Visualizing and Assessing Sea-Ice Motion Below the Continuum Scale

by Cathleen Geiger, Geography, University of Deleware; Gowri Somanath, Vincent Ly, Chandra Kambhamettu, and Mani Thomas, Computer and Information Services, University of Delaware; and Pablo Clemente-Colón, National/Naval Ice Center

Sea-ice cover in the Arctic is retreating and thinning as global temperatures continue to rise. The rise in temperature may eliminate Arctic sea ice during summer months creating the so-called ice-free summer condition. As the ice thins, its strength weakens. A weaker ice cover will encourage more shipping earlier in the spring and later into the fall, thereby, expanding access through polar seas much longer than ice-free periods. It is these ice-engaging activities that will push the Arctic frontier open for the most opportunistic. Hence, the need to not only monitor how much ice is melting, but how the remaining ice is behaving as a dynamic material under new climate conditions.

As a first order approximation, sea ice drifts about 10 km/day and generally follows the isobars of atmospheric pressure fields. These approximations are sufficient when estimating the general circulation of sea ice at scales of 100 km or more. But, to estimate the actual motion, one must track properties of sea ice directly. Since sea ice resides in darkness, roughly half the year, and beneath clouds and fog much of the remaining year, active- and passive-microwave sensors are needed to track ice motion over large areas at regular intervals. Using microwave-frequency instruments, correlation methods identify features in the texture of the ice based on backscatter properties. These methods have been applied to sea ice since the 1990s, with great success down to scales of about 5-10 km in the interior ice pack.
Discontinuities in the ice motion identify the hot spots of dynamic motion. In this context, these are important because they are the locations responsible for sea-ice thickness redistribution. These indicate where open water is exchanging heat between the ocean and atmosphere. The open-water passages provide easy access through pack ice. However, these same open-water areas can close as soon as the wind changes or the ice moves into new regions, constrained by land boundaries or rising sea-floor topography. Discontinuities also identify hazardous navigational areas and vulnerable infrastructure sites. Clearly, such issues require more than scientific inquiry and will need the integration of resources from academia, federal, and industrial partners. One effective approach is to develop new visualization tools at the logistical and tactical scale that track and monitor dynamic ice conditions that impact human activity.

During the International Polar Year, tools were developed to address some of the geophysical issues (Figure 4). These tools were tested in field studies in the spring of 2007 in the Beaufort Sea using RADARSAT-1 SAR-derived motion products. With results scientifically comparing well to 10-m resolution, telemetry-buoy arrays, the next obvious step is developing an open-source application that the National Ice Center can incorporate into their day-to-day operations to support ice charting, logistics, national security agencies and, search-and-rescue operations in need of tactical decision-making tools (Figure 5). Some of these new developments are being supported by a Department of Defense and NASA-sponsored Arctic Collaborative Environment where open-source tools are shared among interdisciplinary groups. This collaborative environment is essential since no one group has the capacity to solve the enormous integrated set of problems faced as people try to understand, visualize, and operate in rapidly changing environmental conditions at high latitudes.

Because of these current and growing issues, the need exists to characterize not only the motion of ice, but also the uncertainties and scale of each resolved motion value. These two critical parameters of scale and uncertainty are essential to resolving the power-law relationships that connect cascading motion through deformation processes. More testing and development is needed and there are still many important details that need to be addressed before observed ice motion can be ingested into regional forecasts and global-climate models. Identification of discontinuities may prove to be a small, but critical part of what will be required for effective assimilation in support of these larger efforts.