Observing and Quantifying Lifetime Stages of Whitecaps using Infrared Imagery

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Wednesday, 7 January 2015
Henry Potter, NRL, Washington, DC; and G. Smith, C. M. Snow, and M. D. Anguelova

Whitecaps formed by waves breaking and air entrainment are dynamic features on the ocean surface which evolve quickly and thus have markedly different properties in different lifetime stages. Active whitecaps (aka stage A or young whitecaps) accompany the turbulent mixing in the moment of wave breaking. In contrast, residual (aka stage B or mature) whitecaps are almost motionless foam patches left behind the breaking wave. The relative contributions of active and residual whitecaps to various air-sea interaction processes differ. Therefore, to properly parameterize modifications of momentum, heat, and mass transfers across the air-sea interface in presence of breaking waves, it is desirable to separately quantify the fraction of the ocean surface covered by active and residual whitecaps.

We pursue the separation of active whitecap fraction WA from total (active plus residual) whitecap fraction W with two approaches. Physical basis of our theoretical approach is the Phillips concept of breaking wave statistics which connects active WA with the energy dissipation rate of breaking waves. Physical basis of our experimental approach is the distinct signature of active and residual whitecaps at infrared (IR) wavelengths. To this end, a multi-instrumental field campaign was conducted in April-May, 2012, on the Floating Instrument Platform (FLIP). The instruments deployed included sensors recording the whitecaps and breaking waves on the surface over wide range of wavelengths: visible (video cameras), infrared (IR camera), microwave (radiometers at two frequencies, 10 GHz and 37 GHz), and acoustic (vertical line array of hydrophones at frequencies of 600, 1200, and 2400 Hz). The main goal of the field campaign was to verify the previous serendipitous observations of active and residual whitecaps as bright and dark patches in IR images and correlate them with time series of other observations.

We will present results from processing and analyzing IR images from the FLIP measurements. We identified 28 events of breaking waves from both visible and IR images in which active and residual whitecaps are clearly distinguished in IR images, but not in the visible. We use the IR images to quantify the lifetime stages and characterize properties of active and residual whitecaps. With such knowledge, we can use IR observations as a tool to identify separation criteria in radiometric and acoustic data which are suitable for remote sensing and long-term monitoring of breaking waves.