  A conceptual model relating the whitecap coverage to the bubble plume
buoyancy is developed following the observation that the entrained bubble plume
buoyancy constitutes a large portion of the breaking wave energy dissipation.
The formulation leads to estimations of an effective or equivalent-buoyancy
depth of bubble entrainment as well as the volume and surface area of bubbles
entrained by surface wave breaking. The results show that the air-water
interface area per unit sea surface area is enhanced dramatically by the
entrained bubbles: on the order of 10 m^2 at about 15 m/s wind speed. The
effective entrainment depth represents the vertical reach of the bubble plume
as if all the bubbles were collected into this depth. Based on empirical
observations of whitecaps and breaking wave energy dissipation, it is about
0.11 m and relatively independent on wind speed. The void fraction of the top
meter ocean layer is related linearly to the whitecap coverage with a
proportionality factor of 0.11. The nearly-constant effective entrainment depth
essentially renders the bubble entrainment process during the active wave
breaking stage into a lateral 2D problem. Published high speed video recordings
of bubble plume evolution appear to support this conclusion. Consistent with
the nearly-constant effective entrainment depth, relevant breaking wave speeds
are within a narrow range between about 2 and 3.5 m/s and depend on wind speed
only weakly. Whitecap observations can also be used to quantify some elusive
breaking properties such as the breaking strength parameter b relating the
breaking energy dissipation rate and length of breaking front.
