174 research outputs found
Target Design for XUV Probing of Radiative Shock Experiments
Radiative shocks are strong shocks characterized by plasma at a high
temperature emitting an important fraction of its energy as radiation.
Radiative shocks are commonly found in many astrophysical systems and are
templates of radiative hydrodynamic flows, which can be studied experimentally
using high-power lasers. This is not only important in the context of
laboratory astrophysics but also to benchmark numerical studies. We present
details on the design of experiments on radiative shocks in xenon gas performed
at the kJ scale PALS laser facility. It includes technical specifications for
the tube targets design and numerical studies with the 1-D radiative
hydrodynamics code MULTI. Emphasis is given to the technical feasibility of an
XUV imaging diagnostic with a 21 nm (~58 eV) probing beam, which allows to
probe simultaneously the post-shock and the precursor region ahead of the
shock. The novel design of the target together with the improved X-ray optics
and XUV source allow to show both the dense post-shock structure and the
precursor of the radiative shock.Comment: 12 pages, 4 figure
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
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Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6
Reducing the uncertainty in the past, present, and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project – phase 6 (CMIP6) focusing on the Greenland and Antarctic ice sheets. In this paper, we describe the framework for ISMIP6 and its relationship with other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice-sheet–climate models as well as standalone ice-sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice-sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change
Sensitivity of the dynamics of Pine Island Glacier, West Antarctica, to climate forcing for the next 50 years
Pine Island Glacier, a major contributor to sea level rise in West
Antarctica, has been undergoing significant changes over the last few
decades. Here, we employ a three-dimensional, higher-order model to simulate
its evolution over the next 50 yr in response to changes in its surface mass
balance, the position of its calving front and ocean-induced ice shelf
melting. Simulations show that the largest climatic impact on ice dynamics is
the rate of ice shelf melting, which rapidly affects the glacier speed over
several hundreds of kilometers upstream of the grounding line. Our
simulations show that the speedup observed in the 1990s and 2000s is
consistent with an increase in sub-ice-shelf melting. According to our
modeling results, even if the grounding line stabilizes for a few decades, we
find that the glacier reaction can continue for several decades longer.
Furthermore, Pine Island Glacier will continue to change rapidly over the
coming decades and remain a major contributor to sea level rise, even if
ocean-induced melting is reduced
The impact of model resolution on the simulated Holocene retreat of the southwestern Greenland ice sheet using the Ice Sheet System Model (ISSM)
Geologic archives constraining the variability of the Greenland
ice sheet (GrIS) during the Holocene provide targets for ice sheet models to
test sensitivities to variations in past climate and model formulation. Even
as data–model comparisons are becoming more common, many models simulating
the behavior of the GrIS during the past rely on meshes with coarse
horizontal resolutions (≥10 km). In this study, we explore the impact of
model resolution on the simulated nature of retreat across southwestern
Greenland during the Holocene. Four simulations are performed using the Ice
Sheet System Model (ISSM): three that use a uniform mesh and horizontal mesh
resolutions of 20, 10, and 5 km, and one that uses a nonuniform mesh with
a resolution ranging from 2 to 15 km. We find that the simulated retreat can
vary significantly between models with different horizontal resolutions based
on how well the bed topography is resolved. In areas of low topographic
relief, the horizontal resolution plays a negligible role in simulated
differences in retreat, with each model instead responding similarly to
retreat driven by surface mass balance (SMB). Conversely, in areas where the bed
topography is complex and high in relief, such as fjords, the lower-resolution models (10 and 20 km) simulate unrealistic retreat that occurs as
ice surface lowering intersects bumps in the bed topography that would
otherwise be resolved as troughs using the higher-resolution grids. Our
results highlight the important role that high-resolution grids play in
simulating retreat in areas of complex bed topography, but also suggest that
models using nonuniform grids can save computational resources through
coarsening the mesh in areas of noncomplex bed topography where the SMB
predominantly drives retreat. Additionally, these results emphasize that care
must be taken with ice sheet models when tuning model parameters to match
reconstructed margins, particularly for lower-resolution models in regions
where complex bed topography is poorly resolved.</p
Toward Improved Understanding of Changes in Greenland Outlet Glacier Shear Margin Dynamics in a Warming Climate
The Greenland Ice Sheet has experienced accelerated mass loss over the last couple decades, in part due to destabilization of marine-terminating outlet glaciers. Retreat and acceleration of outlet glaciers coincides with atmospheric and oceanic warming resulting in a significant contribution to sea-level rise. The relative role of surface meltwater production, runoff and infiltration on the dynamics of these systems is not well-understood. To assess how surface meltwater impacts shear margin dynamics and regional ice flow of outlet glaciers, we investigate the impact of basal lubrication of Jakobshavn Isbræ shear margins due to drainage from water-filled crevasses. We map the areal extent of inundated crevasses during summer (May–August) from 2000 to 2012 using satellite imagery and determined an increasing trend in the total areal extent over this time interval. We use a numerical ice flow model to quantify the potential impact of weakened shear margins due to surface melt derived basal lubrication on regional flow velocities. Ice flow velocities 10 km from the lateral margins of Jakobshavn were amplified by as much as 20%, resulting in an increase of ~0.6 Gt yr−1 in ice-mass discharge through the shear margins into the ice stream. Under future warming scenarios with increased surface melt ponding, simulations indicate up to a 30% increase in extra-marginal ice flow. We conclude that surface meltwater will likely play an important role in the evolving dynamics of glacier shear margins and the future mass flux through Greenland's major marine-terminating outlet glaciers
Simulating ice thickness and velocity evolution of Upernavik Isstrom 1849-2012 by forcing prescribed terminus positions in ISSM
Abstract. Tidewater glacier velocity and mass balance are known to be
highly responsive to terminus position change. Yet it remains challenging
for ice flow models to reproduce observed ice margin changes. Here, using the
Ice Sheet System Model (Larour et al., 2012), we simulate the ice velocity
and thickness changes of Upernavik Isstrøm (north-western Greenland) by prescribing
a collection of 27 observed terminus positions spanning 164 years
(1849–2012). The simulation shows increased ice velocity during the 1930s,
the late 1970s and between 1995 and 2012 when terminus retreat was observed
along with negative surface mass balance anomalies. Three distinct mass
balance states are evident in the reconstruction: (1849–1932) with near zero
mass balance, (1932–1992) with ice mass loss dominated by ice dynamical
flow, and (1998–2012), when increased retreat and negative surface mass
balance anomalies led to mass loss that was twice that of any earlier period. Over
the multi-decadal simulation, mass loss was dominated by thinning and
acceleration responsible for 70 % of the total mass loss induced by
prescribed change in terminus position. The remaining 30 % of the
total ice mass loss resulted directly from prescribed terminus retreat and
decreasing surface mass balance. Although the method can not explain the
cause of glacier retreat, it enables the reconstruction of ice flow and
geometry during 1849–2012. Given annual or seasonal observed terminus front
positions, this method could be a useful tool for evaluating simulations
investigating the effect of calving laws.
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Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors
We present results from new experiments to study the dynamics of radiative
shocks, reverse shocks and radiative precursors. Laser ablation of a solid
piston by the Orion high-power laser at AWE Aldermaston UK was used to drive
radiative shocks into a gas cell initially pressurised between and $1.0 \
bar with different noble gases. Shocks propagated at {80 \pm 10 \ km/s} and
experienced strong radiative cooling resulting in post-shock compressions of {
\times 25 \pm 2}. A combination of X-ray backlighting, optical self-emission
streak imaging and interferometry (multi-frame and streak imaging) were used to
simultaneously study both the shock front and the radiative precursor. These
experiments present a new configuration to produce counter-propagating
radiative shocks, allowing for the study of reverse shocks and providing a
unique platform for numerical validation. In addition, the radiative shocks
were able to expand freely into a large gas volume without being confined by
the walls of the gas cell. This allows for 3-D effects of the shocks to be
studied which, in principle, could lead to a more direct comparison to
astrophysical phenomena. By maintaining a constant mass density between
different gas fills the shocks evolved with similar hydrodynamics but the
radiative precursor was found to extend significantly further in higher atomic
number gases (\sim4$ times further in xenon than neon). Finally, 1-D and 2-D
radiative-hydrodynamic simulations are presented showing good agreement with
the experimental data.Comment: HEDLA 2016 conference proceeding
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