The Arctic sea ice is an important indicator for the climate change. Not only does the size of the area covered by ice play an important role, but also the thickness of the ice. For this reason, detailed data are essential for the assessment of the heat exchange between the ocean and the atmosphere. The heat exchange is especially large in areas covered with thin ice, which strongly influences the weather and climate – even in Northern Europe. Thick sea ice, on the other hand, has an insulating effect and therefore hardly contributes to the heat exchange.
The thickness of sea ice can either be measured at borehole sites, or determined by echo sounding measurements carried out by submarines. In addition, data were retrieved by means of electromagnetic measurements from helicopter sensors. These methods require considerable effort and do not provide data on a large scale.
With the satellite CryoSat it is possible to measure the thickness of the sea ice rising above the water surface, and thus infer the total sea ice thickness. However, this method is not suitable for measuring thin sea ice.
Aboard the satellite SMOS (Soil Moisture & Ocean Salinity) is a sensitive radiometer, measuring the natural heat radiation of the Earth´s and Ocean´s surfaces at a wavelength of 21 centimeters, also known as L-band radiometry. The radiation is in the long-wave microwave frequency range. The range at which electromagnetic waves penetrate into a medium, e.g. sea ice, depends largely on the medium´s electrical conductivity and the wavelength: the lower the medium´s conductivity and the longer the employed wavelength, the greater the penetration depth into the medium. If the electromagnetic waves penetrate deeply, scientists are able to deduce data on deeper layers - such as the thickness of the Arctic sea ice.
"Only by means of satellite-based remote sensing in the microwave frequency range of the spectrum, are we able to continuously observe the thin sea ice on a global scale," says the director of this study, Professor Lars Kaleschke of the KlimaCampus at the University of Hamburg. Furthermore, SMOS delivered for the first time high-quality global L-band data. "The L-band radiometry is a completely new technology in earth observing satellites," explains Kaleschke. "We found that we can measure the thickness of Arctic sea ice up to half a meter. Theory and practice conform well. These new results correspond with the values we previously predicted by means of our model simulations." With these data on sea ice thickness, it might even be possible to improve the weather forecasts in Europe: "Meteorological services have already expressed interest in our data," says Professor Kaleschke.
This study was conducted in cooperation with the University of Hamburg and Bremen, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, the Finnish Meteorological Institute, and the Danish Meteorological Institute.
The project is funded by the European Space Agency ESA. The Hamburg University had won this research assignment in an international bidding process.