Remote Sensing Principles Relevant to Sea Ice - Sea Ice: Physics and Remote Sensing - page 323

conditions (e.g., wet sea ice surface or rough seawater
surface) and (2) the possibility of coexistence of ice and
water in the same footprint of the observation. Moreover,
the radiometric properties (including radar backscatter)
from newly formed ice (<1 cm) mark a transition from the
properties of open water to ice. The significant variation
of some properties of seawater between the VIS/TIR
spectral region on one hand and the microwave region on
the other hand should be understood in order to interpret
the data correctly. For example, while seawater is almost a
perfect blackbody in the infrared region, it is a gray body in
the microwave frequencies with a wide range of frequency‐
dependent emissivity. Understanding radiative processes
of open water is particularly important for quantitative
analysis of sea ice data in the marginal ice zones.

radiation that penetrates the ocean surface is attenuated
by absorption. The average percentage attenuation of
the blue and red wavelengths in 1 m is 6.5% and 46%,
respectively. The nonabsorbed radiation is scattered and
part of it may eventually reach the surface and refracted
back to the air. This and the reflected part determine the
ocean color. The color is detected by satellite sensors such
as the NASA's Sea‐viewing Wide Field‐of‐View Sensor
(SeaWiFS). For clear water, regardless or surface rough-
ness, the spectral albedo is essentially independent of the
wavelength in the optical range [ Perovich , 1991].
Roughness of the ocean surface is manifested in the
form of surface waves, which are mainly triggered by sur-
face wind. It is the key factor in determining the scatter-
ing of EM waves in general, though it plays a more
important role in microwave observations. Because of the
extremely small wavelength of optical radiation, the
ocean surface usually appears smooth. Therefore, specu-
lar (Fresnel) reflection is expected [see equations (7.4) and
(7.5) and Figure 7.17]. If the wave height becomes large
enough, the angular pattern of the reflection will change.
In general, the dominant specular reflection of the solar
radiation from the “smooth” water causes the surface to
appear dark in the images as long as the viewing angle
(angle of reflection) is not equal or close to the angle of
incidence of the light beam. If the two angles are nearly
equal, the received reflectance will be high. This phenom-
enon is called Sun glint (Figure 7.39). Its occurrence

Optical and Thermal Infrared Regions Seawater is a
highly absorbent medium in the optical region of the EM
wave. The reflection coefficient varies between 0.02 and
0.1 in the VIS and about 0.008 in the NIR regions. This
property sustains the average temperature of the Earth
and at 15 °C therefore sustains life. If the entire 71% sea-
water cover (of which 13% is covered with sea ice) freezes,
the energy absorption would drop significantly, and the
average temperature of Earth will drop drastically from
15 to −40 °C. The reflection, absorption, and emission of
seawater in the optical and IR regions are determined by
five surface parameters: salinity, turbidity, temperature,
emissivity, and roughness. Influence of these parameters
on the radiation in the optical spectrum is presented in
several textbooks [e.g., Woźniak and Dera , 2007]. A brief
account of these influences is discussed in the following.
The salinity and turbidity are particularly important
for the VIS and NIR observations. However, the turbidity
of the water is not relevant to the remote sensing of sea
ice because it exists in significant qualities only near
shore, i.e., far from the open ocean where most of the
mobile sea ice exists. The sea surface temperature (SST)
and emissivity in the IR are the two factors that deter-
mine the TIR observations. Since water is almost a black-
body at infrared wavelength, the emissivity of saline
water varies over a narrow range between 0.987 and 0.993
[ Konda et al. , 1994]. It is a weak function of the salinity.
It should be noted that the salinities of the Arctic Ocean
and the Southern Ocean are very close, approximately
35‰ and 34‰, respectively.
Part of the solar radiation that strikes the seawater sur-
face is refracted, but it travels at a slower speed in water
than in air. The refractive index that determines the amount
of refraction is a function of the water salinity and tempera-
ture [ Austin and Halikas , 1976]. It increases with increasing
salinity and decreasing temperature. This relationship is
used to determine the seawater salinity by measuring the
refractive index of a sample at a constant temperature. The

Calm sea

Sun glint

Figure 7.39 MERIS image showing sun glint in a scene in the
South Pacific. Level 1 image (top) and level 2 image masked
to show the areas of moderate glint and high glint (bottom)
[ Kay et al. , 2009].