C11E-01
GLIMS Glacier Database: Status and Challenges
GLIMS (Global Land Ice Measurements from Space) is an international
initiative to map the world's glaciers and to build a GIS database that is
usable via the World Wide Web. The GLIMS programme includes 70
institutions, and 25 Regional Centers (RCs), who analyze satellite imagery
to map glaciers in their regions of expertise. The analysis results are
collected at the National Snow and Ice Data Center (NSIDC) and ingested
into the GLIMS Glacier Database. The database contains approximately
80 000 glacier outlines, half the estimated total on Earth. In
addition, the database contains metadata on approximately 200 000 ASTER
images acquired over glacierized terrain. Glacier data and the ASTER
metadata can be viewed and searched via interactive maps at
http://glims.org/.
As glacier mapping with GLIMS has progressed, various hurdles have arisen
that have required solutions. For example, the GLIMS community has
formulated definitions for how to delineate glaciers with different
complicated morphologies and how to deal with debris cover. Experiments
have been carried out to assess the consistency of the database, and
protocols have been defined for the RCs to follow in their mapping.
Hurdles still remain. In June 2008, a workshop was convened in Boulder,
Colorado to address issues such as mapping debris-covered glaciers, mapping
ice divides, and performing change analysis using two different glacier
inventories. This contribution summarizes the status of the GLIMS Glacier
Database and steps taken to ensure high data quality.
http://glims.org/
C11E-02
Radar Sounding of Fast-Flowing Glaciers
A major challenge in radio glaciology is sounding of fast-flowing glaciers such as Jakobshavn, Helheim, and Kangardlussuq in Greenland. Weak ice-bed echoes from fast-flowing glaciers are masked by off-vertical surface clutter. We need fine resolution both in the along-track and cross-track directions to reduce surface clutter. Synthetic Aperture Radar (SAR) techniques can be used to improve resolution in the along-track direction. However, SAR technique is not useful to synthesize a long-aperture in the cross-track direction unless data can be collected over a grid with a line spacing of about 1 m at 150 MHz. This is impossible to do with an airborne radar. We developed a high-sensitivity radar with array-processing capability in the cross-track direction to reduce clutter. The radar is designed to measure signals as low as 2 nV with a loop sensitivity of about 215 dB with peak power of about 800 W. We have successfully used this radar to obtain ice thickness and bed topography of three outlet glaciers. The radar soundings over Jakobshavn reveal a complex topography with a thickness of about 800 m at calving front and increasing to 2.7 km at about 40 km from the front. In this paper we will provide a brief review of the system that allowed first successful radar soundings of several fast-flowing glaciers, show sample results, and bed topography of a few glaciers.
C11E-03
Hindcasting Gulf of Alaska Glacier Mass Balances using a Degree-Day Model Calibrated to High-Resolution GRACE mascon Solutions
We present an updated mass balance time series for Gulf of Alaska glaciers acquired from Gravity Recovery and Climate Experiment (GRACE) high-resolution mascon solutions, for the period spring 2003 to fall 2008. Our solutions include corrections for gravity signals resulting from glacial isostatic adjustments, earth/ocean tides, and hydrospheric/atmospheric variations. We find that although the 5-year trend is one of overall mass loss, there are strong inter-annual variations in mass balance resulting from extremes in summer temperature (2004) and winter precipitation (2007). Quantifying the climatic causes behind such variations is necessary to improve understanding of key mass balance processes, and to improve models of these systems. We use a degree-day model to investigate links between climate observations and regional GRACE mass balance solutions. Daily temperature and precipitation data from climate stations near the glacier regions are used to drive a zero-dimensional degree-day mass balance model. Model outputs are temporally averaged to match the 10-day resolution GRACE time series. We correlate the ensemble of model outputs against each of 12 GRACE solutions representing 2 arc-degree Gulf of Alaska glacier regions. From this we choose the most representative climate station for each region, and use the correlation analysis to assess the uniqueness of model simulations. This analysis yields a set of 12 degree-day model parameters optimized for each glacier region. The calibrated models are then used to hindcast mass balance trends prior to the start of our GRACE solutions in 2003. We use mass balance measurements from the University of Alaska, Fairbanks aircraft laser altimetry dataset for the period of mid-1990s to present in order to validate our mass balance reconstructions. Preliminary results yield multi-year balances that agree within the range of altimetry and model errors. Differences are attributed to dynamic (tidewater and lacustrine glacier) changes that have cycles which are asynchronous with climate patterns. A series of sensitivity tests are conducted to quantify the reduction in model performance with time prior to the GRACE calibration period, resulting from adjustments in glacier geometry not simulated in the model. An assessment is also made of the utility of the model for forecasting future glacier balance conditions.
C11E-04
Global Ice Sheet Mapping Observatory Concept
We describe field experiments designed to test a spaceborne-radar system-concept envisioned to measure the spatial reflectivity and the 3-dimensional surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. We conducted our experiments during May 2006, September 2007 and July 2008 in Northern and Central Greenland. We operated 150 and 450 MHz radars which we installed on a P-3 and Twin Otter aircraft. We operated at altitudes up to 6800 m above mean sea level and as low as 500 m above the ice sheet surface. We used 2 transmitting antennas and 6-8 receiving antennas so as to be able to acquire nadir data as well as to be able to form interferometric pairs with different baseline combinations. We collected data over different glacial regimes including over the North East Ice Stream which is suspected to be underlain by water and where we detected strong echoes. Our results indicate that without substantial post processing, surface and volume clutter obscures a strong basal echo in the interior ice sheet at 450 MHz and to a lesser degree at 150 MHz when the sensor is flown several kilometers above the ice sheet surface. Comparisons between the strong radar echo at the ice stream location with calibration data acquired over the ocean enable us to estimate attenuation through the ice at both frequencies. We use the data to test several clutter rejection schemes including interferogram filtering and multiaperture beam formation. We successfully tested both approaches and show how the interferogram filtering technique also allows for simultaneous separation of signals from the left and right side of the aircraft. We use the separation scheme to compute the three-dimensional subglacial topography along several swaths each of which is up to 3 km wide. We find that the basal topography of our northwestern Greenland study areas consists of gently undulating hills of about 5 km wavelength that are occasionally cross-cut by narrow valleys.
C11E-05
A detailed new view of ice-sheet change
Changing ice sheet volume directly affects global sea level and the most rapid and dramatic losses are expected to occur along the coastal margins through accelerating glacier flow. Given the existing capability of radar altimetry sensors, this process has to date been only poorly observed and understood. We have developed a new technique to exploit high-resolution, space-borne laser altimetry available from NASA's ICESAT to reveal contemporary ice sheet change in unprecedented detail along satellite tracks, rather than only at orbit cross-over points. In many areas, this improved resolution allows discrimination of the effect of changes in glacier velocity and those in snow accumulation and density. We obtained change measurements over a period of up to 4.5 years for the entire grounded margin of both the Antarctic and Greenland ice sheets. We present evidence of glacier dynamic thinning that now reaches all latitudes in Greenland, has intensified at key Antarctic grounding lines, has endured for decades after ice-shelf collapse, and penetrates far into the interior of both ice sheets. We also identify several areas where these independent measurements allow arbitration between earlier conflicting estimates. The thickness changes we have identified are the starting point for improved estimates of current ice-sheet mass balance and its contribution to sea-level rise.
C11E-06
A Study of Airborne Radar Altimetry Waveforms and Ku Band Ground Penetrating Radar Measurements From the Percolation Zone of the Greenland Ice Sheet
The advent of satellite altimetry has revolutionised the measurement of the mass balance of the world's ice sheets. Reducing errors associated with estimates of ice sheet elevation is an ongoing goal of satellite altimetry programmes, including the European Space Agency's (ESA) current CryoSat mission. The main objective of the CryoSat Validation Experiment (CryoVex) is to understand the penetration of the radar signal from Cryosat, and to identify possible sources of errors in the CryoSat Synthetic Aperture Radar (SAR) elevation estimates. Here we analyze multiple profiles of the power waveform of the ESA's Airborne SAR/Interferometric Radar Altimeter System (ASIRAS) recorded over the percolation zone of the Greenland Ice Sheet during the 2004 and 2006 CryoVex campaigns, and discuss the elevation estimates derived from these profiles. The waveforms are compared with simultaneous ground-based Ku band radar measurements. Density measurements from snowpits are used as input for an electromagnetic model that simulates the radar response to a stratified snowpack. The snow density profiles and the model results reveal the snowpack characteristics that lead to the formation of the radar signals observed. Our observations present conditions under which current altimetry processing methods are unable to detect accurately snow surfaces. The errors could be accentuated by seasonal differences observed in the waveforms recorded.
C11E-07 [WITHDRAWN]
Light Field and Optical Properties Measurements of a Supraglacial Melt Pond, Eastern Greenland
We performed hyperspectral measurements of solar downwelling irradiance and upwelling radiance (as reflected from the bottom) of a melt pond on Helheim Glacier, East Greenland. Simultaneous pond depth measurements were also made, and samples of pond water were acquired for analysis of optical absorption and attenuation characteristics using laboratory multi/hyperspectral spectrophotometers. The downwelling irradiance and upwelling radiance measurements allow us to calculate the widely use remote sensing reflectance parameter, Rrs, once the upwelling radiance is converted to the water-leaving radiance by taking into account effects at the water-air interface. Corrected for atmospheric effects, Rrs is the reflectance measured at the top of the atmosphere (TOA) by satellite sensors, including the visible/near infrared images collected by the ASTER sensor on the Terra satellite. In earlier work, we developed a method for extracting the depth of glacial melt ponds from ASTER TOA reflectance images. Using our recent in situ radiance, irradiance, and depth measurements we test the accuracy of that depth-finding algorithm. The laboratory absorption and attenuation analyses provide further validation of some of the initial fundamental assumptions of our method.
C11E-08
Acoustic monitoring of sea ice and ice sheet off Antarctic Peninsula
Sea ice and ice sheets are under contant thermal stress by temperature changes and mechanical stress by wind and waves. They crack and collide, and as a result emit impulsive acoustic noise into the water column of the sub-polar regions. Under the unique propagation conditions that exist, the low frequency ice noise tends to propagate long distances with little dissipation. Ice noise is intermittent, but occurs frequently and is ubiquitous to all sub-polar regions; it dominates the background noise spectrum. Here we report on the ocean acoustic environment monitoring within the Brasnfield Strait off the Western Antarctic Peninsula (WAP). A 2-year long underwater acoustic monitoring experiment has revealed a surprising seasonal pattern of ocean ambient noise level, which is corrrelated with the seasonal ice breakup and ice coverage of the region. In the Bransfield Strait, where the sea ice extent is shrinking at an alarming rate, the regional ocean ambient noise level may be on the rise as a result of increased melting and fracturing of sea ice of all scales - in the region. Though the experiment itself may not be long enough to implicate a decadal-scale-trend directly, based on the region's well-established warming trend, decreasing sea-ice and its known effect to the acoustic environment lead us to believe that the regional ocean noise level of the WAP is on the rise. Using the same acoustic array data and technique commonly used to detect low-frequency seismic events (T- waves), we also have tracked and mapped sea ice by locating the sources of ice noise in the region. The WAP's seasonal noise levels are compared with the acoustic data collected on acoustic arrays in the Pacific and Atlantic Ocean and contribution of the ice noise to the world-wide low-frequency noise budget is discussed.