The wall-layer dynamics in a weakly stratified tidal bottom boundary layer

Abstract

The application of the classical logarithmic layer model for wall-boundedshear 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 epsilon decreases with the height from the seafloor. much faster than that in a homogeneous stationary BBL. This observation could be well approximated (in terms of r(2)) by an exponentiale epsilon(zeta)=epsilon 0e(-zeta/Lm) or a power law decrease. The mixing length scale L-m = cLhBL, whereh(BL) = 19-20 m is the BBL height and c(L) = 0.17, as well as the characteristic dissipation epsilon(0), should vary in time, depending on the tidal currents and stratification in the BBL. The eddy diffusivity K-N = 0.2 epsilon/N-2 showed an inverse dependence on the Richardson number Ri according to KN = K-0/(1 + Ri/R-c), where R-c is a constant and the diffusivity in nonstratified flow near the seafloor K-0 = u(*)k zeta is specified using u(*) = u(*ml).