SeaWiFS Oceanic Optical Property Retrievals in Low Aerosol Conditions
Algorithm description:
Under very clear atmospheric conditions, the Rayleigh-subtracted radiance in the
NIR approaches zero. When other uncertainties are factored-in, the retrieved aerosol
path radiances in the NIR may even go slightly negative. The impact of this is that
aerosol model selection becomes highly uncertain or the atmospheric correction fails
altogether in clear atrmospheric conditions. As a
result, the current atmospheric correction algorithm often fails to obtain ocean-color
retrievals in the best of atmospheric
conditions. A simple solution to this problem is to fix the aerosol type/model when the
aerosol path radiance in one or both of the
NIR bands approaches zero, and limit the aerosol radiance at 865nm to be greater than
or equal to zero. With these two changes, it
is possible for the atmospheric correction algorithm to procede when the retrieved
aerosol concenteration is effectively zero.
The aerosol type to which low aerosol pixels will be fixed is a simple white aerosol,
i.e.: rho_a(lambda) = rho_a(865). The
threshold below which the aerosol model will be fixed (MSl12 parameter rhoamin) has
been set at a very conservative value of
rho_a(NIR) = 0.0001. For the 765 and 865 nm channels, this reflectance value corresponds
to slightly greater than 1 digital count.
Raising this value will force a larger percentage of the pixels to assume white aerosols,
thus bypassing the Gordon & Wang aerosol
model selection. The threshold of 1-count is sufficient to allow the aerosol type to be
determined with reasonable accurracy,
because the epsilon-targeted NIR noise reduction filtering makes it possible to
distinguish gradations in the relative aerosol
reflectance of the NIR channels at the sub-count level. If this filtering is not
applied, the threshold would need to be increased.
LAC scene over South Africa, S2001171101913.L1A_HPRE
1. AOT(865)
2. angstrom(510,865)
3. chlorophyll-a
4. nLw(412)
5. nLw(443)
6. nLw(490)
7. nLw(510)
8. nLw(555)
Bryan A. Franz
Last modified: Fri Feb 8 11:03:48 EST 2002
