Geology Reference
In-Depth Information
T
T
down
1
(
)
e
(8.19)
b
,
down-ref
b
,
A first guess of 0.9 for ice emissivity would be appropri-
ate for use in equation (8.19) to determine
T
b
,down-ref
. Due
to the proximity of the sensor from the measured surface,
τ
can be assumed to be zero. The downwelling radiation
T
b
,down
should be measured coincidently with the tempera-
ture of the emitting layer. Accurate values can be
determined when the radiometer is pointed to the sky at
an angle equal to the angle of incident radiation from the
radiometer. Values of
T
b
,down
in 37 GHz were found to
vary between 7 and 30 K for clear sky [
Shokr et al.
, 2009].
These are relatively small values compared to
T
b
from ice
surface but not from open water because the latter is
much colder radiometrically. Emissivity can be more
accurately defined by using the temperature of the radiat-
ing layer instead of the surface temperature. The former
can be obtained by measuring the temperature profile in
the snow and ice throughout the penetration depth of the
microwave radiation.
The use of airborne microwave radiometers offers
an advantage over in situ measurements of
T
b
in that
the data can be obtained over a wider area. This approach
can be used to generate regional maps of emissivity.
However, unlike the in situ measurements, variations of
estimated emissivity cannot be accurately interpreted
since surface information may only be known approxi-
mately. Airborne sensors have a typical spatial resolution
of a few tens of meters and can map emitted radiation
over a swath of a few kilometers or tens of kilometers.
This resolution guarantees, to some extent, the homoge-
neous cover of the footprint and is sufficient to resolve ice
types and surface features such as fractures, ridges, hum-
mocks, and melt ponds on MY ice surface. Emissivity can
be estimated accurately from airborne data provided that
appropriate correction for atmospheric influences is per-
formed [
Haggerty and Curry
, 2001]. Satellite sensors, on
the other hand, provide a capability to map the surface at
a very large spatial scale (e.g., the entire Arctic region can
be mapped daily) yet at a much coarser resolution (the
footprint dimensions vary between a few kilometers and
a few tens of kilometers). This does not guarantee the
homogeneity of the surface cover within the footprint.
The heterogeneity of the footprint and the need for accu-
rate atmospheric correction of the observations make the
retrieval of surface emissivity from satellite data a chal-
lenging task. Atmospheric correction of microwave
observations, if needed, is presented in several studies
[e.g.,
Wilheit et al.,
1980;
Yang and Weng,
2011] and
briefly covered in section 7.7.1.
A recent study to retrieve microwave emissivity from
airborne and satellite observations is presented in
Mathew
et al.
[2008] and used in
Mathew et al.
[2009]. The study
Sea ice
Seawater
Figure 8.29
Schematic diagram showing downwelling, reflected
downwelling, and emitted microwave radiation from sea ice
equation (7.28).
radiation, from which the emissivity can be determined.
This has been conducted in field measurements of natu-
ral ice or laboratory experiments on simulated sea ice.
The radiometer can be mounted on a mobile (e.g., sled or
ship) or a fixed platform. SBR instruments are usually
mounted between 2 and 30 m above the surface (higher
mounting may be necessary in case of shipboard meas-
urements). At this height the spatial resolution varies
typically between tens of centimeters to a few meters.
This guarantees the homogeneity of the footprint and
therefore allows identification of the calculated emissiv-
ity with surface parameters if measured coincidently.
With this approach, effects of ice thickness, surface salin-
ity, snow depth, and grain size may be identified if some
controlled experiments are conducted. Another advantage
of using SBR instruments is the elimination of the need to
correct measurements for atmospheric effects except for
the reflected downwelling radiation (Figure 8.29). This is
due to the physical proximity of the instrument to the
surface. If the radiometer is mounted on a fixed platform
and pointed to a stationary ice field, the fluctuations in
the measurements are mostly caused by surface response
to changes in meteorological parameters or precipitation
events.
When using surface‐based radiometers, the emissivity
can be easily retrieved from the measured brightness
temperature using equation (7.28) after subtracting
the reflected downwelling radiation
T
b
,down-ref
from the
observed radiation
T
b
:
