Vertical transport of momentum
in the aqueous boundary-layer.
Dr. Mark A. Bourassa, COAPS/FSU, Tallahassee FL 32306-2840
The dominant mechanism for the vertical
transport of horizontal momentum, in the uppermost 2 m of wave disturbed surfaces, is examined and current profiles are modeled.
The actual mechanism cannot be determined from this analysis; however, the stress and flow profile are shown be to characteristic of gravity waves.
The stress in the aqueous boundary-layer
follows a Charnock-type relation (which is characteristic of gravity waves),
with Charnock's 'constant' (for local equilibrium) equal to 850.
Charnock's 'constant' is dependent on sea state, with smaller values for rising seas, and larger values for falling seas.
The stress partition (the ratio of aqueous
stress to atmospheric stress) is found to be near 20% for local
equilibrium conditions. The stress partition is larger for falling
seas, and it is smaller for growing seas.
The relationship between the log of the roughness length and the log of the friction velocity indicates the source of the mixing. Mixing related to molecular viscosity has a slope of -1, capillary wave related turbulence has a slope of -2, and gravity wave related turbulence has a slope of 2. Friction velocities and roughness lengths determined by Churchill and Csanady (1983) and Bye (1965) are shown (open symbols), as well as new values determined using the same observations (closed symbols). The main improvements in the new calculations are the consideration of displacement height, and restriction that only observations from the regime governed by the physics in question be used in the calculation of zo and u*. Observations with rising seas are circled in red, near local-equilibrium seas in green, and falling seas in blue.
Last Updated April 15, 1997