Chapter 7: APPLICATIONS
7.2.2 Influence of bubbles on optical scattering in the upper ocean
The top few meters of the ocean are most important for remote sensing of
ocean color. Morel’s and Prieur (1977) classification into Case 1 and Case 2
waters, however, ignores the fact that the optical scattering in the nearsurface waters can be largely modified by the presence of bubbles. Air
bubbles efficiently scatter light in water because their refractive index is
considerably less than that of the surrounding water, and their size is
typically large compared to wavelengths of light. The intermittent nature of
bubble entrainment depending on near-surface turbulence and, to some
extent, on biochemical processes leads to strong fluctuations in light
scattering near the ocean's surface (Terrill et al., 2001).
Air bubble concentration and size distribution (i.e., major determinants
of optical scattering) vary greatly in time and space as a function of wind
and wave conditions. Large areas of the ocean surface may experience
significant bubble entrainment at any given time.
Part of the error in the data products generated by existing ocean color
algorithms can be related to variable concentrations of air bubbles in the
near-surface layers; the existing parameterizations of backscattering
coefficient in the semi-analytical algorithms for inverting reflectance
measurements are most likely biased by the presence of the wind-wave
dependent bubble backscatter. In addition, submerged bubbles can influence
the validation of atmospheric correction, which is based on the comparison
of in situ and satellite derived water-leaving radiances (Terrill et al., 2001).
Stramski (1994) assessed the significance of bubbles for the scattering of
light in the ocean in quiescent seas (no breaking waves). He found that even
the biologically formed and stabilized bubbles that dominate the bubble
population during quiescent sea states can contribute to the total scattering
and backscattering coefficients of seawater on the order of 1-10 %.
Zhang et al. (1998) studied the optical properties of bubbles coated with
an organic film. The coatings exerted a small influence on total scattering by
bubbles larger than 10 Pm in size. The estimated effect on backscattering
was found as being much more significant: the backscattering efficiency
enhanced by as much as a factor of 4 for coated bubbles.
Terrill et al. (2001) performed light scattering calculations for acoustically
measured bubble size distributions and size-dependent scattering efficiencies
in rough weather conditions. Light scattering from a single air bubble in
water was determined from the Mie theory (Chandraesekhar, 1950), which
allowed for the solution of the angular distribution of light scattered from the
bubble when illuminated by a monochromatic source. The Mie solution to
the angular distribution of light scattering from a sphere operates with the
radiant energy either completely scattered into all directions, b
Q , or the
453
7.2.2 Influence of bubbles on optical scattering in the upper ocean
The top few meters of the ocean are most important for remote sensing of
ocean color. Morel’s and Prieur (1977) classification into Case 1 and Case 2
waters, however, ignores the fact that the optical scattering in the nearsurface waters can be largely modified by the presence of bubbles. Air
bubbles efficiently scatter light in water because their refractive index is
considerably less than that of the surrounding water, and their size is
typically large compared to wavelengths of light. The intermittent nature of
bubble entrainment depending on near-surface turbulence and, to some
extent, on biochemical processes leads to strong fluctuations in light
scattering near the ocean's surface (Terrill et al., 2001).
Air bubble concentration and size distribution (i.e., major determinants
of optical scattering) vary greatly in time and space as a function of wind
and wave conditions. Large areas of the ocean surface may experience
significant bubble entrainment at any given time.
Part of the error in the data products generated by existing ocean color
algorithms can be related to variable concentrations of air bubbles in the
near-surface layers; the existing parameterizations of backscattering
coefficient in the semi-analytical algorithms for inverting reflectance
measurements are most likely biased by the presence of the wind-wave
dependent bubble backscatter. In addition, submerged bubbles can influence
the validation of atmospheric correction, which is based on the comparison
of in situ and satellite derived water-leaving radiances (Terrill et al., 2001).
Stramski (1994) assessed the significance of bubbles for the scattering of
light in the ocean in quiescent seas (no breaking waves). He found that even
the biologically formed and stabilized bubbles that dominate the bubble
population during quiescent sea states can contribute to the total scattering
and backscattering coefficients of seawater on the order of 1-10 %.
Zhang et al. (1998) studied the optical properties of bubbles coated with
an organic film. The coatings exerted a small influence on total scattering by
bubbles larger than 10 Pm in size. The estimated effect on backscattering
was found as being much more significant: the backscattering efficiency
enhanced by as much as a factor of 4 for coated bubbles.
Terrill et al. (2001) performed light scattering calculations for acoustically
measured bubble size distributions and size-dependent scattering efficiencies
in rough weather conditions. Light scattering from a single air bubble in
water was determined from the Mie theory (Chandraesekhar, 1950), which
allowed for the solution of the angular distribution of light scattered from the
bubble when illuminated by a monochromatic source. The Mie solution to
the angular distribution of light scattering from a sphere operates with the
radiant energy either completely scattered into all directions, b
Q , or the
453