| The ocean surface roughness plays an important role in air-sea interaction. It is the major
cause of wind drag, thus affecting the mass, momentum and energy transfers across the
air-sea interface. It is also an important topic in remote sensing of the ocean environment
because the electromagnetic (EM) emission or scattering is modified by the surface
roughness condition. The primary contribution of the ocean surface roughness is from surface
waves with length scale much shorter than the energetic wave components near the peak of
surface elevation spectrum. The physical mechanisms governing the evolution of short scale
waves are very different from those for the energetic long waves, on which the major
research efforts of surface wave dynamics have been focused. For example, for
long waves, the three major source functions of the wave energy or wave action
conservation equation are wind input, wave breaking dissipation and four-wave nonlinear
resonance interaction. For short waves, nonlinear resonance interaction occurs at
the three-wave level and wave breaking is in fact a generation term more than a
dissipation term. The process of breaking generation of surface roughness can be
envisioned as a combination of impulses of breaking water jets, surface disturbances
and bubble plumes propagating downward from the wave crest and the subsequent
rising of the entrained bubble clouds to the water surface. The complex sequence of
events creates surface roughness covering a broad range of length scales of short
waves.
In this presentation, results from microwave remote sensing are used to help improve our
understanding of the ocean surface roughness properties. In active remote sensing, the
dominant backscattering mechanism is Bragg resonance. The secondary mechanism is the
modification of local incidence angle caused by the longer waves tilting the Bragg resonance
surface roughness components. In passive remote sensing, the situation is much more
complicated because the bistatic nature and mutiple EM sources. The two modes
of remote sensing are connected through the Kirchhoff’s law, which relates the
emissivity and reflectivity of the medium (emissivity = 1 – reflectivity), but for
emissivity computation, the reflectivity is an integral over the upper hemisphere of the
bistatic scattering coefficients in the reciprocal active scattering problem. The surface
roughness contributing to passive remote sensing thus covers a much wider spectral
band than it does in the active mode. Both modes of remote sensing can provide
invaluable information about short scale surface waves in sub-centimeter to decimeter
wavelengths that are the main contributors of ocean surface roughness, and forms
the basis of obtaining global ocean surface wind velocity measurements using the
active and passive microwave sensors. Presently, in the derivation of wind speed
from altimeter, scatterometer, or radiometer output, operational algorithms rely
on empirical relations established from correlating collocated and simultaneous
datasets of in situ wind speeds and backscattering cross sections. The physics of wind
generation of waves and emission or scattering of EM waves from the ocean surface are
totally avoided. With the empirical approach described above, there is little room for
improvement in the accuracy of the derived geophysical parameters even when
major enhancements in sensor hardware and software have been implemented.
Oceanographic community can contribute to improved global wind products, which drive the
surface wave models, by advancing our understanding of the dynamics of short scale
ocean surface waves, especially those in sub-centimeter to meter length scales. |