Satellite validation in Arctic ice

Sea ice scientists lead by the Alfred Wegener Institute of Polar and Marine Sciences (AWI) are presently investigating the accuracy of the environmental satellite “Envisat” and the upcoming ice-satellite “CryoSat” for sea ice measurements in the Arctic. For this purpose, the German research icebreaker “Polarstern”, during its ARK XIX cruise, is currently frozen into the pack, 900 km from the North Pole, with air temperatures down to –30°C.

„Envisat“ and „CryoSat“: Satellites dedicated for climate research
Arctic and Antarctic sea ice plays an important role in the global climate system. Melting and disappearance of sea ice due to climate warming would amplify the temperature rise in the polar regions even more, due to simple feedback mechanisms (“ice-albedo-feedback”). This would have dramatic consequences for the deterioration of ice sheets and permafrost, and would significantly affect sea level variability. The global ocean circulation would change considerably and it could also affect the Gulf Stream, which is responsible for climate conditions in Europe. Satellites are used to monitor changes in global ice masses.

European “Envisat” was launched in March 2002, imaging the types and spatial distribution of polar ice covers. From 2004 onwards, the European Space Agencie’s (ESA) satellite “CryoSat” will deliver complementary data on the thickness of sea ice and ice sheets.

However, to assess the accuracy of “Envisat” and “CryoSat” as well as their potential to monitor climate changes, measurements have to be calibrated and validated. This is particularly important for “CryoSat”, as it is not yet known how various ice properties affect the accuracy of its thickness retrievals.

Simultaneous measurements on the ice, in the air, and from space
The “CryoSat Validation Experiment” (CryoVex 2003) uses “Polarstern”, anchored deep into the Arctic pack ice, as a drifting research platform. From board, scientists go onto the surrounding ice floes to obtain in-situ measurements of ice and snow thickness as well as density. Larger areas around the ship and thin ice are profiled by means of the “EM-Bird”, a new electromagnetic sensor towed by helicopter. Surveys are coordinated with synchronous over-flights by a Danish Geodetic Survey (“KMS”) aircraft carrying a special radar altimeter. The radar altimeter, contributed by John Hopkins University in Washington, measures the distance between aircraft and the ice or water surface, enabling an estimate of ice freeboard and thickness. The altimeter resembles the radar altimeter on board “CryoSat”, thus simulating “CryoSat” measurements prior to its launch.

“Envisat“ supports “Polarstern“ navigating in severe ice conditions
The study is supported to a large extent by three EU projects (GreenICE, SITHOS, IRIS), involving partners from the AWI, Denmark, Norway, Britain, Finland, and the United States. Partners on land receive near-realtime “Envisat” and “Radarsat” radar images and transmit them to “Polarstern” to support navigation through difficult ice conditions. Radar images are particularly suitable for this purpose since they are unaffected by clouds or darkness. On board, the imagery is also used to identify characteristic landmarks in the vicinity of the ship. These can then be visited to study the underlying ice and snow properties responsible for the particular radar signature. During CryoVex 2003, image transmission is hampered by the extreme northerly position of the ship, which is out of range of standardtelecommunication satellites.

Bremerhaven, 11.4.2003