The direct estimation of near-bottom turbulent fluxes in the presence of energetic wave motions

Velocities produced by energetic waves can contaminate direct covariance es timates of near-bottom turbulent shear stress and turbulent heat flux. A ne w adaptive filtering technique is introduced to minimize the contribution o f wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherenc e scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results fr om the oceanic test demonstrate that the technique succeeds at removing sur face-wave contamination from shear stress and heat flux estimates using pai rs of sensors separated in the vertical dimension by a distance of approxim ately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caus ed by internal motions that occur occasionally in the dataset. The internal case is complicated by the facts that the motions are highly intermittent; the internal-wave period is comparable to the Reynolds-averaging period; t he height of the internal-wave boundary layer is on the order of the height of measurement; and, specifically for heat flux estimates, nonlinear effec ts are large. The presence of internal motions does not pose a significant problem for estimating turbulent shear stress, because contamination caused by them is limited to frequencies lower than those of the stress-carrying eddies. In contrast, the presence of internal motions does pose a problem f or estimating turbulent heat flux, because the contamination extends into t he range of the heat flux-carrying eddies.