Methods of the AWI sea ice physicists - an overview
In action on an ice floe
When will the Arctic ocean be ice free? Why did the sea ice cover of the Southern Ocean grow during the last years? And how is marine life changing, when the Arctic sea ice is getting thinner and thinner? To answer these and many other questions, AWI sea ice physicists observe and investigate the sea-ice cover in both hemisphere thoroughly. Which different technique and methods they are using is shown in the following graph, created by AWI designer Martin Künsting.
With a so-called magna probe sea ice physicists measure snow depth along selected tracks on the sea ice. To take measurements, they stick the probe's metal rod vertically into the snow until it reaches the ice. Thereby the probe's metal plate stays on the surface of the snow cover. Its exact position along the metal rod is measured with induction. At the same time the rod's GPS position is recorded.
By measuring sea-ice thickness with the so-called EM-Bird or the Ground-EM, researchers take advantage of the fact that ocean water has a much higher conductivity than sea ice. Therefore both devices are able to measure the distance between the sensor and the bottom side of the sea ice. At the same time the EM-Bird is using a laser to measure the distance to the sea ice's surface. Scientists then use the difference of both distances to calculate the sea-ice thickness along the survey line.
When the German polar research vessel Polarstern is breaking ice, some small ice floes tilt in way that scientists are able to estimate the thickness of the ice and their snow cover. To get better estimations researchers also use a levelling board with markers, whose length is known. Thus they can estimate the sea ice thickness and the volume of the snow cover along the ship's route.
A sensor chain is collecting data about the temperature and heat conductivity of the surrounding snow, ice or water. This data can be used to calculate the sea ice thickness and the thickness of the snow cover. With this method researchers primarily investigate long-term changes of the sea ice.
Four acoustic transmitter and receiver are constantly measuring the distance to the surface of the snow cover. If the distance is changing, researchers are able to see, whether the snow cover is increasing or decreasing and how the snow is scattered on the sea ice. The method is used for observations of changes over time.
Sea ice observations from space are made with data from different satellites. CryoSat-2 for instance is using a radar altimeter to measure the freeboard - the height of the ice surface above the local sea level. With the density of ice and snow and snow depth itself, the freeboard can be converted into ice thickness. Contrary to Cryo-Sat-2 the SMOS satellite is measuring brightness temperatures to determine the thickness of very thin sea ice.
The classic method to measure sea ice thickness is to drill a hole through the ice and use a ruler tape to obtain information about the height of the snow cover, the freeboard (the height of the ice surface above the local sea level), and the sea ice thickness. Researchers use this very simple method for accurate point and reference measurements.
Under ice sonar
An acoustic transmitter and receiver unit is moored to the ocean floor to measure its distance to the sea ice. A changing distance indicates changes of the sea ice thickness. With this method scientists investigate changes in sea ice thickness over time.
Remotely operated vehicle
A remotely operated vehicle (ROV) is equipped with an echo sounder to measure and map its distance to the sea ice in different directions. At the end of its deployment, researchers get a map of the sea ice thickness in the area investigated. Furthermore the ROV is using several other sensors to measure parameters like water temperature, light or salinity.