Jh. Trowbridge, ON A TECHNIQUE FOR MEASUREMENT OF TURBULENT SHEAR-STRESS IN THE PRESENCE OF SURFACE-WAVES, Journal of atmospheric and oceanic technology, 15(1), 1998, pp. 290-298
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.