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Improving Arctic Sea Ice Thickness Measurements

By Edward Lundquist
Principal Science Writer
MCR Federal
Arlington, Virginia

This piece of data was collected using a Riegl LMS-Q560 mounted on a Twin Otter aircraft that was flying approximately 700 feet above the ice at 90 knots. The data shows the topography of the ice, including ice ridges and a crack in the ice that developed into an opening.

Scientists from the Naval Research Laboratory (NRL) are digging into a deep problem: How thick is the Arctic sea ice? While satellites can show where the Arctic Ocean is covered with ice, there is no way to directly determine its thickness, which is essential information for building and operating ships to navigate the Arctic.

With more open water, thinner sea ice due to retreating ice cover and multiyear ice decline comes an increase in oil and gas exploration, fishing, tourism and commercial vessels in the Arctic.

The multiyear ice cover is not only shrinking, it is thinning. As this trend is expected to continue, along with greater activity in the region, it becomes more important to know the extent of ice coverage and thickness.

To date, remote sensing capabilities cannot easily nor accurately measure sea ice thickness. Still, available observations along with modeling can help provide estimates, which can aid forecasters. Some satellite systems can measure the freeboard (the height of ice above the ocean surface) from which total sea ice thickness can be estimated, but they have significant temporal and spatial coverage limitations.

The NRL and its research partners are collecting higher resolution data with aircraft, in-situ measurements and submarines, which can help validate satellite-derived information and ice thickness model output. In 2011, the NRL worked with the U.S. Army Corp of Engineers Cold Regions Research Lab (CRREL), the Navy's Arctic Submarine Lab and NASA. In 2012, the NRL has partnered with the University of Alaska Fairbanks and the Alfred Wegener Institute for Polar and Marine Research. A primary focus of the NRL and its partners is to collect data for validating and calibrating the higher resolution data sets to optimize instrument suites and the development of predictive sea ice models.

Measuring and Calculating Ice
Existing sensors can measure the ice freeboard, which often has an overlying snow cover. The freeboard can be isostatically balanced by the buoyancy of the ice below the water line, which is called ice draft. That buoyancy is dependent on the mean density of ice, but if the freeboard can be measured accurately and the density relationship calculated, then the average thickness of the sea ice can be determined. Understanding the accuracy and relationships of these measurements helps scientists develop new algorithms for regional sea ice models.

The Navy's recent research effort is aimed at evaluating, validating and improving the existing models. Fieldwork by the Office of Naval Research began in October 2011 as a prequel to the NRL's DISTANCE (Determining the Impact of Sea Ice Thickness on the Arctic's Naturally Changing Environment) program, which is basic research that will be funded from fiscal years 2012 to 2016. Scientists from the NRL's Remote Sensing, Oceanography and Marine Geosciences divisions are also involved in the project.

The NRL conducted field research in March 2011 and April 2012 from Barrow, Alaska, using a DHC-6 Twin Otter twin-engine turboprop aircraft, which mounted a Riegl USA (Orlando, Florida) LMS-Q560 2D laser topographic lidar, a precision radar altimeter designed and built by NRL Code 7420 engineers, a high-resolution camera and GPS. Twin Otter International (Grand Junction, Colorado) operated the aircraft.

s Lidar reflects from the top surface of a floating mass, possibly snow sitting on top of ice. Unlike lidar, radar can see through snow. The lidar determined the distance between the aircraft and the top of the snow, and the radar altimeter determined the distance from the aircraft to the top of the ice. The lidar and radar measurements of the freeboard were periodically re-referenced to sea level by measuring distance to the water over openings in the ice. The data collected could not be observed on board the Twin Otter during flight and were analyzed during post-mission analysis. To continue this article please click here.

Edward Lundquist is a retired U.S. Navy captain and a principal science writer for MCR Federal in Arlington, Virginia. He served on active duty as a surface warfare and public affairs officer, with his last assignment as a commanding officer of the Naval Media Center. He writes for naval, maritime, defense and science publications around the world.

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