Defining and quantifying microscale wave breaking with infrared imagery

Breaking without air entrainment of very short wind-forced waves, or microscale wave breaking, is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air-sea interface. However, quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping. In this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air-water interface, producing signatures that can be detected with infrared imagery. Simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process. The infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage, which is substantial under the moderate wind speed conditions investigated. The results imply that ”bursting“ phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts. Oceanic measurements show the ability to quantify microscale wave breaking in the field. Our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air-sea heat and gas fluxes, as well as unprecedented details on the origin and evolution of microscale wave breaking.

[1]  R. P. Cleaver,et al.  Crest instabilities of gravity waves. Part 1. The almost-highest wave , 1994, Journal of Fluid Mechanics.

[2]  William J. Plant,et al.  A model for microwave Doppler sea return at high incidence angles: Bragg scattering from bound, tilted waves , 1997 .

[3]  P. V. Danckwerts.  Significance of Liquid-Film Coefficients in Gas Absorption , 1951 .

[4]  Kristina B. Katsaros,et al.  The aqueous thermal boundary layer , 1980 .

[5]  Andrew T. Jessup,et al.  Modulation of ocean skin temperature by swell waves , 1996 .

[6]  M. Longuet-Higgins,et al.  Periodicity in Whitecaps , 1972, Nature.

[7]  P. M. Saunders,et al.  The Temperature at the Ocean-Air Interface , 1967 .

[8]  S. A. Thorpe,et al.  Dynamical processes of transfer at the sea surface , 1995 .

[9]  M. Longuet-Higgins,et al.  Capillary rollers and bores , 1992, Journal of Fluid Mechanics.

[10]  H. Yeh.  FREE-SURFACE DYNAMICS , 1995 .

[11]  James H. Duncan,et al.  The formation of spilling breaking water waves , 1994 .

[12]  M. Longuet-Higgins.  Shear instability in spilling breakers , 1994, Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences.

[13]  S. Kitaigorodskii.  On the Fluid Dynamical Theory of Turbulent Gas Transfer Across an Air-Sea Interface in the Presence of Breaking Wind-Waves , 1984 .

[14]  W. K. Melville,et al.  The Role of Surface-Wave Breaking in Air-Sea Interaction , 1996 .

[15]  G. Csanady.  The role of breaking wavelets in air‐sea gas transfer , 1990 .

[16]  D. Peregrine,et al.  Wave Breaking in Deep Water , 1993 .

[17]  Hiroshi Kawamura,et al.  Fine structure of laboratory wind-wave surfaces studied using an optical method , 1987 .

[18]  M. Skafel,et al.  Dependence of the Skin Layer Recovery Rate on Heat Flux and Turbulence , 1995 .

[19]  K. Okuda.  Internal flow structure of short wind waves , 1982 .

[20]  B. Jähne,et al.  On the parameters influencing air‐water gas exchange , 1987 .

[21]  William J. Emery,et al.  The Behavior of the Bulk – Skin Sea Surface Temperature Difference under Varying Wind Speed and Heat Flux , 1996 .

[22]  Ian S. Robinson,et al.  Review Article. The sea surface thermal boundary layer and its relevance to the measurement of sea surface temperature by airborne and spaceborne radiometers , 1984 .

[23]  J. Gemmrich,et al.  Small-scale surface streaming under natural conditions as effective in air-sea gas exchange , 1992 .

[24]  Hiroshi Kawamura,et al.  Turbulent structure in water under laboratory wind waves , 1988 .

[25]  C. J. Zappa,et al.  Infrared remote sensing of breaking waves , 1997, Nature.

[26]  H. Kawamura,et al.  Ordered motion in the turbulent boundary layer over wind waves , 1988, Journal of Fluid Mechanics.

[27]  O. M. Phillips,et al.  On the incipient breaking of small scale waves , 1974, Journal of Fluid Mechanics.

[28]  Dennis B. Trizna,et al.  Studies of dual polarized low grazing angle radar sea scatter in nearshore regions , 1996, IEEE Trans. Geosci. Remote. Sens..

[29]  Yasuhiro Murakami,et al.  Turbulence structure and mass transfer across a sheared air–water interface in wind-driven turbulence , 1993, Journal of Fluid Mechanics.

[30]  H. Yeh.  Vorticity Generation at a Fluid Interface , 1992 .

[31]  Y. Toba,et al.  Forced convection accompanying wind waves , 1975 .

[32]  O. M. Phillips,et al.  Wave breaking in the presence of wind drift and swell , 1974, Journal of Fluid Mechanics.