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The Cryosphere (TC)

The impact of ice layers on gas transport through firn at the North Greenland Eemian Ice Drilling (NEEM) site, GreenlandThe Cryosphere, 8, 1801-1806, 2014Author(s): K. Keegan, M. R. Albert, and I. BakerTypically, gas transport through firn is modeled in the context of an
idealized firn column. However, in natural firn, imperfections are present,
which can alter transport dynamics and therefore reduce the accuracy of
reconstructed climate records. For example, ice layers have been found in
several firn cores collected in the polar regions. Here, we examined the
effects of two ice layers found in a NEEM, Greenland firn core on gas
transport through the firn. These ice layers were found to have permeability
values of 3.0 and 4.0 × 10−10 m2, and are therefore not
impermeable layers. However, the shallower ice layer was found to be
significantly less permeable than the surrounding firn, and can therefore
retard gas transport. Large closed bubbles were found in the deeper ice
layer, which will have an altered gas composition than that expected because they
were closed near the surface after the water phase was present. The bubbles
in this layer represent 12% of the expected closed porosity of this firn
layer after the firn-ice transition depth is reached, and will therefore
bias the future ice core gas record. The permeability and thickness of the
ice layers at the North Greenland Eemian Ice Drilling (NEEM) site suggest that they do not disrupt the firn-air
concentration profiles and that they do not need to be accounted for in gas transport
models at NEEM. 2014/10/02 - 10:40

The effect of changing sea ice on the physical vulnerability of Arctic coastsThe Cryosphere, 8, 1777-1799, 2014Author(s): K. R. Barnhart, I. Overeem, and R. S. AndersonSea ice limits the interaction of the land and ocean water
in the Arctic winter and influences this interaction in the summer
by governing the fetch. In many parts of the Arctic, the open-water
season is increasing in duration and summertime sea-ice extents
are decreasing. Sea ice provides a first-order control on the
physical vulnerability of Arctic coasts to erosion, inundation, and damage to
settlements and infrastructures by ocean water. We ask how the changing sea-ice
cover has influenced coastal erosion over the satellite
record. First, we present a pan-Arctic analysis of satellite-based
sea-ice concentration specifically along the Arctic coasts. The
median length of the 2012 open-water season, in comparison to 1979,
expanded by between 1.5 and 3-fold by Arctic Sea sector, which allows
for open water during the stormy Arctic fall. Second, we present
a case study of Drew Point, Alaska, a site on the Beaufort Sea,
characterized by ice-rich permafrost and rapid coastal-erosion rates,
where both the duration of the open-water season and distance to
the sea-ice edge, particularly towards the northwest, have
increased. At Drew Point, winds from the northwest result in
increased water levels at the coast and control the process of
submarine notch incision, the rate-limiting step of coastal
retreat. When open-water conditions exist, the distance to the sea
ice edge exerts control on the water level and wave field through
its control on fetch. We find that the extreme values of water-level
setup have increased consistently with increasing fetch. 2014/09/26 - 18:47

Insights into ice stream dynamics through modelling their response to tidal forcingThe Cryosphere, 8, 1763-1775, 2014Author(s): S. H. R. Rosier, G. H. Gudmundsson, and J. A. M. GreenThe tidal forcing of ice streams at their ocean boundary can serve as a
natural experiment to gain an insight into their dynamics and constrain the
basal sliding law. A nonlinear 3-D viscoelastic full Stokes model of
coupled ice stream ice shelf flow is used to investigate the response of ice
streams to ocean tides. In agreement with previous results based on flow-line
modelling and with a fixed grounding line position, we find that a nonlinear
basal sliding law can qualitatively reproduce long-period modulation of tidal
forcing found in field observations. In addition, we show that the inclusion
of lateral drag, or allowing the grounding line to migrate over the tidal
cycle, does not affect these conclusions. Further analysis of modelled ice
stream flow shows a varying stress-coupling length scale of boundary effects
upstream of the grounding line. We derive a viscoelastic stress-coupling
length scale from ice stream equations that depends on the forcing period and
closely agrees with model output. 2014/09/25 - 15:56

Brief Communication: Trends in sea ice extent north of Svalbard and its impact on cold air outbreaks as observed in spring 2013The Cryosphere, 8, 1757-1762, 2014Author(s): A. Tetzlaff, C. Lüpkes, G. Birnbaum, J. Hartmann, T. Nygård, and T. VihmaAn analysis of Special Sensor Microwave/Imager (SSM/I) satellite data reveals that the Whaler's Bay polynya north of Svalbard was
considerably larger in the three winters from 2012 to 2014 compared to the previous 20
years. This increased polynya size leads to strong atmospheric convection during cold air
outbreaks in a region north of Svalbard that was typically ice-covered in the last decades. The
change in ice cover can strongly influence local temperature conditions. Dropsonde measurements
from March 2013 show that the unusual ice conditions generate extreme convective boundary layer
heights that are larger than the regional values reported in previous studies. 2014/09/25 - 15:56

The length of the world's glaciers – a new approach for the global calculation of center linesThe Cryosphere, 8, 1741-1755, 2014Author(s): H. Machguth and M. HussGlacier length is an important measure of glacier geometry. Nevertheless,
global glacier inventories are mostly lacking length data. Only recently
semi-automated approaches to measure glacier length have been developed and
applied regionally. Here we present a first global assessment of glacier
length using an automated method that relies on glacier surface slope,
distance to the glacier margins and a set of trade-off functions. The method
is developed for East Greenland, evaluated for East Greenland as well as for
Alaska and eventually applied to all ~ 200 000 glaciers around the
globe. The evaluation highlights accurately calculated glacier length where
digital elevation model (DEM) quality is high (East Greenland) and limited accuracy on low-quality DEMs
(parts of Alaska). Measured length of very small glaciers is subject to a
certain level of ambiguity. The global calculation shows that only about
1.5% of all glaciers are longer than 10 km, with Bering Glacier
(Alaska/Canada) being the longest glacier in the world at a length of
196 km. Based on the output of our algorithm we derive global and regional
area–length scaling laws. Differences among regional scaling parameters
appear to be related to characteristics of topography and glacier mass
balance. The present study adds glacier length as a key parameter to global
glacier inventories. Global and regional scaling laws might prove beneficial
in conceptual glacier models. 2014/09/21 - 04:57

Time-evolving mass loss of the Greenland Ice Sheet from satellite altimetryThe Cryosphere, 8, 1725-1740, 2014Author(s): R. T. W. L. Hurkmans, J. L. Bamber, C. H. Davis, I. R. Joughin, K. S. Khvorostovsky, B. S. Smith, and N. SchoenMass changes of the Greenland Ice Sheet may be estimated by the input–output
method (IOM), satellite gravimetry, or via surface elevation change rates
(dH/dt). Whereas the first two have been shown to agree well in
reconstructing ice-sheet wide mass changes over the last decade, there are
few decadal estimates from satellite altimetry and none that provide a
time-evolving trend that can be readily compared with the other methods.
Here, we interpolate radar and laser altimetry data between 1995 and 2009 in
both space and time to reconstruct the evolving volume changes. A firn
densification model forced by the output of a regional climate model is used
to convert volume to mass. We consider and investigate the potential sources
of error in our reconstruction of mass trends, including geophysical biases
in the altimetry, and the resulting mass change rates are compared to other
published estimates. We find that mass changes are dominated by surface mass
balance (SMB) until about 2001, when mass loss rapidly accelerates. The onset
of this acceleration is somewhat later, and less gradual, compared to the
IOM. Our time-averaged mass changes agree well with recently published
estimates based on gravimetry, IOM, laser altimetry, and with radar altimetry
when merged with airborne data over outlet glaciers. We demonstrate that,
with appropriate treatment, satellite radar altimetry can provide reliable
estimates of mass trends for the Greenland Ice Sheet. With the inclusion of
data from CryoSat-2, this provides the possibility of producing a continuous
time series of regional mass trends from 1992 onward. 2014/09/21 - 04:57

Present and future variations in Antarctic firn air contentThe Cryosphere, 8, 1711-1723, 2014Author(s): S. R. M. Ligtenberg, P. Kuipers Munneke, and M. R. van den BroekeA firn densification model (FDM) is used to assess spatial and temporal
(1979–2200) variations in the depth, density and temperature of the firn
layer covering the Antarctic ice sheet (AIS). A time-dependent version of the
FDM is compared to more commonly used steady-state FDM results. Although the
average AIS firn air content (FAC) of both models is similar (22.5 m), large
spatial differences are found: in the ice-sheet interior, the steady-state
model underestimates the FAC by up to 2 m, while the FAC is overestimated by
5–15 m along the ice-sheet margins, due to significant surface melt.
Applying the steady-state FAC values to convert surface elevation to ice
thickness (i.e., assuming flotation at the grounding line) potentially results
in an underestimation of ice discharge at the grounding line, and hence an
underestimation of current AIS mass loss by 23.5% (or 16.7 Gt yr−1)
with regard to the reconciled estimate over the period 1992–2011. The timing of the
measurement is also important, as temporal FAC variations of 1–2 m are
simulated within the 33 yr period (1979–2012). Until 2200, the Antarctic
FAC is projected to change due to a combination of increasing accumulation,
temperature, and surface melt. The latter two result in a decrease of FAC, due
to (i) more refrozen meltwater, (ii) a higher densification rate, and
(iii) a faster firn-to-ice transition at the bottom of the firn layer.
These effects are, however, more than compensated for by increasing snowfall,
leading to a 4–14% increase in FAC. Only in melt-affected regions, future
FAC is simulated to decrease, with the largest changes (−50 to −80%)
on the ice shelves in the Antarctic Peninsula and Dronning Maud Land.
Integrated over the AIS, the increase in precipitation results in a similar
volume increase due to ice and air (both ~150 km3 yr−1 until
2100). Combined, this volume increase is equivalent to a surface elevation
change of +2.1 cm yr−1, which shows that variations in firn depth
remain important to consider in future mass balance studies using satellite
altimetry. 2014/09/21 - 04:57

Sensitivity of the dynamics of Pine Island Glacier, West Antarctica, to climate forcing for the next 50 yearsThe Cryosphere, 8, 1699-1710, 2014Author(s): H. Seroussi, M. Morlighem, E. Rignot, J. Mouginot, E. Larour, M. Schodlok, and A. KhazendarPine 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. 2014/09/21 - 04:57

Projected changes of snow conditions and avalanche activity in a warming climate: the French Alps over the 2020–2050 and 2070–2100 periodsThe Cryosphere, 8, 1673-1697, 2014Author(s): H. Castebrunet, N. Eckert, G. Giraud, Y. Durand, and S. MorinProjecting changes in snow cover due to climate warming is important for
many societal issues, including the adaptation of avalanche risk mitigation
strategies. Efficient modelling of future snow cover requires high
resolution to properly resolve the topography. Here, we introduce results
obtained through statistical downscaling techniques allowing simulations of
future snowpack conditions including mechanical stability estimates for the
mid and late 21st century in the French Alps under three climate
change scenarios. Refined statistical descriptions of snowpack
characteristics are provided in comparison to a 1960–1990 reference period,
including latitudinal, altitudinal and seasonal gradients. These results are
then used to feed a statistical model relating avalanche activity to snow
and meteorological conditions, so as to produce the first projection on
annual/seasonal timescales of future natural avalanche activity based on past observations. The resulting statistical indicators are
fundamental for the mountain economy in terms of anticipation of changes.

Whereas precipitation is expected to remain quite stationary, temperature increase interacting with topography
will constrain the evolution of snow-related variables on all considered spatio-temporal scales and will, in
particular, lead to a reduction of the dry snowpack and an increase of the wet snowpack.
Overall, compared to the reference period, changes are strong for the
end of the 21st century, but already significant for the mid century.
Changes in winter are less important than in spring, but wet-snow conditions
are projected to appear at high elevations earlier in the season. At the
same altitude, the southern French Alps will not be significantly more
affected than the northern French Alps, which means that the snowpack will
be preserved for longer in the southern massifs which are higher on average.

Regarding avalanche activity, a general decrease in mean (20–30%) and
interannual variability is projected. These changes are relatively strong
compared to changes in snow and meteorological variables. The
decrease is amplified in spring and at low altitude. In contrast, an
increase in avalanche activity is expected in winter at high altitude
because of conditions favourable to wet-snow avalanches earlier in the season.
Comparison with the outputs of the deterministic avalanche hazard model
MEPRA (Modèle Expert d'aide à la Prévision du Risque d'Avalanche) shows generally consistent results but suggests that, even if the
frequency of winters with high avalanche activity is clearly projected to
decrease, the decreasing trend may be less strong and smooth than suggested
by the statistical analysis based on changes in snowpack characteristics and
their links to avalanches observations in the past. This important point for
risk assessment pleads for further work focusing on shorter timescales.
Finally, the small differences between different climate change scenarios
show the robustness of the predicted avalanche activity changes. 2014/09/16 - 00:23

Changes in Imja Tsho in the Mount Everest region of NepalThe Cryosphere, 8, 1661-1671, 2014Author(s): M. A. Somos-Valenzuela, D. C. McKinney, D. R. Rounce, and A. C. ByersImja Tsho, located in the Sagarmatha (Everest) National Park of Nepal, is one
of the most studied and rapidly growing lakes in the Himalayan range.
Compared with previous studies, the results of our sonar bathymetric survey
conducted in September of 2012 suggest that its maximum depth has increased
from 90.5 to 116.3 ± 5.2 m since 2002, and that its estimated volume
has grown from 35.8 ± 0.7 to 61.7 ± 3.7 million m3. Most of
the expansion of the lake in recent years has taken place in the glacier
terminus–lake interface on the eastern end of the lake, with the glacier
receding at about 52 m yr−1 and the lake expanding in area by
0.04 km2 yr−1. A ground penetrating radar survey of the
Imja–Lhotse Shar glacier just behind the glacier terminus shows that the ice
is over 200 m thick in the center of the glacier. The volume of water that
could be released from the lake in the event of a breach in the damming
moraine on the western end of the lake has increased to 34.1 ± 1.08
million m3 from the 21 million m3 estimated in 2002. 2014/09/16 - 00:23

Healing of snow surface-to-surface contacts by isothermal sinteringThe Cryosphere, 8, 1651-1659, 2014Author(s): E. A. Podolskiy, M. Barbero, F. Barpi, G. Chambon, M. Borri-Brunetto, O. Pallara, B. Frigo, B. Chiaia, and M. NaaimNatural sintering in ice is a fundamental process determining mechanical
properties of various ice forms. According to the literature, limited data
are available about the complex subjects of snow sintering and bond
formation. Here, through cold laboratory mechanical tests with a new shear
apparatus we demonstrate time-dependent effects of isothermal sintering on
interface strengthening at various normal pressures. Measurements showed that
interfacial strength evolved rapidly, conforming to a power law (mean
exponent ≈ 0.21); higher pressure corresponded to higher initial
strength and sintering rates. Our findings are consistent with observations
on homogeneous snow, provide unique records essential for slope stability
models and indicate the significant importance of normal load on data
interpretation. 2014/09/11 - 20:17

A sea ice concentration estimation algorithm utilizing radiometer and SAR dataThe Cryosphere, 8, 1639-1650, 2014Author(s): J. KarvonenWe have studied the possibility of combining the high-resolution synthetic aperture radar
(SAR) segmentation and ice concentration estimated by radiometer
brightness temperatures. Here we present an algorithm for mapping
a radiometer-based concentration value for each SAR segment. The
concentrations are estimated by a multi-layer perceptron (MLP)
neural network which has the AMSR-2 (Advanced Microwave Scanning Radiometer 2)
polarization ratios and gradient ratios of four
radiometer channels as its inputs. The results have been compared
numerically to the gridded Finnish Meteorological Institute (FMI)
ice chart concentrations and
high-resolution AMSR-2 ASI (ARTIST Sea Ice) algorithm concentrations provided
by the University of Hamburg and also visually to the AMSR-2 bootstrap
algorithm concentrations, which are given in much coarser
resolution. The differences when compared to
FMI daily ice charts were on average small. When compared to ASI ice
concentrations, the differences were a bit larger, but still small on
average. According to our comparisons, the largest differences typically occur near the ice edge
and sea–land boundary. The main advantage of combining radiometer-based ice concentration
estimation and SAR segmentation seems to be a more precise estimation of the boundaries of
different ice concentration zones. 2014/09/06 - 18:14

The effect of snow/sea ice type on the response of albedo and light penetration depth (e-folding depth) to increasing black carbonThe Cryosphere, 8, 1625-1638, 2014Author(s): A. A. Marks and M. D. KingThe optical properties of snow/sea ice vary with age and by the
processes they were formed, giving characteristic types of snow and
sea ice. The response of albedo and light penetration depth
(e-folding depth) to increasing mass ratio of black carbon is shown to
depend on the snow and sea ice type and the thickness of the snow or
sea ice. The response of albedo and e-folding depth of three
different types of snow (cold polar snow, wind-packed snow and
melting snow) and three sea ice (multi-year ice, first-year ice and
melting sea ice) to increasing mass ratio of black carbon is calculated using
a coupled atmosphere–snow/sea ice radiative-transfer model
(TUV-snow), over the optical wavelengths of 300–800 nm.
The snow and sea ice types are effectively defined by a scattering
cross-section, density and asymmetry parameter. The relative change in albedo
and e-folding depth of each of the three snow and three sea ice types
with increasing mass ratio of black carbon is considered relative to a base
case of 1 ng g−1 of black carbon. The relative response of each snow and
sea ice type is intercompared to examine how different types of snow and sea
ice respond relative to each other. The relative change in albedo of a melting
snowpack is a factor of four more responsive to
additions of black carbon compared to cold polar snow over a black
carbon increase from 1 to 50 ng g−1, while the relative
change in albedo of a melting sea ice is a factor of two more
responsive to additions of black carbon compared to multi-year ice
for the same increase in mass ratio of black carbon. The response of
e-folding depth is effectively not dependent on snow/sea ice type. The
albedo of sea ice is more responsive to increasing mass ratios of
black carbon than snow. 2014/09/04 - 23:09

Corrigendum to "Using MODIS land surface temperatures and the Crocus snow model to understand the warm bias of ERA-Interim reanalyses at the surface in Antarctica" published in The Cryosphere, 8, 1361–1373, 2014The Cryosphere, 8, 1623-1623, 2014Author(s): H. Fréville, E. Brun, G. Picard, N. Tatarinova, L. Arnaud, C. Lanconelli, C. Reijmer, and M. van den BroekeNo abstract available. 2014/08/30 - 04:49

Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretationThe Cryosphere, 8, 1607-1622, 2014Author(s): R. Ricker, S. Hendricks, V. Helm, H. Skourup, and M. DavidsonIn the context of quantifying Arctic ice-volume decrease at global scale, the
CryoSat-2 satellite was launched in 2010 and is equipped with the Ku
band synthetic aperture radar
altimeter SIRAL (Synthetic Aperture Interferometric Radar
Altimeter), which we use to derive sea-ice freeboard defined as
the height of the ice surface above the sea level. Accurate CryoSat-2 range
measurements over open water and the ice surface of the order of centimetres
are necessary to achieve the required accuracy of the freeboard-to-thickness
conversion. Besides uncertainties of the actual sea-surface height and
limited knowledge of ice and snow properties, the composition of radar
backscatter and therefore the interpretation of radar echoes is crucial. This
has consequences in the selection of retracker algorithms which are used to
track the main scattering horizon and assign a range estimate to each
measurement. In this study we apply a retracker algorithm with thresholds of
40, 50 and 80% of the first maximum of radar echo power, spanning the
range of values used in the current literature. By using the selected retrackers
and additionally results from airborne validation measurements, we evaluate
the uncertainties of sea-ice freeboard and higher-level products that arise
from the choice of the retracker threshold only, independent of the
uncertainties related to snow and ice properties. Our study shows that the
choice of retracker thresholds does have a significant impact on magnitudes
of estimates of sea-ice freeboard and thickness, but that the spatial
distributions of these parameters are less affected. Specifically we find
mean radar freeboard values of 0.121 m (0.265 m) for the 40% threshold,
0.086 m (0.203 m) for the 50% threshold and 0.024 m (0.092 m) for the
80% threshold, considering first-year ice (multiyear ice) in March 2013.
We show that the main source of freeboard and thickness uncertainty results
from the choice of the retracker and the unknown penetration of the radar
pulse into the snow layer in conjunction with surface roughness effects.
These uncertainties can cause a freeboard bias of roughly 0.06–0.12 m.
Furthermore we obtain a significant rise of 0.02–0.15 m of freeboard from
March 2013 to November 2013 in the area for multiyear sea ice north of
Greenland and Canada. Since this is unlikely, it gives rise to the assumption
that applying different retracker thresholds depending on seasonal properties
of the snow load is necessary in the future. 2014/08/30 - 04:49

Sensitivity of lake ice regimes to climate change in the Nordic regionThe Cryosphere, 8, 1589-1605, 2014Author(s): S. Gebre, T. Boissy, and K. AlfredsenA one-dimensional process-based multi-year lake ice model, MyLake, was used
to simulate lake ice phenology and annual maximum lake ice thickness for the
Nordic region comprising Fennoscandia and the Baltic countries. The model
was first tested and validated using observational meteorological forcing on
a candidate lake (Lake Atnsjøen) and using downscaled ERA-40 reanalysis
data set. To simulate ice conditions for the contemporary period of
1961–2000, the model was driven by gridded meteorological forcings from
ERA-40 global reanalysis data downscaled to a 25 km resolution using the
Rossby Centre Regional Climate Model (RCA). The model was then forced with
two future climate scenarios from the RCA driven by two different general circulation models
(GCMs) based on the Special Report on Emissions Scenarios (SRES) A1B. The two climate scenarios correspond to
two future time periods namely the 2050s (2041–2070) and the 2080s
(2071–2100). To take into account the influence of lake morphometry,
simulations were carried out for four different hypothetical lake depths
(5 m, 10 m, 20 m, 40 m) placed at each of the 3708 grid cells. Based on a
comparison of the mean predictions in the future 30-year periods with the
control (1961–1990) period, ice cover durations in the region will be
shortened by 1 to 11 weeks in 2041–2070, and 3 to 14 weeks in 2071–2100.
Annual maximum lake ice thickness, on the other hand, will be reduced
by a margin of up to 60 cm by 2041–2070 and up to 70 cm by 2071–2100. The
simulated changes in lake ice characteristics revealed that the changes are
less dependent on lake depths though there are slight differences. The
results of this study provide a regional perspective of anticipated changes
in lake ice regimes due to climate warming across the study area by the
middle and end of this century. 2014/08/30 - 04:49

How much snow falls on the Antarctic ice sheet?The Cryosphere, 8, 1577-1587, 2014Author(s): C. Palerme, J. E. Kay, C. Genthon, T. L'Ecuyer, N. B. Wood, and C. ClaudClimate models predict Antarctic precipitation to increase during the 21st
century, but their present day Antarctic precipitation differs. A
model-independent climatology of the Antarctic precipitation characteristics,
such as snowfall rates and frequency, is needed to assess the models, but it is
not yet available. Satellite observations of precipitation by active sensors
has been possible in the polar regions since the launch of CloudSat in 2006.
Here, we use two CloudSat products to generate the first multi-year,
model-independent climatology of Antarctic precipitation. The first product
is used to determine the frequency and the phase of precipitation, while the
second product is used to assess the snowfall rate. The mean snowfall rate
from August 2006 to April 2011 is 171 mm year−1 over the Antarctic ice
sheet, north of 82° S. While uncertainties on individual
precipitation retrievals from CloudSat data are potentially large, the mean
uncertainty should be much smaller, but cannot be easily estimated. There are
no in situ measurements of Antarctic precipitation to directly assess the new
climatology. However, distributions of both precipitation occurrences and
rates generally agree with the European Centre for Medium-Range Weather
Forecasts (ECMWF) ERA-Interim data set, the production of
which is constrained by various in situ and satellite observations, but does
not use any data from CloudSat. The new data set thus offers unprecedented
capability to quantitatively assess Antarctic precipitation statistics and
rates in climate models. 2014/08/23 - 13:50

Dynamic response of Antarctic ice shelves to bedrock uncertaintyThe Cryosphere, 8, 1561-1576, 2014Author(s): S. Sun, S. L. Cornford, Y. Liu, and J. C. MooreAccurate and extensive bedrock geometry data is essential in ice sheet
modelling. The shape of the bedrock on fine scales can influence ice sheet
evolution, for example through the formation of pinning points that alter
grounding line dynamics. Here we test the sensitivity of the
adaptive mesh ice sheet model to small-amplitude height fluctuations on
different spatial scales in the bedrock topography provided by Bedmap2 in
the catchments of Pine Island Glacier, the Amery Ice shelf and a region of
East Antarctica including the Aurora Basin, Law Dome and Totten Glacier. We
generate an ensemble of bedrock topographies by adding random noise to the
Bedmap2 data with amplitude determined by the accompanying estimates of
bedrock uncertainty. We find that the small-amplitude fluctuations result in
only minor changes in the way these glaciers evolve. However, lower-frequency noise, with a broad spatial scale (over tens of kilometres) is more important
than higher-frequency noise even when the features have the same height
amplitudes and the total noise power is maintained. This is cause for optimism
regarding credible sea level rise estimates with presently achievable density of
thickness measurements. Pine Island Glacier and the region around Totten
Glacier and Law Dome undergo substantial retreat and appear to be more
sensitive to errors in bed topography than the Amery Ice shelf region which
remains stable under the present-day observational data uncertainty. 2014/08/23 - 13:50

Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2The Cryosphere, 8, 1539-1559, 2014Author(s): V. Helm, A. Humbert, and H. MillerThis study focuses on the present-day surface elevation of the Greenland and
Antarctic ice sheets. Based on 3 years of CryoSat-2 data acquisition
we derived new elevation models (DEMs) as well as elevation change maps and
volume change estimates for both ice sheets. Here we present the new DEMs and
their corresponding error maps. The accuracy of the derived DEMs for
Greenland and Antarctica is similar to those of previous DEMs obtained by
satellite-based laser and radar altimeters. Comparisons with ICESat data show
that 80% of the CryoSat-2 DEMs have an uncertainty of less than
3 m ± 15 m. The surface elevation change rates between
January 2011 and January 2014 are presented for both ice sheets. We compared
our results to elevation change rates obtained from ICESat data covering the
time period from 2003 to 2009. The comparison reveals that in West Antarctica
the volume loss has increased by a factor of 3. It also shows an anomalous
thickening in Dronning Maud Land, East Antarctica which represents a known
large-scale accumulation event. This anomaly partly compensates for the
observed increased volume loss of the Antarctic Peninsula and West
Antarctica. For Greenland we find a volume loss increased by a factor of 2.5
compared to the ICESat period with large negative elevation changes
concentrated at the west and southeast coasts. The combined volume change of
Greenland and Antarctica for the observation period is estimated to be −503
± 107 km3 yr−1. Greenland contributes nearly 75% to the
total volume change with −375 ± 24 km3 yr−1. 2014/08/23 - 13:50

Surface energy budget on Larsen and Wilkins ice shelves in the Antarctic Peninsula: results based on reanalyses in 1989–2010The Cryosphere, 8, 1519-1538, 2014Author(s): I. Välisuo, T. Vihma, and J. C. KingIce shelves in the Antarctic Peninsula have significantly disintegrated
during recent decades. To better understand the atmospheric contribution
in the process, we have analysed the inter-annual variations in radiative
and turbulent surface fluxes and weather conditions over Larsen C Ice Shelf
(LCIS) and Wilkins Ice Shelf (WIS) in the Antarctic Peninsula in 1989–2010.
Three atmospheric reanalyses were applied: ERA-Interim by ECMWF, Climate
Forecast System Reanalysis (CFSR) by NCEP, and JRA-25/JCDAS by the Japan
Meteorological Agency. In addition, in situ observations from an automatic
weather station (AWS) on LCIS were applied, mainly for validation of the
reanalyses. The AWS observations on LCIS did not show any significant
temperature trend, and the reanalyses showed warming trends only over WIS:
ERA-Interim in winter (0.23 °C yr−1) and JRA-25/JCDAS in
autumn (0.13 °C yr−1). In LCIS from December through
August and in WIS from March through August, the variations of surface net
flux were partly explained by the combined effects of atmospheric pressure,
wind and cloud fraction. The explained variance was much higher in LCIS (up
to 80%) than in WIS (26–27%). Summer melting on LCIS varied between
11 and 58 cm water equivalent (w.e.), which is comparable to previous
results. The mean amount of melt days per summer on LCIS was 69. The high
values of melting in summer 2001–2002 presented in previous studies on the
basis of simple calculations were not supported by our study. Instead, our
calculations based on ERA-Interim yielded strongest melting in summer
1992–1993 on both ice shelves. On WIS the summer melting ranged between 10
and 23 cm w.e., and the peak values coincided with the largest
disintegrations of the ice shelf. The amount of melt on WIS may, however, be
underestimated by ERA-Interim, as previously published satellite
observations suggest that it suffers from a significant bias over WIS. 2014/08/23 - 13:50

The Greenland Ice Mapping Project (GIMP) land classification and surface elevation data setsThe Cryosphere, 8, 1509-1518, 2014Author(s): I. M. Howat, A. Negrete, and B. E. SmithAs part of the Greenland Ice Mapping Project (GIMP) we have produced
three geospatial data sets for the entire ice sheet and periphery. These are
(1) a complete, 15 m resolution image mosaic, (2) ice-covered and ice-free
terrain classification masks, also posted to 15 m resolution, and (3) a
complete, altimeter-registered digital elevation model posted at 30 m. The
image mosaic was created from a combination of Landsat-7 and RADARSAT-1
imagery acquired between 1999 and 2002. Each pixel in the image is stamped
with the acquisition date and geo-registration error to facilitate change
detection. This mosaic was then used to manually produce complete
ice-covered and ice-free land classification masks. Finally, we used
satellite altimetry and stereo-photogrammetric digital elevation models (DEMs) to enhance an existing
DEM for Greenland, substantially improving resolution and accuracy over the
ice margin and periphery. 2014/08/19 - 17:55

Glacier dynamics at Helheim and Kangerdlugssuaq glaciers, southeast Greenland, since the Little Ice AgeThe Cryosphere, 8, 1497-1507, 2014Author(s): S. A. Khan, K. K. Kjeldsen, K. H. Kjær, S. Bevan, A. Luckman, A. Aschwanden, A. A. Bjørk, N. J. Korsgaard, J. E. Box, M. van den Broeke, T. M. van Dam, and A. FitznerObservations over the past decade show significant ice loss associated with
the speed-up of glaciers in southeast Greenland from 2003, followed by a
deceleration from 2006. These short-term, episodic, dynamic perturbations
have a major impact on the mass balance on the decadal scale. To improve the
projection of future sea level rise, a long-term data record that reveals
the mass balance beyond such episodic events is required. Here, we extend
the observational record of marginal thinning of Helheim and Kangerdlugssuaq
glaciers from 10 to more than 80 years. We show that, although the frontal
portion of Helheim Glacier thinned by more than 100 m between 2003 and 2006,
it thickened by more than 50 m during the previous two decades. In contrast,
Kangerdlugssuaq Glacier underwent minor thinning of 40–50 m from 1981 to
1998 and major thinning of more than 100 m after 2003. Extending the record
back to the end of the Little Ice Age (prior to 1930) shows no thinning of
Helheim Glacier from its maximum extent during the Little Ice Age to 1981,
while Kangerdlugssuaq Glacier underwent substantial thinning of 230 to
265 m. Comparison of sub-surface water temperature anomalies and variations
in air temperature to records of thickness and velocity change suggest that
both glaciers are highly sensitive to short-term atmospheric and ocean
forcing, and respond very quickly to small fluctuations. On century
timescales, however, multiple external parameters (e.g. outlet glacier
shape) may dominate the mass change. These findings suggest that special
care must be taken in the projection of future dynamic ice loss. 2014/08/19 - 17:55

A 10 year record of black carbon and dust from a Mera Peak ice core (Nepal): variability and potential impact on melting of Himalayan glaciersThe Cryosphere, 8, 1479-1496, 2014Author(s): P. Ginot, M. Dumont, S. Lim, N. Patris, J.-D. Taupin, P. Wagnon, A. Gilbert, Y. Arnaud, A. Marinoni, P. Bonasoni, and P. LajA shallow ice core was extracted at the summit of Mera Peak at 6376 m a.s.l.
in the southern flank of the Nepalese Himalaya range. From this core, we
reconstructed the seasonal deposition fluxes of dust and refractory black
carbon (rBC) since 1999. This archive presents well preserved seasonal
cycles based on a monsoonal precipitation pattern. According to the seasonal
precipitation regime in which 80% of annual precipitation falls between
June and September, we estimated changes in the concentrations of these
aerosols in surface snow. The analyses revealed that mass fluxes are a few
orders of magnitude higher for dust (10.4 ± 2.8 g m−2 yr−1
than for rBC (7.9 ± 2.8 mg m−2 yr−1). The relative lack of
seasonality in the dust record may reflect a high background level of dust
inputs, whether from local or regional sources. Over the 10-year record, no
deposition flux trends were detected for any of the species of interest. The
data were then used to simulate changes in the surface snow albedo over time
and the potential melting caused by these impurities. Mean potential melting
caused by dust and rBC combined was 713 kg m−2 yr−1, and for rBC
alone, 342 kg m−2 yr−1 for rBC under certain assumptions. Compared
to the melting rate measured using the mass and energy balance at
5360 m a.s.l. on Mera Glacier between November 2009 and October 2010, i.e.
3000 kg m−2 yr−1 and 3690 kg m−2 yr−1 respectively,
the impact of rBC represents less than 16% of annual potential melting
while the contribution of dust and rBC combined to surface melting
represents a maximum of 26%. Over the 10-year period, rBC variability in
the ice core signal primarily reflected variability of the monsoon signal
rather than variations in the intensity of emissions. 2014/08/15 - 12:42

Temporal dynamics of ikaite in experimental sea iceThe Cryosphere, 8, 1469-1478, 2014Author(s): S. Rysgaard, F. Wang, R. J. Galley, R. Grimm, D. Notz, M. Lemes, N.-X. Geilfus, A. Chaulk, A. A. Hare, O. Crabeck, B. G. T. Else, K. Campbell, L. L. Sørensen, J. Sievers, and T. PapakyriakouIkaite (CaCO3 · 6H2O) is a metastable phase of calcium
carbonate that normally forms in a cold environment and/or under high
pressure. Recently, ikaite crystals have been found in sea ice, and it has
been suggested that their precipitation may play an important role in
air–sea CO2 exchange in ice-covered seas. Little is known, however, of
the spatial and temporal dynamics of ikaite in sea ice. Here we present
evidence for highly dynamic ikaite precipitation and dissolution in sea ice
grown at an outdoor pool of the Sea-ice Environmental Research Facility
(SERF) in Manitoba, Canada. During the experiment, ikaite precipitated in
sea ice when temperatures were below −4 °C, creating three distinct
zones of ikaite concentrations: (1) a millimeter-to-centimeter-thin surface layer containing
frost flowers and brine skim with bulk ikaite concentrations of >2000 μmol kg−1,
(2) an internal layer with ikaite concentrations of 200–400 μmol kg−1, and (3) a bottom layer with
ikaite concentrations of <100 μmol kg−1. Snowfall
events caused the sea ice to warm and ikaite crystals to dissolve. Manual
removal of the snow cover allowed the sea ice to cool and brine salinities
to increase, resulting in rapid ikaite precipitation. The observed ikaite
concentrations were on the same order of magnitude as modeled by
FREZCHEM, which further supports the notion that ikaite concentration in sea ice
increases with decreasing temperature. Thus, varying snow conditions may
play a key role in ikaite precipitation and dissolution in sea ice. This
could have a major implication for CO2 exchange with the atmosphere and
ocean that has not been accounted for previously. 2014/08/09 - 13:12

Ice–ocean interaction and calving front morphology at two west Greenland tidewater outlet glaciersThe Cryosphere, 8, 1457-1468, 2014Author(s): N. Chauché, A. Hubbard, J.-C. Gascard, J. E. Box, R. Bates, M. Koppes, A. Sole, P. Christoffersen, and H. PattonWarm, subtropical-originating Atlantic water (AW) has been identified as a
primary driver of mass loss across the marine sectors of the Greenland Ice
Sheet (GrIS), yet the specific processes by which this water mass interacts
with and erodes the calving front of tidewater glaciers is frequently modelled
and much speculated upon but remains largely unobserved. We present a suite of
fjord salinity, temperature, turbidity versus depth casts along with
glacial runoff estimation from Rink and Store glaciers, two major marine
outlets draining the western sector of the GrIS during 2009 and 2010. We
characterise the main water bodies present and interpret their interaction
with their respective calving fronts. We identify two distinct processes of
ice–ocean interaction which have distinct spatial and temporal footprints:
(1) homogenous free convective melting which occurs across the calving front
where AW is in direct contact with the ice mass, and (2) localised upwelling-driven
melt by turbulent subglacial runoff mixing with fjord water which
occurs at distinct injection points across the calving front. Throughout the
study, AW at 2.8 ± 0.2 °C was consistently observed in
contact with both glaciers below 450 m depth, yielding homogenous,
free convective submarine melting up to ~200 m depth. Above
this bottom layer, multiple interactions are identified, primarily controlled
by the rate of subglacial fresh-water discharge which results in localised
and discrete upwelling plumes. In the record melt year of 2010, the Store
Glacier calving face was dominated by these runoff-driven plumes which led
to a highly crenulated frontal geometry characterised by large embayments at
the subglacial portals separated by headlands which are dominated by
calving. Rink Glacier, which is significantly deeper than Store has a larger
proportion of its submerged calving face exposed to AW, which results in a
uniform, relatively flat overall frontal geometry. 2014/08/09 - 13:12

Recent ice dynamic and surface mass balance of Union Glacier in the West Antarctic Ice SheetThe Cryosphere, 8, 1445-1456, 2014Author(s): A. Rivera, R. Zamora, J. A. Uribe, R. Jaña, and J. OberreuterHere we present the results of a comprehensive glaciological investigation of
Union Glacier (79°46' S/83°24' W) in the West
Antarctic Ice Sheet (WAIS), a major outlet glacier within the Ellsworth
Mountains. Union Glacier flows into the Ronne Ice Shelf, where recent models
have indicated the potential for significant grounding line zone (GLZ)
migrations in response to changing climate and ocean conditions. To elaborate
a glaciological base line that can help to evaluate the potential impact of
this GLZ change scenario, we installed an array of stakes on Union Glacier in
2007. The stake network has been surveyed repeatedly for elevation, velocity,
and net surface mass balance. The region of the stake measurements is in
near-equilibrium, and ice speeds are 10 to 33 m a−1.
Ground-penetrating radars (GPR) have been used to map the
topography, internal structure, and crevasse frequency and depth along
surveyed tracks in the stake site area. The bedrock in this area has a
minimum elevation of −858 m a.s.l., significantly deeper than shown by
BEDMAP2 data. However, between this deeper area and the local GLZ, there is a
threshold where the subglacial topography shows a maximum altitude of 190 m.
This subglacial condition implies that an upstream migration of the GLZ will
not have strong effects on Union Glacier until it passes beyond this shallow
ice pinning point. 2014/08/07 - 18:09

Representing moisture fluxes and phase changes in glacier debris cover using a reservoir approachThe Cryosphere, 8, 1429-1444, 2014Author(s): E. Collier, L. I. Nicholson, B. W. Brock, F. Maussion, R. Essery, and A. B. G. BushDue to the complexity of treating moisture in supraglacial debris, surface
energy balance models to date have neglected moisture infiltration and phase
changes in the debris layer. The latent heat flux (QL) is also often excluded
due to the uncertainty in determining the surface vapour pressure. To
quantify the importance of moisture on the surface energy and climatic mass
balance (CMB) of debris-covered glaciers, we developed a simple reservoir
parameterization for the debris ice and water content, as well as an
estimation of the latent heat flux. The parameterization was incorporated
into a CMB model adapted for debris-covered glaciers. We present the results
of two point simulations, using both our new "moist" and the conventional
"dry" approaches, on the Miage Glacier, Italy, during summer 2008 and fall
2011. The former year coincides with available in situ glaciological and
meteorological measurements, including the first eddy-covariance measurements
of the turbulent fluxes over supraglacial debris, while the latter contains
two refreeze events that permit evaluation of the influence of phase changes.
The simulations demonstrate a clear influence of moisture on the glacier
energy and mass-balance dynamics. When water and ice are considered, heat
transmission to the underlying glacier ice is lower, as the effective thermal
diffusivity of the saturated debris layers is reduced by increases in both
the density and the specific heat capacity of the layers. In combination with
surface heat extraction by QL, subdebris ice melt is reduced by 3.1%
in 2008 and by 7.0% in 2011 when moisture effects are included.
However, the influence of the parameterization on the total accumulated mass
balance varies seasonally. In summer 2008, mass loss due to surface vapour
fluxes more than compensates for the reduction in ice melt, such that the
total ablation increases by 4.0%. Conversely, in fall 2011, the
modulation of basal debris temperature by debris ice results in a decrease in
total ablation of 2.1%. Although the parameterization is a simplified
representation of the moist physics of glacier debris, it is a novel attempt
at including moisture in a numerical model of debris-covered glaciers and one
that opens up additional avenues for future research. 2014/08/07 - 18:09

The importance of insolation changes for paleo ice sheet modelingThe Cryosphere, 8, 1419-1428, 2014Author(s): A. Robinson and H. GoelzerThe growth and retreat of continental ice sheets in the past has largely been
a response to changing climatic forcing. Since ablation is the principal
component of mass loss for land-based ice sheets, the calculation of surface
melt is an important aspect of paleo ice sheet modeling. Changes in
insolation are often not accounted for in calculations of surface melt, under
the assumption that the near-surface temperature transmits the majority of
the climatic forcing to the ice sheet. To assess how this could affect paleo
simulations, here we investigate the importance of different orbital
configurations for estimating melt on the Greenland ice sheet. We find that
during peak Eemian conditions, increased insolation contributes 20–50% to
the surface melt anomaly. However, this percentage depends strongly on the
temperature anomaly at the time. For higher temperature anomalies, the role
of insolation changes is less important. This relationship is not homogenous
over the ice sheet, since the contribution of insolation to melt is modulated
by the local surface albedo. In coupled simulations, the additional
insolation-induced melt translates into up to threefold more ice volume loss,
compared to output using a model that does not account for insolation
changes. We also introduce a simple correction factor that allows reduced-complexity
melt models to account for changes in insolation. 2014/08/07 - 18:09

The influence of edge effects on crack propagation in snow stability testsThe Cryosphere, 8, 1407-1418, 2014Author(s): E. H. Bair, R. Simenhois, A. van Herwijnen, and K. BirkelandThe Extended Column Test (ECT) and the Propagation Saw Test (PST)
are two commonly used tests to assess the likelihood of crack propagation in
a snowpack. Guidelines suggest beams with lengths of around 1 m, yet little is
known about how test length affects propagation. Thus, we performed 163 ECTs
and PSTs 1.0–10.0 m long. On days with full crack propagation in 1.0–1.5 m
tests, we then made videos of tests 2.0–10.0 m long. We inserted markers
for particle tracking to measure collapse amplitude, propagation speed, and
wavelength. We also used a finite element (FE) model to simulate the strain
energy release rate at fixed crack lengths. We find that (1) the proportion
of tests with full propagation decreased with test length; (2) collapse was
greater at the ends of the beams than in the centers; (3) collapse amplitude
was independent of beam length and did not reach a constant value; (4)
collapse wavelengths in the longer tests were around 3 m, two times greater than
what is predicted by the anticrack model. We also confirmed the prediction
that centered PSTs had double the critical length of edge PSTs. Based on our
results, we conclude that cracks propagated more frequently in the shorter
tests because of increased stress concentration from the far edge. The FE
model suggests this edge effect occurs for PSTs of up to 2 m long or a crack to
beam length ratio ≥ 0.20. Our results suggest that ECT and PST length
guidelines may need to be revisited. 2014/08/07 - 18:09

Importance of basal processes in simulations of a surging Svalbard outlet glacierThe Cryosphere, 8, 1393-1405, 2014Author(s): R. Gladstone, M. Schäfer, T. Zwinger, Y. Gong, T. Strozzi, R. Mottram, F. Boberg, and J. C. MooreThe outlet glacier of Basin 3 (B3) of Austfonna ice cap, Svalbard, is one of
the fastest outlet glaciers in Svalbard, and shows dramatic changes since
1995. In addition to previously observed seasonal summer speed-up associated
with the melt season, the winter speed of B3 has accelerated approximately
fivefold since 1995. We use the Elmer/Ice full-Stokes model for ice dynamics
to infer spatial distributions of basal drag for the winter seasons of 1995,
2008 and 2011. This "inverse" method is based on minimising discrepancy
between modelled and observed surface velocities, using satellite remotely
sensed velocity fields. We generate steady-state temperature distributions
for 1995 and 2011. Frictional heating caused by basal sliding contributes
significantly to basal temperatures of the B3 outlet glacier, with heat
advection (a longer-timescale process than frictional heating) also being
important in the steady state.

We present a sensitivity experiment consisting of transient simulations under
present-day forcing to demonstrate that using a temporally fixed basal drag
field obtained through inversion can lead to thickness change errors of the
order of 2 m year−1. Hence it is essential to incorporate the evolution of
basal processes in future projections of the evolution of B3. Informed by a
combination of our inverse method results and previous studies, we
hypothesise a system of processes and feedbacks involving till deformation
and basal hydrology to explain both the seasonal accelerations (short
residence time pooling of meltwater at the ice–till interface) and the
ongoing interannual speed-up (gradual penetration of water into the till,
reducing till strength). 2014/08/04 - 19:04

Constraining the recent mass balance of Pine Island and Thwaites glaciers, West Antarctica, with airborne observations of snow accumulationThe Cryosphere, 8, 1375-1392, 2014Author(s): B. Medley, I. Joughin, B. E. Smith, S. B. Das, E. J. Steig, H. Conway, S. Gogineni, C. Lewis, A. S. Criscitiello, J. R. McConnell, M. R. van den Broeke, J. T. M. Lenaerts, D. H. Bromwich, J. P. Nicolas, and C. LeuschenIn Antarctica, uncertainties in mass input and output translate directly
into uncertainty in glacier mass balance and thus in sea level impact. While
remotely sensed observations of ice velocity and thickness over the major
outlet glaciers have improved our understanding of ice loss to the ocean,
snow accumulation over the vast Antarctic interior remains largely
unmeasured. Here, we show that an airborne radar system, combined with
ice-core glaciochemical analysis, provide the means necessary to measure the
accumulation rate at the catchment-scale along the Amundsen Sea coast of
West Antarctica. We used along-track radar-derived accumulation to generate
a 1985–2009 average accumulation grid that resolves moderate- to
large-scale features (>25 km) over the Pine Island–Thwaites
glacier drainage system. Comparisons with estimates from atmospheric models
and gridded climatologies generally show our results as having less
accumulation in the lower-elevation coastal zone but greater accumulation in the
interior. Ice discharge, measured over discrete time intervals between 1994
and 2012, combined with our catchment-wide accumulation rates provide an
18-year mass balance history for the sector. While Thwaites Glacier lost the
most ice in the mid-1990s, Pine Island Glacier's losses increased
substantially by 2006, overtaking Thwaites as the largest regional
contributor to sea-level rise. The trend of increasing discharge for both
glaciers, however, appears to have leveled off since 2008. 2014/08/01 - 21:20

Using MODIS land surface temperatures and the Crocus snow model to understand the warm bias of ERA-Interim reanalyses at the surface in AntarcticaThe Cryosphere, 8, 1361-1373, 2014Author(s): H. Fréville, E. Brun, G. Picard, N. Tatarinova, L. Arnaud, C. Lanconelli, C. Reijmer, and M. van den BroekeModerate-Resolution Imaging spectroradiometer (MODIS) land surface
temperatures in Antarctica were processed in order to produce a gridded data
set at 25 km resolution, spanning the period 2000–2011 at an hourly
time step. The Aqua and Terra orbits and MODIS swath width, combined with
frequent clear-sky conditions, lead to very high availability of
quality-controlled observations: on average, hourly data are available
14 h per day at the grid points around the South Pole and more than 9 h over a
large area of the Antarctic Plateau. Processed MODIS land surface
temperatures, referred to hereinafter as MODIS Ts values, were compared with in
situ hourly measurements of surface temperature collected over the
entirety of the year 2009 by seven stations from the Baseline Surface Radiation Network (BSRN)
and automatic weather stations (AWSs). In spite of an occasional failure in
the detection of clouds, MODIS Ts values exhibit a good performance, with a
bias ranging from −1.8 to 0.1 °C and errors ranging from 2.2 to 4.8 °C root
mean square at the five stations located on the plateau. These results show that
MODIS Ts values can be used as a precise and accurate reference to test other
surface temperature data sets. Here, we evaluate the performance of surface
temperature in the European Centre for Medium-Range Weather Forecasts (ECMWF)
reanalysis known as ERA-Interim reanalysis. During conditions detected as
cloud free by MODIS, ERA-Interim shows a widespread warm bias in Antarctica
in every season, ranging from +3 to +6 °C on the plateau. This confirms a
recent study which showed that the largest discrepancies in 2 m air
temperature between ERA-Interim and the global temperature data set HadCRUT4
compiled by the Met Office Hadley Centre and the University of East Anglia's
Climatic Research Unit occur in Antarctica. A comparison with in situ surface
temperature shows that this bias is not strictly limited to clear-sky
conditions. A detailed comparison with stand-alone simulations by the Crocus
snowpack model, forced by ERA-Interim, and with the ERA-Interim/land
simulations, shows that the warm bias may be due primarily to an
overestimation of the surface turbulent fluxes in very stable conditions.
Numerical experiments with Crocus show that a small change in the
parameterization of the effects of stability on the surface exchange
coefficients can significantly impact the snow surface temperature. The
ERA-Interim warm bias appears to be likely due to an overestimation of the
surface exchange coefficients under very stable conditions. 2014/08/01 - 21:20

Modelling the evolution of the Antarctic ice sheet since the last interglacialThe Cryosphere, 8, 1347-1360, 2014Author(s): M. N. A. Maris, B. de Boer, S. R. M. Ligtenberg, M. Crucifix, W. J. van de Berg, and J. OerlemansWe present the effects of changing two sliding parameters, a deformational
velocity parameter and two bedrock deflection parameters on the evolution of
the Antarctic ice sheet over the period from the last interglacial until the
present. These sensitivity experiments have been conducted by running the
dynamic ice model ANICE forward in time. The temporal climatological forcing is established by interpolating between two temporal climate states created with a
regional climate model. The interpolation is done in such a way
that both temperature and surface mass balance follow the European Project for Ice Coring in Antarctica (EPICA) Dome C
ice-core proxy record for temperature. We have determined an optimal set of
parameter values, for which a realistic grounding-line retreat history and
present-day ice sheet can be simulated; the simulation with this set of
parameter values is defined as the reference simulation. An increase of
sliding with respect to this reference simulation leads to a decrease of the
Antarctic ice volume due to enhanced ice velocities on mainly the West
Antarctic ice sheet. The effect of changing the deformational velocity
parameter mainly yields a change in east Antarctic ice volume. Furthermore,
we have found a minimum in the Antarctic ice volume during the mid-Holocene,
in accordance with observations. This is a robust feature in our model
results, where the strength and the timing of this minimum are both dependent
on the investigated parameters. More sliding and a slower responding bedrock
lead to a stronger minimum which emerges at an earlier time. From the model
results, we conclude that the Antarctic ice sheet has contributed
10.7 ± 1.3 m of eustatic sea level to the global ocean from the last
glacial maximum (about 16 ka for the Antarctic ice sheet) until the
present. 2014/08/01 - 21:20

Two independent methods for mapping the grounding line of an outlet glacier – an example from the Astrolabe Glacier, Terre Adélie, AntarcticaThe Cryosphere, 8, 1331-1346, 2014Author(s): E. Le Meur, M. Sacchettini, S. Garambois, E. Berthier, A. S. Drouet, G. Durand, D. Young, J. S. Greenbaum, J. W. Holt, D. D. Blankenship, E. Rignot, J. Mouginot, Y. Gim, D. Kirchner, B. de Fleurian, O. Gagliardini, and F. Gillet-ChauletThe grounding line is a key element of coastal outlet glaciers, acting on
their dynamics. Accurately knowing its position is fundamental for both
modelling the glacier dynamics and establishing a benchmark for later change
detection. Here we map the grounding line of the Astrolabe Glacier in East
Antarctica (66°41' S, 140°05' E), using both hydrostatic
and tidal methods. The first method is based on new surface and ice thickness
data from which the line of buoyant floatation is found. The second method
uses kinematic GPS measurements of the tidal response of the ice surface. By
detecting the transitions where the ice starts to move vertically in response
to the tidal forcing we determine control points for the grounding line
position along GPS profiles. Employing a two-dimensional elastic plate model,
we compute the rigid short-term behaviour of the ice plate and estimate the
correction required to compare the kinematic GPS control points with the
previously determined line of floatation. These two approaches show
consistency and lead us to propose a grounding line for the Astrolabe Glacier
that significantly deviates from the lines obtained so far from satellite
imagery. 2014/07/25 - 10:31

Debris thickness of glaciers in the Everest area (Nepal Himalaya) derived from satellite imagery using a nonlinear energy balance modelThe Cryosphere, 8, 1317-1329, 2014Author(s): D. R. Rounce and D. C. McKinneyDebris thickness is an important characteristic of debris-covered glaciers
in the Everest region of the Himalayas. The debris thickness controls the
melt rates of the glaciers, which has large implications for hydrologic
models, the glaciers' response to climate change, and the development of
glacial lakes. Despite its importance, there is little knowledge of how the
debris thickness varies over these glaciers. This paper uses an energy
balance model in conjunction with Landsat7 Enhanced Thematic Mapper Plus (ETM+) satellite imagery to
derive thermal resistances, which are the debris thickness divided by the
thermal conductivity. Model results are reported in terms of debris
thickness using an effective thermal conductivity derived from field data.
The developed model accounts for the nonlinear temperature gradient in the
debris cover to derive reasonable debris thicknesses. Fieldwork performed on
Imja–Lhotse Shar Glacier in September 2013 was used to compare to the
modeled debris thicknesses. Results indicate that accounting for the
nonlinear temperature gradient is crucial. Furthermore, correcting the
incoming shortwave radiation term for the effects of topography and
resampling to the resolution of the thermal band's pixel is imperative to
deriving reasonable debris thicknesses. Since the topographic correction is
important, the model will improve with the quality of the digital elevation model (DEM). The main
limitation of this work is the poor resolution (60 m) of the satellite's
thermal band. The derived debris thicknesses are reasonable at this
resolution, but trends related to slope and aspect are unable to be modeled
on a finer scale. Nonetheless, the study finds this model derives reasonable
debris thicknesses on this scale and was applied to other debris-covered
glaciers in the Everest region. 2014/07/24 - 00:49

Tracing glacier changes since the 1960s on the south slope of Mt. Everest (central Southern Himalaya) using optical satellite imageryThe Cryosphere, 8, 1297-1315, 2014Author(s): S. Thakuri, F. Salerno, C. Smiraglia, T. Bolch, C. D'Agata, G. Viviano, and G. TartariThis contribution examines glacier changes on the south side of Mt. Everest
from 1962 to 2011 considering five intermediate periods using optical
satellite imagery. The investigated glaciers cover ~ 400 km2
and present among the largest debris coverage (32%)
and the highest elevations (5720 m) of the world. We found an overall
surface area loss of 13.0 ± 3.1% (median 0.42 ± 0.06 % a−1),
an upward shift of 182 ± 22 m (3.7 ± 0.5 m a−1)
in snow-line altitude (SLA), a terminus retreat of 403 ± 9 m (median
6.1 ± 0.2 m a−1), and an increase of 17.6 ± 3.1%
(median 0.20 ± 0.06% a−1) in debris coverage between 1962 and
2011. The recession process of glaciers has been relentlessly continuous
over the past 50 years. Moreover, we observed that (i) glaciers that have
increased the debris coverage have experienced a reduced termini retreat (r = 0.87,
p < 0.001). Furthermore, more negative mass balances (i.e.,
upward shift of SLA) induce increases of debris coverage (r = 0.79,
p < 0.001); (ii) since early 1990s, we observed a slight but statistically
insignificant acceleration of the surface area loss (0.35 ± 0.13% a−1
in 1962–1992 vs 0.43 ± 0.25% a−1 in 1992–2011),
but an significant upward shift of SLA which increased almost three times
(2.2 ± 0.8 m a−1 in 1962–1992 vs 6.1 ± 1.4 m a−1 in
1992–2011). However, the accelerated shrinkage in recent decades (both in
terms of surface area loss and SLA shift) has only significantly affected
glaciers with the largest sizes (> 10 km2), presenting
accumulation zones at higher elevations (r = 0.61, p < 0.001) and
along the preferable south–north direction of the monsoons. Moreover, the
largest glaciers present median upward shifts of the SLA (220 m) that are
nearly double than that of the smallest (119 m); this finding leads to a hypothesis
that Mt. Everest glaciers are shrinking, not only due to warming
temperatures, but also as a result of weakening Asian monsoons registered
over the last few decades. We conclude that the shrinkage of the glaciers in
south of Mt. Everest is less than that of others in the western and eastern
Himalaya and southern and eastern Tibetan Plateau. Their position in higher
elevations have likely reduced the impact of warming on these glaciers, but
have not been excluded from a relentlessly continuous and slow recession
process over the past 50 years. 2014/07/22 - 22:51

A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated?The Cryosphere, 8, 1289-1296, 2014Author(s): I. Eisenman, W. N. Meier, and J. R. NorrisRecent estimates indicate that the Antarctic sea ice cover is expanding at
a statistically significant rate with a magnitude one-third as large as the
rapid rate of sea ice retreat in the Arctic. However, during the mid-2000s,
with several fewer years in the observational record, the trend in Antarctic
sea ice extent was reported to be considerably smaller and statistically
indistinguishable from zero. Here, we show that much of the increase in the
reported trend occurred due to the previously undocumented effect of a change
in the way the satellite sea ice observations are processed for the
widely used Bootstrap algorithm data set, rather than a physical increase in
the rate of ice advance. Specifically, we find that a change in the
intercalibration across a 1991 sensor transition when the data set was
reprocessed in 2007 caused a substantial change in the long-term trend.
Although our analysis does not definitively identify whether this change
introduced an error or removed one, the resulting difference in the trends
suggests that a substantial error exists in either the current data set or the
version that was used prior to the mid-2000s, and numerous studies that have
relied on these observations should be reexamined to determine the
sensitivity of their results to this change in the data set. Furthermore,
a number of recent studies have investigated physical mechanisms for the
observed expansion of the Antarctic sea ice cover. The results of this
analysis raise the possibility that much of this expansion may be a spurious
artifact of an error in the processing of the satellite observations. 2014/07/22 - 22:51

Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS)The Cryosphere, 8, 1275-1287, 2014Author(s): P. Vallelonga, K. Christianson, R. B. Alley, S. Anandakrishnan, J. E. M. Christian, D. Dahl-Jensen, V. Gkinis, C. Holme, R. W. Jacobel, N. B. Karlsson, B. A. Keisling, S. Kipfstuhl, H. A. Kjær, M. E. L. Kristensen, A. Muto, L. E. Peters, T. Popp, K. L. Riverman, A. M. Svensson, C. Tibuleac, B. M. Vinther, Y. Weng, and M. WinstrupThe Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic
ice stream. Fast flow initiates near the summit dome, and the ice stream
terminates approximately 1000 km downstream in three large outlet glaciers
that calve into the Greenland Sea. To better understand this important
system, in the summer of 2012 we drilled a 67 m firn core and conducted
ground-based radio-echo sounding (RES) and active-source seismic surveys at
a site approximately 150 km downstream from the onset of streaming flow
(NEGIS firn core, 75°37.61' N, 35°56.49' W).
The site is representative of the upper part of the ice stream, while
also being in a crevasse-free area for safe surface operations.

Annual cycles were observed for insoluble dust, sodium and ammonium
concentrations and for electrolytic conductivity, allowing a seasonally
resolved chronology covering the past 400 yr. Annual layer thicknesses
averaged 0.11 m ice equivalent (i.e.) for the period 1607–2011, although
accumulation varied between 0.08 and 0.14 m i.e., likely due to flow-related
changes in surface topography. Tracing of RES layers from the NGRIP (North Greenland Ice Core Project) ice core
site shows that the ice at NEGIS preserves a climatic record of at least the
past 51 kyr. We demonstrate that deep ice core drilling in this location
can provide a reliable Holocene and late-glacial climate record, as well as
helping to constrain the past dynamics and ice–lithosphere interactions of
the Greenland Ice Sheet. 2014/07/22 - 22:51

A high-resolution bedrock map for the Antarctic PeninsulaThe Cryosphere, 8, 1261-1273, 2014Author(s): M. Huss and D. FarinottiAssessing and projecting the dynamic response of glaciers on the Antarctic
Peninsula to changed atmospheric and oceanic forcing requires high-resolution
ice thickness data as an essential geometric constraint for ice flow models.
Here, we derive a complete bedrock data set for the Antarctic Peninsula north
of 70° S on a 100 m grid. We calculate distributed ice thickness
based on surface topography and simple ice dynamic modelling. Our approach is
constrained with all available thickness measurements from Operation
IceBridge and gridded ice flow speeds for the entire study region. The new
data set resolves the rugged subglacial topography in great detail, indicates
deeply incised troughs, and shows that 34% of the ice volume is grounded
below sea level. The Antarctic Peninsula has the potential to raise global
sea level by 69 ± 5 mm. In comparison to Bedmap2, covering all
Antarctica on a 1 km grid, a significantly higher mean ice thickness
(+48%) is found. 2014/07/19 - 01:15

Parameterization of basal friction near grounding lines in a one-dimensional ice sheet modelThe Cryosphere, 8, 1239-1259, 2014Author(s): G. R. Leguy, X. S. Asay-Davis, and W. H. LipscombIce sheets and ice shelves are linked by the transition zone, the region
where flow dominated by vertical shear stress makes a transition to flow
dominated by extensional stress. Adequate resolution of the transition zone
is necessary for numerically accurate ice sheet–ice shelf simulations. The
required resolution depends on how the basal physics is parameterized. We
propose a~new, simple parameterization of the effective pressure near the
grounding line, combined with an existing friction law linking effective
pressure to basal stress and sliding, in a one-dimensional, fixed-grid,
vertically integrated model. This parameterization represents connectivity
between the basal hydrological system and the ocean in the transition zone.
Our model produces a smooth transition between finite basal friction in the
ice sheet and zero basal friction in the ice shelf. In a set of experiments
based on the Marine Ice Sheet Model Intercomparison Project (MISMIP), we show
that with a smoother basal shear stress, the model yields accurate
steady-state results at a fixed-grid resolution of ~1 km. 2014/07/19 - 01:15