The snow radar was integrated into a smaller ‘mini-bird’ to fly out over the Ross Sea on a Twin Otter aircraft. Photo: Wolfgang Rack
Airborne snow measurements over pack ice
Researchers have successfully achieved airborne snow measurements over pack ice. This missing piece of the puzzle is critical for understanding past, present and future trends in sea-ice behaviour.
Antarctic sea ice has experienced an unexpected and rapid decline in the past couple of years. It tracks all-year round well below the expected levels observed since satellite monitoring began during the late 1970s. Understanding why this is happening is a key question for Antarctic researchers globally.
Sea ice trends are poorly understood because satellites can only reliably map the area of sea ice, not its thickness. And while a few thickness measurements are taken from the ground every year, they cover a relatively small area. Researchers from the Antarctic Science Platform are developing a tailored satellite-derived method so that the sea ice thickness can be measured remotely.
Flying over sea ice. Photo: Wolfgang Rack
As part of the ASP’s Project 4, researchers aim to quantify the distribution of ice thickness and snow, and the ice drift by combining airborne geophysics, ground-based sea-ice measurements and satellite data analysis. The Platform team, with collaborators from the Alfred Wegener Institute for Polar and Marine Research (AWI) in Germany, have been developing an airborne system to measure the thickness of snow and ice.
The first part of this system is an ‘EM Bird’: an electromagnetic induction device suspended 20m under a helicopter. This device sends an electromagnetic induction signal that measures the distance from the instrument to the underside of the sea ice. At the same time a laser measures the distance from the instrument to the surface. But this device measures the combined thickness of ice and snow. Therefore, in 2021, it was tested with a snow radar to get both measurements over stable fast ice near the coast. The snow radar was developed with collaborators at Lincoln Agritech and integrated with the EM Bird system at University of Canterbury and AWI.
But, flying over freely floating pack ice in the Ross Sea requires a fixed wing aircraft. In early 2024, the snow radar was integrated into a smaller ‘mini-bird’ to fly out into the Ross Sea on a Twin Otter aircraft. This allows, for the first time, mapping of the snow over large distances, which is a fundamental step to reference and validate satellite data for obtaining large scale Antarctic thickness maps.
The 2023/24 field team led by Wolfgang Rack (University of Canterbury) included Adrian Tan (Lincoln Agritech, radar technology development), Christian Haas (Alfred Wegener Institute, Germany; airborne geophysics and sea ice expertise), Pauline Barras (University of Canterbury), Inga Smith (University of Otago), and Sean Chua (Australian Antarctic Division; snow measurements on sea ice and sea ice expertise).
With the first test flight successful, two further flights were made and a total of about 2000km of radar measurements were collected over different types of sea ice.
The mini-bird suspended below a fixed-wing aircraft, flying over sea ice. Photo: Anthony Powell
Snow thickness measurements were also made on the ground to validate the airborne snow radar. The reference data (depth, density, layers) were acquired along a straight profile with variable snow conditions. Near-simultaneous radar measurements were conducted from the ground (on a sledge) and from the air over this validation line. The initial interpretation of the radar waveforms showed excellent results, with the snow radar performing as expected.
The reference measurements from ground and aircraft team are also used to evaluate the sea ice freeboard - the proportion of sea ice above the waterline, which is measured by satellites to derive thickness.
Large scale observation of snow on sea ice is an important field of research for understanding the sea-ice mass balance. These datasets also improve computer simulations and will allow better forecasting of future trends of sea-ice formation, drift and decay. Both snow and sea-ice thickness data enhances the understanding of the cryosphere-ocean–atmosphere interactions along the Antarctic coast, where polynya and sea ice processes fundamentally influence the Southern Ocean.
As part of a multi-nation collaboration, the longer term goal is to use this airborne system to conduct an aerial survey of up to 5000km of Antarctica’s coastline.
Snow thickness measurements were also made on the ground to validate the airborne snow radar. Photo: Wolfgang Rack