Abstract:
This study presents observations of turbulence dynamics made during the low winds portion
of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-Low).
Observations were made of turbulent fluxes, turbulent kinetic energy, and the length scales
of flux-carrying and energy-containing eddies in the ocean surface boundary layer. A new
technique was developed to separate wave and turbulent motions spectrally, using ideas for
turbulence spectra that were developed in the study of the bottom boundary layer of the
atmosphere.
The observations of turbulent fluxes allowed the closing of heat and momentum budgets
across the air-sea interface. The observations also show that flux-carrying eddies are
similar in size to those expected in rigid-boundary turbulence, but that energy-containing
eddies are smaller than those in rigid-boundary turbulence. This suggests that the relationship
between turbulent kinetic energy, depth, and turbulent diffusivity are different in the
ocean surface boundary layer than in rigid-boundary turbulence.
The observations confirm previous speculation that surface wave breaking provides a
surface source of turbulent kinetic energy that is transported to depth where it dissipates. A
model that includes the effects of shear production, wave breaking and dissipation is able
to reproduce the enhancement of turbulent kinetic energy near the wavy ocean surface.
However, because of the different length scale relations in the ocean surface boundary
layer, the empirical constants in the energy model are different from the values that are
used to model rigid-boundary turbulence.
The ocean surface boundary layer is observed to have small but finite temperature
gradients that are related to the boundary fluxes of heat and momentum, as assumed by
closure models. However, the turbulent diffusivity of heat in the surface boundary layer is
larger than predicted by rigid-boundary closure models. Including the combined effects of
wave breaking, stress, and buoyancy forcing allows a closure model to predict the turbulent
diffusivity for heat in the ocean surface boundary layer.
