Geology Reference
In-Depth Information
10
0.9
8
0.8
6
Site 1 FY ice
Site 2 FY ice
Instantaneous albedo
0.7
4
0.6
2
0.5
0
0.4
-2
June 7
-4
135 40
0.3
145 50 155 60
Day of the year
165
170 75
Figure 7.42
Time series of the deviation of backscatter coefficient of snow‐covered, smooth FY ice in the Arctic
from the typical winter values ( ). Backscatter was measured using Radarsat‐1. The coincident measured
albedo is also shown. Inverse correlation between
and the albedo is visible after 7 June [
Yackle et al.
, 2007,
Figure 9, with permission from Wiley].
the increase of dielectric constant near the surface follow-
ing an increase in snow wetness. This leads to an increase
in surface scattering, although the wetness attenuates the
volume scattering.
Thermal infrared data are also used to discriminate
between open water and sea ice based on the thermal con-
trast between the two objects. This is particularly true in
the polar region where the contrast becomes sharper. While
the surface temperature of the bare ice surface is mainly
determined by the air temperature, the surface temperature
of the open water is nearly constant in winter (not far
above the freezing temperature of the seawater; −1.8 °C).
This may create a sharp enough temperature contrast to
allow discrimination between ice and water. Nevertheless it
will not be as sharp as the contrast in albedo. Moreover,
thickness‐based ice types cannot be distinguished in TIR
images as there is not enough thermal contrast between
their surfaces. The only exception is the new ice types
(<10 cm thick), which has its surface temperature typically
not far below the freezing temperature. In the presence of
snow, thermal information of the underlying ice is masked.
caused mainly by the snow wetness and the relatively high
salinity at the snow base. For all operational passive
microwave frequencies, the penetration depth of micro-
wave radiation in the snow decreases as the snow wetness
increase (see section 8.5). Beyond a certain upper limit of
snow wetness or grain size the snowpack masks to a great
extent the information about the underlying ice. That
affects the retrieval of ice concentration, thickness, and
surface temperature. Effects of the above‐mentioned fac-
tors on the microwave emission and scattering from snow
cover sea ice are discussed in the following. Details on
microwave emission and radar backscatter from snow on
sea ice are presented in
Markus and Cavalieri
[1998] and
Langilois and Barber
[2007]; respectively.
Dry snow is not a lossy medium, so it allows more emis-
sion from the underlying ice to penetrate the snowpack;
however, it also causes more scattering of the signal within
the snow volume. The net effect is that fresh snowfall
causes a decrease in brightness temperature for both H and
V polarizations, especially in lower frequencies [
Grenfell
and Comiso
, 1986]. However, the polarization difference
remains generally small and independent of snow depth.
A related observation was reported in
Shokr et al
. [2009]
in a laboratory study of microwave emission from simulated
thin ice grown in an outdoor tank. In two independent
experiments in 2001 and 2005 they observed a sharp drop
in brightness temperature when fresh snow falls on thin ice
(<5 cm thick) surface (Figure 10.24). In both experiments
the surface temperature was relatively high (>−5 °C). The
drop was sharper from the lower frequency channels
(19 GHz). It was followed by a gradual increase as the
snow settled until the brightness temperature regained its
typical value for thin ice. Unlike the observations by
Grenfell and Comiso,
[1986], the sharp drop in brightness
temperature found in
Shokr et al
. [2009] was associated
with an increase in the polarization difference. This implies
7.7.3.2. Microwave Region
The factors that influence the microwave interaction
with the snowpack over sea ice (passive or active) include
snow density, grain size (which is related to density), snow
wetness and salinity, and ice layering at the surface or
within the snowpack (ice crust, lenses, and glaze). Snow
depth can be added to the list, but it has to be qualified
using other properties of the snow, i.e., the wetness or the
dominant grain size within the given depth.
As microwave signal propagates in the snowpack, it
may undergo scattering and absorption. Scattering is
generated by dielectrically mismatched elements of
millimeter scales in the case of passive microwave and
centimeter scale in the case of radar. The absorption is
