[A-OS11] Ocean Mixing Frontiers

[AOS11-15] Verifying the parameterization of vertical eddy viscosity and diffusivity in the bottom boundary layer

*Takahiro Endoh¹, Takuya Hirooka¹ (1.RIAM Research Institute for Applied Mechanics)

Keywords:Bottom boundary layer, Vertical eddy viscosity, Vertical eddy diffusivity, Flux Richardson number, Gradient Richardson number, Turbulent kinetic energy budget

By estimating the terms of the steady-state turbulent kinetic energy (TKE) budget from microstructure data obtained over the continental shelf of the East China Sea, we verify a recently proposed parameterization of vertical eddy viscosity and diffusivity in the bottom boundary layer (BBL), where mixing efficiency is expressed as a function of the gradient Richardson number (Ri). The dissipation term (ε) is calculated from the micro-scale vertical shear measured with a freely-falling microstructure profiler, whereas the production term (P) is calculated from Reynolds stress and the fine-scale vertical shear measured with an acoustic Doppler current profiler (ADCP) mounted on the seabed. If the proportional relationship between the flux Richardson number (Rf) and Ri holds, which is assumed in the proposed BBL parameterization, the buoyancy flux term (B) as well as the transport term (T) can be estimated from P, ε, and Ri. Therefore, we use the iterative procedure to find a constant of proportionality which is close to a slope of the regression line for Rf (=-B/(P+T)) and Ri. The resulting constant is within the range suggested by laboratory experiments, large eddy simulations, and observations of the atmospheric boundary layer. The parameterized eddy viscosity agrees remarkably well with the observed one, whereas the parameterization underestimates the observed eddy diffusivity by a factor of two to three presumably because of the uncertainty involved in the estimated values of B.