907 research outputs found
Evaluation of Regional Climate Models Using Regionally Optimized GRACE Mascons in the Amery and Getz Ice Shelves Basins, Antarctica
We develop regionally optimized Gravity Recovery and Climate Experiment (GRACE) solutions to evaluate the mass balance of the drainage basins of Amery Ice Shelf, East Antarctica, and Getz Ice Shelf, West Antarctica. We find that the Amery region is near balance, while the Getz region is rapidly losing mass. We compare the results with the mass budget method (MBM) combining ice discharge along the periphery with surface mass balance derived from three regional climate models: (1) Regional Atmospheric Climate Model (RACMO) 2.3p1 and (2) 2.3p2, and (3) Modèle Atmosphérique Régional 3.6.41. For Amery, MBM/RACMO2.3p1 agrees with GRACE, while MBM/RACMO2.3p2 and MBM/MAR3.6.41 suggest a positive mass balance. For Getz, all estimates agree with a mass loss and the GRACE results are robust to uncertainties in glacial isostatic adjustment derived from an ensemble 128,000 forward models. Over the period April 2002 to November 2015, the mass loss of the Getz drainage basin is 22.9 ± 10.9 Gt/yr with an acceleration of 1.6 ± 0.9 Gt/yr2
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Mass Loss of Totten and Moscow University Glaciers, East Antarctica, Using Regionally Optimized GRACE Mascons
Totten and Moscow University glaciers, in the marine-based sector of East Antarctica, contain enough ice to raise sea level by 5 m. Obtaining precise measurements of their mass balance is challenging owing to large area of the basins and the small mass balance signal compared to West Antarctic glaciers. Here we employ a locally optimized processing of Gravity Recovery and Climate Experiment (GRACE) harmonics to evaluate their mass balance at the sub-basin scale and compare the results with mass budget method (MBM) estimates using regional atmospheric climate model version 2.3 (RACMO2.3) or Modèle Atmosphérique Régional version 3.6.4 (MAR3.6.4). The sub-basin mass loss estimate for April 2002 to November 2015 is 14.8 ± 4.3 Gt/yr, which is weakly affected by glacial isostatic adjustment uncertainties (±1.4 Gt/yr). This result agrees with MBM/RACMO2.3 (15.8 ± 2.0 Gt/yr), whereas MBM/MAR3.6.4 underestimates the loss (6.6 ± 1.6 Gt/yr). For the entire drainage, the mass loss for April 2002 to August 2016 is 18.5 ± 6.6 Gt/yr, or 15 ± 4% of its ice flux. These results provide unequivocal evidence for mass loss in this East Antarctic sector
High-resolution ice-thickness mapping in South Greenland
Airborne radar sounding is difficult in South Greenland because of the presence of englacial water, which prevents the signal from reaching the bed. Data coverage remains suboptimal for traditional methods of ice-thickness and bed mapping that rely on geostatistical techniques, such as kriging, because important features are missing. Here we apply two alternative approaches of highresolution (̃300 m) ice-thickness mapping, that are based on the conservation of mass, to two regions of South Greenland: (1) Qooqqup Sermia and Kiattuut Sermiat, and (2) Ikertivaq. These two algorithms solve optimization problems, for which the conservation of mass is either enforced as a hard constraint, or as a soft constraint. For the first region, very few measurements are available but there is no gap in ice motion data, whereas for Ikertivaq, more ice-thickness measurements are available, but there are gaps in ice motion data. We show that mass-conservation algorithms can be used as validation tools for radar sounding. We also show that it is preferable to apply mass conservation as a hard constraint, rather than a soft constraint, as it better preserves elongated features, such as glacial valleys and ridges
Retreat of Thwaites Glacier, West Antarctica, over the next 100 years using various ice flow models, ice shelf melt scenarios and basal friction laws
Thwaites Glacier (TG), West Antarctica, has experienced rapid,
potentially irreversible grounding line retreat and mass loss in response to
enhanced ice shelf melting. Results from recent numerical models suggest a
large spread in the evolution of the glacier in the coming decades to a
century. It is therefore important to investigate how different
approximations of the ice stress balance, parameterizations of basal
friction and ice shelf melt parameterizations may affect projections. Here,
we simulate the evolution of TG using ice sheet models of varying levels of
complexity, different basal friction laws and ice shelf melt to quantify
their effect on the projections. We find that the grounding line retreat and
its sensitivity to ice shelf melt are enhanced when a full-Stokes model is
used, a Budd friction is used and ice shelf melt is applied on partially
floating elements. Initial conditions also impact the model results. Yet, all
simulations suggest a rapid, sustained retreat of the glacier along the same
preferred pathway. The fastest retreat rate occurs on the eastern side of the
glacier, and the slowest retreat occurs across a subglacial ridge on the
western side. All the simulations indicate that TG will undergo an
accelerated retreat once the glacier retreats past the western subglacial
ridge. Combining all the simulations, we find that the uncertainty of the
projections is small in the first 30Â years, with a cumulative contribution to
sea level rise of 5 mm, similar to the current rate. After 30Â years, the
contribution to sea level depends on the model configurations, with
differences up to 300 % over the next 100Â years, ranging from 14 to 42 mm.</p
Quantifying mass balance processes on the Southern Patagonia Icefield
ArtÃculo de publicación ISIWe present surface mass balance simulations of
the Southern Patagonia Icefield (SPI) driven by downscaled
reanalysis data. The simulations were evaluated and interpreted
using geodetic mass balances, measured point balances
and a complete velocity field of the icefield for spring
2004. The high measured accumulation of snow of up to
15.4 m w.e. yr−1
(meters water equivalent per year) as well
as the high measured ablation of up to 11 m w.e. yr−1
is reproduced
by the model. The overall modeled surface mass
balance was positive and increasing during 1975–2011. Subtracting
the surface mass balance from geodetic balances,
calving fluxes were inferred. Mass losses of the SPI due to
calving were strongly increasing from 1975–2000 to 2000–
2011 and higher than losses due to surface melt. Calving
fluxes were inferred for the individual glacier catchments and
compared to fluxes estimated from velocity data. Measurements
of ice thickness and flow velocities at the glaciers’
front and spatially distributed accumulation measurements
can help to reduce the uncertainties of the different terms in
the mass balance of the Southern Patagonia Icefield.FONDECYT
3140135
European Union
22637
Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb-Wills Ice Shelf, Antarctica
Ice shelves play a major role in buttressing ice sheet flow into the ocean, hence the importance of accurate numerical modeling of their stress regime. Commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features (crevasses, rifts, and faults) that significantly affect the overall flow patterns. Here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves. The model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following: (1) a spatial distribution of contact and friction points along detected faults and rifts, (2) a more realistic spatial pattern of ice shelf rheology, and (3) a better representation of the stress balance in the immediate vicinity of faults and rifts. Thus, applying the model to the Brunt/Stancomb-Wills Ice Shelf, Antarctica, we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf. We further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice mélange in rifts and the change in fracture configurations. All are major applications for better insight into the important question of ice shelf stability
Modeling the response of northwest Greenland to enhanced ocean thermal forcing and subglacial discharge
Calving-front dynamics is an important
control on Greenland's ice mass balance. Ice front retreat of
marine-terminating glaciers may, for example, lead to a loss in resistive
stress, which ultimately results in glacier acceleration and thinning. Over
the past decade, it has been suggested that such retreats may be triggered by
warm and salty Atlantic Water, which is typically found at a depth below
200–300 m. An increase in subglacial water discharge at glacier ice fronts
due to enhanced surface runoff may also be responsible for an intensification
of undercutting and calving. An increase in ocean thermal forcing or
subglacial discharge therefore has the potential to destabilize
marine-terminating glaciers along the coast of Greenland. It remains unclear
which glaciers are currently stable but may retreat in the future and how far
inland and how fast they will retreat. Here, we quantify the sensitivity and
vulnerability of marine-terminating glaciers along the northwest coast of
Greenland (from 72.5 to 76∘ N) to ocean forcing and subglacial
discharge using the Ice Sheet System Model (ISSM). We rely on a
parameterization of undercutting based on ocean thermal forcing and
subglacial discharge and use ocean temperature and salinity from
high-resolution ECCO2 (Estimating the Circulation and Climate of the Ocean,
Phase II) simulations at the fjord mouth to constrain the ocean thermal
forcing. The ice flow model includes a calving law based on a tensile von
Mises criterion. We find that some glaciers, such as Dietrichson Gletscher or
Alison Glacier, are sensitive to small increases in ocean thermal forcing,
while others, such as Illullip Sermia or Cornell
Gletscher, are remarkably stable,
even in a +3 ∘C ocean warming scenario. Under the most intense
experiment, we find that Hayes Gletscher retreats by more than 50 km inland by 2100 into a deep trough,
and its velocity increases by a factor of 3 over only 23Â years. The model
confirms that ice–ocean interactions can trigger extensive and rapid glacier
retreat, but the bed controls the rate and magnitude of the retreat. Under
current oceanic and atmospheric conditions, we find that this sector of the
Greenland ice sheet alone will contribute more than 1 cm to sea level rise
and up to 3 cm by 2100 under the most extreme scenario.</p
Unusual radar echoes from the Greenland ice sheet
In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) instrument collected the first calibrated data set of multifrequency, polarimetric, radar observations of the Greenland ice sheet. At the time of the AIRSAR overflight, ground teams recorded the snow and firn (old snow) stratigraphy, grain size, density, and temperature at ice camps in three of the four snow zones identified by glaciologists to characterize four different degrees of summer melting of the Greenland ice sheet. The four snow zones are: (1) the dry-snow zone, at high elevation, where melting rarely occurs; (2) the percolation zone, where summer melting generates water that percolates down through the cold, porous, dry snow and then refreezes in place to form massive layers and pipes of solid ice; (3) the soaked-snow zone where melting saturates the snow with liquid water and forms standing lakes; and (4) the ablation zone, at the lowest elevations, where melting is vigorous enough to remove the seasonal snow cover and ablate the glacier ice. There is interest in mapping the spatial extent and temporal variability of these different snow zones repeatedly by using remote sensing techniques. The objectives of the 1991 experiment were to study changes in radar scattering properties across the different melting zones of the Greenland ice sheet, and relate the radar properties of the ice sheet to the snow and firn physical properties via relevant scattering mechanisms. Here, we present an analysis of the unusual radar echoes measured from the percolation zone
Near real-time monitoring of flow velocity and direction in the floating ice tongue of the Shirase Glacier using low-cost GPS buoys
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