From on board, 10 October 2019, by Stefan Hendricks for the ICE and Remote Sensing Teams |
How we found the right floe for the ice drift through the Arctic Ocean
First of all, the floe had to reflect the typical conditions in our region; this aspect is indispensable to arriving at representative conclusions. Secondly, it had to be sufficiently stable for us to work on its surface for an entire year. These two requirements weren’t easy to reconcile, since the typical sea ice we encountered was predominantly thin and showed signs of intensive melting.
The search began with an analysis of the satellite data. On board Polarstern, the Sea Ice (ICE) and Remote Sensing Teams regularly analysed satellite images updated on a daily basis, just as our colleagues on our partner ship Akademik Fedorov did. These analyses had been going on for some time, but we still needed to know about the current status and positions of the ice floes. To get that information, we relied on daily updated radar images from the satellites Sentinel-1 (ESA/Copernicus) and TerraSAR-X (DLR), since they can be used as reliable ice maps, regardless of the cloud cover or lack of sunlight. Using these maps, we then identified a number of candidates that met the basic criteria in terms of their size and structure. Particularly the size requirement narrowed the choices down to just a handful, even though we surveyed a region measuring well over 100 kilometres.
In the next step, we visited the respective floes, so as to examine them first-hand, a task shared between the Polarstern and Akademik Fedorov. The goal of these initial scouting trips was to gain a first impression of each floe’s thickness. To do so, we took advantage of our extensive scientific equipment, e.g. a sledge with built-in electromagnetic ice-thickness sensor. By combining our findings onsite with the radar images from the satellites, we arrived at an overview of the ice situation in the region. What we found: the vast majority of the floes surveyed consisted of very thin (30 – 70 cm) sea ice and had meltwater pools that had since frozen over – a clear indicator of intensive melting in the summer – on much of their surface area. Floes of this type can easily break apart or be pushed together, which makes long-term, continuous monitoring and measuring work highly problematic. Yet one of the floes that Polarstern surveyed, despite some thin ice, proved to also contain a substantially thicker and more stable area, as could be recognised both in the satellite data and the onsite thickness measurements. After comparing notes with our colleagues on the Akademik Fedorov we decided on this floe, which will be home to the Polarstern and countless researchers for the year to come.
Then a new phase, the detailed floe mapping, began, since we needed to find suitable sites for all the research and logistical installations. Moreover, we had to do so in a way that would have a minimal influence on the floe itself. In this regard we used a sensor mounted on one of Polarstern’s two on-board helicopters: the Airborne Laser Scanner (ALS), which can detect fine-scale changes in surface height below the helicopter. The sensor can cover a ca. 300-metre-wide swath, so we had to sweep back and forth several times to map the entire floe: no mean feat when it comes to moving sea ice, since the GPS coordinates can quickly become meaningless above the drifting ice. Thankfully, the scientists on board were able to see the laser scanner’s coverage of the ice surface in real time. By combining this input with the pilot’s trained eye, we were able to almost seamlessly map the entire floe and its immediate vicinity. Sometime in the future we’ll repeat these measurements so as to gauge the changes in the ice and its snow cover.
The laser scanner produces a tremendous amount of data, some of which we used on board to create a preliminary map of the ice surface, one far more detailed than the satellite images. This map will help us determine which areas are best suited for specific types of scientific testing, and which ones need to be set aside for logistical installations like paths and power cables. But a map isn’t truly a map until it has a name on it; therefore, from very early on, the floe’s most stable area was dubbed the “fortress”.
In the past few days, heavy winds have produced a number of cracks in the ice, as a result of which our map is no longer entirely accurate. It’s precisely events like this one that we plan to spend the next year investigating, so our mapping work has just begun!