A Comprehensive Ice Velocity Map of Greenland from RADARSAT-1 and Other Satellite Data
by Ian Joughin and Ted Scambos
The rate at which Greenland discharges ice to the ocean has accelerated dramatically over the last several years. These changes are significant and likely indicate a response to climate change (e.g., enhanced basal lubrication from surface melt draining to the bed or dynamic response due to the loss of resistance from grounded and floating ice as calving fronts retreat). With the near-doubling in speed of many of Greenland’s outlet glaciers, the magnitude of the change is startling and has overturned the conventional wisdom that ice sheets respond sluggishly, with response times measured in centuries to millennia.
Field-based investigations can observe only a few locations on the ice sheet each year. Therefore, satellite remote sensing is essential both for detecting change and for understanding the physical processes that govern the ice sheet’s response to climate change. Through its Program for Arctic Regional Climate Assessment, NASA in collaboration with NSF, has been investigating the ice sheet’s mass balance for more than 15 years. As part of this effort, in late 2000, CSA and NASA conducted the Arctic Ice Mapping Mission at the same time as the Modified Antarctic Mapping Mission to collect multiple fine-beam mode acquisitions suitable for interferometry/speckle tracking over nearly the entire Greenland ice sheet. As rapid change began to unfold, the space agencies acquired four, back-to-back repeat mappings of the ice sheet during the winters of 2005-2006 and 2006-2007.
In March 2006, NASA funded a 2-year investigation at the Universities of Washington (I. Joughin, B. Smith, and I. Howat) and Colorado (T. Scambos and I. Howat) to process the 2000 and 2006 datasets. The main goal of this project is to produce comprehensive maps of velocity for the Greenland Ice Sheet. The project also is uses ASTER data (15 m) from NASA’s Terra platform to determine summer velocities for many of Greenland’s outlet glaciers. In addition, the project is producing high-resolution (20 and 100 m) SAR image mosaics of the ice sheet.
During the project’s first year, the available data were processed and preliminary velocity maps were produced for both 2000 and 2006 (see Figure 1). Fine-beam mode data yielded velocity estimates for roughly two thirds of the ice sheet’s nearly featureless interior. Coverage is even better on outlet glaciers, where the presence of crevasses and other features improves the performance of the algorithms used for feature tracking. As a result, velocities are available on nearly all outlet glaciers where fine-beam mode coverage is available. Some large gaps in coverage occur along the southern portions of Greenland where ScanSAR commercial acquisitions have higher priority for acquisition. ASTER processing has largely focused on the southeast portion of the ice sheet, and time series of summer velocity now exist for many of the rapidly changing glaciers in this region.
Processing during the second year will focus on additional tracking in the fast moving regions, where the large displacements (>3 km per year) fall outside the window of the speckle-tracking algorithm. In addition, the interferometric phase data will be included in the velocity estimates to improve accuracy in the interior of the ice sheet.
The University of Colorado effort at the National Snow and Ice Data Center has focused on developing a Minnesota Map-Server based system for web-distribution of the data. Users will be able to browse the image and velocity data sets and select data for their regions of interest. The data can then be retrieved as GeoTiff products, which can be read by a number of packages (e.g., IDL-ENVI and ArcGIS). The data will also be available through direct ftp access.
The velocity and image mosaic products will be ready for release early in the summer of 2008. The high-resolution (20 m) image products will be subject to the same restrictions as those that apply to the high-resolution Antarctic Mapping Mission products, which require user approval from NASA.
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