Optical measurement of melting ice
The Kiel geographer Natascha Oppelt took part in a Polarstern expedition to the Arctic, to gather size, depth and optical data of melt ponds. The aim is to gather such data, which is important for climate modelling, by means of remote sensing in the future.
Well-protected in bright red survival suits, Natascha Oppelt (left) and two colleagues measure, amongst other things, the depth of the melt ponds on Arctic ice, the thickness of the ice below the ponds, as well as the optical characteristics of the pond water and the surrounding ice surface. Photo: Niels Fuchs, AWI Bremen
The physical geographer heads the Earth Observation and Modelling working group at Kiel University’s Department of Geography, and was on board the research vessel Polarstern when it drifted with an ice floe in the central Arctic in June 2017. »We were anchored directly to an ice floe for two weeks, and when the weather allowed it, we went onto the floe to take measurements. There were several ponds, and in the course of two weeks some new ponds also formed.«
Exploring the ice floe was not without risk. »You are exploring unknown terrain and could fall through the ice at any time. That is why we were only allowed to leave the ship and explore the ice floe wearing a survival suit, and took along special metal hooks which you can use to pull yourself to the edge.« It becomes particularly adventurous if the largest land carnivore crosses the researchers’ path. Each research group was also only allowed onto the ice with an armed guard in case of polar bears . »On the second day we were there, a polar bear mother and her cub visited the research station. She investigated all the equipment and then moved on again. We could watch everything from the ship - not even 100 meters away,« reported the Dean of the Faculty of Mathematics and Natural Sciences.
Natascha Oppelt and her team member Marcel König investigated the melt ponds using various methods, together with colleagues from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven and the German Aerospace Center (DLR). They determined, amongst other things, the extent and depth of the pond, the thickness of the underlying ice, the optical characteristics of the pond water and the surrounding sea ice surface.
In addition to measurements on the ground, a helicopter equipped with special cameras flew over the area to capture hyperspectral data, which provides very accurate images of the reflected light. Whereas the human eye only sees white snow or turquoise-blue melt ponds, the special camera records the reflected light from very many closely-spaced wavelengths. Correctly interpreting the colour signal of the different colours of ice and water of varying depths, or calibrating them, requires on-site measurements. Thus, the ground measurements are used to validate the data recorded with the helicopter.
This data should be used to develop a method that allows important parameters of melt ponds (extent of coverage, depth) to be derived from satellite data. The aim is ultimately to use satellite data to document how the melt ponds develop spatially and chronologically, to better understand their role in the Arctic climate and ecosystem.
»The fact is, the Arctic ice is shrinking. A logical consequence would be that the melt ponds become bigger and deeper. We can’t yet prove this is true everywhere,« said Oppelt, who has taken on this challenge, and will also participate in the expedition to the Arctic again next year.
Apparently the melting ice sets off a self-perpetuating chain of events, which leads to more melting and more heat. One reason for this is that a bright ice surface reflects almost all the incoming solar energy. A dark ocean, on the other hand, absorbs the energy and heats up, which makes the formation of new ice more difficult. (ne)
Zuständig für die Pflege dieser Seite: unizeit-Redaktion ► firstname.lastname@example.org