Sea ice concentration is a useful variable for climatescientists and nautical navigators. It is defined as the area ofsea ice relative to the total at a given point in the ocean.This article will deal primarily with its determination from remote sensing measurements.
Sea ice concentration helps determine a number of other important climatevariables. 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 determinesseveral other important fluxes between the air and sea,such as salt and fresh-water fluxes between the polar oceans(see for instance bottom water) as well asheat transfer between the atmosphere.Maps of sea ice concentration can be used to determineSea ice area andSea ice extent, both of which are importantmarkers of climate change.
Ice concentration charts are also used by navigators to determinepotentially passable regions—see icebreaker.
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 sensingmeasurements.
Both synthetic aperture radar and visible sensors (such as Landsat)are normally high enough resolution that each pixel is simply classifiedas a distinct surface type, i.e. water versus ice. The concentration can then bedetermined by counting the number of ice pixels in a given area whichis useful for validating concentration estimates from lower resolutioninstruments such as microwave radiometers. Since SAR images are normallymonochrome and the backscatter of ice can vary quite considerably,classification is normally done based on texture using groups ofpixels—see pattern recognition.
Visible sensors have the disadvantage of being quite weather sensitive—images are obscured by clouds—while SAR sensors, especially in thehigher resolution modes, have a limited coverage and must be pointed.This is why the tool of choice for determining ice concentration isoften a passive microwave sensor.[1] [2]
All warm bodies emit electro-magnetic radiation: see 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 emittedradiation—see spectroscopy. This principle underlies all passivemicrowave sensors and most passive infrared sensors. Passive is used in thesense that the sensor only measures radiation that has been emitted by otherobjects 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, especiallyat lower frequencies, microwave radiometers are quite weather insensitive.Since most microwave radiometers operate along a polar orbit witha broad, sweeping scan, full ice maps of the polar regions wherethe 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 directlyproportional to the wavelengthand inversely proportional to the effective aperture area.Thus we need a large deflector dish to compensate for a low frequency.[1]
Most ice concentration algorithms based on microwave radiometryare predicated on the dual observation that: 1. different surface typeshave different, strongly clustered, microwave signatures and2. the radiometric signature at the instrument head is a linearcombination of that of the different surface types, with the weightstaking on the values of the relative concentrations.If we form a vector space from each of the instrument channelsin which all but one of the signatures of the different surface typesare linearly independent, then it is straightforward to solve forthe relative concentrations:
\vecTb=\vecTb0+
| n(\vec | |
| \sum | |
| i=1 |
Tbi-\vecTb0)Ci
where
\vecTb
\vecTb0
\vecTbi
Every operational ice concentration algorithm is predicated on thisprinciple or a slight variation.The NASA team algorithm, for instance, works by taking thedifference of two channels and dividing by their sum.This makes the retrieval slightly nonlinear, but withthe advantage that the influence of temperature is mitigated.This is because brightness temperature varies roughly linearlywith physical temperature when all other things are equal—see emissivity—and because the sea ice emissivity at different microwavechannels is strongly correlated.[3] As the equation suggests, concentrations of multiple icetypes can potentially be detected, with NASA team distinguishing betweenfirst-year and multi-year ice (see image above).[6] [7]
Accuracies of sea ice concentration derived from passive microwave sensors may be expected to be on the order of 5\% (absolute).[6] [8] [9] 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.[4]