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.

Three general mechanisms for transferring momentum are shown. For wave-perturbed surfaces, dashed paths indicate relatively small importance. The mechanism for momentum transport in the aqueous boundary-layer is not clearly identified, and could be due to vertical shear or wave related transport.

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.

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Last Updated April 15, 1997