Geology Reference
In-Depth Information
Accurate mapping of albedo over the Arctic has become
increasingly important for climate models as evidence of
global warming are being confirmed. Based on the increas-
ing mean atmospheric temperature in the Arctic with more
temperature fluctuations around the melting point of the
ice, more snow metamorphism is expected. This affects
albedo as well as TIR and microwave emission. Albedo
decreases slowly with increasing wavelength in the visible
region and more rapidly in the near‐infrared region. It can
reach low values below 0.1 for NIR wavelengths between
1.5 and 2.0
μ
m. For that reason, NIR data are used to dis-
criminate between snow and ice on one hand (with their
low albedo) and clouds on the other hand (with their rela-
tively high albedo) [
Sandven and Johannessen
, 2006]. The
albedo of a thin snow layer depends on the albedo of the
underlying ice surface. When the snow becomes optically
thick, the effects of the underlying surface can be ignored.
In addition to the wavelength of the incident radiation, the
optical depth of the snow depends primarily on the snow
grain size. A snow layer of 10 cm depth would make it
impossible to identify the underlying ice type in the visible
or NIR wavelengths [
Perovich
, 1991].
Snow density indirectly affects the optical properties of
snow through the grain size. As described above, the grain
size increases as the snowpack ages. It increases from
about 50
μ
m for new snow to about 1 mm for old snow
that has gone through wetness and re-freezing. This results
in an increase of density and a decrease in albedo. A use-
ful relationship that governs the growth of grain diameter
(
D
) with time (
t
), starting from an initial diameter (
D
0
) is
presented and verified in
Wiscombe and Warren
[1980]:
more sensitive to the grain size at the very top snow layer
(<5 cm), while the albedo of the visible bands is sensitive to
the grain size of a much thicker snow layer. Therefore, the
NIR solar irradiance plays a major role in the energy bal-
ance at a snow surface and consequently snowmelt.
Liquid water in snow does not affect the albedo as much
it affects the emission in the TIR. It decreases the albedo
slightly, but when the snow starts to melt the albedo starts
to decrease sharply and that leads to more snowmelt (i.e.,
a positive feedback). Wet snow is encountered more in
the snow cover on young ice during fall or on thicker ice
in the spring. Liquid water in snow is a catalyst for snow
grain clustering. Upon refreezing, larger effective grain
sizes of the snow causes more decrease of albedo. This
was verified using field measurements obtained in the
Antarctic during the austral summer of 1999 [
Zhou et al
.,
2001]. It is worth mentioning that the albedo of the snow
on glaciers and ice sheets can be reduced due to the pres-
ence of powdery wind‐blown dust made of a combination
of small rock particles and soot, called cryoconite.
The long periods of low Sun elevation in the polar areas
limit the possibility to estimate the surface albedo using
optical data. On this account,
Yackle et al.
[2007] explored
the correlation between albedo and the backscatter from
snow‐covered sea ice, hoping to develop a capability to
retrieve albedo from SAR backscatter measurements.
The study used an extensive data set collected during the
SIMMS field program from 1992 to 2001 except, years
1996, and the Collaborative‐Interdisciplinary Cryospheric
Experiments (C‐ICE) from 1999 to 2001. All data were
obtained from Lancaster Sound near Resolute Bay in the
Canadian central Arctic. The authors modeled the stable
winter ice backscatter
DD te
bT
2
2
/
(7.106)
0
as a function of the radar inci-
dence angle and measured the seasonally evolving back-
scatter
σ
0
. They found an inverse correlation, valid during
the early melt season, between the optical albedo and
the deviation of the backscatter from its typical winter
values. That deviation is expressed as
0
w
where
T
is the snow temperature in Kelvin and
a
and
b
are
constants. The grain size of the snow is the most effective fac-
tor for determining the scattering pattern of the optical and
infrared radiation. In an early study on the optical properties
of snowpack,
Bohren and Barkstorm
[1974] found that most
of scattering of the light beam in the snow is the result of
change in direction upon transmission through snow grains
rather than reflection. They derived a relationship between
the diffuse albedo
α
d
and the radius of the grain size
r
:
0 0 0
() ()
, where
θ
is the radar incidence angle. Figure 7.42 displays
this relationship. It shows time evolution of
W
w
0
meas-
ured from Radarsat‐1 images over two test sites of smooth
FY ice and the “instantaneous” insitu measurements
from the snow‐covered sea ice within 15 min of the satel-
lite overpass. The satellite measurements were obtained
from a variety of beam modes using ascending passes
only. The incidence angle was accounted for. The inverse
correlation between
W
cr
/
12
1
(7.107)
d
where
c
is a constant.
Wiscombe and Warren
[1980] exam-
ined the validity of this equation and concluded that it
held for a visible wavelength (0.4-0.8
μ
m) but not for longer
wavelengths. They presented plots showing the effect of
snow grain size and wetness on albedo (see section 8.3).
They also presented a model for calculating the spectral
albedo of snow in the VIS/IR spectrum. It accounts for the
direct or the diffuse incident radiation at any zenith angle.
Zhou
[2002] found that the albedo in the NIR bands is
0
and the albedo measurements
is visible in the data of the early melt season (7 June-21
June). The backscatter difference increases during this
period by up to 8 dB, and the albedo decreases from about
0.8 to 0.3 during the same period.
Yackle et al.
[2007]
attributed the increase in backscatter to two factors: the
growth of snow grains at the snow basal plane, which
causes large volume scattering. It can also be attributed to
W
