Abstract

The application of the classical logarithmic layer model for wall-bounded shear flows to marine bottom boundary layer (BBL) usually leads to an overestimation of the friction velocity u* due possibly to the influence of form drag, stratification, and rotation of the flow vector. To gain insights on the BBL velocity scaling, acoustic Doppler current profiler (ADCP) measurements taken in the East China Sea were analyzed (a total of 270 sixteen-minute averaged velocity profiles). Single and double log-layer models, a log-wake model, and a modified log-layer (MLL) model that accounts for stratification in the upper part of the BBL (Perlin, Moum, Klymak, Levine et al. 2005) were explored. Although the first three models fit well for a majority of the profiles, the friction velocities appeared to be substantially overestimated, leading to unreasonably high drag coefficients. The friction velocity u*ml inferred from a slightly modified MLL, however, is half of that estimated using the classical log-layer assumption u*l. In a weakly stratified extended BBL, the dissipation rate ε decreases with the height from the seafloor ζ much faster than that in a homogeneous stationary BBL. This observation could be well approximated (in terms of r2) by an exponential ε (ζ) = ε0e–ζ/Lm or a power law decrease. The mixing length scale Lm = cLhBL, where hBL = 19–20 m is the BBL height and cL = 0.17, as well as the characteristic dissipation ε0, should vary in time, depending on the tidal currents and stratification in the BBL. The eddy diffusivity KN = 0.2ε/N2 showed an inverse dependence on the Richardson number Ri according to KN = K0/ (1 + Ri/Rc), where Rc is a constant and the diffusivity in nonstratified flow near the seafloor K0 = u*κζ is specified using u* = u*ml.

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