Determining sea ice thickness with an airborne scanning laser
Sea ice is regarded by many as the 'canary in the coal mine' for climate researchers. Model climate predictions show that an expected increase in air temperature will be most dramatic in the polar regions and that summer Arctic sea ice extent will decline further in the next 10 to 20 years. While Antarctic sea ice extent and concentration is routinely monitored from space, it is currently unknown whether changes are occurring in its thickness distribution, and therefore its mass balance, in response to environmental change.To address this issue we used recent advances in technology to take a close look at sea ice off the coast of East Antarctica during the Sea Ice Physics and Ecosystem eXperiment (SIPEX). A range of different instruments were mounted in a Eurocopter AS 350 'Squirrel' helicopter for long-distance (up to 250 nautical miles) aerial surveys of sea ice.
- a scanning laser system (a recent acquisition of the Australian Antarctic Division) in combination with radar, to determine sea ice and snow cover properties (ultimately thickness);
- a digital camera to take aerial photographs for sea ice classification and concentration estimates; and
- a pyrometer for measuring surface temperature.
The latter helps to identify sea ice type, particularly for thin (new) ice. An inertial navigation system (INS) also provided precise information on the helicopter's location and attitude during operation. All together the aircraft was named 'RAPPLS', which stands for Radar-Aerial Photography-Pyrometer-Laser Scanner.The centimetre-precision scanning laser was used for the first time during SIPEX in the East Antarctic sea ice zone to determine the surface roughness and elevation of the sea ice. These estimates of sea ice freeboard (the part of the ice that projects above the water) can then be converted into total thickness if certain physical properties of the ice are known, such as ice density and snow cover thickness and density. Important information on the latter is derived from the airborne radar system. The in situ measurements were made at 14 ice stations. Every time the Aurora Australis was anchored in the pack ice and helicopter operations commenced, scientists measured sea ice thickness by drilling holes along a 200 m transect. Sea ice cores were also taken for analysis of ice structure and density, while snow properties were measured during snow pit sampling. After flying over these 200 m transects
the airborne data were calibrated for the long range surveys.
The laser scanner produces an across-track scanning pattern of the underlying surface (see image above). This pattern can be adapted to different applications, flying altitudes and speed. During SIPEX, we used a setup which allowed us to scan with a swath width of about 450 m. We successfully used the laser system on 28 flights for over 50 hours. The data are currently being processed at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) in close cooperation with the Australian Antarctic Division.The image at right shows an example of 1 million surface elevation points (approximately 3000 scanner lines out of 4.5 million lines collected in total, over about a 2 km flight track) over a section of the Dalton Iceberg Tongue at 66.21°S, 122.35°E. A prominent feature in the north-west corner of the image is a sea ice enclosed iceberg with an edge height of about 30 m and two smaller icebergs further down to the south-east. Some distinctive floes can be seen within the sea ice matrix and even wind-blown features like snow dunes can be identified in the data.
This example demonstrates the great potential for sea ice monitoring and validation of space-borne sea ice thickness estimates (such as those from NASA's ICESat - a laser altimeter satellite - or ESA's upcoming CryoSat-II - a dedicated cryosphere radar mission). It also demonstrates the capabilities of high precision range measurements for digital elevation mapping of glaciers, ice shelves, icebergs, and islands. In combination with the simultaneously taken digital aerial photographs, we will produce high resolution 3-D digital elevation models. This work represents the first in a series of airborne sea ice monitoring efforts to be carried out by the Australian Antarctic Division, and will make an important contribution to improving our understanding of the thickness of East Antarctic sea ice and its spatio-temporal variability.
JAN L. LIESER