Wave Breaking Dissipation in a Young Wind Sea

Abstract

Coupled in situ and remote sensing measurements of young, strongly forced wind waves are applied to assess the role of breaking in an evolving wave field. In situ measurements of turbulent energy dissipation from wave-following Surface Wave Instrument Float with Tracking (SWIFT) drifters and a tethered acoustic Doppler sonar system are consistent with wave evolution and wind input (as estimated using the radiative transfer equation). The Phillips breaking crest distribution Λ(c) is calculated using stabilized shipboard video recordings and the Fourier-based method of Thomson and Jessup, with minor modifications. The resulting Λ(c) are unimodal distributions centered around half of the phase speed of the dominant waves, consistent with several recent studies. Breaking rates from Λ(c) increase with slope, similar to in situ dissipation. However, comparison of the breaking rate estimates from the shipboard video recordings with the SWIFT video recordings show that the breaking rate is likely underestimated in the shipboard video when wave conditions are calmer and breaking crests are small. The breaking strength parameter b is calculated by comparison of the fifth moment of Λ(c) with the measured dissipation rates. Neglecting recordings with inconsistent breaking rates, the resulting b data do not display any clear trends and are in the range of other reported values. The Λ(c) distributions are compared with the Phillips equilibrium range prediction and previous laboratory and field studies, leading to the identification of several inconsistencies.

Surface Wave Effects on the Translation of Wind Stress across the Air–Sea Interface in a Fetch-Limited, Coastal Embayment

“…Measured distributions of breaking rate show a peak at a phase speed approximately half that of the spectral peak with dissipation of high-frequency, short waves composing a significant fraction of the total breaking rate (Gemmrich et al 2008;Thomson et al 2009;Schwendeman et al 2014). Schwendeman et al (2014) also noted that a regime shift occurs in young wind seas where large whitecaps replace, not add to, small-scale breakers as forcing becomes stronger. Furthermore, field observations of the dissipation of turbulent kinetic energy (TKE) beneath surface gravity waves exceed wallbounded shear flow scaling (Kitaigorodskii et al 1983;Agrawal et al 1992;Drennan et al 1992;Terray et al 1996;Drennan et al 1996,;Gemmrich and Farmer 1999,;Gemmrich 2010;Scully et al 2016).…”

“…Wave breaking in deep water is the result of wind-wave, wave-wave, and wave-current interactions (Melville 1996). Measured distributions of breaking rate show a peak at a phase speed approximately half that of the spectral peak with dissipation of high-frequency, short waves composing a significant fraction of the total breaking rate (Gemmrich et al 2008;Thomson et al 2009;Schwendeman et al 2014). Schwendeman et al (2014) also noted that a regime shift occurs in young wind seas where large whitecaps replace, not add to, small-scale breakers as forcing becomes stronger.…”

Breaking-onset, energy and momentum flux in unsteady focused wave packets

“…Although some of the available estimates of the non-dimensional breaking strength parameter, b, in the field (Thomson, Gemmrich & Jessup 2009;Schwendeman et al 2014) are comparable to those in spillers due to dispersive energy focusing (Drazen et al 2008;Tian et al 2010), most of the field estimations of b (Phillips, Posner & Hansen 2001;Gemmrich, Banner & Garrett 2008;Gemmrich et al 2013) tend to be more consistent with the observed b values in weak spillers due to modulational instability (Banner & Peirson 2007;Allis 2013). Here, we briefly establish that the present model is capable of capturing breaking waves due to modulational instability.…”

“…Beside dispersive energy focusing, other mechanisms such as modulational instability (Benjamin & Feir 1967;Melville 1982) and wind forcing (Grare et al 2013;Schwendeman, Thomson & Gemmrich 2014) can induce wave breaking. Although some of the available estimates of the non-dimensional breaking strength parameter, b, in the field (Thomson, Gemmrich & Jessup 2009;Schwendeman et al 2014) are comparable to those in spillers due to dispersive energy focusing (Drazen et al 2008;Tian et al 2010), most of the field estimations of b (Phillips, Posner & Hansen 2001;Gemmrich, Banner & Garrett 2008;Gemmrich et al 2013) tend to be more consistent with the observed b values in weak spillers due to modulational instability (Banner & Peirson 2007;Allis 2013).…”

A Horizon-Tracking Method for Shipboard Video Stabilization and Rectification

“…In Melville (2010, 2011) and Romero et al (2012), an IMU was used to gather the positional data needed to rectify images taken from an airborne system. In Schwendeman et al (2014), a shipboard camera was actively stabilized with an inertial pan-tilt system. There are two disadvantages of this strategy.…”