Deriving Ocean Color From Satellite
(Correcting For Atmospheric Aerosols)
As water changes from clean mid-ocean conditions to coastal areas
(with soil particles) or highly productive waters (with phytoplankton),
the ocean color changes from blue to green. By observing this change in
color it is possible to estimate phytoplankton concentrations (in cases
where soil particle amounts are known to be small). In both blue
and green waters, the ocean appears fairly dark when viewed from
satellite and light scattering from atmospheric aerosols contributes a
significant portion of the light seen by the satellite. In
order to derive phytopankton concentrations from satellite,
the aerosol contribution to the satellite radiance must be removed.
This is traditionally accomplished by measuring the satellite radiance
at two longer wavelengths (~760 and 870 nm) ,where the ocean is assumed
to be dark (due to increased absorption by water). , Next the
slope of the aerosol radiance at these two longer wavelengths is
used to select an aerosol model from some relatively small set of
aerosol possibilities. Using the selected aerosol model, the aerosol
radiance at shorter wavelengths is calculated allowing one
to obtain the water leaving radiance in regions where the water is less
absorbing (~450-550 nm). Atmospheric correction reamins a challenge in
current NASA products. This is evident in SeaWifs images processed
near Hawaii. When the Hawaii volcano plume is present in some
area, then the water leaving radiances predicted by the ocean
color algorithm are significantly higher. This is
illustrated in the figures below. The top figure shows two
regions of large aerosol loading (downwind of Hawaii Island and
north of Oahu). The bottom figure shows the water leaving radiance
calculated from the SeaWifs algorithm (after removing the aerosol
contribution). The regions with large aerosol loading are also the
regions with large water leaving radiance. This problem is
discussed in Porter et al., (2002) explaining the problem which occurs
due the selection of poor aerosol models. A likely solution is
also discussed.
