Geology Reference
In-Depth Information
hypothesis of the increased surface scattering and
decreased volume scattering from the snow as wetness
increases was verified in a study by
Koskinen et al
. [2000]
using a snow backscattering model combined with the
Helsinki University of Technology (HUT) semiempirical
forest backscattering model [
Pulliainen et al.
, 1999]. The
study incorporated ground‐based measurements from a
forest test site in Finland. The measured parameters
include physical properties of snow and the underlying
soil. Results are shown in Figure 7.45. Although the data
are not for snow on ice, they provide useful clues in the
absence of a similar study for snow on ice. As snow wet-
ness increases, the contribution of surface scattering will
eventually exceed the contribution of volume scattering.
The value of snow wetness at which the surface scattering
starts to exceed volume scattering is about 4.6%. The
range of the snow wetness up to 7% defines what is known
as the pendular snow regime. This is observed when water
remains in the interstices of the snow before it starts to
drain. Note the sharp linear decrease of the backscatter
coefficient over the lower range of the snow wetness
(<1.5%), which is followed by a nearly constant value.
An example that shows the effect of snow metamor-
phism on the emitted microwave and radar backscatter is
presented in
Shokr and Agnew
[2013] and reproduced in
Figure 7.46. The top two panels show mosaics of bright-
ness temperature
T
b
,36
h
from AMSR‐E and backscatter
hh
typical values. The short duration of the anomaly con-
firms that snow on MY ice restores its properties as soon
as the cold temperature is restored. Snow on MY ice has
no salinity (so there will be no loss of brine during the
warm temperature) and has already metamorphosed into
snow grains so the duration of the warm spell will not add
or alter the metamorphism.
The same study by
Shokr and Agnew
[2013] presents
another anomaly of the microwave signature of snow on
FY ice caused by warm air temperatures during the
spring. Unlike snow on MY ice, the snow on FY ice usu-
ally acquires salinity by wicking up brine from the under-
lying ice surface through capillary action [
Drinkwater
and Crocker
, 1988]. Warm atmospheric temperature will
not only increase the snow wetness but will also cause
brine drainage from the snow on FY ice.
Shokr and
Agnew
[2013] found that this scenario lead to a decrease
of brightness temperature and increase of backscatter
from QuikSCAT. They also noted that these changes are
not reversed after the return of the cold temperature. This
is unlike the changes of the same parameters (namely,
increase of brightness temperature and decrease of back-
scatter) from the snow on MY ice in response to warm
temperature. Warm temperature causes irreversible
changes of snow properties on FY ice because it removes
some brine contents, and consequently the changes in
brightness temperature and backscatter are not recover-
able. The snow on MY ice is not saline, and therefore the
microwave signatures are restored after the restoration of
the cold temperature.
In general, the impact of the interacting physical
processes in the snow on the microwave emission and
scattering remains to be understood. Findings from the
many studies of snow on sea ice continue to raise ques-
tions more than providing answers. For example, the
refreezing of liquid water in the snow decreases the
attenuation and increases the scattering of microwave
emission, leading to an increase in
T
b
[
Tonboe et al.,
2003].
Yet, refreezing of liquid water may also lead to the for-
mation of ice layering within the snowpack and ice crust
near the surface of the snow base. This causes significant
scattering and consequently a decrease of
T
b
especially at
lower frequencies (≤37 GHz) [
Comiso et al.
, 1997]. This
effect is more pronounced for the horizontal polarization
at high incidence angles due to the lower penetration
depth and stronger snow layering effects [
Hallikainen
,
1989;
Langlois
&
Barber
, 2007].
Montpetit et al
. [2013]
studied the effect of ice lenses within a snowpack on
microwave emission.
0
from QuikSCAT, constructed from all available orbits
on 18 September 2007. An anomaly signature of the MY
ice at the central Arctic is indicated by the solid arrows. It
features higher‐than‐normal
T
b
,36
h
in the AMSR‐E image
and a corresponding lower‐than‐normal
0
in the
QuikSCAT image. These anomalies are linked to a north-
ward heat wave caused by a large cyclone just north of
Greenland that brought warm southerly air (temperature
around 0 °C) into the region as shown in the near‐surface
temperature map (12 m above the surface). The map is
produced using the Canadian Global Environmental
Multiscale (GEM) weather model. The figure shows the
warm spill coinciding with the location of the anomalies
in the images. The warm air raises the physical tempera-
ture of the snow, but this increases the snow wetness and
leads to a decrease in the emissivity. The net effect in this
case is an increase in brightness temperature. The snow
wetness also attenuates the radar backscatter as shown
in the QuikSCAT image in Figure 7.46. Three days later
(on 21 September 2007) the atmospheric temperature at
the same location of the anomaly decreased to values
below −10 ° C and the satellite signatures returned to their
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