ON A TECHNIQUE FOR MEASUREMENT OF TURBULENT SHEAR-STRESS IN THE PRESENCE OF SURFACE-WAVES

Surface waves can produce large biases in estimates of turbulent shear stress obtained from single-sensor measurements of velocity if there is even a small uncertainty in the orientation of either the velocity sensor or the principal axes of the wave-induced velocity field. The w ave-induced bias can be diminished substantially by differencing measu rements obtained from two velocity sensors separated by a distance lar ger than the correlation scale of the turbulence but small in comparis on to the inverse wavenumber of the surface waves. If the scale separa tion is sufficiently large, then minus the density times half of the c ovariance between horizontal and vertical velocity differences is a ne arly wave-free estimate of the average of the turbulent shear stresses at the two sensors. A theoretical analysis determines the bias associ ated with this technique under simplified conditions, in which waves a nd turbulence are uncorrelated and the waves are weakly nonlinear and narrow-banded in both frequency and direction. Order-of-magnitude esti mates indicate that the technique can be used to obtain nearly unbiase d estimates of near-bottom turbulent shear stress on continental shelv es. A brief set of oceanic mea surements demonstrates the success of t he technique in practice.