|Title||The saturation of fluid turbulence in breaking laboratory waves and implications for whitecaps|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Deane GB, Stokes MD, Callaghan AH|
|Journal||Journal of Physical Oceanography|
|Type of Article||Article|
|Keywords||air entrainment; Atm/Ocean Structure/ Phenomena; Atmosphere-ocean interaction; Boundary layer; bubble-size distributions; deep-water; energy-dissipation; Microscale processes/variability; near-surface; ocean; Oceanic mixed layer; sea; sound; turbulence; void-fraction measurements; zone|
Measurements of energy dissipated in breaking laboratory waves, averaged over time and space and directly visualized with a bioluminescent technique, are presented. These data show that the energy dissipated in the crest of the breaking waves is constrained: average turbulence intensity within the crest saturates at around 0.5-1.2 W kg(-1), whereas breaking crest volume scales with wave energy lost. These results are consistent with laboratory and field observations of the Hinze scale, which is the radius of the largest bubble entrained within a breaking crest that is stabilized against turbulent fragmentation. The Hinze scale depends on turbulence intensity but lies in the restricted range 0.7-1.7 mm over more than two orders of magnitude variation in underlying unbroken wave energy. The results have important implications for understanding the energetics of breaking waves in the field, the injection of turbulence into the upper ocean, and air-sea exchange processes in wind-driven seas.
Our laboratory observations have implications for breaking ocean waves. If the turbulence saturation effect carries over into the field then bubble creation physics and the densities and scale of bubbles in breaking wave crests can be expected to be largely independent of wave scale and slope. A constant Hinze scale has implications for any air–sea property or exchange processes that depend on bubble size, such as the production of marine aerosols, the generation of underwater ambient noise, and changes in ocean albedo due to whitecap foam.