Geology Reference
In-Depth Information
(band 32). The Japanese counterpart, the Multifunctional
Transport Satellite (MTSAT), carries two channels with
10.3-11.3 and 11.5-12.5
μ
m wavelengths.
Thermal IR observations are suitable for discriminating
sea ice from the surrounding water because the emissivities
of water, ice, and snow are similar (around 0.96) in this
spectral region. This makes the observed radiation a
strong function of physical temperature. Since the emitted
radiation is proportional to
T
4
(equation 7.23), therefore a
small change in surface temperature causes a large change
in the radiated energy. If the ice surface temperature is
lower than the surrounding OW temperature by only 5%
(e.g., if the ice temperature is 260 K compared to the water
temperature of nearly 273 K), the measured
T
b
from the
ice will be lower by 21% [
Stringer et al.,
1984]. The differ-
ence between surface temperature of ice and water in
winter is usually high; typically >20 °C in the polar regions
and between 5 and 15 °C in subpolar regions. Snow cover
hardly affects the capability of ice‐water discrimination
based on TIR data because it has a slightly lower emissiv-
ity than ice, and a physical temperature not very far from
that of ice. This capability is lost in the summer when the
surface temperature of ice and OW becomes almost equal.
The most commonly retrieved parameter from the TIR
observations is surface temperature (also referred to as
skin temperature). Sea surface temperature (SST) can be
retrieved fairy accurately because the seawater emissivity
in the TIR region is very stable (around 0.98). Retrieval
of ice surface temperature (IST), on the other hand, can
be problematic because of possible changes of surface
emissivity in the presence of metamorphozed snow
(though the change is not significant). The surface tem-
perature is defined as the weighted mean temperature of
the radiating layer (i.e., penetration depth). For TIR this
layer is usually less than 1 mm deep. Interest in IST is not
limited to discrimination between ice and water. A cli-
matic record of ice surface temperature in polar regions is
an important indicator for revealing the decade‐scale
changes in sea ice extent and volume.
In order to obtain accurate estimates of IST from TIR
data, the observations have to be corrected to account for
atmospheric influences. The simplest equation that
describes the received radiance
L
(
T
b
) in the TIR region
can be written as [
Yu et al.
, 1995]
is nearly transparent to the 11.0
μ
m [
Yu et al.,
1995].
Therefore equation (7.52) can be reduced to
(7.53)
LT LT
b
() ()
sfc
s
If the satellite observation is presented in terms of bright-
ness temperature, the radiance in the left‐hand side (LHS)
can be replaced by the observation and in the right‐hand
side (RHS) becomes simply the physical temperature of
the radiating layer. Moreover, since the emissivity of sea
ice or seawater is virtually constant and very close to 1, the
estimation of ice surface temperature should be nearly
equal to the brightness temperature at 11.0
μ
m. This
approximation is confirmed in
Key et al.
[1997]. Equation
(7.53) is the simplest, first‐cut estimate of SST and IST
when the emissivity of the snow is known.
Yu et al.
[1995] estimated the IST from equation (7.53)
using data from the 11
μ
m AVHRR channel, assuming
appropriate values of the snow cover emissivity that var-
ied with the scan angle of the sensor. Assumptions also
include cold and dry ice surface under clear sky. The
authors also developed an empirical equation to calculate
the IST directly from the AVHRR TIR channel 4:
T
4 349
.
0 863
.
T
(7.54)
s
4
They verified the estimates from both methods against
measurements from drifting buoys and ice stations in the
central Arctic throughout the entire year of 1989 except
May. Figure 7.22 shows the scatterplot of
T
s
from using
equation (7.54) versus in situ surface air temperature
T
air
.
-10
Buoy
lce station
-20
-30
Correlation coeff.=0.96
Mean (
T
s
-
T
air
)=0.0
Stand. dev. (
T
s
-
T
air
)=1.68 (°C)
-40
LT LTTL TL
b
() ()
(
1
)
(7.52)
sfc
s
atm
atm
-40
-30
-20
-10
where
L
sfc
is the radiance emitted from the radiating layer,
L
atm
is the radiance emitted by the atmosphere,
T
s
is sur-
face temperature,
ε
is the surface emissivity, and
T
is the
atmospheric transmittance (equation (7.13)). The third
term represents the reflected downwelling radiation off
the surface. It is reasonable to assume that in the cold
season under clear sky conditions, the polar atmosphere
T
air
(°C)
Figure 7.22
Plot of surface temperature derived from AVHRR
using equation (7.54) (solid line) and the measured surface air
temperature from buoys and ice stations. Data obtained in the
central Arctic throughout 1989 (except May) [
Yu et al.,
1995,
Figure 3, with permission from AGU].
