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Passive microwave sensors (radiometers)

Radiometers

Year-round, daily satellite monitoring of the ice extent is based on measuring the microwave radiation emitted by the ice and the ocean’s surface. For the past five decades, passive microwave sensors (radiometers) on satellites (ESMR, SMMR, SSM/I, and AMSR-E/AMSR2) have regularly supplied the requisite data. As such, satellite-based microwave radiometers are the undisputed “beasts of burden” in this regard.

Every object on Earth emits not only infrared radiation, but also microwaves. A microwave radiometer measures the radiation naturally emitted in the microwave spectrum by the Earth and by every object with a temperature above absolute zero (–273.15 degrees Celsius). Particularly in the microwave range, the atmosphere is transparent for many frequencies. At the microwave frequencies used (less than ca. 100 GHz, i.e., a wavelength of over 3 mm), clouds emit and absorb virtually no microwave radiation; they are quasi-transparent, making it possible to measure the sea ice without any interference from them. In addition, passive microwave sensors do not depend on sunlight, as they measure the thermal microwave radiation emitted, an aspect that is particularly vital in the polar regions and represents a clear advantage over optical sensors.

Further, unlike infrared radiation, microwave emissions are linked not only to the temperature of a given object, but also its material. In this regard, physical properties like the molecular structure and crystalline structure are decisive for the amount of microwave radiation emitted. The crystalline structure of ice typically emits more microwave radiation than the liquid water of the ocean, making it easy for sensors to tell the difference. Further, microwave radiometers can reliably distinguish first-year ice from multiyear ice. Radiometers can tell the difference even when the sea ice is covered with snow, since, in the frequency range used, fresh snow is effectively transparent for the electromagnetic radiation emitted by the ice.

As previously mentioned, passive microwave sensors used to monitor sea ice operate at frequencies between 100 GHz and 1 GHz, i.e., a wavelength of between 0.3 and 30 centimetres. At such a large wavelength, the achievable spatial resolution is low. Depending on the frequency (7 to 89 GHz), the spatial resolution of the AMSR sensor varies from 50 to three kilometres. As such, details like leads in the ice are only sometimes recognisable. For other purposes, e.g. determining the mean temperature in a given area as an initial parameter for a weather forecasting model, spatial averaging is advantageous in comparison to taking point measurements with an instrument on-site.

Thanks to their ability to measure sea ice through clouds and round the clock, passive microwave sensors can deliver a virtually complete representation of the sea-ice cover in the polar regions. They are ideally suited to providing a broad-scale overview of global sea-ice cover, and to reflecting processes in the sea ice with considerable temporal variability; moreover, they allow us to observe long-term changes in the sea-ice cover. The daily ice concentration maps provided on SEA ICE PORTAL are based on data from the AMSR2 sensor.

Since the launch of the Nimbus-5 satellite (sensor: ESMR – Electrically Scanning Microwave Radiometer) in 1972, we have had access to consistent and detailed information on the large-scale properties of and changes in global sea-ice cover. In 1978, NASA added the Scanning Multichannel Microwave Radiometer (SMMR) and, starting in 1987, a range of DMSP Special Sensor Microwave / Imager (SSM/I) sensors.

The launch of the nearly identical systems AMSR (on board Japan’s ADEOS-II, Advanced Earth Observing Satellite II) and AMSR-E (on board NASA’s Earth Observing Systems (EOS) Aqua) in 2002 marked the beginning of a new era in passive microwave observation. AMSR-E measures microwaves emitted from the Earth in a frequency range from 7 to 89 GHz. The introduction of AMSR and AMSR-E meant improved temporal and spatial resolution (to 5 km) and a broader usable spectral range (which makes it possible to e.g. measure additional parameters like the ice temperature). In October 2011, AMSR-E monitoring was discontinued. Its improved successor AMSR2 was put into orbit on board GCOM-W1 on 18 May 2012 and has been providing observational data since August 2012. Just in time for AMSR2’s tenth birthday, the launch of its successor AMSR3 – which will tentatively continue its predecessor’s time series sometime between April 2023 and March 2024 on board the GOSAT-GW satellite – was recently announced.

But Europe also has plans to put a passive microwave sensor in orbit in the near future. The mission, dubbed Copernicus Imaging Microwave Radiometer (CIMR), will most likely take place in the late 2020s, with the goal e.g. of filling potential observational gaps.

The current sea-ice extent on the basis of AMSR2 data can be viewed at our website, while the corresponding data and data products can be found at the Data Portal. Additional data and regional maps of the current sea-ice extent can be found on the website of the University of Bremen’s Institute of Environmental Physics (https://seaice.uni-bremen.de).

Since late 2010, the ESA satellite SMOS (Soil Moisture and Ocean Salinity) has been monitoring the Earth at 1.4 GHz (L Band), the equivalent of a 21-cm wavelength. At this low microwave frequency, an aperture synthesis of the measurements taken by 69 individual radiometers, arranged along a triad, is required in order to achieve a surface resolution of 35 to 50 km. In addition to the original mission objectives (measuring soil moisture and the salinity of the ocean), SMOS observations are used to determine the daily thickness of thin sea ice (up to 50 cm). When combined with ice thicknesses derived from Cryosat-2 data, it’s the best of both worlds, making it possible to cover virtually the entire spectrum of sea-ice thickness distribution with sufficient accuracy.

Thin sea ice manifests during the freezing season. In the melting season, the sea-ice thickness varies considerably. In addition, in the Arctic the emission properties change in step with surface moisture and the formation of melt ponds. Accordingly, data on the thickness of thin sea ice is only calculated during the freezing season from October to April in the Arctic and from March to September in the Antarctic. During the melting season, no reliable findings on sea-ice thickness can be derived from SMOS data. Given that the resolution of SMOS data at the observation angle used is ca. 40 km, only measurements for larger areas of thin sea ice can be accurately derived.