Institute of Oceanography

University of Hamburg

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Deutsche version
Institute of Oceanography
University of Hamburg
Bundesstraße 53
D-20146 Hamburg
Tel.: +49 40 42838-2605 / -5449
Fax: +49 40 42838-7488
E-Mail:  waltraut.domke-sommer(at)zmaw.de

Introduction

Introduction

One of the parameters which is observed regularly from space is the areal sea-ice cover. This page briefly gives an overview about the terminology and about which portion of the electromagntic spectrum can be used for which aspect of sea-ice monitoring.


Terminology
The sea-ice concentration is the coverage of a unit area with sea ice. It is expressed either with values 0 ... 1 or in per cent of the unit area, i.e. 0% means the area is totally covered by open water and 100% means that the area is totally covered by sea ice.
The sea-ice extent is the cumulative area covered with sea ice inside a specifically selected ice-concentration boundary (for instance 15%). Since many sea-ice concentration products are calculated or provided on a polarstereographic grid with a certain grid-cell size, the sea-ice extent is often simply the number of grid cells wherein ice concentrations are at least as high as the sea-ice concentration boundary value times the area of the grid cell.
The sea-ice area is the sea-ice extent weighted by the ice concentration, i.e. it is the sum of the number of grid cells with ice concentrations above the chosen ice-concentration threshold times the ice concentration (0 ... 1).


Sensors & Techniques
Most widely used for sea-ice monitoring (concentration, area, extent, drift, type, snow-cover) is data from microwave radiometry because of its global coverage, relative insensitivity to clouds, and all-daylight measuring capabilities. Microwave radiometry takes advantage of the fact that sea ice & snow are radiometrically warmer than open water in most cases at one frequency and polarization and/or that at least the polarization difference (vertical minus horizontal polarized brightness temperature) differs by roughly one order of magnitude.

The Special Sensor Microwave/Imager (SSM/I) aboard the satellites of the Defense Meteorological Satellite Program (DMSP) has offered almost continuous data coverage since 1987. The SSM/I is equipped with seven channels operating at frequencies of 19, 37, and 85GHz (horizontal + vertical polarization) and 22GHz (just vertical polarization), has a conical swath with along-scan equal-area footprints at the surface, a swath-width of approx. 1400km; footprint sizes vary between 69km x 43km at 19GHz and 15km x 13km at 85GHz. Its orbit permits to monitor the high latitudes almost completely - except a disc 4 degree in diameter at the poles. At times more than two sensors of the same kind are in orbit providing excellent coverage of the high latitudes. SSM/I data have been and are used in a still growing number of sea-ice concentration algorithms to calculate the sea-ice concentration and to obtain information about the sea-ice type.
Take a look at some typical winter brightness temperature values in the ARCTIC [February 24, 2000, 9UTC, DMSP-f13-SSM/I]:


Using values of one brightness temperature or of a combination of brightness temperatures at different frequencies or of the polarization difference that are typical for distinct surface types, i.e. open water and 100% sea ice, so-called tie points, one is able to derive the sea-ice concentration in a usually relatively simple way. Difficulties can arise from the fact that
  • tie points often represent only sea ice/snow properties of a certain region or surface type [multiyear ice <==> first-year ice; multiyear ice <==> brash ice]
  • tie points often represent only sea ice/snow properties of a certain season [melt ponding in summer in Arctic, melt-refreeze cycles due to heavy weather in Antarctic]
  • brightness temperatures are influenced directly and/or indirectly by the atmosphere and/or atmospheric processes [large changes in atmospheric water content, precipitation, weather-related surface property changes]
  • radiation layer properties and temperatures can change due to changes in sea ice/snow morphology
Different approaches and algorithms have been developed to mitigate these difficulties, for instance, the NASA-Team algorithm, the Comiso-Bootstrap algorithm (see NSIDC-webpage for data collections obtained with these algorithms), and the ARTIST Sea-Ice (ASI) algorithm.


Data of the SSM/Is' precedessors, the Electrically Scanning Microwave Radiometer (ESMR) and the Scanning Multichannel Microwave Radiometer (SMMR), which have been operated at similar frequencies and orbits, however, with a less frequent data coverage, have also been extensively used for sea-ice monitoring.
Data of the SSM/Is' successor, the Advanced Microwave Scanning Radiometer (AMSR-E) are available since late summer 2002 and have improved the degree of detail in the obtained sea-ice concentration maps and are already used to derive standard products.
Take a look at National Snow and Ice Data Center (NSIDC) webpage. Here you will find a variety of satellite data and products from microwave radiometry relevant for sea-ice remote sensing.



Another possibility to monitor sea ice is to use active microwave remote sensing, i.e. to measure the backscattered power of microwave radiation which has been transmitted by the sensor towards the surface nad is scattered there. This can be done using different techniques and yields the surface backscatter in case of looking at a certain angle different from nadir, for instance, with Real Aperture Radar (RAR) [e.g. aboard the satellite OKEAN], Synthetic Aperture Radar (SAR) [e.g. aboard satellites ERS1/2, Radarsat-1, Envisat], and Scatterometry [e.g. aboard satellite QuikScat], and can be used to infer the sea-ice freeboard in case of looking from nadir, for instance, with an altimeter [e.g. aboard satellit IceSat]. Obtained surface backscatter values permit to distinguish between different surface roughnesses and by that between open water and ice, different sea-ice types (smooth <==> rough), and different surface properties (dry <==> wet). Difficulties arise particularly due to the fact that
  • under certain viewing angles and/or wind speeds sea ice and open water look the same
  • different sea-ice (surface) types can exhibit the same backscatter
Active microwave sensors are also independent of daylight and atmospheric conditions. For technical reasons for SAR the spatial coverage is limited; swath-widths range between 50km (Envisat) and 500km (Radarsat) and the revisit time is not better than two days in polar latitudes. However, a unique spatial resolution between 10m and 1000m (Envisat) is achieved. For scatterometry, the spatial coverage is by far better with a swath-width of about 1800km permitting a complete coverage of polar latitudes every day. The spatial resolution is about 25km.
Take a look at the webpage of the Alaska SAR Facility in case that you're interested to learn more about SAR and at webpages QuikScat views Icebergs and CERSAT at IFREMER to get an impression about QuikScat.


Finally, although dependent upon daylight and hampered seriously by clouds data obtained in the visible and infrared portion of the electromagnetic spectrum also permit to monitor sea ice. Sensors operating in this frequency range measure the solar radiation reflected at the surface (open water: low reflectivity, sea ice: medium to high reflectivity, snow: high reflectivity) and/or the infrared emission of the surface which depends merely on the surface temperature. Data of this kind have been obtained since 1978 by the Advanced Very High Resolution Radiometer (AVHRR) aboard the Polar Pathfinder satellites (TIROS-N through NOAA-17) and have been used for sea-ice monitoring and in particular sea-ice drift estimation (see e.g. Polar Pathfinder Daily 25 km EASE-Grid Sea Ice Motion Vectors. Difficulties can arise - among the constraints due to clouds and daylight - due to the fact that
  • particularly during summer, sea ice and open water may exhibit the same surface temperature
  • particularly during summer, sea ice may be covered extensively by melt ponds
However, under clear-sky conditions data of this kind permit to easily monitor the ice edge, leads, polynyas, and the surface temperature. The spatial resolution is between 1km and 4km across a swath-width of approx. 2000km. Daily coverage of the entire polar latitudes is achieved.
Recently, data from MODIS aboard EOS-TERRA and MERIS aboard Envisat have demonstrated and further improved the sea-ice monitoring capabilities in this frequency range.