16 research outputs found
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Wave breaking in developing and mature seas
In response to the growing need for robust validation data for Phillips (1985) breaking wave spectral framework, we contribute new field results observed from R/P FLIP for the breaking crest length distributions, Λ, during two different wind-wave conditions, and breaking strength during one wind-wave condition. The first experiment in Santa Barbara Channel had developing seas and the second experiment in the central Pacific Ocean south of Hawaii had mature seas. These are among the first experiments to use dissipation rate measurements probing up into the breaking crest together with simultaneous measurements of breaking crest length distributions. We directly measured the effective breaking strength parameter to be inline image in mature seas with wave age, inline image, of 40–47. We also found that the velocity scale of the breaking dissipation rate peak decreases with increasing wave age. Further, the breaking crest length spectrum falls off slower than the inline image behavior predicted by Phillips (1985). The integrated dissipation rate was consistently higher for mature seas compared to developing seas due to higher energy and momentum fluxes from the wind
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On the Variation of the Effective Breaking Strength in Oceanic Sea States
A spectral framework for quantifying the geometric/kinematic and dynamic/energetic properties of breaking ocean waves was proposed by Phillips in 1985. Phillips assumed a constant breaking strength coefficient to link the kinematic/geometric breaking crest properties to the associated excess energy and momentum fluxes from the waves to the upper ocean. However, a scale-dependent (spectral) breaking strength coefficient is needed, but is unavailable from measurements. In this paper, the feasibility of a parametric mean effective breaking strength coefficient valid for a wide range of sea states is investigated. All available ocean breaking wave datasets were analyzed and complemented with wave model behavior. Robust evidence is found supporting a single linear parameter relationship between the effective breaking strength and wave age or significant wave steepness. Envisaged applications for the effective breaking strength are described
An overview of sea state conditions and air-sea fluxes during RaDyO
Refining radiative-transfer modeling capabilities for light transmission through the sea surface requires a more detailed prescription of the sea surface roughness beyond the probability density function of the sea surface slope field. To meet this need, exciting new measurement methodologies now provide the opportunity to enhance present knowledge of sea surface roughness, especially at the microscale. In this context, two intensive field experiments using R/P Floating Instrument Platform were staged within the Office of Naval Research's Radiance in a Dynamic Ocean (RaDyO) field program in the Santa Barbara Channel and in the central Pacific Ocean south of Hawaii. As part of this program, our team gathered and analyzed a comprehensive suite of sea surface roughness measurements designed to provide optimal coverage of fundamental optical distortion processes associated with the air-sea interface. This contribution describes the ensemble of instrumentation deployed. It provides a detailed documentation of the ambient environmental conditions that prevailed during the RaDyO field experiments. It also highlights exciting new sea surface roughness measurement capabilities that underpin a number of the scientific advances resulting from the RaDyO program. For instance, a new polarimetric imaging camera highlights the complex interplay of wind and surface currents in shaping the roughness of the sea surface that suggests the traditional Cox-Munk framework is not sufficient. In addition, the breaking crest length spectral density derived from visible and infrared imagery is shown to be modulated by the development of the wavefield (wave age) and alignment of wind and surface currents at the intermediate (dominant) scale of wave breaking
An energetic signature for breaking inception in surface gravity waves
A dynamical understanding of the physical process of surface gravity wave
breaking remains an unresolved problem in fluid dynamics. Conceptually,
breaking can be described by inception and onset, where breaking inception is
the initiation of unknown irreversible processes within a wave crest that
precede the visible manifestation of breaking onset. In the search for an
energetic indicator of breaking inception, we use an ensemble of non-breaking
and breaking crests evolving within unsteady wave packets simulated in a
numerical wave tank to investigate the evolution of each term in the kinetic
energy balance equation. We observe that breaking onset is preceded by around
one quarter of a wave period by a rapid increase in the rate of convergence of
kinetic energy that triggers an irreversible acceleration of the kinetic energy
growth rate. This energetic signature, which is present only for crests that
subsequently break, arises when the kinetic energy growth rate exceeds a
critical threshold. At this point the additional kinetic energy convergence
cannot be offset by converting excess kinetic energy to potential energy or by
dissipation through friction. Our results suggest that the ratio of the leading
terms of the kinetic energy balance equation at the time of this energetic
signature is proportional to the strength of the breaking crest. Hence this
energetic inception point both predicts the occurrence of breaking onset and
indicates the strength of the breaking event.Comment: Submitted for publication to the Journal of Fluid Mechanic