Sea ice concentration
Encyclopedia
Sea ice concentration is a useful variable for climate
scientists and nautical navigators. It is defined as the area of
sea ice
relative to the total at a given point in the ocean
.
This article will deal primarily with its determination from remote sensing
measurements.
variables. Since the albedo
of ice is much higher than that of water,
ice concentration will regulate insolation
in the polar oceans.
When combined with ice thickness
, it determines
several other important flux
es between the air and sea,
such as salt and fresh-water fluxes between the polar oceans
(see for instance bottom water
) as well as
heat transfer
between the atmosphere.
Maps of sea ice concentration can be used to determine
ice area and
ice extent, both of which are important
markers of climate change
.
Ice concentration charts are also used by navigators to determine
potentially passable regions—see icebreaker
.
the relative area of ice versus water visible within the scene.
This can be done using photographs or by eye.
In situ measurements are used to validate remote sensing
measurements.
and visible sensors (such as Landsat)
are normally high enough resolution that each pixel is simply classified
as a distinct surface type, i.e. water versus ice. The concentration can then be
determined by counting the number of ice pixels in a given area which
is useful for validating concentration estimates from lower resolution
instruments such as microwave radiometers. Since SAR images are normally
monochrome and the backscatter
of ice can vary quite considerably,
classification is normally done based on texture using groups of
pixels—see pattern recognition
.
Visible sensors have the disadvantage of being quite weather sensitive—images are obscured by clouds—while SAR sensors, especially in the
higher resolution modes, have a limited coverage and must be pointed.
This is why the tool of choice for determining ice concentration is
often a passive microwave sensor.
.
Since different objects will emit differently at different frequencies,
we can often determine what type of object we are looking at based on its emitted
radiation—see spectroscopy
. This principle underlies all passive
microwave sensors and most passive infrared sensors. Passive is used in the
sense that the sensor only measures radiation that has been emitted by other
objects but does not emit any of its own.
(A SAR sensor, by contrast, is active.) SSMR and SSMI radiometers were flown on the Nimbus program
and DMSP
series of satellites.
Because clouds are translucent in the microwave regime, especially
at lower frequencies, microwave radiometers are quite weather insensitive.
Since most microwave radiometers operate along a polar orbit
with
a broad, sweeping scan, full ice maps of the polar regions where
the swaths are largely overlapping can usually be obtained within one day.
This frequency and reliability comes at the cost of a poor resolution:
the angular field of view
of an antenna
is directly
proportional
to the wavelength
and inversely proportional to the effective aperture
area.
Thus we need a large deflector dish to compensate for a low frequency
.
Most ice concentration algorithms based on microwave radiometry
are predicated on the dual observation that: 1. different surface types
have different, strongly clustered, microwave signatures and
2. the radiometric signature at the instrument head is a linear
combination of that of the different surface types, with the weights
taking on the values of the relative concentrations.
If we form a vector space from each of the instrument channels
in which all but one of the signatures of the different surface types
are linearly independent, then it is straightforward to solve for
the relative concentrations:
where
is the radiometric signature at the
instrument head (normally measured as a brightness temperature
),
is the signature of the nominal
background surface type (normally water),
is the signature of the ith
surface type while Ci are the relative
concentrations.
Every operational ice concentration algorithm is predicated on this
principle or a slight variation.
The NASA team algorithm, for instance, works by taking the
difference of two channels and dividing by their sum.
This makes the retrieval slightly nonlinear, but with
the advantage that the influence of temperature is mitigated.
This is because brightness temperature varies roughly linearly
with physical temperature when all other things are equal—see emissivity
--and because the sea ice emissivity at different microwave
channels is strongly correlated.
As the equation suggests, concentrations of multiple ice
types can potentially be detected, with NASA team distinquishing between
first-year and multi-year ice.
Accuracies of sea ice concentration derived from passive microwave sensors may be expected to be on the order of 5\% (absolute).
A number of factors act to reduce the accuracy of the retrievals, the most obvious being variations in the microwave signatures produced by a given surface type.
For sea ice, the presence of snow, variations in salt and moisture content, the presence of melt ponds as well as variations in surface temperature will all produce strong variations in the microwave signature of a given ice type. New and thin ice in particular will often have a microwave signature closer to that of open water. This is normally because of its high salt content, not because of radiation being transmitted from the water through the ice—see sea ice emissivity modelling
.
The presence of waves and surface roughness will change the signature over open water. Adverse weather conditions, clouds and humidity
in particular, will also tend to reduce the accuracy of retrievals.
Climatology
Climatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences...
scientists and nautical navigators. It is defined as the area of
sea ice
Sea ice
Sea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....
relative to the total at a given point in the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
.
This article will deal primarily with its determination from remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
measurements.
Significance
Sea ice concentration helps determine a number of other important climatevariables. Since the albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of ice is much higher than that of water,
ice concentration will regulate insolation
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...
in the polar oceans.
When combined with ice thickness
Sea ice thickness
Sea ice thickness is an important climate-related variable whose determination from satellite measurements is still an unsolved problem. While ice concentration is often used as a marker for climate change, the more important variable is sea ice volume which can be determined by multiplying...
, it determines
several other important flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
es between the air and sea,
such as salt and fresh-water fluxes between the polar oceans
(see for instance bottom water
Bottom water
Bottom water is the lowermost water mass in a water body, by its bottom, with distinct characteristics, in terms of physics, chemistry, and ecology.-Oceanology:In oceanology, bottom water is by the ocean floor...
) as well as
heat transfer
Heat flux
Heat flux or thermal flux is the rate of heat energy transfer through a given surface. The SI derived unit of heat rate is joule per second, or watt. Heat flux is the heat rate per unit area. In SI units, heat flux is measured in W/m2]. Heat rate is a scalar quantity, while heat flux is a vectorial...
between the atmosphere.
Maps of sea ice concentration can be used to determine
ice area and
ice extent, both of which are important
markers of climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
.
Ice concentration charts are also used by navigators to determine
potentially passable regions—see icebreaker
Icebreaker
An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters. Although the term usually refers to ice-breaking ships, it may also refer to smaller vessels .For a ship to be considered an icebreaker, it requires three traits most...
.
In situ
Measurements from ships and aircraft are based on simply calculatingthe relative area of ice versus water visible within the scene.
This can be done using photographs or by eye.
In situ measurements are used to validate remote sensing
measurements.
SAR inteferometry and visible
Both synthetic aperture radarSynthetic aperture radar
Synthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
and visible sensors (such as Landsat)
are normally high enough resolution that each pixel is simply classified
as a distinct surface type, i.e. water versus ice. The concentration can then be
determined by counting the number of ice pixels in a given area which
is useful for validating concentration estimates from lower resolution
instruments such as microwave radiometers. Since SAR images are normally
monochrome and the backscatter
Backscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
of ice can vary quite considerably,
classification is normally done based on texture using groups of
pixels—see pattern recognition
Pattern recognition
In machine learning, pattern recognition is the assignment of some sort of output value to a given input value , according to some specific algorithm. An example of pattern recognition is classification, which attempts to assign each input value to one of a given set of classes...
.
Visible sensors have the disadvantage of being quite weather sensitive—images are obscured by clouds—while SAR sensors, especially in the
higher resolution modes, have a limited coverage and must be pointed.
This is why the tool of choice for determining ice concentration is
often a passive microwave sensor.
Microwave radiometry
All warm bodies emit electro-magnetic radiation: see thermal radiationThermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
.
Since different objects will emit differently at different frequencies,
we can often determine what type of object we are looking at based on its emitted
radiation—see spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
. This principle underlies all passive
microwave sensors and most passive infrared sensors. Passive is used in the
sense that the sensor only measures radiation that has been emitted by other
objects but does not emit any of its own.
(A SAR sensor, by contrast, is active.) SSMR and SSMI radiometers were flown on the Nimbus program
Nimbus program
The Nimbus satellites were second-generation U.S. robotic spacecraft used for meteorological research and development. The spacecraft were designed to serve as stabilized, Earth-oriented platforms for the testing of advanced systems to sense and collect atmospheric science data...
and DMSP
DMSP
DMSP may stand for:* Defense Meteorological Satellite Program, a United States Department of Defense satellite system* Dimethylsulfoniopropionate, an important component of the organic sulfur cycle*...
series of satellites.
Because clouds are translucent in the microwave regime, especially
at lower frequencies, microwave radiometers are quite weather insensitive.
Since most microwave radiometers operate along a polar orbit
Polar orbit
A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the equator...
with
a broad, sweeping scan, full ice maps of the polar regions where
the swaths are largely overlapping can usually be obtained within one day.
This frequency and reliability comes at the cost of a poor resolution:
the angular field of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
of an antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
is directly
proportional
Proportionality (mathematics)
In mathematics, two variable quantities are proportional if one of them is always the product of the other and a constant quantity, called the coefficient of proportionality or proportionality constant. In other words, are proportional if the ratio \tfrac yx is constant. We also say that one...
to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
and inversely proportional to the effective aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
area.
Thus we need a large deflector dish to compensate for a low frequency
.
Most ice concentration algorithms based on microwave radiometry
are predicated on the dual observation that: 1. different surface types
have different, strongly clustered, microwave signatures and
2. the radiometric signature at the instrument head is a linear
combination of that of the different surface types, with the weights
taking on the values of the relative concentrations.
If we form a vector space from each of the instrument channels
in which all but one of the signatures of the different surface types
are linearly independent, then it is straightforward to solve for
the relative concentrations:
where
is the radiometric signature at theinstrument head (normally measured as a brightness temperature
Brightness temperature
Brightness temperature is the temperature a black body in thermal equilibrium with its surroundings would have to be to duplicate the observed intensity of a grey body object at a frequency \nu....
),
is the signature of the nominalbackground surface type (normally water),
is the signature of the ithsurface type while Ci are the relative
concentrations.
Every operational ice concentration algorithm is predicated on this
principle or a slight variation.
The NASA team algorithm, for instance, works by taking the
difference of two channels and dividing by their sum.
This makes the retrieval slightly nonlinear, but with
the advantage that the influence of temperature is mitigated.
This is because brightness temperature varies roughly linearly
with physical temperature when all other things are equal—see emissivity
Emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
--and because the sea ice emissivity at different microwave
channels is strongly correlated.
As the equation suggests, concentrations of multiple ice
types can potentially be detected, with NASA team distinquishing between
first-year and multi-year ice.
Accuracies of sea ice concentration derived from passive microwave sensors may be expected to be on the order of 5\% (absolute).
A number of factors act to reduce the accuracy of the retrievals, the most obvious being variations in the microwave signatures produced by a given surface type.
For sea ice, the presence of snow, variations in salt and moisture content, the presence of melt ponds as well as variations in surface temperature will all produce strong variations in the microwave signature of a given ice type. New and thin ice in particular will often have a microwave signature closer to that of open water. This is normally because of its high salt content, not because of radiation being transmitted from the water through the ice—see sea ice emissivity modelling
Sea ice emissivity modelling
With increased interest in sea ice and its effects on the global climate, efficient methods are required to monitor both its extent and exchange processes. Satellite-mounted, microwave radiometers, such SSMI, AMSR and AMSU, are an ideal tool for the task because they can see through cloud cover,...
.
The presence of waves and surface roughness will change the signature over open water. Adverse weather conditions, clouds and humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
in particular, will also tend to reduce the accuracy of retrievals.
External links
- High-resolution sea ice concentration charts derived from AMSR-E 89 GHz channel