A Parameter Describing Overall Conditions of Wave Breaking, Whitecapping, Sea-Spray Production and Wind Stress

The dim ensionless parameter u * 2 /vσ can be used widely as a parameter to describe the overall conditions of air-sea boundary processes, where u* is the friction velocity of air, v is the kinematic viscosity of air and s is the spectral peak frequency of the wind waves. A critical value of this parameter for the appreciable commencement of breaking of wind waves is 103. Beyond this value, the percentage of waves passing a fixed point that are breaking, α, and the percentage of whitecap coverage, P, are both approximately proportional to this parameter. The number concentration of sea-salt particles containing salt in the vicinity of 10−10 g at the 6-m level is also proportional to this parameter. The dimensionless roughness length associated with the air flow over water, u*Z0 /V, also correlates better with this parameter than with a parameter which does not contain the spectral peak frequency. This gives an approximate relation of z0 σ/u* = 0.025, and a corresponding formula for the drag coefficient is proposed. The dimensional and physical interpretation of the parameteru */2/vσ is presented.

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