Geology Reference
In-Depth Information
where
E
TOA
is the solar spectral radiance incident at the
TOA,
ρ
s
is the surface reflectance integrated over the sen-
sor's footprint,
T
is the total transmittance of the atmos-
phere due to scattering and absorption, and and
s
is the
reflectance of isotropic light entering from the base of the
atmosphere (also called atmospheric spherical albedo).
Combining equations (7.47) and (7.48) and by normaliz-
ing the radiance received by the satellite with respect to
the incident solar radiation (
E
TOA
T
cos
θ
), the reflectance
received at the TOA (
ρ
TOA
) can be written as
1. 0
0.8
Surface reflectance
0.6
Normalized irradiance
at 0 m
0.4
0.2
Normalized
irradiance at 1 m
T
(
,,,
)
TOA
,,,
(
)
T
(
, ,,
)
ts i
i oo
0.0
i
i oo g
a
i
i oo
1
(
,,,)
i oo
s
s
i
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
(7.49)
Figure 7.21
Measurements of surface reflectance from sea ice
surface, normalized underice irradiance taken directly below
the ice (0 m), and normalized underice irradiance taken 1 m
below the underside of the ice. Measurements were conducted
on fast ice in Kongfjorden, Svalbard, in March 2002 [
Winther
et al.
, 2004, with permission from the auhor].
where
ρ
a
is the contribution of the atmospheric reflec-
tance measured at TOA, and
T
t
and
T
g
are two‐way trans-
mittance due to scattering and gaseous absorption effects,
respectively. These parameters are examined in details in
Tanré et al.
[1992]. The unknown in equation (7.49) is the
surface reflectance
ρ
s
. This can be obtained by rewriting
the equation in the form
of phytoplankton blooms, upwelling bottom sediments,
and particle transportation from rivers [
Doron et al.,
2011]. In March 2002
Winther et al.
[2004] measured the
spectral reflectance in the VIS and NIR bands from fast
ice in Kongsfjorden, Svalbard. They present ice surface
albedo as well as the normalized irradiance from seawater
at two locations: directly below the ice sheet and 1 m
below the sheet (Figure 7.21). The surface reflectance
from sea ice does not change much between the VIS and
NIR spectral region (ice was covered by 1-2 cm snow
layer, hence albedo values in the figure are relatively
high). However, results show nearly total absorption of
the NIR radiation (wavelength >700 nm) by water. One
purpose of measuring the underice irradiance was to
demonstrate the intensities of the photosynthetically
active radiation (PAR) (400-700 nm). Figure 7.21 dem-
onstrates that the PAR at 1 m depth is about 50% of the
intensities measured directly under the sea ice interface.
y
/
1
y s
/
(7.50)
s
where
(
,, ,
T
TT
)
(
, ,,
)
TOA
i
i oo gai
i oo
y
(7.51)
ga
If the reflectance from the atmosphere and the two‐way
atmospheric transmittance of the radiation are given,
then equation (7.51) can be used to retrieve the surface
reflectance from the measured TOA reflectance. The
transmittance is determined mainly from the scattering
and absorption properties of the aerosol, ozone, and
water vapor. For a successful atmospheric correction the
ozone amount, water vapor content, and aerosol optical
depth should be accurately determined.
De Abreu
[1996]
presented details on atmospheric transmissivity for
AVHRR visible channels using the radiative transfer
model LOWTRAN 7 [
Kneizys et al.,
1989].
The albedo of seawater typically falls between 0.04 and
0.08, depending on the incidence angle of the incoming
solar radiation [
Ivanov
, 1979]. This is much lower than
the albedo from sea ice or snow, which falls between 0.4
and 0.8 (although small values around 0.08 are observed
for dark Nilas). Similarly, the reflectivity in the NIR spec-
trum shows the same contrast between open seawater and
sea ice. The NIR reflectivity of OW is near zero, although
it increases when the amount of suspended particles
becomes significant. This usually happens during events
7.5. tHermal infrared sensinG
Since TIR sensors can acquire data independent of
daylight, they become particularly useful in polar regions
during the dark winter seasons. However, similar to the VIS
sensors, they cannot acquire data about the surface under a
cloudy sky. Clouds emit and reflect significant amount of
TIR radiation. Most of the operating TIR sensors record
emitted radiation in the spectral band 10-12
μ
m wave-
length. For example, the radiometers onboard AVHRR
record observations in two bands: 10.3-11.3
μ
m (channel 4)
and 11.5-12.5
μ
m (channel 5). Similar channels exist on
MODIS: 10.78-11.28
μ
m (band 31) and 11.77-12.27
μ
m
