Airborne observations of the kinematics and statistics of breaking waves

Breaking waves play an important role in air-sea interaction : enhancing momentum flux from the atmosphere to the ocean, dissipating wave energy that is then available for turbulent mixing, injecting aerosols and sea spray into the atmosphere, and affecting air-sea gas transfer due to air entrainment. In this thesis observations are presented of the occurrence of breaking waves under conditions of strong winds (10-25ms⁻¹) and fetch-limited seas (0-500km) in the Gulf of Tehuantepec Experiment in 2004. An airborne nadir-looking video camera, along with a global positioning system (GPS) and inertial motion unit (IMU), provided digital videos of the breaking sea surface in an earth frame. In particular, we present observations of \[Lambda\] (c)dc, which is the distribution of breaking wave crest lengths per unit sea surface area, binned by breaking velocity increments dc. Methods of analyzing visible images of the sea surface to yield \[Lambda\](c) are studied. The results from previous field observations of \[Lambda\](c) give qualitatively different results : Melville and Matusov (2002) find an exponential form for \[Lambda\](c), while Gemmrich et al. (2008) obtain a function that peaks at intermediate speeds and falls of for high and low speeds, and at higher speeds is up to an order of magnitude higher than Melville and Matusov (2002). Two independent processing techniques for determining \[Lambda\](c) from video images are implemented, and it is shown that the results from both studies can qualitatively be obtained when the assumptions built into the processing of the previous studies are used. The effects of surface currents, long wave orbital velocity, and the relationship between the speed of the foam patch and the underlying intrinsic phase speed are addressed. The appropriateness and limitations of comparison of the first moment of \[Lambda\](c) to the breaking rate are explored. The spectrally resolved \[Lambda\](c) as well as bulk measurements of whitecap coverage and breaking rate are presented for a range of wind speeds and sea states. Observations are compared to Phillips' (1985) model of \[Lambda\](c) in the equilibrium range of the wave spectrum. The observed \[Lambda\](c) distributions are described well by the Rayleigh distribution for slow and intermediate speeds, yet fall above the Rayleigh distribution for the fastest breaking speeds. The dimensionless width of the \[Lambda\](c) distribution increases weakly with dimensionless fetch, while the total length of breaking per unit sea surface area decreases with dimensionless fetch for intermediate to fully-developed seas. The integral and the first two moments of \[Lambda\](c) show a strong correlation with the active whitecap coverage and active breaking rate. The active whitecap coverage falls in the range of previous observations, but with a stronger dependence on wind speed. The breaking rate plotted against the spectral peak steepness is in agreement with previous observations (Banner et al., 2000)