High Wavenumber Ocean Wave Spectra Determined through Polarimetric Imaging

The small-scale sea surface roughness is fundamental to air-sea gas exchange processes as well as to satellite-borne microwave backscatter techniques for recovering global marine winds and waves. Additionally, refining radiative-transfer modeling 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. In this context, exciting new measurement methodologies now provide the opportunity to enhance present knowledge of sea surface roughness, especially at the microscale. Polarimetric Slope Sensing (PSS) is a passive optical remote sensing technique for recovering shape information about a water surface, in the form of a two-dimensional slope map. The PSS method uses the relationship between surface orientation and the change in polarization of reflected light to infer the instantaneous 2-dimensional slope across the field-of-view of an imaging polarimeter. For unpolarized skylight, the polarization orientation and degree of linear polarization of the reflected skylight provide sufficient information to determine the local surface slope vectors. We have demonstrated that the two-dimensional slope field of short gravity waves could be recovered accurately without interfering with the fluid dynamics of the air or water, and water surface features appear remarkably realistic.

Two intensive field experiments using R/P FLIP were staged within the ONR 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 highlights exciting new sea surface roughness measurement capabilities that underpin a number of the scientific advances resulting from the RaDyO program.

The observations gathered comprised standard marine environmental variables (winds, waves and fluxes). Our roughness measurements were highlighted by the first field deployment of our polarimeter for detecting wave fields down to 1.5 mm wavelengths at sample rates up to 60 Hz. These high resolution measurements were complemented by co-located scanning and fixed lidars. In addition, co-located video and infrared imagery gave an unprecedented coverage of the sea surface microstructure including breaking waves, over a range of wind speeds and underlying sea states.

We present open ocean wavenumber spectra for wavelengths from 3 mm to 1,000 mm obtained from our polarimetric imaging camera for a range of sea state conditions. We present results on the mean wavenumber spectrum and the modulation properties of the short-wave spectrum by the dominant waves.

These results highlight the complex interplay of wind and surface currents in shaping the small-scale roughness of the sea surface that suggests the traditional Cox-Munk framework may not be sufficient. These small-scale slope observations demonstrate the possible oversimplification of the classic Cox-Munk view that mss increases linearly with wind speed. The small-scale slope observations suggest the potential importance of upper-ocean currents in addition to the wind. These results are timely considering issues raised in the recent review article on the conundrums of capillary-gravity waves [Munk, 2009]. These field results demonstrate that the polarimetric camera gives a robust characterization of the fine-scale surface wave features intrinsic to wind-driven air-sea interaction processes.