Geology Reference
In-Depth Information
kilometers. This is too large to have a uniform sea rough-
ness scale.
Ocean foam increases the microwave emissivity in both
polarizations over the entire range of incidence angle.
This has been shown in a study by
Camps et al.
[2005] on
the emissivity of foam‐covered water surface at the micro-
wave L‐band. They found that the foam‐induced emissiv-
ity increases at a rate of approximately 0.007 per millimeter
of foam thickness when extrapolated to the nadir direc-
tion. They also found that the foam cover increases the
polarization difference with respect to the foam‐free
surface. This is contrary to a finding by
Stogryn
[1972],
which confirmed that foam causes a significant decrease
in polarization difference.
Ulaby et al
. [1986] present data
showing emissivity increase due to the presence of foam at
20 GHz frequency as a function of the ratio of foam layer
thickness to the wavelength of the passive microwave
emission. At this frequency, the emissivity increases from
0.42 over foam‐free open water to 0.92 over a foam layer
with the above‐mentioned ratio equalling 1. The emissiv-
ity saturates at this level for a thicker foam layer.
bond together, squeezing the air between them. The density
of the metamorphosed snow increases accordingly. Snow
metamorphism takes one or more of the following forms:
snow grains, ice lenses, surface crust layer, and surface
glaze. Except for the surface crust, which is wind driven, all
other formations are thermodynamically driven.
Snow grains are formed as a result of a high‐temperature
gradient within the snowpack; the higher the gradient the
faster the growth rate of the grain. High‐temperature
gradient (in excess of 0.3°/cm) generates vapor diffusion
(snow sublimation). The vapor transport from the snow
base toward the surface, resulting in a fast growth rate of
grains with dendritic structure and faceted shapes. This
kind of growth takes place more often at the snow base
and is responsible for forming what is known as the hoar
layer. In areas where temperature gradient is low, usually
near the snow surface, grains tend to form into well‐
rounded tiny grains with fine texture. Snow may develop
into grains also as a result of midwinter melt‐refreeze
cycles of the snowpack. Another form of snow metamor-
phism is ice lenses. These are frozen layers of water that
accumulate at the snow surface or within the snowpack
through drainage. They tend to reduce the bulk tempera-
ture of the snow and create impermeable ice barrier lay-
ers. Snow crust is wind‐generated snow packing formed
at the surface. Compacted crusts may form a wavy pat-
tern known as sastrugi, but this has no impact on remote
sensing observations. Glaze is a smooth impermeable sur-
face, which is developed at the snow surface when rainwa-
ter freezes. All forms of snow metamorphism affect the
microwave scattering and emission properties, in some
cases significantly. Empirical data and modeling of these
phenomena are needed to understand and interpret
remote sensing observations.
The most significant metamorphism takes place at the
snow base. This influences all physical parameters in that
region. An example is shown in Figure 7.41 from a study
conducted by
Barber and Nghiem
[1999] that aimed at
exploring the effect of snow thermodynamic processes on
radar backscatter in the C‐band. The data in the figure are
representative of the shown parameters in the presence of
a metamorphosed saline layer (hoar layer) of the snow at
the ice interface. This layer (the bottom 4 cm in the figure)
features higher salinity due to the brine wicking mecha-
nism by the snow from the highly saline ice surface. The
hoar layer is also characterized by relatively large snow
grains (~4 mm in diameter). This is usually associated
with relatively low density since the large grains cannot be
packed as tightly as the fine grains. Snow density varies
significantly with its age and compactness as shown in
Table 7.7. Note that sea ice density occupies the range
830-917 kg/m
3
. The two components of the complex die-
lectric constant at the snow base increase following the
increase of brine volume at the ice/snow interface.
7.7.3. Snow on Sea Ice: Physical and Radiative
Processes
Snow is one of the most complex and changeable
substances on Earth. While ice exists in nature near its
melting point, snow exists near its “triple point,” meaning
that the transition between its three phases of solid, liquid,
and vapor can take place very fast. Dramatic and rapid
changes of snow occur from the instant snow hits the
ground. This starts a long process of metamorphism. The
process is triggered by changes in atmospheric tempera-
ture, pressure, humidity, and other forms of precipitation
that fall at the snow surface. Even a subtle change in any
of these parameters can have a significant effect on the
sow parameters and snow grains. That is how the presence
of snow modifies observations from the underlying sea
ice and complicates the ice parameter retrievals. Unless
the snowpack on sea ice is fresh and dry, its presence can
impact the remote sensing observations significantly.
Langlois and Barber
[2007] present a creditable review on
the current state of knowledge pertaining to the geophys-
ical, thermal, and dielectric properties of snow on sea ice
as well as the different microwave emission and scattering
mechanisms associated with it. The factors that affect the
reflection, emission, and microwave scattering include
snow metamorphism (which implies grain size and snow
density), depth, salinity, and wetness. Impacts of those fac-
tors on the observations are discussed in the following.
Snow metamorphoses under its own weight or when it
becomes wet in response to some meteorological effects
and then refreezes. In this case the snowflakes no longer
exist and are replaced by ice crystals (grains) that may
