments. The Boat acc urately follows that part of the velocity field with scales much large r than its own size (1 m) but does not follow components with scales s maller than itself. A model of this dependence is presented for turbul ent flows. Several unique measurements are possible with these Boots. They measure vertical displacement using pressure and therefore accura tely filter out the vertical velocity of surface waves, since linear s urface waves have no pressure fluctuations along Lagrangian trajectori es. Accurate measurements of vertical velocity in the oceanic mixed la yer are therefore possible. This, combined with temperature, can be us ed to measure vertical heat flux. A compass measures the spin rate of the Boat and thus the vertical vorticity. In fully turbulent Bows with outer scales much larger than the Boat size, the spectra of both vert ical velocity and vorticity scale with epsilon (the turbulent kinetic energy dissipation) over a wide range of epsilon values, thus allowing epsilon to be measured. The floats can, in principle. therefore measu re many important properties of turbulent Bows even in the presence of surface waves.