Geology Reference
In-Depth Information
Since
T
sp
is extremely small (approximately 2.7 K), the
fourth term in equation (7.60) is usually neglected. In the
case of thick ice (say, ice thicker than 30 cm),
T
RL
can be
approximated as 0.9
T
air
, but this is not always a valid
assumption. Given an observation of
T
b
, equation (7.60)
can be solved for
T
RL
or
RL
. If a relation between
T
RL
and
T
air
is available, the equation can be solved for
RL
.
Svendsen et al.
[1983] suggested the following relation;
temperature is presented in section 10.2.2. The presence of
clouds usually does not affect the received radiation emit-
ted from the surface. Over consolidated ice fields in the
Arctic, the atmosphere is usually dry with prevailing stra-
tus clouds that contain mostly ice crystals. These clouds
are almost fully transparent in the microwave region.
However,
Gloersen et al.
[1973] mention that clouds increase
the satellite observation from Arctic sea ice, in general, by
about 7 K.
)
(7.62)
TT
RL
272 (
air
7.6.2. Active Microwave
The authors obtained
T
air
using drifting buoys in an area
north of Ellesmere Island in the Arctic. They estimated
α
to be roughly 0.4 from using SMMR measurements.
Zwally et al.
[1983] used a different equation:
Active microwave is also known as radar, a term coined
in 1941 by the U.S. Navy as an acronym for radio detec-
tion and ranging. Radar systems are traditionally used
for object detection (i.e., aircrafts, ships, guided missiles,
heavy rain, etc.). In remote sensing, there are three types
of radar systems, all used in sea ice applications: imaging
radar, profile radar (or scatterometer), and radar altime-
try. Imagery data can also be generated from scatterome-
ter observations. Radar systems measure the scattering of
an incident radar signal that travels back to the receiving
antenna. This is known as backscatter. If the same
antenna is used to transmit and receive the data, the sys-
tem is called monostatic. Generally speaking, strong
backscatter is produced by rough surfaces or from a vol-
ume that has numerous scattering elements. Data from
imaging radar systems are frequently used in monitoring
sea ice. The systems offer several modes of imaging with
a variety of resolutions and swath widths. The Canadian
Ice Service uses the Radarsat ScanSAR mode as a prime
data source in its sea ice operational monitoring program.
This mode features a swath width of 500 km and a spatial
resolution of 100 m. Scatterometer systems, on the other
hand, are designed to measure wind speed and directions
over the ocean. However, they have proven very valuable
in monitoring sea ice in the polar regions and relate the
annual variability to climate change. Most scatterometers
operate in the Ku‐band (13.4 GHz). A scatterometer sys-
tem that has been used frequently in sea ice applications
was NASA's QuikSCAT. Radar altimeters are nadir‐
looking instruments designed to generate topographic
mapping of glaciers and ice shelves. They have been used
to map ice thickness in polar regions. An example of such
systems is ESA's Cryosat. Another type of altimeter,
which is also available for sea ice applications, is the laser
altimeter onboard ICESat (see section 7.2).
)
(7.63)
TTfT T
w
(
RL
air
air
where
T
w
is the freezing point of seawater, and
f
is an
empirical parameter determined using brightness temper-
ature observations.
Zwally et al.
[1983] examined ice con-
centration values derived from ESMR data and adjusted
f
' until the correct concentration value was reached. They
found that 0.25 is a reasonable value for use in both Arctic
and Antarctic regions. In reality, the magnitude of
f
varies
with the thickness of the ice and snow.
Due to their coarse resolution (a few kilometers or tens
of kilometers), footprints of space‐borne passive micro-
wave radiometers are almost always heterogeneous. A
basic linear model that decomposes the observed bright-
ness from a heterogeneous footprint into its components
from each surface in the heterogeneous footprint is com-
monly used. If a footprint is composed, for example, of
OW, FY ice and MY ice, with concentrations
C
OW
,
C
FY
,
and
C
MY
, respectively, then
T
b
can be written as
TCTCTCT
b
(7.64)
OW
b
,
OW
FY
b
,
FY
MY MY
b
,
where
T
b,OW
,
T
b,FY
and
T
b,MY
are typical brightness tempera-
tures (tie points) from the relevant surface type. The above
equation is used in many algorithms for retrieval of ice
concentration. The estimation of the tie points is usually
achieved by sampling brightness temperature from areas
of uniform surface cover (ice type or OW). However, some
ice types, such as young ice, exhibit a wide range of bright-
ness temperature under different atmospheric tempera-
tures or other meteorological conditions. The brightness
of OW is a weak function of water salinity and tempera-
ture. Ocean foam, wind‐driven surface roughness, or rain-
fall cause changes to emissivity or the scattering pattern of
the surface, which widen the range of brightness tempera-
ture too. Representing a surface by a single tie point may
not be appropriate in this case. An appropriate approach
that takes into consideration variability of brightness
7.6.2.1. Imaging Radar Principles
An imaging radar system transmits pulses from its
antenna at a certain pulse repetition frequency, designed to
give an appropriate ground resolution of the imaged area.
The pulses illuminate continuous strips of the surface, cov-
ering the entire swath, to one side of the orbit direction
