G33C-01 INVITED
Simultaneous GPS and teleseismic monitoring of glacial stick-slip of Whillans Ice Stream, West Antarctica.
The TIDES experiment is a coordinated effort to measure and model the Siple Coast ice streams using GPS instruments, local and teleseismic seismographs, radio echo sounding data, and active-source seismic data. The experiment grew out of the discovery that the flow speed and the local seismicity of these ice streams (Whillans, Kamb, Bindschadler, and MacAyeal, originally ice streams B, C, D, and E, respectively) are controlled or modulated by the local ocean tide beneath the Ross Ice Shelf. Arrays of GPS and seismic instruments were deployed on the ice streams. Radio echo sounding of the subglacial environment, allied to numerical modeling of the base as a frictional material, have resulted in a new view of the stability of these important conduits of West Antarctic ice to the ocean. In addition, analysis of broadband seismic data shows that changes in the motion of the Whillans Ice Stream (ice stream B) can be detected at great distances. The ice streams of the Siple Coast are grossly similar in size, surface slope, and average flow speed, but remarkably different in detail. The downstream part of ice stream B flows primarily by stick slip motion, with the times and magnitude of slip related to the tides at the grounding line. The partitioning between rapid slip and slow inter-slip deformation is variable between different parts of the ice stream. The slip timing and to a lesser extent the slip magnitude is strongly correlated to the tide cycle, with a complex relationship to the tide height. Ice stream D and E do not undergo stick-slip, however there is significant modulation of the flow speed of these glaciers with the tide. In all these ice streams, the tidal effect can be detected far inland behind the grounding line. The slip of ice stream B can be detected at seismographs around the Antarctic continent, and even in Australia. Alhough each slip is ~25 minutes long, we detect three main packets of seismic energy of durations of ~100 seconds at periods of 20-100 seconds. Time correlation with the GPS observations identifies the first packet with the initial rupture nucleation and the final packet with rupture termination at the grounding line. The seismic amplitudes are correlated with the spring-to-neap tidal cycle and with rupture velocity. We suggest that the slip events can be monitored using permanent seismic stations, allowing us to detect changes over a longer time period than is possible with in-situ measurements. More generally, other glaciers and ice streams can be remotely monitored for fast glacial slip using seismic detection techniques.
G33C-02 INVITED
Geodetic Measurements and Analysis of Glacier Kinematics in East Greenland
The harsh environment on the highly crevassed surfaces of fast-flowing tidewater glaciers makes the acquisition of in situ geodetic measurements, such as GPS, extremely difficult. These measurements, however, provide key information pertaining to glacier dynamics, including glacial earthquakes, and help improve our understanding of the complex interactions between the cryosphere, the atmosphere, the lithosphere, and the oceans. The kinematic analysis of high-rate GPS data acquired in these environments is also challenging. Although this analysis shares many features with the analysis of high-rate GPS data in the application known as "GPS seismology," it also has its own peculiarities. These derive largely, though not exclusively, from the fast background velocities that are characteristic of ice flow but absent in tectonically induced seismic motions. In a concerted multidisciplinary effort, we acquired, along with other geophysical observations, continuous, high- rate GPS measurements at several locations on Helheim Glacier, East Greenland, for about two months during the Arctic summers of 2006 and 2007. Analysis of these GPS observations indicates that Helheim Glacier velocities ranged between ~25~m~d-1 at the calving front to ~5~m~d-1 at an upglacier distance of ~35~km, with significant departures from simple, linear motion at both inter- and intra-day timescales. This presentation will focus on the geodetic approach to data acquisition and data analysis from these glaciers, with emphasis on the accuracy and temporal resolution that can be achieved. Furthermore, we will discuss ongoing hardware-directed efforts to develop accurate, robust, and inexpensive GPS systems that may serve to ameliorate some of the observational difficulties associated with "GPS glaciology" without compromising its resultant accuracy.
G33C-03 INVITED
GPS and Conventional Surveying Measurement of Glacier and Iceberg Motion in the Jakobshavns Isbrae System
The lower reach of Jakobshavns Isbrae, a large outlet glacier of the Greenland ice sheet, has changed dramatically in the last ten years. Work to characterize the present patterns and time variation of ice flow in this system has involved several satellite and surface based measurement techniques. Our field efforts over the past two summers have attempted to document time variations in ice flow in the main channel of the glacier, and to characterize motion of the glacier and icebergs in the fjord during large calving events. We have used conventional geodetic GPS systems on the ice where deployment and recovery are reasonable, radio and iridium linked GPS in more difficult areas, and a set of conventional survey reflectors tracked by automatic total station near the terminus to recover a record of ice flow. The glacier experiences large-scale calving retreat events through the summer; our measurements are able to characterize the changes in glacier flow near the terminus across several of these events. In addition, flow in the main branch of the glacier upstream has been followed through two melt seasons; here flow variations appear to be in sync with surface water inputs in the basin. The time series of motion from different parts of the glacier show responses to different forcings; near the terminus the ice flow does not show the strong tidal fluctuations in speed that were observed twenty years earlier by Echelmeyer; while at higher elevations subtle diurnal variations in flow are observed later in the melt season
G33C-04
Recent ice sheet and glacier elevation changes in Greenland from aircraft laser altimetry
The Arctic Ice Mapping group (Project AIM) at the NASA Goddard Space Flight Center Wallops Flight Facility has been conducting systematic topographic surveys of the Greenland Ice Sheet (GIS) since 1993, using scanning airborne laser altimeters combined with Global Positioning System (GPS) technology. Earlier surveys showed the ice sheet above 2000-m elevation to be in balance, but with localized regions of thickening or thinning. Thinning predominates at lower elevations and thinning rates have recently increased, resulting in a negative mass balance for the entire ice sheet. In May 2007 critical segments of near-coastal flight lines in Greenland were re-surveyed. Results from the new data will be presented.
G33C-05
An assessment of high latitude land ice mass flux estimates derived from GRACE
Understanding the evolution of the ice sheets and glacier systems is of paramount importance due to the vulnerability of the Earth's cryosphere to climate change and its contribution to sea level. In recent years we have obtained a wealth of information regarding the remote land ice regions mainly through spaceborne and airborne observations. One such very important data set is that obtained from the NASA/DLR Gravity Recovery and Climate Experiment (GRACE) mission. The data from GRACE provide new opportunities to observe the complex spatial and temporal evolution of the cryosphere's land ice. However, use of these data for the understanding of climate change and its impacts requires that we fully characterize and quantify the limitations and accuracies of the mass flux solutions derived from these GRACE observations. In this talk we will discuss the latest Greenland, Antarctica and Alaska ice mass flux solutions derived from GRACE. We will explore the spatial and temporal resolution of these solutions and their accuracies, and discuss their limitations. We will present results of simulations, technique comparison, error analysis and comparisons to other data sets such as surface elevation change observations from laser altimetry, ICESat laser altimetry, and surface melt data from MODIS. The purpose is to understand the fundamental accuracy, resolution, limitations and strengths of the GRACE mass flux observations in order to better utilize them for climate change research.
G33C-06
Recent results from GRACE in Antarctica and Greenland: comparison with Insar mass balance estimates.
Using measurements of time variable gravity from GRACE satellites we determine long term and seasonal mass variations of the Antarctic and Greenland ice sheets for over a five year period starting in Apr 2002.We use monthly GRACE gravity fields to estimate the linear trends in Greenland and Antarctic ice mass. Both ice sheets display a large mass imbalance during the analyzed period. This presentation will examine uncertainties associated with GRACE ice sheet mass balance. Particularly we will focus on the interpretation of Antarctic mass balance by comparing GRACE and InSAR derived estimates. The mass of the Antarctic ice sheet decreases significantly during the observation period. Most of this mass loss is generated by the West Antarctic Ice Sheet. We will compare ice mass balance from GRACE with InSAR derived estimates in Antarctica and with particular attention to the West Antarctic ice sheet. This comparison will provide an independent verification of the quality of the new snow accumulation model adopted for the InSAR derived estimate as well as of the post glacial rebound estimates.
G33C-07
Geodetic Constraints on Steric Sea Level Rise: Using Earth Oblateness as a Metric for Land Ice Ablation
Recent sea level observations reported by the IPCC and other sources show a global rise of about 3mm/yr, since the advent of satellite altimetry in the early 1990's. Attribution of this rise has been controversial, with some studies indicating that it can be fully explained by thermal expansion while others have shown a net heat loss by the oceans in recent years. Here we examine the combination of radar altimetry with geodetic observations of the Earth's dynamic oblateness (J2), as a means of providing independent constraints on changes in the steric component of sea level rise. In this presentation we focus on global sea level rise and subsequent fall during the 1997-98 El Nino. Steric sea level rise computed by a number of recent studies fails to account for the global anomaly observed during this event, and nonsteric contributions implied by changes in snow and ground water storage derived from the LaD hydrological model, while producing sea level anomalies that are well-correlated with the altimeter data, also fail to reproduce the full amplitude of the global sea level change. By using SLR-derived J2 data, which also showed a large anomaly during the late 1990's, we derive independent constraints on latitudinal shifts in water mass substance and hence on changes in high-latitude land ice ablation, and use these constraints to evaluate the contribution of mass balance changes in polar ice sheets and sub-polar glaciers to the large sea level anomaly observed during the 1997-98 ENSO.
G33C-08
Assessment of Current Estimates of Global and Regional Mean Sea Level Estimates from TOPEX and Jason-1 Altimetry Based on Revised Reference Frame and Orbits
The measurement of mean sea-level change from satellite altimetry requires extreme stability of the altimeter measurement system. In particular, the orbit and reference frame within which the altimeter measurements are situated, as well as the associated altimeter corrections, must be stable and accurate enough to permit robust mean sea level (MSL) estimates over an extended time period. The terrestrial reference frame is linked inseparably to the measurement of global mean sea level estimates from satellite altimetry and provides the context for the interpretation of the causes of current mean sea level trends. In an effort to adhere to cross mission consistency, we have generated the full time series of orbits for both TOPEX/Poseidon (TP) and Jason-1 through reduced dynamic methods based on the GGM02C GRACE derived gravity field within a consistent well defined ITRF2005 terrestrial reference frame. The recent release of the entire revised Jason-1 Geophysical Data Record (GDRB), and recalibration of the TOPEX microwave radiometer correction also require the further re-examination of TP/Jason-1 consistency issues. Here we present an assessment of these recent improvements to the accuracy of the TP/Jason-1 sea surface height time series, and evaluate the subsequent impact on global and regional mean sea level estimates.