Shedding light in the dark: radar satellites lead the way

Shedding light in the dark: radar satellites lead the way
The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition will make a major contribution to Arctic climate science. Spearheaded by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), it is the biggest polar expedition of all time. It involves the Polarstern German research icebreaker spending a year trapped in the sea ice so that scientists from around the world can study the Arctic as the epicentre of global warming and gain fundamental insights that are key to better understand global climate change – and ESA is contributing with a range of experiments. During the polar winter, researchers are subjected to temperatures as low as –45°C and the perpetual darkness. Credit: Alfred-Wegener-Institute/Esther Horvath , CC BY-SA 3.0 IGO

Spare a thought this Christmas for researchers hunkered down on their Polarstern icebreaker, adrift in the frozen Arctic Ocean. Subjected to temperatures as low as –45°C and the perpetual darkness of the polar winter, they are willing participants in MOSAiC – the world's largest and longest polar research expedition. Despite the darkness, however, the researchers and crew remain aware of what is happening close by. How? With the help of radar imaging satellites.

Over the course of one year, a total of some 600 researchers from 20 countries will take part in different stages of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (or MOSAiC) expedition.

After entering the Arctic Ocean in October, the Polarstern has been drifting across the central Arctic at about 7 km per day with the wind and currents expected to carry it close to the geographic North Pole before exiting next spring or summer.

On board, the scientists are carrying out multiple experiments on the sea ice around the ship to better understand the impact of climate change on sea ice and the Arctic environment. The team has now established hundreds of instruments on the sea ice surrounding the ship within a distance of 50 km.

Despite the darkness currently enveloping the ship as it drifts through the frozen sea, the researchers and crew are not blind and remain aware of what is happening thanks to radar imaging satellites of Europe's Copernicus programme, Canada, Germany and Japan.

Shedding light in the dark: radar satellites lead the way
In September 2019, the German research icebreaker Polarstern has set sail from Tromsø, Norway, to spend a year drifting through the Arctic Ocean – trapped in ice. The goal of the MOSAiC expedition is to take the closest look ever at the Arctic as the epicentre of global warming and to gain fundamental insights that are key to better understand global climate change. Hundreds of researchers from 20 countries take part in this exceptional endeavour. This animation shows Polarstern’s route and drift as well as the growth of the winter sea ice. Credit: MOSAiC team/US National Snow & Ice Data Center for sea-ice extent

Sea ice near the MOSAiC Arctic expedition

The crew and scientists monitor the sea ice and generate remarkable maps of the sea-ice floes surrounding the ship. These radar satellites cross the Arctic on a daily basis and carry with them their own source of illumination, which allows them to pierce through the Arctic winter darkness, continuously sensing and mapping the sea-ice conditions below.

Suman Singha, from the German Aerospace Center's Remote Sensing Technology Institute, helps coordinate the acquisition of images from different satellites and is responsible for relaying the precious information further to the ship.

"This information is very much needed at Polarstern, especially at the beginning of the expedition, when the challenge was to find the right kind of ice floe able to harbour both the Polarstern and the deployment of all the scientific instruments on the ice around the ice breaker," says Dr Singha.

Shedding light in the dark: radar satellites lead the way
Radar image from Japan’s ALOS-2 satellite of the sea ice near the Polarstern icebreaker. Polarstern is drifting in the Arctic sea ice for a year for the MOSAiC polar research expedition. During the polar winter, the researchers use radar satellite images such as this to monitor the sea ice in the surrounding area. In this false-colour image, which was acquired on 19 November 2019, dark blue cracks show open water leads or thin ice between the ice floes. The white filament-like structures are typically sea-ice ridges or other deformed sea ice. Credit: JAXA

"Here we made use of high-resolution radar images from the German TerraSAR-X to help locate the best-possible floe, which has since been given the name Fortress. Monitoring the safety of the floe thus remains a constant challenge."

Also contributing to the international mapping effort are Europe's Copernicus Sentinel-1 satellites which provide continual wide-area coverage of the site, helping to follow and predict the ever-changing drift of the sea ice up to 300 km away from the ship.

The Japanese ALOS-2 satellite with its PalSAR-2 sensor uses a much longer wavelength than both Copernicus Sentinel-1 and TerraSAR-X to map sea-ice floes and conditions below.

ESA's Malcolm Davidson said, "Wavelength matters when it comes to radar satellites as a particular wavelength greatly influences the information provided by the satellite.

This video is based on data acquisitions from the Copernicus Sentinel-1 mission between 3 October and 31 October 2019. It remains constantly centred on the Polarstern (bright dot starting at the centre of the grid). Polarstern is a German research icebreaker spending a year trapped and drifting in the Arctic sea ice so that scientists from around the world can study the Arctic as the epicentre of global warming and gain fundamental insights that are key to better understand global climate change. The video shows how the initial grid distorts over time by the uneven ice drift over time within the grid array. This results in opening (ice divergence) and closing (ice compression and ridging), shear and vorticity. This shear caused a massive crack to form through the experiment ice floe, disrupting the experiments and forcing movement of some of the instrumentation. Credit: contains modified Copernicus Sentinel data (2019), processed by R. Kwok (JPL)

"In Europe we are very interested in the additional information that ALOS-2 can provide on sea-ice conditions especially now that we are developing our own long-wavelength radar satellite called the L-band Synthetic Aperture mission,  ROSE-L – which is one of the six Copernicus high-priority candidate missions.

Shedding light in the dark: radar satellites lead the way
The researchers participating in the MOSAiC expedition not only have to keep an eye on the ever-changing sea ice, but also on visitors. These polar bears seem to be enjoying playing with the marker flags. Spearheaded by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research MOSAiC is the biggest shipborne polar expedition of all time. It involves the Polarstern German research icebreaker spending a year trapped and drifting in the sea ice so that scientists from around the world can study the Arctic as the epicentre of global warming and gain fundamental insights that are key to better understand global climate change. Credit: Alfred-Wegener-Institute/Esther Horvath, CC BY-SA 3.0 IGO

For instance, detecting sea-ice ridges is critical for safe navigation in the Arctic and these are much easier to identify with ALOS-2 than with the existing European satellites."

While Polarstern now drifts through the frozen and dark ocean for the coming months, the eyes in the sky will continue to monitor its progress through the Arctic and accompany the researchers through the rest of this remarkable expedition.


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