Almost every geophysical process leaves it signature in the terrestrial gravity field or its changes. If the process is "big'' enough, it can be measured by GRACE, a twin satellite mission that is sensitive to the Earth's time-variable gravity field, which has been yielding data for about seven years now. Among its many other accomplishments, models based on GRACE have provided evidence that the Earth's polar regions are losing mass due to progressive melting of their ice cover, and GRACE modeling has also helped constrain crustal deformation resulting from large earthquakes, thereby inaugurating an era of a new class of earthquake observations. The GRACE data are noisy and require extensive processing, filtering, and statistical analysis to yield signals of this kind, which are buried deeply beneath the noise and contaminated by the geophysical signatures of ocean currents, the hydrological cycle, or post-glacial rebound.
Our methods development will enable us, and the scientific community, to make the most of today's available GRACE data. Well-constrained mass flux rates from Earth's polar regions, and their errors, and the robust detection, analysis and modeling of the coseismic deformation from large earthquakes will stand alone as scientific results, but they will also feed back into climate research, glaciology, seismology, and geodesy. Even more broadly than that, the problems we solve are relevant in providing new estimation methods for noisy and incomplete data distributed on a sphere, in the most general sense, e.g. as used in biomedical and statistical research, in physics, cosmology and computer science.
We develop a new mathematical technique for the inversion of GRACE data, which are noisy and incomplete. Central to this effort is the development of "noise-cognizant Slepian functions'', a function basis, alternative to spherical harmonics, that is eminently suited to represent and analyze geographically localized, bandlimited, signals on the sphere. No longer producing only spatial averages of the signal, nor reprocessing global models based on spherical harmonics, we perform inversions on time series of the inter-satellite potential difference derived from GRACE, directly in this basis. From this we recover estimates of the entire spatial dependence of the mass gain/loss in ice-covered regions and hydrological basins, as well as estimates of the gravity perturbations accompanying large earthquakes. The former are important to science and society per se, the latter add information on a different temporal and spatial scale to that which can be had from GPS measurements or seismological data.
This project is supported by the Geophysics, Arctic Natural Sciences, and Antarctic Earth Sciences Programs.
Embracing data 'noise' brings Greenland's complex ice melt into focus by Morgan Kelly, edited by Frederik J Simons An enhanced approach to capturing changes on the Earth's surface via satellite could provide a more accurate account of how ice sheets, river basins and other geographic areas are changing as a result of natural and human factors. The technique revealed sharper-than-ever details about Greenland's massive ice sheet. The technique comprises a mathematical framework and a computer code to accurately capture ground-level conditions collected on particular geographic regions by the GRACE satellites. GRACE measures gravity to depict how mass such as ice or water is distributed over the Earth's surface. Typically, GRACE data are recorded for the whole globe and processed to remove large regional differences, said lead author Christopher Harig. The result is a coarse image that can provide a general sense of mass change, but not details such as various mass fluctuations within an area. With our method, Harig and co-author Frederik Simons, can clean up "noise" — the signal variations and distortions that can obscure satellite readings — and then recover the finer surface details hidden within. From this, they can configure regional information into a high-resolution map that depicts the specific areas where mass change is happening. "We do very little processing to the data and stay closer to the real signal," Simons said. "GRACE data contain a lot of signals and a lot of noise. Our technique learns enough about the noise to effectively recover the signal, and at much finer spatial scales than was possible before." The researchers tested their method on GRACE data for Greenland recorded from 2003 to 2012 and brought the complexities of the island's glaciers into clearer focus. While overall ice loss on Greenland consistently increased between 2003 and 2012, Harig and Simons found that it was in fact very patchy from region to region. Douglas MacAyeal, a professor at the University of Chicago, said that the research provides a standardized and accurate method for translating GRACE data, particularly for ice sheets. The sprawling, incomplete nature of the satellite's information has spawned a myriad of approaches to interpreting it, some unique to specific scientists, he said. "Each particular investigator ends up getting a different individual number for the net change in mass," he said. "What this research does is figure out a way to be more thoughtful and purposeful, allowing researchers to standardize a bit more and also to understand more precisely where they are, and where they are not, able to resolve ice changes." Greenland lost roughly 200 billion tons of ice each year during the period studied, which falls within the range reported by other studies. As expected, ice loss occurred in the lower, warmer coastal areas — as opposed to the higher and colder interior, which gained ice mass — but the melt was concentrated on the southeast and northwest coasts for most of the period studied. Indeed, many coastal areas showed no ice-mass loss, while the ice sheet on the southwest coast actually thickened slightly from 2003 to 2006. But these trends were more complex when Harig and Simons got into the details. Surprisingly, the location of the greatest melt activity migrated around the island, shifting from the southeast to the northwest coast in just a few years. Ice loss on the southeast coast built up starting in 2003 and hit a highpoint in 2007. In 2008, loss on this coast began to recede and shift toward the northwest coast; by 2010, the southeast coast displayed only minor ice loss, while nearly the entire western coast exhibited the most severe melt. During this transition, melt also receded then picked up again on the northeastern coast with seemingly little overlap with activity elsewhere. Details such as these can help scientists better understand the interplay between Greenland's glaciers and factors that influence melt such as ocean temperature, daily sunshine and cloud coverage, Harig said. That understanding can in turn help researchers determine how the Greenland ice sheet responds to climate change — and how much more ice loss to expect.
