Forecasting of ice cover and movement

Sea ice occupies about 7% of the whole planet surface: this amounts to 14-16 million square kilometers in late winter in the Arctic and 17-20 million square kilometers around the Antarctic. Seasonal changes are much pronounced in the Antarctic region where ice cover decreases until 3-4 million square kilometers during summertime (versus the 7-9 million square kilometers in the Arctic): this depends mainly on the Southern Ocean not being centered on the Pole (1). 

Several variables have to be taken into account when studying sea ice, such as its thickness, its spatial extent and the fraction of open water within the ice pack (1). Ice cover change is an important variable for our understanding of climate change and to forecast future scenarios. An important consequence of ice cover reduction, related to the high albedo of sea ice, would be lower reflection of  incoming solar short-wave radiation during summer time: this would have as a consequence the increase of water temperature, but also increased evaporation resulting in larger cloud cover and reflectance.

Ice can also create dangerous conditions for ocean transportation in polar sea and generate hazardous situations for off-shore infrastructures. It can also modify submarine acoustics which is a sensible parameter for vessels depending on sonars. For all these reasons precise detection and forecasting of ice cover are very important (2).

Sea ice is characterised by a bulk salinity, which is maximum in newly formed ice and is rejected while the ice body ages. It has a key role in regulating ecosystems and global climate: it prevents the exchange of heat between the sea and the atmosphere, reducing temperature extremes. It has impacts on ocean currents, by affecting water salinity and temperature. Fast ice grows outward from the coast and it's anchored to sea bed, while drift ice is continuously moving in direct relation with wind speed and direction. Sea ice concentration can be very different in different ice bodies: this is one of the parameters that can be qualitatively estimated via satellite imagery and help ice forecasting (2).

Since 1978 satellites have monitored sea ice growth and retreat, and they have detected an overall decline in Arctic sea ice. The rate of decline steepened in the twenty-first century, with a series of record or near record lows from 1990s onwards: this situation is likely due to the combination of natural variability and greenhouse gas emissions, together with increased global temperatures (3).

Panchromatic photography, infrared imagery and radar imagery are remote sensing satellite techniques which yield information on sea ice. Relevant observations came fror example from microwave images and multichannel microwave scanning radiometers which measure microwave energy radiated from the Earth surface and let scientists identify sea ice and open water area to map sea ice concentrations. Satellite sensor capabilities are continuously improving, but limitations still remain especially in relation to weather and the mixing of land (coast) and water in satellite sensor's field of view (3).

Ice Movement detection of Austfonna derived by Sentinel-1A data. Credits: ESA/Univeristy of Leeds

Sentinel 1A, the first radar satellite of the Copernicus fleet, launched in April 2014, can overcome cloud coverage limitations due to its day-and-night all weather data gathering. With the very first images acquired, European experts have already shown how to obtain ice charts, very useful for mariners. Moreover Sentinel-1A data, combined wit TerraSAR-X ones ,  have been used to study the movement of Austfonna, the largest ice cap in Svalbard: an extraordinary acceleration in ice motion has been detected (the southeastern section is flowing at more than 3km per year), which is still to be attributed either to natural causes or to climate change.



(2) Conway E.D., An Introduction to Satellite Image Interpretation





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