Geology Reference
In-Depth Information
depends on the Sun's position, the viewing angle of the
sensor, and the local tilt of the ocean surface. The orienta-
tion and the extent of Sun glint, as seen in the VIS/IR
imagery, have been used to identify regions of calm sea
surface (Sun glint has a better chance to occur in calm
seawater). A few methods that correct the reflectance for
Sun glint have been developed. The underlying theme is
to estimate the contribution of the Sun glint to the radi-
ance reaching the sensor, and then subtract it from the
observation. A review of the methods is presented in
Kay
et al.
[2009]. Another common approach utilizes the fact
that the NIR radiation from the sea surface is negligible
unless water is very shallow, turbid, or with vegetation
reaching the surface. In this case any appreciable reflec-
tion in the NIR observation can be attributed to the Sun
glint. It should be noted that Sun glint hardly affects the
visual discrimination between ice and seawater in optical
remote sensing images because its texture is much finer
than that caused by reflection off the ice surface.
Ocean foam (or whitecaps) is another phenomenon
that affects the reflection of the solar radiation from sea-
water. It is generated during active breaking of the ocean
waves when the ratio of the wave height to its wavelength
is greater than 0.1. At a wind speed of 20 m/s foam covers
one‐third of the open ocean [
Ross and Cardone
, 1974].
Ocean foam (with its heavily bubbly contents) changes
the optical and microwave observations of the surface.
Since bubbles scatter light, reflection from foam is usually
high, and therefore the foam‐covered surfaces appear
white (hence the name whitecap). The reflection is iso-
tropic and independent of the wavelength in the VIS
region but decreases with wavelength in the IR region.
This phenomenon does not also affect the interpretation
of sea ice imagery data because it happens in the open
ocean, far from the ice fields. It rather impacts a few air‐
sea processes at the ocean surface, including gas fluxes
and turbulent mixing. Spectral reflectance of whitecaps is
reviewed in
Kokhanovsky
[2004].
the incident electromagnetic wave should be twice the
wavelength of the ocean wave (measured in the slant
range). This relationship is presented later in this section,
but first a brief presentation on categories of ocean waves
in relation to the scattering mechanism is introduced.
Detailed information on this subject can be found in a few
text books on microwave remote sensing of the ocean [e.g.,
LeBlond and Mysak
, 1981;
Robinson
, 2004;
Comiso
, 2010].
Ocean waves are categorized into four classes based on
their wavelengths: (1) capillary waves, (2) gravity waves, (3)
swells, and (4) tsunamis. The first three are generated by
wind in open ocean, and they generally coexist as their
driving forces act simultaneously. Tsunamis, which are not
relevant to this discussion, are caused by geological effects
in deep water. Capillary and gravity waves are particularly
relevant to the identification of the microwave scattering
mechanisms. Capillary waves, also known as ripples, are
small waves with a height of about 20 mm and a typical
wavelength of a few millimeters though it can reach 17 mm.
They are generated by fresh wind (light breezes) blowing
on a smooth water surface. Their dynamics are dominated
by surface tension. The friction caused by the wind stretches
the water surface, causing waves to be generated as surface
tension tries to restore it to smoothness. Capillary waves
are usually found where a boundary is placed around the
water body such that it restricts the growth of the waves.
This is frequently observed in narrow leads and polynyas.
Beyond the wavelength of 17.3 mm the gravity supersedes
capillary action as the dominant restoring force of the wave
(i.e., to restore the displaced water element toward equilib-
rium). The oscillation about the equilibrium state is known
as gravity waves. The wavelength of a gravity wave is typi-
cally a few meters but it can reach tens of meters. There
is a range of overlap between capillary and gravity wave
called capillary‐gravity wave. As gravity waves build up,
their wavelength tends to lengthen and the speed increases
until it matches the speed of the wind, at which point they
can no longer extract energy from the wind.
The next category of wind‐generated ocean surface
waves is the swell. It is not generated by the local wind but
rather by a series of gravity waves generated elsewhere or
sometime ago by the wind action. Swell waves have a
wide range of wavelengths measured in hundreds of
meters and depend on the size of the water body and the
wind event that originate them. In most severe storms
swells are occasionally longer than 700 m. Gravity waves
and swells penetrate sea ice at its edges, leaving its imprint
as a wavy pattern in radar images of the ice sheet. This is
usually observed when the ice is thin enough. The last
category of tsunamis, though not relevant to remote sens-
ing of ocean and certainly not of ice, are huge waves
measured in hundreds of kilometers of wavelength yet
relatively small in height. They are generated by earth-
quakes. They are not visible in the ocean, but they grow
Microwave Region
Compared to the optical and infra-
red observations, knowledge about microwave scattering
mechanisms from ocean surface is more critical for the
interpretation of microwave remote sensing observations.
The ocean generates waves with a wide range of wave-
lengths in response to the wind forcing but none of these
scales “resonate” with the extremely small wavelength of
the optical radiation. In the microwave region, different
scales of wavelength of ocean surface activate different
scattering mechanisms. A particularly important mecha-
nism is the well‐known Bragg scattering, which is responsi-
ble for high scattering values. It represents the constructive
addition of the radar scattering from successive ocean
waves to generate a dominant value of backscattering. To
fulfill the condition for Bragg scattering the wavelength of
