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- Title
Using Energy Dissipation Rate to Obtain Active Whitecap Fraction.
- Authors
Anguelova, Magdalena D.; Hwang, Paul A.
- Abstract
Active and total whitecap fractions quantify the spatial extent of oceanic whitecaps in different lifetime stages. Total whitecap fraction W includes both the dynamic foam patches of the initial breaking and the static foam patches during whitecap decay. Dynamic air-sea processes in the upper ocean are best parameterized in terms of active whitecap fraction W A associated with actively breaking crests. The conventional intensity threshold approach used to extract W A from photographs is subjective, which contributes to the wide spread of W A data. A novel approach of obtaining W A from energy dissipation rate ε is proposed. An expression for W A is derived in terms of energy dissipation rate W A( ε) on the basis of the Phillips concept of breaking crest length distribution. This approach allows more objective determination of W A using the breaker kinematic and dynamic properties yet avoids the use of measuring breaking crest distribution from photographs. The feasibility of using W A( ε) is demonstrated with one possible implementation using buoy data and a parametric model for the energy dissipation rate. Results from W A( ε) are compared to W A from photographic data. Sensitivity analysis quantifies variations in W A estimates caused by different parameter choices in the W A( ε) expression. The breaking strength parameter b has the greatest influence on the W A( ε) estimates, followed by the breaker minimal speed and bubble persistence time. The merits and caveats of the novel approach, possible improvements, and implications for using the W A( ε) expression to extract W A from satellite-based radiometric measurements of W are discussed.
- Subjects
ENERGY dissipation; OCEAN-atmosphere interaction; OCEAN dynamics; CRESTS (Hydrology); BUOYANT ascent (Hydrodynamics); SENSITIVITY analysis
- Publication
Journal of Physical Oceanography, 2016, Vol 46, Issue 2, p461
- ISSN
0022-3670
- Publication type
Article
- DOI
10.1175/JPO-D-15-0069.1