ALBERT

Description: Coupling of climate models and ice sheet models by surface mass balance gradients: application to the Greenland Ice Sheet The Cryosphere, 6, 255-272, 2012 Author(s): M. M. Helsen, R. S. W. van de Wal, M. R. van den Broeke, W. J. van de Berg, and J. Oerlemans It is notoriously difficult to couple surface mass balance (SMB) results from climate models to the changing geometry of an ice sheet model. This problem is traditionally avoided by using only accumulation from a climate model, and parameterizing the meltwater run-off as a function of temperature, which is often related to surface elevation ( H s ). In this study, we propose a new strategy to calculate SMB, to allow a direct adjustment of SMB to a change in ice sheet topography and/or a change in climate forcing. This method is based on elevational gradients in the SMB field as computed by a regional climate model. Separate linear relations are derived for ablation and accumulation, using pairs of H s and SMB within a minimum search radius. The continuously adjusting SMB forcing is consistent with climate model forcing fields, also for initially non-glaciated areas in the peripheral areas of an ice sheet. When applied to an asynchronous coupled ice sheet – climate model setup, this method circumvents traditional temperature lapse rate assumptions. Here we apply it to the Greenland Ice Sheet (GrIS). Experiments using both steady-state forcing and glacial-interglacial forcing result in realistic ice sheet reconstructions.

Description: Estimating ice phenology on large northern lakes from AMSR-E: algorithm development and application to Great Bear Lake and Great Slave Lake, Canada The Cryosphere, 6, 235-254, 2012 Author(s): K.-K. Kang, C. R. Duguay, and S. E. L. Howell Time series of brightness temperatures ( T B ) from the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E) are examined to determine ice phenology variables on the two largest lakes of northern Canada: Great Bear Lake (GBL) and Great Slave Lake (GSL). T B measurements from the 18.7, 23.8, 36.5, and 89.0 GHz channels (H- and V- polarization) are compared to assess their potential for detecting freeze-onset/melt-onset and ice-on/ice-off dates on both lakes. The 18.7 GHz (H-pol) channel is found to be the most suitable for estimating these ice dates as well as the duration of the ice cover and ice-free seasons. A new algorithm is proposed using this channel and applied to map all ice phenology variables on GBL and GSL over seven ice seasons (2002–2009). Analysis of the spatio-temporal patterns of each variable at the pixel level reveals that: (1) both freeze-onset and ice-on dates occur on average about one week earlier on GBL than on GSL (Day of Year (DY) 318 and 333 for GBL; DY 328 and 343 for GSL); (2) the freeze-up process or freeze duration (freeze-onset to ice-on) takes a slightly longer amount of time on GBL than on GSL (about 1 week on average); (3) melt-onset and ice-off dates occur on average one week and approximately four weeks later, respectively, on GBL (DY 143 and 183 for GBL; DY 135 and 157 for GSL); (4) the break-up process or melt duration (melt-onset to ice-off) lasts on average about three weeks longer on GBL; and (5) ice cover duration estimated from each individual pixel is on average about three weeks longer on GBL compared to its more southern counterpart, GSL. A comparison of dates for several ice phenology variables derived from other satellite remote sensing products (e.g. NOAA Interactive Multisensor Snow and Ice Mapping System (IMS), QuikSCAT, and Canadian Ice Service Database) show that, despite its relatively coarse spatial resolution, AMSR-E 18.7 GHz provides a viable means for monitoring of ice phenology on large northern lakes.

Description: Large surface meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during the record-warm year 2010 explained by detailed energy balance observations The Cryosphere, 6, 199-209, 2012 Author(s): D. van As, A. L. Hubbard, B. Hasholt, A. B. Mikkelsen, M. R. van den Broeke, and R. S. Fausto This study uses data from six on-ice weather stations, calibrated MODIS-derived albedo and proglacial river gauging measurements to drive and validate an energy balance model. We aim to quantify the record-setting positive temperature anomaly in 2010 and its effect on mass balance and runoff from the Kangerlussuaq sector of the Greenland ice sheet. In 2010, the average temperature was 4.9 °C (2.7 standard deviations) above the 1974–2010 average in Kangerlussuaq. High temperatures were also observed over the ice sheet, with the magnitude of the positive anomaly increasing with altitude, particularly in August. Simultaneously, surface albedo was anomalously low in 2010, predominantly in the upper ablation zone. The low albedo was caused by high ablation, which in turn profited from high temperatures and low winter snowfall. Surface energy balance calculations show that the largest melt excess (∼170%) occurred in the upper ablation zone (above 1000 m), where higher temperatures and lower albedo contributed equally to the melt anomaly. At lower elevations the melt excess can be attributed to high atmospheric temperatures alone. In total, we calculate that 6.6 ± 1.0 km 3 of surface meltwater ran off the ice sheet in the Kangerlussuaq catchment in 2010, exceeding the reference year 2009 (based on atmospheric temperature measurements) by ∼150%. During future warm episodes we can expect a melt response of at least the same magnitude, unless a larger wintertime snow accumulation delays and moderates the melt-albedo feedback. Due to the hypsometry of the ice sheet, yielding an increasing surface area with elevation, meltwater runoff will be further amplified by increases in melt forcings such as atmospheric heat.

Description: Greenland ice sheet surface mass balance: evaluating simulations and making projections with regional climate models The Cryosphere, 6, 1275-1294, 2012 Author(s): J. G. L. Rae, G. Aðalgeirsdóttir, T. L. Edwards, X. Fettweis, J. M. Gregory, H. T. Hewitt, J. A. Lowe, P. Lucas-Picher, R. H. Mottram, A. J. Payne, J. K. Ridley, S. R. Shannon, W. J. van de Berg, R. S. W. van de Wal, and M. R. van den Broeke Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its contribution to sea level rise, with greater accuracy than is possible from coarser-resolution general circulation models (GCMs). This is the first time an intercomparison has been carried out of RCM results for Greenland climate and SMB. Output from RCM simulations for the recent past with the four RCMs is evaluated against available observations. The evaluation highlights the importance of using a detailed snow physics scheme, especially regarding the representations of albedo and meltwater refreezing. Simulations with three of the RCMs for the 21st century using SRES scenario A1B from two GCMs produce trends of between −5.5 and −1.1 Gt yr −2 in SMB (equivalent to +0.015 and +0.003 mm sea level equivalent yr −2 ), with trends of smaller magnitude for scenario E1, in which emissions are mitigated. Results from one of the RCMs whose present-day simulation is most realistic indicate that an annual mean near-surface air temperature increase over Greenland of ~ 2°C would be required for the mass loss to increase such that it exceeds accumulation, thereby causing the SMB to become negative, which has been suggested as a threshold beyond which the ice sheet would eventually be eliminated.

Description: Simulating snow maps for Norway: description and statistical evaluation of the seNorge snow model The Cryosphere, 6, 1323-1337, 2012 Author(s): T. M. Saloranta Daily maps of snow conditions have been produced in Norway with the seNorge snow model since 2004. The seNorge snow model operates with 1 × 1 km resolution, uses gridded observations of daily temperature and precipitation as its input forcing, and simulates, among others, snow water equivalent (SWE), snow depth (SD), and the snow bulk density (ρ). In this paper the set of equations contained in the seNorge model code is described and a thorough spatiotemporal statistical evaluation of the model performance from 1957–2011 is made using the two major sets of extensive in situ snow measurements that exist for Norway. The evaluation results show that the seNorge model generally overestimates both SWE and ρ, and that the overestimation of SWE increases with elevation throughout the snow season. However, the R 2 -values for model fit are 0.60 for (log-transformed) SWE and 0.45 for ρ, indicating that after removal of the detected systematic model biases (e.g. by recalibrating the model or expressing snow conditions in relative units) the model performs rather well. The seNorge model provides a relatively simple, not very data-demanding, yet nonetheless process-based method to construct snow maps of high spatiotemporal resolution. It is an especially well suited alternative for operational snow mapping in regions with rugged topography and large spatiotemporal variability in snow conditions, as is the case in the mountainous Norway.

Description: Ground penetrating radar detection of subsnow slush on ice-covered lakes in interior Alaska The Cryosphere, 6, 1435-1443, 2012 Author(s): A. Gusmeroli and G. Grosse Lakes are abundant throughout the pan-Arctic region. For many of these lakes ice cover lasts for up to two thirds of the year. The frozen cover allows human access to these lakes, which are therefore used for many subsistence and recreational activities, including water harvesting, fishing, and skiing. Safe traveling condition onto lakes may be compromised, however, when, after significant snowfall, the weight of the snow acts on the ice and causes liquid water to spill through weak spots and overflow at the snow-ice interface. Since visual detection of subsnow slush is almost impossible our understanding on overflow processes is still very limited and geophysical methods that allow water and slush detection are desirable. In this study we demonstrate that a commercially available, lightweight 1 GHz, ground penetrating radar system can detect and map extent and intensity of overflow. The strength of radar reflections from wet snow-ice interfaces are at least twice as much in strength than returns from dry snow-ice interface. The presence of overflow also affects the quality of radar returns from the base of the lake ice. During dry conditions we were able to profile ice thickness of up to 1 m, conversely, we did not retrieve any ice-water returns in areas affected by overflow.

Description: The footprint of Asian monsoon dynamics in the mass and energy balance of a Tibetan glacier The Cryosphere, 6, 1445-1461, 2012 Author(s): T. Mölg, F. Maussion, W. Yang, and D. Scherer Determinations of glacier-wide mass and energy balance are still scarce for the remote mountains of the Tibetan Plateau, where field measurements are challenging. Here we run and evaluate a physical, distributed mass balance model for Zhadang Glacier (central Tibet, 30° N) based on in-situ measurements over 2009–2011 and an uncertainty estimate by Monte Carlo and ensemble strategies. The model application aims to provide the first quantification of how the Indian Summer Monsoon (ISM) impacts an entire glacier over the various stages of the monsoon's annual cycle. We find a strong and systematic ISM footprint on the interannual scale. Early (late) monsoon onset causes higher (lower) accumulation, and reduces (increases) the available energy for ablation primarily through changes in absorbed shortwave radiation. By contrast, only a weak footprint exists in the ISM cessation phase. Most striking though is the core monsoon season: local mass and energy balance variability is fully decoupled from the active/break cycle that defines large-scale atmospheric variability during the ISM. Our results demonstrate quantitatively that monsoon onset strongly affects the ablation season of glaciers in Tibet. However, we find no direct ISM impact on the glacier in the main monsoon season, which has not been acknowledged so far. This result also adds cryospheric evidence that, once the monsoon is in full swing, regional atmospheric variability prevails on the Tibetan Plateau in summer.

Description: The first complete inventory of the local \newline glaciers and ice caps on Greenland The Cryosphere, 6, 1483-1495, 2012 Author(s): P. Rastner, T. Bolch, N. Mölg, H. Machguth, R. Le Bris, and F. Paul Glacier inventories provide essential baseline information for the determination of water resources, glacier-specific changes in area and volume, climate change impacts as well as past, potential and future contribution of glaciers to sea-level rise. Although Greenland is heavily glacierised and thus highly relevant for all of the above points, a complete inventory of its glaciers was not available so far. Here we present the results and details of a new and complete inventory that has been compiled from more than 70 Landsat scenes (mostly acquired between 1999 and 2002) using semi-automated glacier mapping techniques. A digital elevation model (DEM) was used to derive drainage divides from watershed analysis and topographic attributes for each glacier entity. To serve the needs of different user communities, we assigned to each glacier one of three connectivity levels with the ice sheet (CL0, CL1, CL2; i.e. no, weak, and strong connection) to clearly, but still flexibly, distinguish the local glaciers and ice caps (GIC) from the ice sheet and its outlet glaciers. In total, we mapped ~ 20 300 glaciers larger than 0.05 km 2 (of which ~ 900 are marine terminating), covering an area of 130 076 ± 4032 km 2 , or 89 720 ± 2781 km 2 without the CL2 GIC. The latter value is about 50% higher than the mean value of more recent previous estimates. Glaciers smaller than 0.5 km 2 contribute only 1.5% to the total area but more than 50% (11 000) to the total number. In contrast, the 25 largest GIC (〉 500 km 2 ) contribute 28% to the total area, but only 0.1% to the total number. The mean elevation of the GIC is 1700 m in the eastern sector and around 1000 m otherwise. The median elevation increases with distance from the coast, but has only a weak dependence on mean glacier aspect.

Description: Glacier changes from 1966–2009 in the Gongga Mountains, on the south-eastern margin of the Qinghai-Tibetan Plateau and their climatic forcing The Cryosphere, 6, 1087-1101, 2012 Author(s): B. T. Pan, G. L. Zhang, J. Wang, B. Cao, H. P. Geng, J. Wang, C. Zhang, and Y. P. Ji In order to monitor the changes of the glaciers in the Gongga Mountain region on the south-eastern margin of the Qinghai-Tibetan Plateau, 74 monsoonal temperate glaciers were investigated by comparing the Chinese Glacier Inventory (CGI), recorded in the 1960s, with Landsat MSS in 1974, Landsat TM in 1989, 1994, 2005, and ASTER data in 2009. The remote sensing data have been applied to map the glacier outline by threshold ratio images (TM4/TM5). Moreover, the glacier outlines were verified by GPS survey on four large glaciers (Hailuogou (HLG), Mozigou (MZG), Yanzigou (YZG), and Dagongba (DGB)) in 2009. The results show that the area dominated by the 74 glaciers has shrunk by 11.3% (29.2 km 2 ) from 1966 to 2009. Glacier area on the eastern and western slopes of the Gongga Mountains decreased by 9.8% and 14.6% since 1966, respectively. The loss in glacier area and length is, respectively, 0.8 km 2 and 1146.4 m for the HLG Glacier, 2.1 km 2 and 501.8 m for the MZG Glacier, 0.8 km 2 and 724.8 m for the YZG Glacier, and 2.4 km 2 and 1002.3 m for the DGB Glacier. Decades of climate records obtained from three meteorological stations in the Gongga Mountains were analyzed to evaluate the impact of the temperature and precipitation on glacier retreat. The mean annual temperatures over the eastern and western slopes of the Gongga Mountains have been increasing by 0.34 K decade −1 and 0.24 K decade −1 (1988–2009), respectively. Moreover, mean annual precipitation has only increased by 1% in the past 50 yr. The increasing amount of precipitation could not compensate for the glacier mass loss due to the temperature increase in the Gongga Mountains. This suggests that the warming of the climate is probably also responsible for the glacier retreat in the study region. At the region scale, glacier changes were also controlled by local topographical factors.

Description: Accelerated contributions of Canada's Baffin and Bylot Island glaciers to sea level rise over the past half century The Cryosphere, 6, 1103-1125, 2012 Author(s): A. Gardner, G. Moholdt, A. Arendt, and B. Wouters Canadian Arctic glaciers have recently contributed large volumes of meltwater to the world's oceans. To place recently observed glacier wastage into a historical perspective and to determine the region's longer-term (~50 years) contribution to sea level, we estimate mass and volume changes for the glaciers of Baffin and Bylot Islands using digital elevation models generated from airborne and satellite stereoscopic imagery and elevation postings from repeat airborne and satellite laser altimetry. In addition, we update existing glacier mass change records from GRACE satellite gravimetry to cover the period from 2003 to 2011. Using this integrated approach, we find that the rate of mass loss from the region's glaciers increased from 11.1 ± 3.4 Gt a −1 (271 ± 84 kg m −2 a −1 ) for the period 1963–2006 to 23.8 ± 6.1 Gt a −1 (581 ± 149 kg m −2 a −1 ) for the period 2003–2011. The doubling of the rate of mass loss is attributed to higher temperatures in summer with little change in annual precipitation. Through both direct and indirect effects, changes in summer temperatures accounted for 70–98% of the variance in the rate of mass loss, to which the Barnes Ice Cap was found to be 1.7 times more sensitive than either the Penny Ice Cap or the region's glaciers as a whole. This heightened sensitivity is the result of a glacier hypsometry that is skewed to lower elevations, which are shown to have a higher mass change sensitive to temperature compared to glacier surfaces at higher elevations. Between 2003 and 2011 the glaciers of Baffin and Bylot Islands contributed 0.07 ± 0.02 mm a −1 to sea level rise accounting for 16% of the total contribution from glaciers outside of Greenland and Antarctica, a rate much higher than the longer-term average of 0.03 ± 0.01 mm a −1 (1963 to 2006).

Description: Ice velocity changes in the Ross and Ronne sectors observed using satellite radar data from 1997 and 2009 The Cryosphere, 6, 1019-1030, 2012 Author(s): B. Scheuchl, J. Mouginot, and E. Rignot We report changes in ice velocity of a 6.5 million km 2 region around South Pole encompassing the Filchner-Ronne and Ross Ice Shelves and a significant portion of the ice streams and glaciers that constitute their catchment areas. Using the first full interferometric synthetic aperture radar (InSAR) coverage of the region completed in 2009 and partial coverage acquired in 1997, we processed the data to assemble a comprehensive map of ice speed changes between those two years. On the Ross Ice Shelf, our results confirm a continued deceleration of Mercer and Whillans Ice Streams with a 12-yr velocity difference of −50 m yr −1 (−16.7%) and −100 m yr −1 (−25.3%) at their grounding lines. The deceleration spreads 450 km upstream of the grounding line and more than 500 km onto the shelf, beyond what was previously known. Ross and Filchner Ice Shelves exhibit signs of pre-calving events, representing the largest observed changes, with an increase in speed in excess of +100 m yr −1 in 12 yr. Other changes in the Ross Ice Shelf region are less significant. The observed changes in glacier speed extend on the Ross Ice Shelf along the ice streams' flow lines. Most tributaries of the Filchner-Ronne Ice Shelf show a modest deceleration or no change between 1997 and 2009. Slessor Glacier shows a small deceleration over a large sector. No change is detected on the Bailey, Rutford, and Institute Ice Streams. On the Filchner Ice Shelf itself, ice decelerated rather uniformly with a 12-yr difference in speed of −50 m yr −1 , or −5% of its ice front speed, which we attribute to a 12 km advance in its ice front position. Our results show that dynamic changes are present in the region. They highlight the need for continued observation of the area with a primary focus on the Siple Coast. The dynamic changes in Central Antarctica between 1997 and 2009 are generally second-order effects in comparison to losses on glaciers in the Bellingshausen and Amundsen Seas region and on the Antarctic Peninsula. We therefore conclude that the dynamic changes shown here do not have a strong impact on the mass budget of the Antarctic continent.

Description: Uncertainties in the global temperature change caused by carbon release from permafrost thawing The Cryosphere, 6, 1063-1076, 2012 Author(s): E. J. Burke, I. P. Hartley, and C. D. Jones Under climate change thawing permafrost will cause old carbon which is currently frozen and inert to become vulnerable to decomposition and release into the climate system. This paper develops a simple framework for estimating the impact of this permafrost carbon release on the global mean temperature (P-GMT). The analysis is based on simulations made with the Hadley Centre climate model (HadGEM2-ES) for a range of representative CO 2 concentration pathways. Results using the high concentration pathway (RCP 8.5) suggest that by 2100 the annual methane (CH 4 ) emission rate is 2–59 Tg CH 4 yr −1 and 50–270 Pg C has been released as CO 2 with an associated P-GMT of 0.08–0.36 °C (all 5th–95th percentile ranges). P-GMT is considerably lower – between 0.02 and 0.11 °C – for the low concentration pathway (RCP2.6). The uncertainty in climate model scenario causes about 50% of the spread in P-GMT by the end of the 21st century. The distribution of soil carbon, in particular how it varies with depth, contributes to about half of the remaining spread, with quality of soil carbon and decomposition processes contributing a further quarter each. These latter uncertainties could be reduced through additional observations. Over the next 20–30 yr, whilst scenario uncertainty is small, improving our knowledge of the quality of soil carbon will contribute significantly to reducing the spread in the, albeit relatively small, P-GMT.

Description: Refreezing on the Greenland ice sheet: a comparison of parameterizations The Cryosphere, 6, 743-762, 2012 Author(s): C. H. Reijmer, M. R. van den Broeke, X. Fettweis, J. Ettema, and L. B. Stap Retention and refreezing of meltwater are acknowledged to be important processes for the mass budget of polar glaciers and ice sheets. Several parameterizations of these processes exist for use in energy and mass balance models. Due to a lack of direct observations, validation of these parameterizations is difficult. In this study we compare a set of 6 refreezing parameterizations against output of two Regional Climate Models (RCMs) coupled to an energy balance snow model, the Regional Atmospheric Climate Model (RACMO2) and the Modèle Atmosphérique Régional (MAR), applied to the Greenland ice sheet. In both RCMs, refreezing is explicitly calculated in a snow model that calculates vertical profiles of temperature, density and liquid water content. Between RACMO2 and MAR, the ice sheet-integrated amount of refreezing differs by only 4.9 mm w.e yr −1 (4.5 %), and the temporal and spatial variability are very similar. For consistency, the parameterizations are forced with output (surface temperature, precipitation and melt) of the RCMs. For the ice sheet-integrated amount of refreezing and its inter-annual variations, all parameterizations give similar results, especially after some tuning. However, the spatial distributions differ significantly and the spatial correspondence between the RCMs is better than with any of the parameterizations. Results are especially sensitive to the choice of the depth of the thermally active layer, which determines the cold content of the snow in most parameterizations. These results are independent of which RCM is used to force the parameterizations.

Description: Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography The Cryosphere, 6, 1141-1155, 2012 Author(s): B. R. Pinzer, M. Schneebeli, and T. U. Kaempfer Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during temperature gradient metamorphism (TGM) under a constant gradient of 50 K m −1 , using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, in addition to the exact locations of these phase changes. We calculated the average time that an ice volume stayed in place before it sublimated and found a characteristic residence time of 2–3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snowpack where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60% of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of {temperature gradient metamorphism} produced by directly observing the changing microstructure sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow.

Description: Transition in the fractal geometry of Arctic melt ponds The Cryosphere, 6, 1157-1162, 2012 Author(s): C. Hohenegger, B. Alali, K. R. Steffen, D. K. Perovich, and K. M. Golden During the Arctic melt season, the sea ice surface undergoes a remarkable transformation from vast expanses of snow covered ice to complex mosaics of ice and melt ponds. Sea ice albedo, a key parameter in climate modeling, is determined by the complex evolution of melt pond configurations. In fact, ice–albedo feedback has played a major role in the recent declines of the summer Arctic sea ice pack. However, understanding melt pond evolution remains a significant challenge to improving climate projections. By analyzing area–perimeter data from hundreds of thousands of melt ponds, we find here an unexpected separation of scales, where pond fractal dimension D transitions from 1 to 2 around a critical length scale of 100 m 2 in area. Pond complexity increases rapidly through the transition as smaller ponds coalesce to form large connected regions, and reaches a maximum for ponds larger than 1000 m 2 , whose boundaries resemble space-filling curves, with D ≈ 2. These universal features of Arctic melt pond evolution are similar to phase transitions in statistical physics. The results impact sea ice albedo, the transmitted radiation fields under melting sea ice, the heat balance of sea ice and the upper ocean, and biological productivity such as under ice phytoplankton blooms.

Description: Albedo of the ice covered Weddell and Bellingshausen Seas The Cryosphere, 6, 479-491, 2012 Author(s): A. I. Weiss, J. C. King, T. A. Lachlan-Cope, and R. S. Ladkin This study investigates the surface albedo of the sea ice areas adjacent to the Antarctic Peninsula during the austral summer. Aircraft measurements of the surface albedo, which were conducted in the sea ice areas of the Weddell and Bellingshausen Seas show significant differences between these two regions. The averaged surface albedo varied between 0.13 and 0.81. The ice cover of the Bellingshausen Sea consisted mainly of first year ice and the sea surface showed an averaged sea ice albedo of α i = 0.64 ± 0.2 (± standard deviation). The mean sea ice albedo of the pack ice area in the western Weddell Sea was α i = 0.75 ± 0.05. In the southern Weddell Sea, where new, young sea ice prevailed, a mean albedo value of α i = 0.38 ± 0.08 was observed. Relatively warm open water and thin, newly formed ice had the lowest albedo values, whereas relatively cold and snow covered pack ice had the highest albedo values. All sea ice areas consisted of a mixture of a large range of different sea ice types. An investigation of commonly used parameterizations of albedo as a function of surface temperature in the Weddell and Bellingshausen Sea ice areas showed that the albedo parameterizations do not work well for areas with new, young ice.

Description: Seasonal speed-up of two outlet glaciers of Austfonna, Svalbard, inferred from continuous GPS measurements The Cryosphere, 6, 453-466, 2012 Author(s): T. Dunse, T. V. Schuler, J. O. Hagen, and C. H. Reijmer A large part of the ice discharge from ice caps and ice sheets occurs through spatially limited flow units that may operate in a mode of steady flow or cyclic surge behaviour. Changes in the dynamics of distinct flow units play a key role in the mass balance of Austfonna, the largest ice cap on Svalbard. The recent net mass loss of Austfonna was dominated by calving from marine terminating outlet glaciers. Previous ice-surface velocity maps of the ice cap were derived by satellite radar interferometry (InSAR) and rely on data acquired in the mid-1990s with limited information concerning the temporal variability. Here, we present continuous Global Positioning System (GPS) observations along the central flowlines of two fast flowing outlet glaciers over 2008–2010. The data show prominent summer speed-ups with ice-surface velocities as high as 240% of the pre-summer mean. Acceleration follows the onset of the summer melt period, indicating enhanced basal motion due to input of surface meltwater into the subglacial drainage system. In 2008, multiple velocity peaks coincide with successive melt periods. In 2009, the major melt was of higher amplitude than in 2008. Flow velocities appear unaffected by subsequent melt periods, suggesting a transition towards a hydraulically more efficient drainage system. The observed annual mean velocities of Duvebreen and Basin-3 exceed those from the mid-1990s by factors two and four, respectively, implying increased ice discharge at the calving front. Measured summer velocities up to 2 m d −1 for Basin-3 are close to those of Kronebreen, often referred to as the fastest glacier on Svalbard.

Description: Repeat optical satellite images reveal widespread and long term decrease in land-terminating glacier speeds The Cryosphere, 6, 467-478, 2012 Author(s): T. Heid and A. Kääb By matching of repeat optical satellite images it is now possible to investigate glacier dynamics within large regions of the world and also between regions to improve knowledge about glacier dynamics in space and time. In this study we investigate whether the negative glacier mass balance seen over large parts of the world has caused the glaciers to change their speeds. The studied regions are Pamir, Caucasus, Penny Ice Cap, Alaska Range and Patagonia. In addition we derive speed changes for Karakoram, a region assumed to have positive mass balance and that contains many surge-type glaciers. We find that the mapped glaciers in the five regions with negative mass balance have over the last decades decreased their velocity at an average rate per decade of: 43 % in the Pamir, 8 % in the Caucasus, 25 % on Penny Ice Cap, 11 % in the Alaska Range and 20 % in Patagonia. Glaciers in Karakoram have generally increased their speeds, but surging glaciers and glaciers with flow instabilities are most prominent in this area. Therefore the calculated average speed change is not representative for this area.

Description: Remote sensing of sea ice: advances during the DAMOCLES project The Cryosphere, 6, 1411-1434, 2012 Author(s): G. Heygster, V. Alexandrov, G. Dybkjær, W. von Hoyningen-Huene, F. Girard-Ardhuin, I. L. Katsev, A. Kokhanovsky, T. Lavergne, A. V. Malinka, C. Melsheimer, L. Toudal Pedersen, A. S. Prikhach, R. Saldo, R. Tonboe, H. Wiebe, and E. P. Zege In the Arctic, global warming is particularly pronounced so that we need to monitor its development continuously. On the other hand, the vast and hostile conditions make in situ observation difficult, so that available satellite observations should be exploited in the best possible way to extract geophysical information. Here, we give a résumé of the sea ice remote sensing efforts of the European Union's (EU) project DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies). In order to better understand the seasonal variation of the microwave emission of sea ice observed from space, the monthly variations of the microwave emissivity of first-year and multi-year sea ice have been derived for the frequencies of the microwave imagers like AMSR-E (Advanced Microwave Scanning Radiometer on EOS) and sounding frequencies of AMSU (Advanced Microwave Sounding Unit), and have been used to develop an optimal estimation method to retrieve sea ice and atmospheric parameters simultaneously. In addition, a sea ice microwave emissivity model has been used together with a thermodynamic model to establish relations between the emissivities from 6 GHz to 50 GHz. At the latter frequency, the emissivity is needed for assimilation into atmospheric circulation models, but is more difficult to observe directly. The size of the snow grains on top of the sea ice influences both its albedo and the microwave emission. A method to determine the effective size of the snow grains from observations in the visible range (MODIS) is developed and demonstrated in an application on the Ross ice shelf. The bidirectional reflectivity distribution function (BRDF) of snow, which is an essential input parameter to the retrieval, has been measured in situ on Svalbard during the DAMOCLES campaign, and a BRDF model assuming aspherical particles is developed. Sea ice drift and deformation is derived from satellite observations with the scatterometer ASCAT (62.5 km grid spacing), with visible AVHRR observations (20 km), with the synthetic aperture radar sensor ASAR (10 km), and a multi-sensor product (62.5 km) with improved angular resolution (Continuous Maximum Cross Correlation, CMCC method) is presented. CMCC is also used to derive the sea ice deformation, important for formation of sea ice leads (diverging deformation) and pressure ridges (converging). The indirect determination of sea ice thickness from altimeter freeboard data requires knowledge of the ice density and snow load on sea ice. The relation between freeboard and ice thickness is investigated based on the airborne Sever expeditions conducted between 1928 and 1993.

Description: Ikaite crystals in melting sea ice – implications for p CO 2 and pH levels in Arctic surface waters The Cryosphere, 6, 901-908, 2012 Author(s): S. Rysgaard, R. N. Glud, K. Lennert, M. Cooper, N. Halden, R. J. G. Leakey, F. C. Hawthorne, and D. Barber A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air–sea CO 2 exchange. This has been complicated by the recent discoveries of ikaite (a polymorph of CaCO 3 ·6H 2 O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO 2 and pH conditions in surface waters. Here, we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from a melting 1.7 km 2 (0.5–1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice floe thickness by 0.2 m per week and resulted in an estimated 3.8 ppm decrease of p CO 2 in the ocean surface mixed layer. This corresponds to an air–sea CO 2 uptake of 10.6 mmol m −2 sea ice d −1 or to 3.3 ton km −2 ice floe week −1 . This is markedly higher than the estimated primary production within the ice floe of 0.3–1.3 mmol m −2 sea ice d −1 . Finally, the presence of ikaite in sea ice and the dissolution of the mineral during melting of the sea ice and mixing of the melt water into the surface oceanic mixed layer accounted for half of the estimated p CO 2 uptake.

Description: Drifting snow climate of the Greenland ice sheet: a study with a regional climate model The Cryosphere, 6, 891-899, 2012 Author(s): J. T. M. Lenaerts, M. R. van den Broeke, J. H. van Angelen, E. van Meijgaard, and S. J. Déry This paper presents the drifting snow climate of the Greenland ice sheet, using output from a high-resolution (∼11 km) regional climate model. Because reliable direct observations of drifting snow do not exist, we evaluate the modeled near-surface climate instead, using automatic weather station (AWS) observations from the K-transect and find that RACMO2 realistically simulates near-surface wind speed and relative humidity, two variables that are important for drifting snow. Integrated over the ice sheet, drifting snow sublimation (SU ds ) equals 24 ± 3 Gt yr −1 , and is significantly larger than surface sublimation (SU s , 16 ± 2 Gt yr −1 ). SU ds strongly varies between seasons, and is only important in winter, when surface sublimation and runoff are small. A rapid transition exists between the winter season, when snowfall and SU ds are important, and the summer season, when snowmelt is significant, which increases surface snow density and thereby limits drifting snow processes. Drifting snow erosion (ER ds ) is only important on a regional scale. In recent decades, following decreasing wind speed and rising near-surface temperatures, SU ds exhibits a negative trend (0.1 ± 0.1 Gt yr −1 ), which is compensated by an increase in SU s of similar magnitude.

Description: Antarctic sea ice variability and trends, 1979–2010 The Cryosphere, 6, 871-880, 2012 Author(s): C. L. Parkinson and D. J. Cavalieri In sharp contrast to the decreasing sea ice coverage of the Arctic, in the Antarctic the sea ice cover has, on average, expanded since the late 1970s. More specifically, satellite passive-microwave data for the period November 1978–December 2010 reveal an overall positive trend in ice extents of 17 100 ± 2300 km 2 yr −1 . Much of the increase, at 13 700 ± 1500 km 2 yr −1 , has occurred in the region of the Ross Sea, with lesser contributions from the Weddell Sea and Indian Ocean. One region, that of the Bellingshausen/Amundsen Seas, has (like the Arctic) instead experienced significant sea ice decreases, with an overall ice extent trend of −8200 ± 1200 km 2 yr −1 . When examined through the annual cycle over the 32-yr period 1979–2010, the Southern Hemisphere sea ice cover as a whole experienced positive ice extent trends in every month, ranging in magnitude from a low of 9100 ± 6300 km 2 yr −1 in February to a high of 24 700 ± 10 000 km 2 yr −1 in May. The Ross Sea and Indian Ocean also had positive trends in each month, while the Bellingshausen/Amundsen Seas had negative trends in each month, and the Weddell Sea and western Pacific Ocean had a mixture of positive and negative trends. Comparing ice-area results to ice-extent results, in each case the ice-area trend has the same sign as the ice-extent trend, but the magnitudes of the two trends differ, and in some cases these differences allow inferences about the corresponding changes in sea ice concentrations. The strong pattern of decreasing ice coverage in the Bellingshausen/Amundsen Seas region and increasing ice coverage in the Ross Sea region is suggestive of changes in atmospheric circulation. This is a key topic for future research.

Description: Improved modelling of Siberian river flow through the use of an alternative frozen soil hydrology scheme in a land surface model The Cryosphere, 6, 859-870, 2012 Author(s): D. L. Finney, E. Blyth, and R. Ellis A parameterisation to incorporate the effects of frozen soil on modelled hydrology is described and implemented within a land surface model, the Joint UK Land Surface Environment Simulator. It is shown to generally improve the modelled flow of Siberian rivers compared to observations, specifically in seasons of freezing or thawing soil. Most noticeably, the revised model increases the snowmelt flow peak by 26–100% compared to the control model, thereby better matching observed flows. The model physics resulting in the changes to river flow are discussed and attention is given to the effect of inaccuracies in snowfall driving data which can hinder the comparison of new model processes.

Description: Arctic sea ice variability and trends, 1979–2010 The Cryosphere, 6, 881-889, 2012 Author(s): D. J. Cavalieri and C. L. Parkinson Analyses of 32 yr (1979–2010) of Arctic sea ice extents and areas derived from satellite passive microwave radiometers are presented for the Northern Hemisphere as a whole and for nine Arctic regions. There is an overall negative yearly trend of −51.5 ± 4.1 × 10 3 km 2 yr −1 (−4.1 ± 0.3% decade −1 ) in sea ice extent for the hemisphere. The yearly sea ice extent trends for the individual Arctic regions are all negative except for the Bering Sea: −3.9 ± 1.1 × 10 3 km 2 yr −1 (−8.7 ± 2.5% decade −1 ) for the Seas of Okhotsk and Japan, +0.3 ± 0.8 × 10 3 km 2 yr −1 (+1.2 ± 2.7% decade −1 ) for the Bering Sea, −4.4 ± 0.7 × 10 3 km 2 yr −1 (−5.1 ± 0.9% decade −1 ) for Hudson Bay, −7.6 ± 1.6 × 10 3 km 2 yr −1 (−8.5 ± 1.8% decade −1 ) for Baffin Bay/Labrador Sea, −0.5 ± 0.3 × 10 3 km 2 yr −1 (−5.9 ± 3.5% decade −1 ) for the Gulf of St. Lawrence, −6.5 ± 1.1 × 10 3 km 2 yr −1 (−8.6 ± 1.5% decade −1 ) for the Greenland Sea, −13.5 ± 2.3 × 10 3 km 2 yr −1 (−9.2 ± 1.6% decade −1 ) for the Kara and Barents Seas, −14.6 ± 2.3 × 10 3 km 2 yr −1 (−2.1 ± 0.3% decade −1 ) for the Arctic Ocean, and −0.9 ± 0.4 × 10 3 km 2 yr −1 (−1.3 ± 0.5% decade −1 ) for the Canadian Archipelago. Similarly, the yearly trends for sea ice areas are all negative except for the Bering Sea. On a seasonal basis for both sea ice extents and areas, the largest negative trend is observed for summer with the next largest negative trend being for autumn. Both the sea ice extent and area trends vary widely by month depending on region and season. For the Northern Hemisphere as a whole, all 12 months show negative sea ice extent trends with a minimum magnitude in May and a maximum magnitude in September, whereas the corresponding sea ice area trends are smaller in magnitude and reach minimum and maximum values in March and September.

Description: Borehole temperatures reveal details of 20th century warming at Bruce Plateau, Antarctic Peninsula The Cryosphere, 6, 675-686, 2012 Author(s): V. Zagorodnov, O. Nagornov, T. A. Scambos, A. Muto, E. Mosley-Thompson, E. C. Pettit, and S. Tyuflin Two ice core boreholes of 143.18 m and 447.73 m (bedrock) were drilled during the 2009–2010 austral summer on the Bruce Plateau at a location named LARISSA Site Beta (66°02' S, 64°04' W, 1975.5 m a.s.l.). Both boreholes were logged with thermistors shortly after drilling. The shallow borehole was instrumented for 4 months with a series of resistance thermometers with satellite uplink. Surface temperature proxy data derived from an inversion of the borehole temperature profiles are compared to available multi-decadal records from weather stations and ice cores located along a latitudinal transect of the Antarctic Peninsula to West Antarctica. The LARISSA Site Beta profiles show temperatures decreasing from the surface downward through the upper third of the ice, and warming thereafter to the bed. The average temperature for the most recent year is −14.78°C (measured at 15 m depth, abbreviated T 15 ). A minimum temperature of −15.8°C is measured at 173 m depth, and basal temperature is estimated to be −10.2°C. Current mean annual temperature and the gradient in the lower part of the measured temperature profile have a best fit with an accumulation rate of 1.9×10 3 kg m −2 a −1 and basal heat flux ( q ) of 88 mW m −2 , if steady-state conditions are assumed. However, the mid-level temperature variations show that recent temperature has varied significantly. Reconstructed surface temperatures ( T s = T 15 ) over the last 200 yr are derived by an inversion technique (Tikhonov and Samarskii, 1990). From this, we find that cold temperatures (minimum T s =−16.2°C) prevailed from ~1920 to ~1940, followed by a gradual rise of temperature to −14.2°C around 1995, then cooling over the following decade and warming in the last few years. The coldest period was preceded by a relatively warm 19th century at T 15 ≥−15°C. To facilitate regional comparisons of the surface temperature history, we use our T 15 data and nearby weather station records to refine estimates of lapse rates (altitudinal, adjusted for latitude: Γ a(l) ). Good temporal and spatial consistency of Γ a(l) over the last 35 yr are observed, implying that the climate trends observed here are regional and consistent over a broad altitude range.

Description: Statistical adaptation of ALADIN RCM outputs over the French Alps – application to future climate and snow cover The Cryosphere, 6, 785-805, 2012 Author(s): M. Rousselot, Y. Durand, G. Giraud, L. Mérindol, I. Dombrowski-Etchevers, M. Déqué, and H. Castebrunet In this study, snowpack scenarios are modelled across the French Alps using dynamically downscaled variables from the ALADIN Regional Climate Model (RCM) for the control period (1961–1990) and three emission scenarios (SRES B1, A1B and A2) for the mid- and late 21st century (2021–2050 and 2071–2100). These variables are statistically adapted to the different elevations, aspects and slopes of the Alpine massifs. For this purpose, we use a simple analogue criterion with ERA40 series as well as an existing detailed climatology of the French Alps (Durand et al., 2009a) that provides complete meteorological fields from the SAFRAN analysis model. The resulting scenarios of precipitation, temperature, wind, cloudiness, longwave and shortwave radiation, and humidity are used to run the physical snow model CROCUS and simulate snowpack evolution over the massifs studied. The seasonal and regional characteristics of the simulated climate and snow cover changes are explored, as is the influence of the scenarios on these changes. Preliminary results suggest that the snow water equivalent (SWE) of the snowpack will decrease dramatically in the next century, especially in the Southern and Extreme Southern parts of the Alps. This decrease seems to result primarily from a general warming throughout the year, and possibly a deficit of precipitation in the autumn. The magnitude of the snow cover decline follows a marked altitudinal gradient, with the highest altitudes being less exposed to climate change. Scenario A2, with its high concentrations of greenhouse gases, results in a SWE reduction roughly twice as large as in the low-emission scenario B1 by the end of the century. This study needs to be completed using simulations from other RCMs, since a multi-model approach is essential for uncertainty analysis.

Description: Sensitivity of basal conditions in an inverse model: Vestfonna ice cap, Nordaustlandet/Svalbard The Cryosphere, 6, 771-783, 2012 Author(s): M. Schäfer, T. Zwinger, P. Christoffersen, F. Gillet-Chaulet, K. Laakso, R. Pettersson, V. A. Pohjola, T. Strozzi, and J. C. Moore The dynamics of Vestfonna ice cap (Svalbard) are dominated by fast-flowing outlet glaciers. Its mass balance is poorly known and affected dynamically by these fast-flowing outlet glaciers. Hence, it is a challenging target for ice flow modeling. Precise knowledge of the basal conditions and implementation of a good sliding law are crucial for the modeling of this ice cap. Here we use the full-Stokes finite element code Elmer/Ice to model the 3-D flow over the whole ice cap. We use a Robin inverse method to infer the basal friction from the surface velocities observed in 1995. Our results illustrate the importance of the basal friction parameter in reproducing observed velocity fields. We also show the importance of having variable basal friction as given by the inverse method to reproduce the velocity fields of each outlet glacier – a simple parametrization of basal friction cannot give realistic velocities in a forward model. We study the robustness and sensitivity of this method with respect to different parameters (mesh characteristics, ice temperature, errors in topographic and velocity data). The uncertainty in the observational parameters and input data proved to be sufficiently small as not to adversely affect the fidelity of the model.

Description: Glacier dynamics over the last quarter of a century at Helheim, Kangerdlugssuaq and 14 other major Greenland outlet glaciers The Cryosphere, 6, 923-937, 2012 Author(s): S. L. Bevan, A. J. Luckman, and T. Murray The Greenland ice sheet is experiencing increasing rates of mass loss, the majority of which results from changes in discharge from tidewater glaciers. Both atmospheric and ocean drivers have been implicated in these dynamic changes, but understanding the nature of the response has been hampered by the lack of measurements of glacier flow rates predating the recent period of warming. Here, using Landsat-5 data from 1985 onwards, we extend back in time the record of surface velocities and ice-front position for 16 of Greenland's fastest-flowing tidewater glaciers, and compare these to more recent data from Landsat-7 and satellite-borne synthetic-aperture radar. Climate re-analysis data and sea surface temperatures from 1982 show that since 1995 most of Greenland and its surrounding oceans have experienced significant overall warming, and a switch to a warming trend. During the period from 1985 to 1995 when Greenland and the surrounding oceans were not warming, major tidewater outlet glaciers around Greenland, including Kangerdlugssuaq and Helheim, were dynamically stable. Since the mid-1990s, glacier discharge has consistently been both greater and more variable. Together, these observations support the hypothesis that recent dynamic change is a rapid response to climate forcing. Both air and ocean temperatures in this region are predicted to continue to warm, and will therefore likely drive further change in outlet glacier discharge.

Description: A simple inverse method for the distribution of basal sliding coefficients under ice sheets, applied to Antarctica The Cryosphere, 6, 953-971, 2012 Author(s): D. Pollard and R. M. DeConto Variations in intrinsic bed conditions that affect basal sliding, such as the distribution of deformable sediment versus hard bedrock, are important boundary conditions for large-scale ice-sheet models, but are hard to observe and remain largely uncertain below the modern Greenland and Antarctic ice sheets. Here a very simple model-based method is described for deducing the modern spatial distribution of basal sliding coefficients. The model is run forward in time, and the basal sliding coefficient at each grid point is periodically increased or decreased depending on whether the local ice surface elevation is too high or too low compared to observed in areas of unfrozen bed. The method considerably reduces large-scale errors in Antarctic ice elevation, from several 100s to several 10s of meters in most regions. Remaining ice elevation errors over mountain ranges such as the Transantarctics are further improved by parameterizing the possible effect of sub-grid topography in the basal sliding law, representing sliding in deep valleys. Results are compared with modern velocity data, and various sensitivity tests are described in Appendices.

Description: 3-D image-based numerical computations of snow permeability: links to specific surface area, density, and microstructural anisotropy The Cryosphere, 6, 939-951, 2012 Author(s): N. Calonne, C. Geindreau, F. Flin, S. Morin, B. Lesaffre, S. Rolland du Roscoat, and P. Charrier We used three-dimensional (3-D) images of snow microstructure to carry out numerical estimations of the full tensor of the intrinsic permeability of snow ( K ). This study was performed on 35 snow samples, spanning a wide range of seasonal snow types. For several snow samples, a significant anisotropy of permeability was detected and is consistent with that observed for the effective thermal conductivity obtained from the same samples. The anisotropy coefficient, defined as the ratio of the vertical over the horizontal components of K , ranges from 0.74 for a sample of decomposing precipitation particles collected in the field to 1.66 for a depth hoar specimen. Because the permeability is related to a characteristic length, we introduced a dimensionless tensor K *= K / r es 2 , where the equivalent sphere radius of ice grains ( r es ) is computed from the specific surface area of snow (SSA) and the ice density (ρ i ) as follows: r es =3/(SSA×ρ i . We define K and K * as the average of the diagonal components of K and K *, respectively. The 35 values of K * were fitted to snow density (ρ s ) and provide the following regression: K = (3.0 ± 0.3) r es 2 exp((−0.0130 ± 0.0003)ρ s ). We noted that the anisotropy of permeability does not affect significantly the proposed equation. This regression curve was applied to several independent datasets from the literature and compared to other existing regression curves or analytical models. The results show that it is probably the best currently available simple relationship linking the average value of permeability, K , to snow density and specific surface area.

Description: Evaluation of the criticality of cracks in ice shelves using finite element simulations The Cryosphere, 6, 973-984, 2012 Author(s): C. Plate, R. Müller, A. Humbert, and D. Gross The ongoing disintegration of large ice shelf parts in Antarctica raise the need for a better understanding of the physical processes that trigger critical crack growth in ice shelves. Finite elements in combination with configurational forces facilitate the analysis of single surface fractures in ice under various boundary conditions and material parameters. The principles of linear elastic fracture mechanics are applied to show the strong influence of different depth dependent functions for the density and the Young's modulus on the stress intensity factor K I at the crack tip. Ice, for this purpose, is treated as an elastically compressible solid and the consequences of this choice in comparison to the predominant incompressible approaches are discussed. The computed stress intensity factors K I for dry and water filled cracks are compared to critical values K Ic from measurements that can be found in literature.

Description: Limitations of a coupled regional climate model in the reproduction of the observed Arctic sea-ice retreat The Cryosphere, 6, 985-998, 2012 Author(s): W. Dorn, K. Dethloff, and A. Rinke The effects of internal model variability on the simulation of Arctic sea-ice extent and volume have been examined with the aid of a seven-member ensemble with a coupled regional climate model for the period 1948–2008. Beyond general weaknesses related to insufficient representation of feedback processes, it is found that the model's ability to reproduce observed summer sea-ice retreat depends mainly on two factors: the correct simulation of the atmospheric circulation during the summer months and the sea-ice volume at the beginning of the melting period. Since internal model variability shows its maximum during the summer months, the ability to reproduce the observed atmospheric summer circulation is limited. In addition, the atmospheric circulation during summer also significantly affects the sea-ice volume over the years leading to a limited ability to start with reasonable sea-ice volume into the melting period. Furthermore, the sea-ice volume pathway shows notable decadal variability that varies in amplitude among the ensemble members. The scatter is particularly large in periods when the ice volume increases, indicating limited skill in reproducing high-ice years.

Description: Thin-layer effects in glaciological seismic amplitude-versus-angle (AVA) analysis: implications for characterising a subglacial till unit, Russell Glacier, West Greenland The Cryosphere, 6, 909-922, 2012 Author(s): A. D. Booth, R. A. Clark, B. Kulessa, T. Murray, J. Carter, S. Doyle, and A. Hubbard Seismic amplitude-versus-angle (AVA) methods are a powerful means of quantifying the physical properties of subglacial material, but serious interpretative errors can arise when AVA is measured over a thinly-layered substrate. A substrate layer with a thickness less than 1/4 of the seismic wavelength, λ, is considered "thin", and reflections from its bounding interfaces superpose and appear in seismic data as a single reflection event. AVA interpretation of subglacial till can be vulnerable to such thin-layer effects, since a lodged (non-deforming) till can be overlain by a thin (metre-scale) cap of dilatant (deforming) till. We assess the potential for misinterpretation by simulating seismic data for a stratified subglacial till unit, with an upper dilatant layer between 0.1–5.0 m thick (λ / 120 to 〉 λ / 4, with λ = 12 m). For dilatant layers less than λ / 6 thick, conventional AVA analysis yields acoustic impedance and Poisson's ratio that indicate contradictory water saturation. A thin-layer interpretation strategy is proposed, that accurately characterises the model properties of the till unit. The method is applied to example seismic AVA data from Russell Glacier, West Greenland, in which characteristics of thin-layer responses are evident. A subglacial till deposit is interpreted, having lodged till (acoustic impedance = 4.26±0.59 × 10 6 kg m −2 s −1 ) underlying a water-saturated dilatant till layer (thickness 〈 2 m, Poisson's ratio ~ 0.5). Since thin-layer considerations offer a greater degree of complexity in an AVA interpretation, and potentially avoid misinterpretations, they are a valuable aspect of quantitative seismic analysis, particularly for characterising till units.

Description: Laboratory study of initial sea-ice growth: properties of grease ice and nilas The Cryosphere, 6, 729-741, 2012 Author(s): A. K. Naumann, D. Notz, L. Håvik, and A. Sirevaag We investigate initial sea-ice growth in an ice-tank study by freezing an NaCl solution of about 29 g kg −1 in three different setups: grease ice grew in experiments with waves and in experiments with a current and wind, while nilas formed in a quiescent experimental setup. In this paper we focus on the differences in bulk salinity, solid fraction and thickness between these two ice types. The bulk salinity of the grease-ice layer in our experiments remained almost constant until the ice began to consolidate. In contrast, the initial bulk-salinity evolution of the nilas is well described by a linear decrease of about 2.1 g kg −1 h −1 independent of air temperature. This rapid decrease can be qualitatively understood by considering a Rayleigh number that became maximum while the nilas was still less than 1 cm thick. Comparing three different methods to measure solid fraction in grease ice based on (a) salt conservation, (b) mass conservation and (c) energy conservation, we find that the method based on salt conservation does not give reliable results if the salinity of the interstitial water is approximated as being equal to the salinity of the underlying water. Instead the increase in salinity of the interstitial water during grease-ice formation must be taken into account. In our experiments, the solid fraction of grease ice was relatively constant with values of 0.25, whereas it increased to values as high as 0.50 as soon as the grease ice consolidated at its surface. In contrast, the solid fraction of the nilas increased continuously in the first hours of ice formation and reached an average value of 0.55 after 4.5 h. The spatially averaged ice thickness was twice as large in the first 24 h of ice formation in the setup with a current and wind compared to the other two setups, since the wind kept parts of the water surface ice free and therefore allowed for a higher heat loss from the water. The development of the ice thickness can be reproduced well with simple, one dimensional models that only require air temperature or ice surface temperature as input.

Description: Extrapolating glacier mass balance to the mountain-range scale: the European Alps 1900–2100 The Cryosphere, 6, 713-727, 2012 Author(s): M. Huss This study addresses the extrapolation of in-situ glacier mass balance measurements to the mountain-range scale and aims at deriving time series of area-averaged mass balance and ice volume change for all glaciers in the European Alps for the period 1900–2100. Long-term mass balance series for 50 Swiss glaciers based on a combination of field data and modelling, and WGMS data for glaciers in Austria, France and Italy are used. A complete glacier inventory is available for the year 2003. Mass balance extrapolation is performed based on (1) arithmetic averaging, (2) glacier hypsometry, and (3) multiple regression. Given a sufficient number of data series, multiple regression with variables describing glacier geometry performs best in reproducing observed spatial mass balance variability. Future mass changes are calculated by driving a combined model for mass balance and glacier geometry with GCM ensembles based on four emission scenarios. Mean glacier mass balance in the European Alps is −0.31 ± 0.04 m w.e. a −1 in 1900–2011, and −1 m w.e. a −1 over the last decade. Total ice volume change since 1900 is −96 ± 13 km 3 ; annual values vary between −5.9 km 3 (1947) and +3.9 km 3 (1977). Mean mass balances are expected to be around −1.3 m w.e. a −1 by 2050. Model results indicate a glacier area reduction of 4–18% relative to 2003 for the end of the 21st century.

Description: Spatial patterns of North Atlantic Oscillation influence on mass balance variability of European glaciers The Cryosphere, 6, 661-673, 2012 Author(s): B. Marzeion and A. Nesje We present and validate a set of minimal models of glacier mass balance variability. The most skillful model is then applied to reconstruct 7735 individual time series of mass balance variability for all glaciers in the European Alps and Scandinavia. Subsequently, we investigate the influence of atmospheric variability associated with the North Atlantic Oscillation (NAO) on the glaciers' mass balances. We find a spatial coherence in the glaciers' sensitivity to NAO forcing which is caused by regionally similar mechanisms relating the NAO forcing to the mass balance: in southwestern Scandinavia, winter precipitation causes a correlation of mass balances with the NAO. In northern Scandinavia, temperature anomalies outside the core winter season cause an anti-correlation between NAO and mass balances. In the western Alps, both temperature and winter precipitation anomalies lead to a weak anti-correlation of mass balances with the NAO, while in the eastern Alps, the influences of winter precipitation and temperature anomalies tend to cancel each other, and only on the southern side a slight anti-correlation of mass balances with the NAO prevails.

Description: Brief communication "Can recent ice discharges following the Larsen-B ice-shelf collapse be used to infer the driving mechanisms of millennial-scale variations of the Laurentide ice sheet?" The Cryosphere, 6, 687-693, 2012 Author(s): J. Alvarez-Solas, A. Robinson, and C. Ritz The effects of an ice-shelf collapse on inland glacier dynamics have recently been widely studied, especially since the breakup of the Antarctic Peninsula's Larsen-B ice shelf in 2002. Several studies have documented acceleration of the ice streams that were flowing into the former ice shelf. The mechanism responsible for such a speed-up lies with the removal of the ice-shelf backforce. Independently, it is also well documented that during the last glacial period, the Northern Hemisphere ice sheets experienced large discharges into the ocean, likely reflecting ice flow acceleration episodes on the millennial time scale. The classic interpretation of the latter is based on the existence of an internal thermo-mechanical feedback with the potential to generate oscillatory behavior in the ice sheets. Here we would like to widen the debate by considering that Larsen-B-like glacial analog episodes could have contributed significantly to the registered millennial-scale variablity.

Description: Impact of spatial resolution on the modelling of the Greenland ice sheet surface mass balance between 1990–2010, using the regional climate model MAR The Cryosphere, 6, 695-711, 2012 Author(s): B. Franco, X. Fettweis, C. Lang, and M. Erpicum With the aim to force an ice dynamical model, the Greenland ice sheet (GrIS) surface mass balance (SMB) was modelled at different spatial resolutions (15–50 km) for the period 1990–2010, using the regional climate model MAR (Modèle Atmosphérique Régional) forced by the ERA-INTERIM reanalysis. This comparison revealed that (i) the inter-annual variability of the SMB components is consistent within the different spatial resolutions investigated, (ii) the MAR model simulates heavier precipitation on average over the GrIS with decreasing spatial resolution, and (iii) the SMB components (except precipitation) can be derived from a simulation at lower resolution with an "intelligent" interpolation. This interpolation can also be used to approximate the SMB components over another topography/ice sheet mask of the GrIS. These results are important for the forcing of an ice dynamical model needed to enable future projections of the GrIS contribution to sea level rise over the coming centuries.

Description: Results of the Marine Ice Sheet Model Intercomparison Project, MISMIP The Cryosphere, 6, 573-588, 2012 Author(s): F. Pattyn, C. Schoof, L. Perichon, R. C. A. Hindmarsh, E. Bueler, B. de Fleurian, G. Durand, O. Gagliardini, R. Gladstone, D. Goldberg, G. H. Gudmundsson, P. Huybrechts, V. Lee, F. M. Nick, A. J. Payne, D. Pollard, O. Rybak, F. Saito, and A. Vieli Predictions of marine ice-sheet behaviour require models that are able to robustly simulate grounding line migration. We present results of an intercomparison exercise for marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no effects of lateral buttressing). Unique steady state grounding line positions exist for ice sheets on a downward sloping bed, while hysteresis occurs across an overdeepened bed, and stable steady state grounding line positions only occur on the downward-sloping sections. Models based on the shallow ice approximation, which does not resolve extensional stresses, do not reproduce the approximate analytical results unless appropriate parameterizations for ice flux are imposed at the grounding line. For extensional-stress resolving "shelfy stream" models, differences between model results were mainly due to the choice of spatial discretization. Moving grid methods were found to be the most accurate at capturing grounding line evolution, since they track the grounding line explicitly. Adaptive mesh refinement can further improve accuracy, including fixed grid models that generally perform poorly at coarse resolution. Fixed grid models, with nested grid representations of the grounding line, are able to generate accurate steady state positions, but can be inaccurate over transients. Only one full-Stokes model was included in the intercomparison, and consequently the accuracy of shelfy stream models as approximations of full-Stokes models remains to be determined in detail, especially during transients.

Description: An assessment of key model parametric uncertainties in projections of Greenland Ice Sheet behavior The Cryosphere, 6, 589-606, 2012 Author(s): P. J. Applegate, N. Kirchner, E. J. Stone, K. Keller, and R. Greve Lack of knowledge about the values of ice sheet model input parameters introduces substantial uncertainty into projections of Greenland Ice Sheet contributions to future sea level rise. Computer models of ice sheet behavior provide one of several means of estimating future sea level rise due to mass loss from ice sheets. Such models have many input parameters whose values are not well known. Recent studies have investigated the effects of these parameters on model output, but the range of potential future sea level increases due to model parametric uncertainty has not been characterized. Here, we demonstrate that this range is large, using a 100-member perturbed-physics ensemble with the SICOPOLIS ice sheet model. Each model run is spun up over 125 000 yr using geological forcings and subsequently driven into the future using an asymptotically increasing air temperature anomaly curve. All modeled ice sheets lose mass after 2005 AD. Parameters controlling surface melt dominate the model response to temperature change. After culling the ensemble to include only members that give reasonable ice volumes in 2005 AD, the range of projected sea level rise values in 2100 AD is ~40 % or more of the median. Data on past ice sheet behavior can help reduce this uncertainty, but none of our ensemble members produces a reasonable ice volume change during the mid-Holocene, relative to the present. This problem suggests that the model's exponential relation between temperature and precipitation does not hold during the Holocene, or that the central-Greenland temperature forcing curve used to drive the model is not representative of conditions around the ice margin at this time (among other possibilities). Our simulations also lack certain observed physical processes that may tend to enhance the real ice sheet's response. Regardless, this work has implications for other studies that use ice sheet models to project or hindcast the behavior of the Greenland Ice Sheet.

Description: Multi-decadal marine- and land-terminating glacier recession in the Ammassalik region, southeast Greenland The Cryosphere, 6, 625-639, 2012 Author(s): S. H. Mernild, J. K. Malmros, J. C. Yde, and N. T. Knudsen Landsat imagery was applied to elucidate glacier fluctuations of land- and marine-terminating outlet glaciers from the Greenland Ice Sheet (GrIS) and local land-terminating glaciers and ice caps (GIC) peripheral to the GrIS in the Ammassalik region, Southeast Greenland, during the period 1972–2011. Data from 21 marine-terminating glaciers (including the glaciers Helheim, Midgaard, and Fenris), the GrIS land-terminating margin, and 35 GIC were examined and compared to observed atmospheric air temperatures, precipitation, and reconstructed ocean water temperatures (at 400 m depth in the Irminger Sea). Here, we document that net glacier recession has occurred since 1972 in the Ammassalik region for all glacier types and sizes, except for three GIC. The land-terminating GrIS and GIC reflect lower marginal and areal changes than the marine-terminating outlet glaciers. The mean annual land-terminating GrIS and GIC margin recessions were about three to five times lower than the GrIS marine-terminating recession. The marine-terminating outlet glaciers had an average net frontal retreat for 1999–2011 of 0.098 km yr −1 , which was significantly higher than in previous sub-periods 1972–1986 and 1986–1999. For the marine-terminating GrIS, the annual areal recession rate has been decreasing since 1972, while increasing for the land-terminating GrIS since 1986. On average for all the observed GIC, a mean net frontal retreat for 1986–2011 of 0.010 ± 0.006 km yr −1 and a mean areal recession of around 1% per year occurred; overall for all observed GIC, a mean recession rate of 27 ± 24% occurred based on the 1986 GIC area. Since 1986, five GIC melted away in the Ammassalik area.

Description: Simulating melt, runoff and refreezing on Nordenskiöldbreen, Svalbard, using a coupled snow and energy balance model The Cryosphere, 6, 641-659, 2012 Author(s): W. J. J. van Pelt, J. Oerlemans, C. H. Reijmer, V. A. Pohjola, R. Pettersson, and J. H. van Angelen A distributed energy balance model is coupled to a multi-layer snow model in order to study the mass balance evolution and the impact of refreezing on the mass budget of Nordenskiöldbreen, Svalbard. The model is forced with output from the regional climate model RACMO and meteorological data from Svalbard Airport. Extensive calibration and initialisation are performed to increase the model accuracy. For the period 1989–2010, we find a mean net mass balance of −0.39 m w.e. a −1 . Refreezing contributes on average 0.27 m w.e. a −1 to the mass budget and is most pronounced in the accumulation zone. The simulated mass balance, radiative fluxes and subsurface profiles are validated against observations and are generally in good agreement. Climate sensitivity experiments reveal a non-linear, seasonally dependent response of the mass balance, refreezing and runoff to changes in temperature and precipitation. It is shown that including seasonality in climate change, with less pronounced summer warming, reduces the sensitivity of the mass balance and equilibrium line altitude (ELA) estimates in a future climate. The amount of refreezing is shown to be rather insensitive to changes in climate.

Description: Relation between surface topography and sea-salt snow chemistry from Princess Elizabeth Land, East Antarctica The Cryosphere, 6, 505-515, 2012 Author(s): K. Mahalinganathan, M. Thamban, C. M. Laluraj, and B. L. Redkar Previous studies on Antarctic snow have established an unambiguous correlation between variability of sea-salt records and site specific features like elevation and proximity to the sea. On the other hand, variations of Cl − /Na + ratios in snow have been attributed to the reaction mechanisms involving atmospheric acids. In the present study, the annual records of Na + , Cl − and SO 4 2− were investigated using snow cores along a 180 km coast to inland transect in Princess Elizabeth Land, East Antarctica. Exceptionally high Na + concentrations and large variations in Cl − /Na + ratios were observed up to 50 km (∼1100 m elevation) of the transect. The steepest slope in the entire transect (49.3 m km −1 ) was between 20 and 30 km and the sea-salt records in snow from this area revealed extensive modifications, with Cl − /Na + ratios as low as 0.2. Statistical analysis showed a strong association between the slope and variations in Cl − /Na + ratios along the transect ( r = −0.676, 99% confidence level). While distance from the coast accounted for some variability, the altitude by itself has no significant control over the sea-salt ion variability. However, the steep slopes influence the deposition of sea-salt aerosols in snow. The wind redistribution of snow due to the steep slopes on the coastal escarpment increases the concentration of Na + , resulting in a low Cl − /Na + ratios. We propose that the slope variations in the coastal regions of Antarctica could significantly influence the sea-salt chemistry of snow.

Description: Influence of surface and subsurface heterogeneity on observed borehole temperatures at a mountain permafrost site in the Upper Engadine, Swiss Alps The Cryosphere, 6, 517-531, 2012 Author(s): S. Schneider, M. Hoelzle, and C. Hauck Compared to lowland (polar) regions, permafrost in high mountain areas occurs in a large variety of surface and subsurface materials and textures. This work presents an eight-year (2002–2010) data set of borehole temperatures for five different (sub-) surface materials from a high alpine permafrost area, Murtèl-Corvatsch, Switzerland. The influence of the material on the thermal regime was investigated by borehole temperature data, the temperature at the top of the permafrost (TTOP-concept) and the apparent thermal diffusivity (ATD). The results show that during the last eight years, material-specific temperature changes were more significant than climate-induced temperature trends. At coarse blocky, ice-rich sites, no changes in active layer depth were observed, whereas the bedrock and the fine-grained sites appear to be highly sensitive to changes in the microclimate. The results confirm that the presence and growth of ice as well as a thermally driven air circulation within the subsurface are the key factors for the occurence and preservation of alpine permafrost.

Description: Thermal state of the active layer and permafrost along the Qinghai-Xizang (Tibet) Railway from 2006 to 2010 The Cryosphere, 6, 607-612, 2012 Author(s): Q. Wu, T. Zhang, and Y. Liu In this study, we investigated changes in active layer thickness (ALT) and permafrost temperatures at different depths using data from the permafrost monitoring network along the Qinghai-Xizang (Tibet) Railway (QXR) since 2005. Among these sites, mean ALT is ~3.1 m, with a range of ~1.1 to 5.9 m. From 2006 through 2010, ALT has increased at a rate of ~6.3 cm a −1 . The mean rate of permafrost temperature rise at the depth of 6.0 m is ~0.02 °C a −1 , estimated by linear regression using 5 yr of data, and the mean rate of mean annual ground temperature (MAGT) rise at a depth of zero amplitude is ~0.012 °C a −1 . Changes for colder permafrost (MAGT −1.0 °C). This is consistent with results observed in the Arctic and subarctic.

Description: Numerical modeling of permafrost dynamics in Alaska using a high spatial resolution dataset The Cryosphere, 6, 613-624, 2012 Author(s): E. E. Jafarov, S. S. Marchenko, and V. E. Romanovsky Climate projections for the 21st century indicate that there could be a pronounced warming and permafrost degradation in the Arctic and sub-Arctic regions. Climate warming is likely to cause permafrost thawing with subsequent effects on surface albedo, hydrology, soil organic matter storage and greenhouse gas emissions. To assess possible changes in the permafrost thermal state and active layer thickness, we implemented the GIPL2-MPI transient numerical model for the entire Alaska permafrost domain. The model input parameters are spatial datasets of mean monthly air temperature and precipitation, prescribed thermal properties of the multilayered soil column, and water content that are specific for each soil class and geographical location. As a climate forcing, we used the composite of five IPCC Global Circulation Models that has been downscaled to 2 by 2 km spatial resolution by Scenarios Network for Alaska Planning (SNAP) group. In this paper, we present the modeling results based on input of a five-model composite with A1B carbon emission scenario. The model has been calibrated according to the annual borehole temperature measurements for the State of Alaska. We also performed more detailed calibration for fifteen shallow borehole stations where high quality data are available on daily basis. To validate the model performance, we compared simulated active layer thicknesses with observed data from Circumpolar Active Layer Monitoring (CALM) stations. The calibrated model was used to address possible ground temperature changes for the 21st century. The model simulation results show widespread permafrost degradation in Alaska could begin between 2040–2099 within the vast area southward from the Brooks Range, except for the high altitude regions of the Alaska Range and Wrangell Mountains.

Description: A numerical model for meltwater channel evolution in glaciers The Cryosphere, 6, 493-503, 2012 Author(s): A. H. Jarosch and M. T. Gudmundsson Meltwater channels form an integral part of the hydrological system of a glacier. Better understanding of how meltwater channels develop and evolve is required to fully comprehend supraglacial and englacial meltwater drainage. Incision of supraglacial stream channels and subsequent roof closure by ice deformation has been proposed in recent literature as a possible englacial conduit formation process. Field evidence for supraglacial stream incision has been found in Svalbard and Nepal. In Iceland, where volcanic activity provides meltwater with temperatures above 0 °C, rapid enlargement of supraglacial channels has been observed. Supraglacial channels provide meltwater through englacial passages to the subglacial hydrological systems of big ice sheets, which in turn affects ice sheet motion and their contribution to eustatic sea level change. By coupling, for the first time, a numerical ice dynamic model to a hydraulic model which includes heat transfer, we investigate the evolution of meltwater channels and their incision behaviour. We present results for different, constant meltwater fluxes, different channel slopes, different meltwater temperatures, different melt rate distributions in the channel as well as temporal variations in meltwater flux. The key parameters governing incision rate and depth are channel slope, meltwater temperature loss to the ice and meltwater flux. Channel width and geometry are controlled by melt rate distribution along the channel wall. Calculated Nusselt numbers suggest that turbulent mixing is the main heat transfer mechanism in the meltwater channels studied.

Description: Brief communication Greenland's shrinking ice cover: "fast times" but not that fast The Cryosphere, 6, 533-537, 2012 Author(s): J. S. Kargel, A. P. Ahlstrøm, R. B. Alley, J. L. Bamber, T. J. Benham, J. E. Box, C. Chen, P. Christoffersen, M. Citterio, J. G. Cogley, H. Jiskoot, G. J. Leonard, P. Morin, T. Scambos, T. Sheldon, and I. Willis A map of Greenland in the 13th edition (2011) of the Times Comprehensive Atlas of the World made headlines because the publisher's media release mistakenly stated that the permanent ice cover had shrunk 15% since the previous 10th edition (1999) revision. The claimed shrinkage was immediately challenged by glaciologists, then retracted by the publisher. Here we show: (1) accurate maps of ice extent based on 1978/87 aerial surveys and recent MODIS imagery; and (2) shrinkage at 0.019% a −1 in ~50 000 km 2 of ice in a part of east Greenland that is shown as ice-free in the Times Atlas .

Description: Sensitivity of a distributed temperature-radiation index melt model based on AWS observations and surface energy balance fluxes, Hurd Peninsula glaciers, Livingston Island, Antarctica The Cryosphere, 6, 539-552, 2012 Author(s): U. Y. Jonsell, F. J. Navarro, M. Bañón, J. J. Lapazaran, and J. Otero We use an automatic weather station and surface mass balance dataset spanning four melt seasons collected on Hurd Peninsula Glaciers, South Shetland Islands, to investigate the point surface energy balance, to determine the absolute and relative contribution of the various energy fluxes acting on the glacier surface and to estimate the sensitivity of melt to ambient temperature changes. Long-wave incoming radiation is the main energy source for melt, while short-wave radiation is the most important flux controlling the variation of both seasonal and daily mean surface energy balance. Short-wave and long-wave radiation fluxes do, in general, balance each other, resulting in a high correspondence between daily mean net radiation flux and available melt energy flux. We calibrate a distributed melt model driven by air temperature and an expression for the incoming short-wave radiation. The model is calibrated with the data from one of the melt seasons and validated with the data of the three remaining seasons. The model results deviate at most 140 mm w.e. from the corresponding observations using the glaciological method. The model is very sensitive to changes in ambient temperature: a 0.5 °C increase results in 56 % higher melt rates.

Description: Use of a thermal imager for snow pit temperatures The Cryosphere, 6, 287-299, 2012 Author(s): C. Shea, B. Jamieson, and K. W. Birkeland Weak snow of interest to avalanche forecasting often forms and changes as thin layers. Thermometers, the current field technology for measuring the temperature gradients across such layers – and for thus estimating the expected vapour flux and future type of crystal metamorphism – are difficult to use at distances shorter than 1 cm. In contrast, a thermal imager can provide thousands of simultaneous temperature measurements across small distances with better accuracy. However, a thermal imager only senses the exposed surface, complicating its methods for access and accuracy of buried temperatures. This paper presents methods for exposing buried layers on pit walls and using a thermal imager to measure temperatures on these walls, correct for lens effects with snow, adjust temperature gradients, adjust time exposed, and calculate temperature gradients over millimetre distances. We find lens error on temperature gradients to be on the order of 0.03 °C between image centre and corners. We find temperature gradient change over time to usually decrease – as expected with atmospheric equalization as a strong effect. Case studies including thermal images and visual macro photographs of crystals, collected during the 2010–2011 winter, demonstrate large temperature differences over millimetre-scale distances that are consistent with observed kinetic metamorphism. Further study is needed to use absolute temperatures independently of supporting gradient data.

Description: Relative effect of slope and equilibrium line altitude on the retreat of Himalayan glaciers The Cryosphere, 6, 301-311, 2012 Author(s): T. N. Venkatesh, A. V. Kulkarni, and J. Srinivasan It has been observed that a majority of glaciers in the Himalayas have been retreating. In this paper, we show that there are two major factors which control the advance/retreat of the Himalayan glaciers. They are the slope of the glacier and changes in the equilibrium line altitude. While it is well known, that these factors are important, we propose a new way of combining them and use it to predict retreat. The functional form of this model has been derived from numerical simulations using an ice-flow code. The model has been successfully applied to the movement of eight Himalayan glaciers during the past 25 years. It explains why the Gangotri glacier is retreating while Zemu of nearly the same length is stationary, even if they are subject to similar environmental changes. The model has also been applied to a larger set of glaciers in the Parbati basin, for which retreat based on satellite data is available, though over a shorter time period.

Description: Inter-annual variations of snow days over Switzerland from 2000–2010 derived from MODIS satellite data The Cryosphere, 6, 331-342, 2012 Author(s): N. Foppa and G. Seiz Snow cover plays a vital role in the Swiss Alps and therefore it is of major interest to determine and understand its variability on different spatiotemporal scales. Within the activities of the National Climate Observing System (GCOS Switzerland) inter-annual variations of snow days over Switzerland were derived from 2000 to 2010 based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite. To minimize the impact of cloud cover on the MODIS snow product MOD10C1, we implemented a post-processing technique based on a forward and backward gap-filling approach. Using the proposed methodology it was possible to determine the total number of annual snow days over Switzerland from 2000 to 2010 (SCD MODIS ). The accuracy of the calculated snow days per year were quantitatively evaluated against three in situ snow observation sites representing different climatological regimes (SCD in_situ ). Various statistical indices were computed and analysed over the entire period. The overall accuracy between SCD MODIS and SCD in_situ on a daily basis over 10 yr is 88% to 94%, depending on the regional characteristics of each validation site. Differences between SCD MODIS and SCD in_situ vary during the snow accumulation period in autumn and smaller differences after spring, in particularly for the Central Alps.

Description: An algorithm to detect sea ice leads by using AMSR-E passive microwave imagery The Cryosphere, 6, 343-352, 2012 Author(s): J. Röhrs and L. Kaleschke Leads are major sites of energy fluxes and brine releases at the air-ocean interface of sea-ice covered oceans. This study presents an algorithm to detect leads wider than 3 km in the entire Arctic Ocean. The algorithm detects 50 % of the lead area that was visible in optical MODIS satellite images. Passive microwave imagery from the Advanced Microwave Scanning Radiometer – Earth Observation System (AMSR-E) is used, allowing daily observations due to the fact that AMSR-E does not depend on daylight or cloud conditions. Using the unique signatures of thin ice in the brightness temperature ratio between the 89 GHz and 19 GHz channels, the algorithm is able to detect thin ice areas in the ice cover and is optimized to detect leads. Leads are mapped for the period from 2002 to 2011 excluding the summer months, and validated qualitatively by using MODIS, Envisat ASAR, and CryoSat-2 data. Several frequently recurring large scale lead patterns are found, especially in regions where sea ice is known to drift out of the Arctic Ocean.

Description: Near-surface climate and surface energy budget of Larsen C ice shelf, Antarctic Peninsula The Cryosphere, 6, 353-363, 2012 Author(s): P. Kuipers Munneke, M. R. van den Broeke, J. C. King, T. Gray, and C. H. Reijmer Data collected by two automatic weather stations (AWS) on the Larsen C ice shelf, Antarctica, between 22 January 2009 and 1 February 2011 are analyzed and used as input for a model that computes the surface energy budget (SEB), which includes melt energy. The two AWSs are separated by about 70 km in the north–south direction, and both the near-surface meteorology and the SEB show similarities, although small differences in all components (most notably the melt flux) can be seen. The impact of subsurface absorption of shortwave radiation on melt and snow temperature is significant, and discussed. In winter, longwave cooling of the surface is entirely compensated by a downward turbulent transport of sensible heat. In summer, the positive net radiative flux is compensated by melt, and quite frequently by upward turbulent diffusion of heat and moisture, leading to sublimation and weak convection over the ice shelf. The month of November 2010 is highlighted, when strong westerly flow over the Antarctic Peninsula led to a dry and warm föhn wind over the ice shelf, resulting in warm and sunny conditions. Under these conditions the increase in shortwave and sensible heat fluxes is larger than the decrease of net longwave and latent heat fluxes, providing energy for significant melt.

Description: Corrigendum to "An algorithm to detect sea ice leads by using AMSR-E passive microwave imagery" published in The Cryosphere, 6, 343–352, 2012 The Cryosphere, 6, 365-365, 2012 Author(s): J. Röhrs, L. Kaleschke, D. Bröhan, and P. K. Siligam No abstract available.

Description: Stable water isotopes of precipitation and firn cores from the northern Antarctic Peninsula region as a proxy for climate reconstruction The Cryosphere, 6, 313-330, 2012 Author(s): F. Fernandoy, H. Meyer, and M. Tonelli In order to investigate the climate variability in the northern Antarctic Peninsula region, this paper focuses on the relationship between stable isotope content of precipitation and firn, and main meteorological variables (air temperature, relative humidity, sea surface temperature, and sea ice extent). Between 2008 and 2010, we collected precipitation samples and retrieved firn cores from several key sites in this region. We conclude that the deuterium excess oscillation represents a robust indicator of the meteorological variability on a seasonal to sub-seasonal scale. Low absolute deuterium excess values and the synchronous variation of both deuterium excess and air temperature imply that the evaporation of moisture occurs in the adjacent Southern Ocean. The δ 18 O-air temperature relationship is complicated and significant only at a (multi)seasonal scale. Backward trajectory calculations show that air-parcels arriving at the region during precipitation events predominantly originate at the South Pacific Ocean and Bellingshausen Sea. These investigations will be used as a calibration for ongoing and future research in the area, suggesting that appropriate locations for future ice core research are located above 600 m a.s.l. We selected the Plateau Laclavere, Antarctic Peninsula as the most promising site for a deeper drilling campaign.

Description: Multi-scale validation of a new soil freezing scheme for a land-surface model with physically-based hydrology The Cryosphere, 6, 407-430, 2012 Author(s): I. Gouttevin, G. Krinner, P. Ciais, J. Polcher, and C. Legout Soil freezing is a major feature of boreal regions with substantial impact on climate. The present paper describes the implementation of the thermal and hydrological effects of soil freezing in the land surface model ORCHIDEE, which includes a physical description of continental hydrology. The new soil freezing scheme is evaluated against analytical solutions and in-situ observations at a variety of scales in order to test its numerical robustness, explore its sensitivity to parameterization choices and confront its performance to field measurements at typical application scales. Our soil freezing model exhibits a low sensitivity to the vertical discretization for spatial steps in the range of a few millimetres to a few centimetres. It is however sensitive to the temperature interval around the freezing point where phase change occurs, which should be 1 °C to 2 °C wide. Furthermore, linear and thermodynamical parameterizations of the liquid water content lead to similar results in terms of water redistribution within the soil and thermal evolution under freezing. Our approach does not allow firm discrimination of the performance of one approach over the other. The new soil freezing scheme considerably improves the representation of runoff and river discharge in regions underlain by permafrost or subject to seasonal freezing. A thermodynamical parameterization of the liquid water content appears more appropriate for an integrated description of the hydrological processes at the scale of the vast Siberian basins. The use of a subgrid variability approach and the representation of wetlands could help capture the features of the Arctic hydrological regime with more accuracy. The modeling of the soil thermal regime is generally improved by the representation of soil freezing processes. In particular, the dynamics of the active layer is captured with more accuracy, which is of crucial importance in the prospect of simulations involving the response of frozen carbon stocks to future warming. A realistic simulation of the snow cover and its thermal properties, as well as the representation of an organic horizon with specific thermal and hydrological characteristics, are confirmed to be a pre-requisite for a realistic modeling of the soil thermal dynamics in the Arctic.

Description: Longitudinal surface structures (flowstripes) on Antarctic glaciers The Cryosphere, 6, 383-391, 2012 Author(s): N. F. Glasser and G. H. Gudmundsson Longitudinal surface structures ("flowstripes") are common on many glaciers but their origin and significance are poorly understood. In this paper we present observations of the development of these longitudinal structures from four different Antarctic glacier systems; the Lambert Glacier/Amery Ice Shelf area, the Taylor and Ferrar Glaciers in the Ross Sea sector, Crane and Jorum Glaciers (ice-shelf tributary glaciers) on the Antarctic Peninsula, and the onset zone of a tributary to the Recovery Glacier Ice Stream in the Filchner Ice Shelf area. Mapping from optical satellite images demonstrates that longitudinal surface structures develop in two main situations: (1) as relatively wide flow stripes within glacier flow units and (2) as relatively narrow flow stripes where there is convergent flow around nunataks or at glacier confluence zones. Our observations indicate that the confluence features are narrower, sharper, and more clearly defined features. They are characterised by linear troughs or depressions on the ice surface and are much more common than the former type. Longitudinal surface structures within glacier flow units have previously been explained as the surface expression of localised bed perturbations but a universal explanation for those forming at glacier confluences is lacking. Here we propose that these features are formed at zones of ice acceleration and extensional flow at glacier confluences. We provide a schematic model for the development of longitudinal surface structures based on extensional flow that can explain their ridge and trough morphology as well as their down-ice persistence.

Description: Autonomous detection of calving-related seismicity at Kronebreen, Svalbard The Cryosphere, 6, 393-406, 2012 Author(s): A. Köhler, A. Chapuis, C. Nuth, J. Kohler, and C. Weidle We detect and cluster waveforms of seismic signals recorded close to the calving front of Kronebreen, Svalbard, to identify glacier-related seismic events and to investigate their relation to calving processes. Single-channel geophone data recorded over several months in 2009 and 2010 are combined with eleven days of direct visual observations of the glacier front. We apply a processing scheme which combines conventional seismic event detection using a sensitive trigger algorithm and unsupervised clustering of all detected signals based on their waveform characteristics by means of Self-Organizing Maps (SOMs). About 10% of the directly observed calving events close to the geophone (

Description: Thermal remote sensing of ice-debris landforms using ASTER: an example from the Chilean Andes The Cryosphere, 6, 367-382, 2012 Author(s): A. Brenning, M. A. Peña, S. Long, and A. Soliman Remote sensors face challenges in characterizing mountain permafrost and ground thermal conditions or mapping rock glaciers and debris-covered glaciers. We explore the potential of thermal imaging and in particular thermal inertia mapping in mountain cryospheric research, focusing on the relationships between ground surface temperatures and the presence of ice-debris landforms on one side and land surface temperature (LST) and apparent thermal inertia (ATI) on the other. In our case study we utilize ASTER daytime and nighttime imagery and in-situ measurements of near-surface ground temperature (NSGT) in the Mediterranean Andes during a snow-free and dry observation period in late summer. Spatial patterns of LST and NSGT were mostly consistent with each other both at daytime and at nighttime. Daytime LST over ice-debris landforms was decreased and ATI consequently increased compared to other debris surfaces under otherwise equal conditions, but NSGT showed contradictory results, which underlines the complexity and possible scale dependence of ATI in heterogeneous substrates with the presence of a thermal mismatch and a heat sink at depth. While our results demonstrate the utility of thermal imaging and ATI mapping in a mountain cryospheric context, further research is needed for a better interpretation of ATI patterns in complex thermophysical conditions.

Description: Kinematic first-order calving law implies potential for abrupt ice-shelf retreat The Cryosphere, 6, 273-286, 2012 Author(s): A. Levermann, T. Albrecht, R. Winkelmann, M. A. Martin, M. Haseloff, and I. Joughin Recently observed large-scale disintegration of Antarctic ice shelves has moved their fronts closer towards grounded ice. In response, ice-sheet discharge into the ocean has accelerated, contributing to global sea-level rise and emphasizing the importance of calving-front dynamics. The position of the ice front strongly influences the stress field within the entire sheet-shelf-system and thereby the mass flow across the grounding line. While theories for an advance of the ice-front are readily available, no general rule exists for its retreat, making it difficult to incorporate the retreat in predictive models. Here we extract the first-order large-scale kinematic contribution to calving which is consistent with large-scale observation. We emphasize that the proposed equation does not constitute a comprehensive calving law but represents the first-order kinematic contribution which can and should be complemented by higher order contributions as well as the influence of potentially heterogeneous material properties of the ice. When applied as a calving law, the equation naturally incorporates the stabilizing effect of pinning points and inhibits ice shelf growth outside of embayments. It depends only on local ice properties which are, however, determined by the full topography of the ice shelf. In numerical simulations the parameterization reproduces multiple stable fronts as observed for the Larsen A and B Ice Shelves including abrupt transitions between them which may be caused by localized ice weaknesses. We also find multiple stable states of the Ross Ice Shelf at the gateway of the West Antarctic Ice Sheet with back stresses onto the sheet reduced by up to 90 % compared to the present state.

Description: The impact of a seasonally ice free Arctic Ocean on the temperature, precipitation and surface mass balance of Svalbard The Cryosphere, 6, 35-50, 2012 Author(s): J. J. Day, J. L. Bamber, P. J. Valdes, and J. Kohler The observed decline in summer sea ice extent since the 1970s is predicted to continue until the Arctic Ocean is seasonally ice free during the 21st Century. This will lead to a much perturbed Arctic climate with large changes in ocean surface energy flux. Svalbard, located on the present day sea ice edge, contains many low lying ice caps and glaciers and is expected to experience rapid warming over the 21st Century. The total sea level rise if all the land ice on Svalbard were to melt completely is 0.02 m. The purpose of this study is to quantify the impact of climate change on Svalbard's surface mass balance (SMB) and to determine, in particular, what proportion of the projected changes in precipitation and SMB are a result of changes to the Arctic sea ice cover. To investigate this a regional climate model was forced with monthly mean climatologies of sea surface temperature (SST) and sea ice concentration for the periods 1961–1990 and 2061–2090 under two emission scenarios. In a novel forcing experiment, 20th Century SSTs and 21st Century sea ice were used to force one simulation to investigate the role of sea ice forcing. This experiment results in a 3.5 m water equivalent increase in Svalbard's SMB compared to the present day. This is because over 50 % of the projected increase in winter precipitation over Svalbard under the A1B emissions scenario is due to an increase in lower atmosphere moisture content associated with evaporation from the ice free ocean. These results indicate that increases in precipitation due to sea ice decline may act to moderate mass loss from Svalbard's glaciers due to future Arctic warming.

Description: Reformulating the full-Stokes ice sheet model for a more efficient computational solution The Cryosphere, 6, 21-34, 2012 Author(s): J. K. Dukowicz The first-order or Blatter-Pattyn ice sheet model, in spite of its approximate nature, is an attractive alternative to the full Stokes model in many applications because of its reduced computational demands. In contrast, the unapproximated Stokes ice sheet model is more difficult to solve and computationally more expensive. This is primarily due to the fact that the Stokes model is indefinite and involves all three velocity components, as well as the pressure, while the Blatter-Pattyn discrete model is positive-definite and involves just the horizontal velocity components. The Stokes model is indefinite because it arises from a constrained minimization principle where the pressure acts as a Lagrange multiplier to enforce incompressibility. To alleviate these problems we reformulate the full Stokes problem into an unconstrained, positive-definite minimization problem, similar to the Blatter-Pattyn model but without any of the approximations. This is accomplished by introducing a divergence-free velocity field that satisfies appropriate boundary conditions as a trial function in the variational formulation, thus dispensing with the need for a pressure. Such a velocity field is obtained by vertically integrating the continuity equation to give the vertical velocity as a function of the horizontal velocity components, as is in fact done in the Blatter-Pattyn model. This leads to a reduced system for just the horizontal velocity components, again just as in the Blatter-Pattyn model, but now without approximation. In the process we obtain a new, reformulated Stokes action principle as well as a novel set of Euler-Lagrange partial differential equations and boundary conditions. The model is also generalized from the common case of an ice sheet in contact with and sliding along the bed to other situations, such as to a floating ice shelf. These results are illustrated and validated using a simple but nontrivial Stokes flow problem involving a sliding ice sheet.

Description: Geochemical characterization of supraglacial debris via in situ and optical remote sensing methods: a case study in Khumbu Himalaya, Nepal The Cryosphere, 6, 85-100, 2012 Author(s): K. A. Casey, A. Kääb, and D. I. Benn Surface glacier debris samples and field spectra were collected from the ablation zones of Nepal Himalaya Ngozumpa and Khumbu glaciers in November and December 2009. Geochemical and mineral compositions of supraglacial debris were determined by X-ray diffraction and X-ray fluorescence spectroscopy. This composition data was used as ground truth in evaluating field spectra and satellite supraglacial debris composition and mapping methods. Satellite remote sensing methods for characterizing glacial surface debris include visible to thermal infrared hyper- and multispectral reflectance and emission signature identification, semi-quantitative mineral abundance indicies and spectral image composites. Satellite derived supraglacial debris mineral maps displayed the predominance of layered silicates, hydroxyl-bearing and calcite minerals on Khumbu Himalayan glaciers. Supraglacial mineral maps compared with satellite thermal data revealed correlations between glacier surface composition and glacier surface temperature. Glacier velocity displacement fields and shortwave, thermal infrared false color composites indicated the magnitude of mass flux at glacier confluences. The supraglacial debris mapping methods presented in this study can be used on a broader scale to improve, supplement and potentially reduce errors associated with glacier debris radiative property, composition, areal extent and mass flux quantifications.

Description: Basal crevasses in Larsen C Ice Shelf and implications for their global abundance The Cryosphere, 6, 113-123, 2012 Author(s): A. Luckman, D. Jansen, B. Kulessa, E. C. King, P. Sammonds, and D. I. Benn Basal crevasses extend upwards from the base of ice bodies and can penetrate more than halfway through the ice column under conditions found commonly on ice shelves. As a result, they may locally modify the exchange of mass and energy between ice shelf and ocean, and by altering the shelf's mechanical properties could play a fundamental role in ice shelf stability. Although early studies revealed that such features may be abundant on Antarctic ice shelves, their geometrical properties and spatial distribution has gained little attention. We investigate basal crevasses in Larsen C Ice Shelf using field radar survey, remote sensing and numerical modelling. We demonstrate that a group of features visible in MODIS imagery are the surface expressions of basal crevasses in the form of surface troughs, and find that basal crevasses can be generated as a result of stresses well downstream of the grounding line. We show that linear elastic fracture mechanics modelling is a good predictor of basal crevasse penetration height where stresses are predominantly tensile, and that measured surface trough depth does not always reflect this height, probably because of snow accumulation in the trough, marine ice accretion in the crevasse, or stress bridging from the surrounding ice. We conclude that all features visible in MODIS imagery of ice shelves and previously labelled simply as "crevasses", where they are not full thickness rifts, must be basal crevasse troughs, highlighting a fundamental structural property of many ice shelves that may have been previously overlooked.

Description: Comparison of MODIS-derived land surface temperatures with ground surface and air temperature measurements in continuous permafrost terrain The Cryosphere, 6, 51-69, 2012 Author(s): S. Hachem, C. R. Duguay, and M. Allard Obtaining high resolution records of surface temperature from satellite sensors is important in the Arctic because meteorological stations are scarce and widely scattered in those vast and remote regions. Surface temperature is the primary climatic factor that governs the existence, spatial distribution and thermal regime of permafrost which is a major component of the terrestrial cryosphere. Land Surface (skin) Temperatures (LST) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra and Aqua satellite platforms provide spatial estimates of near-surface temperature values. In this study, LST values from MODIS are compared to ground-based near-surface air ( T air ) and ground surface temperature (GST) measurements obtained from 2000 to 2008 at herbaceous and shrub tundra sites located in the continuous permafrost zone of Northern Québec, Nunavik, Canada, and of the North Slope of Alaska, USA. LSTs (temperatures at the surface materials-atmosphere interface) are found to be better correlated with T air (1–3 m above the ground) than with available GST (3–5 cm below the ground surface). As T air is most often used by the permafrost community, this study focused on this parameter. LSTs are in stronger agreement with T air during the snow cover season than in the snow free season. Combining Aqua and Terra LST-Day and LST-Nigh acquisitions into a mean daily value provides a large number of LST observations and a better overall agreement with T air . Comparison between mean daily LSTs and mean daily T air , for all sites and all seasons pooled together yields a very high correlation ( R = 0.97; mean difference (MD) = 1.8 °C; and standard deviation of MD (SD) = 4.0 °C). The large SD can be explained by the influence of surface heterogeneity within the MODIS 1 km 2 grid cells, the presence of undetected clouds and the inherent difference between LST and T air . Retrieved over several years, MODIS LSTs offer a great potential for monitoring surface temperature changes in high-latitude tundra regions and are a promising source of input data for integration into spatially-distributed permafrost models.

Description: A minimal model for reconstructing interannual mass balance variability of glaciers in the European Alps The Cryosphere, 6, 71-84, 2012 Author(s): B. Marzeion, M. Hofer, A. H. Jarosch, G. Kaser, and T. Mölg We present a minimal model of the glacier surface mass balance. The model relies solely on monthly precipitation and air temperatures as forcing. We first train the model individually for 15 glaciers with existing mass balance measurements. Based on a cross validation, we present a thorough assessment of the model's performance outside of the training period. The cross validation indicates that our model is robust, and our model's performance compares favorably to that from a less parsimonious model based on seasonal sensitivity characteristics. Then, the model is extended for application on glaciers without existing mass balance measurements. We cross validated the model again by withholding the mass balance information from each of the 15 glaciers above during the model training, in order to measure its performance on glaciers not included in the model training. This cross validation indicates that the model retains considerable skill even when applied on glaciers without mass balance measurements. As an exemplary application, the model is then used to reconstruct time series of interannual mass balance variability, covering the past two hundred years, for all glaciers in the European Alps contained in the extended format of the world glacier inventory. Based on this reconstruction, we present a spatially detailed attribution of the glaciers' mass balance variability to temperature and precipitation variability.

Description: A statistical approach to modelling permafrost distribution in the European Alps or similar mountain ranges The Cryosphere, 6, 125-140, 2012 Author(s): L. Boeckli, A. Brenning, S. Gruber, and J. Noetzli Estimates of permafrost distribution in mountain regions are important for the assessment of climate change effects on natural and human systems. In order to make permafrost analyses and the establishment of guidelines for e.g. construction or hazard assessment comparable and compatible between regions, one consistent and traceable model for the entire Alpine domain is required. For the calibration of statistical models, the scarcity of suitable and reliable information about the presence or absence of permafrost makes the use of large areas attractive due to the larger data base available. We present a strategy and method for modelling permafrost distribution of entire mountain regions and provide the results of statistical analyses and model calibration for the European Alps. Starting from an integrated model framework, two statistical sub-models are developed, one for debris-covered areas (debris model) and one for steep bedrock (rock model). They are calibrated using rock glacier inventories and rock surface temperatures. To support the later generalization to surface characteristics other than those available for calibration, so-called offset terms have been introduced into the model that allow doing this in a transparent and traceable manner. For the debris model a generalized linear mixed-effect model (GLMM) is used to predict the probability of a rock glacier being intact as opposed to relict. It is based on the explanatory variables mean annual air temperature (MAAT), potential incoming solar radiation (PISR) and the mean annual sum of precipitation (PRECIP), and achieves an excellent discrimination (area under the receiver-operating characteristic, AUROC = 0.91). Surprisingly, the probability of a rock glacier being intact is positively associated with increasing PRECIP for given MAAT and PISR conditions. The rock model is based on a linear regression and was calibrated with mean annual rock surface temperatures (MARST). The explanatory variables are MAAT and PISR. The linear regression achieves a root mean square error (RMSE) of 1.6 °C. The final model combines the two sub-models and accounts for the different scales used for model calibration. The modelling approach provides a theoretical basis for estimating mountain permafrost distribution over larger mountain ranges and can be expanded to more surface types and sub-models than considered, here. The analyses performed with the Alpine data set further provide quantitative insight into larger-area patterns as well as the model coefficients for a later spatial application. The transfer into a map-based product, however, requires further steps such as the definition of offset terms that usually contain a degree of subjectivity.

Description: A three-dimensional full Stokes model of the grounding line dynamics: effect of a pinning point beneath the ice shelf The Cryosphere, 6, 101-112, 2012 Author(s): L. Favier, O. Gagliardini, G. Durand, and T. Zwinger The West Antarctic ice sheet is confined by a large area of ice shelves, fed by inland ice through fast flowing ice streams. The dynamics of the grounding line, which is the line-boundary between grounded ice and the downstream ice shelf, has a major influence on the dynamics of the whole ice sheet. However, most ice sheet models use simplifications of the flow equations, as they do not include all the stress components, and are known to fail in their representation of the grounding line dynamics. Here, we present a 3-D full Stokes model of a marine ice sheet, in which the flow problem is coupled with the evolution of the upper and lower free surfaces, and the position of the grounding line is determined by solving a contact problem between the shelf/sheet lower surface and the bedrock. Simulations are performed using the open-source finite-element code Elmer/Ice within a parallel environment. The model's ability to cope with a curved grounding line and the effect of a pinning point beneath the ice shelf are investigated through prognostic simulations. Starting from a steady state, the sea level is slightly decreased to create a contact point between a seamount and the ice shelf. The model predicts a dramatic decrease of the shelf velocities, leading to an advance of the grounding line until both grounded zones merge together, during which an ice rumple forms above the contact area at the pinning point. Finally, we show that once the contact is created, increasing the sea level to its initial value does not release the pinning point and has no effect on the ice dynamics, indicating a stabilising effect of pinning points.

Description: Corrigendum to "The impact of a seasonally ice free Arctic Ocean on the temperature, precipitation and surface mass balance of Svalbard" published in The Cryosphere, 6, 35–50, 2012 The Cryosphere, 6, 141-141, 2012 Author(s): J. J. Day, J. L. Bamber, P. J. Valdes, and J. Kohler No abstract available.

Description: Derivation and analysis of a high-resolution estimate of global permafrost zonation The Cryosphere, 6, 221-233, 2012 Author(s): S. Gruber Permafrost underlies much of Earth's surface and interacts with climate, eco-systems and human systems. It is a complex phenomenon controlled by climate and (sub-) surface properties and reacts to change with variable delay. Heterogeneity and sparse data challenge the modeling of its spatial distribution. Currently, there is no data set to adequately inform global studies of permafrost. The available data set for the Northern Hemisphere is frequently used for model evaluation, but its quality and consistency are difficult to assess. Here, a global model of permafrost extent and dataset of permafrost zonation are presented and discussed, extending earlier studies by including the Southern Hemisphere, by consistent data and methods, by attention to uncertainty and scaling. Established relationships between air temperature and the occurrence of permafrost are re-formulated into a model that is parametrized using published estimates. It is run with a high-resolution (

Description: Influence of sea ice lead-width distribution on turbulent heat transfer between the ocean and the atmosphere The Cryosphere, 6, 143-156, 2012 Author(s): S. Marcq and J. Weiss Leads are linear-like structures of open water within the sea ice cover that develop as the result of fracturing due to divergence or shear. Through leads, air and water come into contact and directly exchange latent and sensible heat through convective processes driven by the large temperature and moisture differences between them. In the central Arctic, leads only cover 1 to 2% of the ocean during winter, but account for more than 70% of the upward heat fluxes. Furthermore, narrow leads (several meters) are more than twice as efficient at transmitting turbulent heat than larger ones (several hundreds of meters). We show that lead widths are power law distributed, P(X) ~ X −a with a 〉1, down to very small spatial scales (20 m or below). This implies that the open water fraction is by far dominated by very small leads. Using two classical formulations, which provide first order turbulence closure for the fetch-dependence of heat fluxes, we find that the mean heat fluxes (sensible and latent) over open water are up to 55% larger when considering the lead-width distribution obtained from a SPOT satellite image of the ice cover, compared to the situation where the open water fraction constitutes one unique large lead and the rest of the area is covered by ice, as it is usually considered in climate models at the grid scale. This difference may be even larger if we assume that the power law scaling of lead widths extends down to smaller (~1 m) scales. Such estimations may be a first step towards a subgrid scale parameterization of the spatial distribution of open water for heat fluxes calculations in ocean/sea ice coupled models.

Description: Changes in the marine-terminating glaciers of central east Greenland, 2000–2010 The Cryosphere, 6, 211-220, 2012 Author(s): K. M. Walsh, I. M. Howat, Y. Ahn, and E. M. Enderlin Marine-terminating outlet glaciers of the Greenland Ice Sheet have undergone substantial changes over the past decade. The synchronicity of these changes suggest a regional external forcing, such as changes in coastal ocean heat transport and/or increased surface melt and subglacial runoff. A distinct contrast in rates of ice front retreat has been observed between glaciers north and south of 69° N latitude on along the East Greenland coast. This latitude corresponds with the northward limit of subtropical waters carried by the Irminger Current, suggesting variability in ocean heat transport as the dominant forcing. Glacier surging, however, is yet another mechanism of change in this region. In order to provide further spatial and temporal constraint on glacier change across this important oceanographic transition zone, we construct time series of thinning, retreat and flow speed of 37 marine-terminating glaciers along the central east Greenland coast from 2000 to 2010. We assess this dataset for spatial and temporal patterns that may elucidate the mechanisms of glacier change. We confirm that glacial retreat, dynamical thinning, and acceleration have been more pronounced south of 69° N, with a high degree of variability along the Blosseville Coast and little inter-annual change in Scoresby Sound. Our results support the conclusion that variability in coastal ocean heat transport is the primary driver of regional glacier change, but that local factors, such as surging and/or individual glacier morphology, are overprinted on this regional signal.

Description: Cornice dynamics and meteorological control at Gruvefjellet, Central Svalbard The Cryosphere, 6, 157-171, 2012 Author(s): S. Vogel, M. Eckerstorfer, and H. H. Christiansen Cornice fall avalanches endanger life and infrastructure in Nybyen, a part of Svalbard's main settlement Longyearbyen, located at 78° N in the High Arctic. Thus, cornice dynamics – accretion, cracking and eventual failure – and their controlling meteorological factors were studied along the ridgeline of the Gruvefjellet plateau mountain above Nybyen in the period 2008–2010. Using two automatic time-lapse cameras and hourly meteorological data in combination with intensive field observations on the Gruvefjellet plateau, cornice process dynamics were investigated in larger detail than previously possible. Cornice accretion starts directly following the first snowfall in late September and October, and proceeds throughout the entire snow season under a wide range of air temperature conditions that the maritime winter climate of Svalbard provides. Cornice accretion is particularly controlled by distinct storm events, with a prevailing wind direction perpendicular to the ridge line and average wind speeds from 12 m s −1 . Particularly high wind speeds in excess of 30 m s −1 towards the plateau ridgeline lead to cornice scouring and reduce the cornice mass both vertically and horizontally. Induced by pronounced air temperature fluctuations which might reach above freezing and lead to midwinter rainfall events, tension cracks develop between the cornice mass and the plateau. Our measurements indicate a linear crack opening due to snow creep and tilt of the cornice around a pivot point. Four to five weeks elapsed between the first observations of a cornice crack until cornice failure. Throughout the two snow seasons studied, 180 cornice failures were recorded, of which 70 failures were categorized as distinctive cornice fall avalanches. A clear temporal pattern with the majority of cornice failures in June was found. Thus only daily air temperature could determine avalanche from non-avalanche days. Seven large cornice fall avalanches reached the avalanche fans on which the Nybyen settlement is located. The size of the avalanches was primarily determined by the size of the cornice that detached. The improved process understanding of the cornice dynamics provides a first step towards a better predictability of this natural hazard, but also highlights that any type of warning based on meteorological factors is not an adequate measure to ensure safety of the housing at risk.

Description: Laboratory study of frazil ice accumulation under wave conditions The Cryosphere, 6, 173-191, 2012 Author(s): S. De la Rosa and S. Maus Ice growth in turbulent seawater is often accompanied by the accumulation of frazil ice crystals at its surface, forming a grease ice layer. The thickness and volume fraction of this ice layer play an important role in shaping the gradual transition from a loose to a solid ice cover, however, observations are very sparse. Here we analyse an extensive set of observations of frazil ice, grown in two parallel tanks with controlled wave conditions and thermal forcing, focusing on the first one to two days of grease ice accumulation. The following unresolved issues are addressed: (i) at which volume fraction the frazil crystals' rising process starts and how densely they accumulate at the surface, (ii) how the grease ice solid fraction and salinity evolve with time until solid ice starts to form and (iii) how do these conditions affect, and are affected by, waves and heat loss from the ice. We obtained estimates of the minimum initial grease ice solid fraction (0.03–0.05) and the maximum solid fraction to which it accumulates before freezing into pancakes (0.23–0.31). The equivalent thickness of solid ice that needs to be accumulated until grease ice packs close to maximum (95% of the compaction accomplished), was estimated as 0.4 to 1.2 cm. Comparison of grease ice thickness and wave observations indicates that a grease ice layer first begins to affect the wave field significantly when its thickness exceeds the initial wave amplitude. These results are relevant for modelling frazil ice accumulation and freeze-up of leads, polynyas and along the seasonal ice zone.

Description: How reversible is sea ice loss? The Cryosphere, 6, 193-198, 2012 Author(s): J. K. Ridley, J. A. Lowe, and H. T. Hewitt It is well accepted that increasing atmospheric CO 2 results in global warming, leading to a decline in polar sea ice area. Here, the specific question of whether there is a tipping point in the sea ice cover is investigated. The global climate model HadCM3 is used to map the trajectory of sea ice area under idealised scenarios. The atmospheric CO 2 is first ramped up to four times pre-industrial levels (4 × CO 2 ), then ramped down to pre-industrial levels. We also examine the impact of stabilising climate at 4 × CO 2 prior to ramping CO 2 down to pre-industrial levels. Against global mean temperature, Arctic sea ice area is reversible, while the Antarctic sea ice shows some asymmetric behaviour – its rate of change slower, with falling temperatures, than its rate of change with rising temperatures. However, we show that the asymmetric behaviour is driven by hemispherical differences in temperature change between transient and stabilisation periods. We find no irreversible behaviour in the sea ice cover.

Description: Radar diagnosis of the subglacial conditions in Dronning Maud Land, East Antarctica The Cryosphere, 6, 1203-1219, 2012 Author(s): S. Fujita, P. Holmlund, K. Matsuoka, H. Enomoto, K. Fukui, F. Nakazawa, S. Sugiyama, and S. Surdyk In order to better understand the spatial distribution of subglacial environments, ground-based radar profiling data were analyzed for a total distance of ~ 3300 km across Dronning Maud Land, East Antarctica. The relationship between geometrically corrected bed returned power [ P c bed ] dB in decibels and ice thickness H was examined. When H is smaller than a~critical value that varies according to location, [ P c bed ] dB tends to decrease relatively smoothly with increasing H , which is explicable primarily by the cumulative effect of dielectric attenuation within the ice. However, at locations where H is larger than the critical H values, anomalous increases and fluctuations in [ P c bed ] dB were observed, regardless of the choice of radar frequency or radar-pulse width. In addition, the amplitude of the fluctuations often range 10 ~ 20 dB. We argue that the anomalous increases are caused by higher bed reflectivity associated with the existence of subglacial water. We used these features to delineate frozen and temperate beds. Approximately two-thirds of the investigated area was found to have a temperate bed. The beds of the inland part of the ice sheet tend to be temperate, with the exception of subglacial high mountains. In contrast, the beds of coastal areas tend to be frozen, with the exception of fast-flowing ice on the subglacial lowland or troughs. We argue that this new analytical method can be applied to other regions.

Description: Sea ice inertial oscillations in the Arctic Basin The Cryosphere, 6, 1187-1201, 2012 Author(s): F. Gimbert, D. Marsan, J. Weiss, N. C. Jourdain, and B. Barnier An original method to quantify the amplitude of inertial motion of oceanic and ice drifters, through the introduction of a non-dimensional parameter M defined from a spectral analysis, is presented. A strong seasonal dependence of the magnitude of sea ice inertial oscillations is revealed, in agreement with the corresponding annual cycles of sea ice extent, concentration, thickness, advection velocity, and deformation rates. The spatial pattern of the magnitude of the sea ice inertial oscillations over the Arctic Basin is also in agreement with the sea ice thickness and concentration patterns. This argues for a strong interaction between the magnitude of inertial motion on one hand, the dissipation of energy through mechanical processes, and the cohesiveness of the cover on the other hand. Finally, a significant multi-annual evolution towards greater magnitudes of inertial oscillations in recent years, in both summer and winter, is reported, thus concomitant with reduced sea ice thickness, concentration and spatial extent.

Description: Hydrologic controls on coastal suspended sediment plumes around the Greenland Ice Sheet The Cryosphere, 6, 1-19, 2012 Author(s): V. W. Chu, L. C. Smith, A. K. Rennermalm, R. R. Forster, and J. E. Box Rising sea levels and increased surface melting of the Greenland ice sheet have heightened the need for direct observations of meltwater release from the ice edge to ocean. Buoyant sediment plumes that develop in fjords downstream of outlet glaciers are controlled by numerous factors, including meltwater runoff. Here, Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery is used to average surface suspended sediment concentration (SSC) in fjords around ∼80% of Greenland from 2000–2009. Spatial and temporal patterns in SSC are compared with positive-degree-days (PDD), a proxy for surface melting, from the Polar MM5 regional climate model. Over this decade significant geographic covariance occurred between ice sheet PDD and fjord SSC, with outlet type (land- vs. marine-terminating glaciers) also important. In general, high SSC is associated with high PDD and/or a high proportion of land-terminating glaciers. Unlike previous site-specific studies of the Watson River plume at Kangerlussuaq, temporal covariance is low, suggesting that plume dimensions best capture interannual runoff dynamics whereas SSC allows assessment of meltwater signals across much broader fjord environments around the ice sheet. Remote sensing of both plume characteristics thus offers a viable approach for observing spatial and temporal patterns of meltwater release from the Greenland ice sheet to the global ocean.

Description: Permafrost distribution in the European Alps: calculation and evaluation of an index map and summary statistics The Cryosphere, 6, 807-820, 2012 Author(s): L. Boeckli, A. Brenning, S. Gruber, and J. Noetzli The objective of this study is the production of an Alpine Permafrost Index Map (APIM) covering the entire European Alps. A unified statistical model that is based on Alpine-wide permafrost observations is used for debris and bedrock surfaces across the entire Alps. The explanatory variables of the model are mean annual air temperatures, potential incoming solar radiation and precipitation. Offset terms were applied to make model predictions for topographic and geomorphic conditions that differ from the terrain features used for model fitting. These offsets are based on literature review and involve some degree of subjective choice during model building. The assessment of the APIM is challenging because limited independent test data are available for comparison and these observations represent point information in a spatially highly variable topography. The APIM provides an index that describes the spatial distribution of permafrost and comes together with an interpretation key that helps to assess map uncertainties and to relate map contents to their actual expression in the terrain. The map can be used as a first resource to estimate permafrost conditions at any given location in the European Alps in a variety of contexts such as research and spatial planning. Results show that Switzerland likely is the country with the largest permafrost area in the Alps, followed by Italy, Austria, France and Germany. Slovenia and Liechtenstein may have marginal permafrost areas. In all countries the permafrost area is expected to be larger than the glacier-covered area.

Description: Significant contribution to total mass from very small glaciers The Cryosphere, 6, 763-770, 2012 Author(s): D. B. Bahr and V. Radić A single large glacier can contain tens of millions of times the mass of a small glacier. Nevertheless, very small glaciers (with area ≤1 km 2 ) are so numerous that their contribution to the world's total ice volume is significant and may be a notable source of error if excluded. With current glacier inventories, total global volume errors on the order of 10% are possible. However, to reduce errors to below 1% requires the inclusion of glaciers that are smaller than those recorded in most inventories. At the global scale, 1% accuracy requires a list of all glaciers and ice caps (GIC, exclusive of the ice sheets) larger than 1 km 2 , and for regional estimates requires a complete list of all glaciers down to the smallest possible size. For this reason, sea-level rise estimates and other total mass and total volume analyses should not omit the world's smallest glaciers. In particular, upscaling GIC inventories has been common practice in sea level estimates, but downscaling may also be necessary to include the smallest glaciers.

Description: Surge dynamics on Bering Glacier, Alaska, in 2008–2011 The Cryosphere, 6, 1251-1262, 2012 Author(s): E. W. Burgess, R. R. Forster, C. F. Larsen, and M. Braun A surge cycle of the Bering Glacier system, Alaska, is examined using observations of surface velocity obtained using synthetic aperture radar (SAR) offset tracking, and elevation data obtained from the University of Alaska Fairbanks LiDAR altimetry program. After 13 yr of quiescence, the Bering Glacier system began to surge in May 2008 and had two stages of accelerated flow. During the first stage, flow accelerated progressively for at least 10 months and reached peak observed velocities of ~ 7 m d −1 . The second stage likely began in 2010. By 2011 velocities exceeded 9 m d −1 or ~ 18 times quiescent velocities. Fast flow continued into July 2011. Surface morphology indicated slowing by fall 2011; however, it is not entirely clear if the surge is yet over. The quiescent phase was characterized by small-scale acceleration events that increased driving stresses up to 70%. When the surge initiated, synchronous acceleration occurred throughout much of the glacier length. Results suggest that downstream propagation of the surge is closely linked to the evolution of the driving stress during the surge, because driving stress appears to be tied to the amount of resistive stress provided by the bed. In contrast, upstream acceleration and upstream surge propagation is not dependent on driving stress evolution.

Description: An ice flow modeling perspective on bedrock adjustment patterns of the Greenland ice sheet The Cryosphere, 6, 1263-1274, 2012 Author(s): M. Olaizola, R. S. W. van de Wal, M. M. Helsen, and B. de Boer Since the launch in 2002 of the Gravity Recovery and Climate Experiment (GRACE) satellites, several estimates of the mass balance of the Greenland ice sheet (GrIS) have been produced. To obtain ice mass changes, the GRACE data need to be corrected for the effect of deformation changes of the Earth's crust. Recently, a new method has been proposed where ice mass changes and bedrock changes are simultaneously solved. Results show bedrock subsidence over almost the entirety of Greenland in combination with ice mass loss which is only half of the currently standing estimates. This subsidence can be an elastic response, but it may however also be a delayed response to past changes. In this study we test whether these subsidence patterns are consistent with ice dynamical modeling results. We use a 3-D ice sheet–bedrock model with a surface mass balance forcing based on a mass balance gradient approach to study the pattern and magnitude of bedrock changes in Greenland. Different mass balance forcings are used. Simulations since the Last Glacial Maximum yield a bedrock delay with respect to the mass balance forcing of nearly 3000 yr and an average uplift at present of 0.3 mm yr −1 . The spatial pattern of bedrock changes shows a small central subsidence as well as more intense uplift in the south. These results are not compatible with the gravity based reconstructions showing a subsidence with a maximum in central Greenland, thereby questioning whether the claim of halving of the ice mass change is justified.

Description: Past and future sea-level change from the surface mass balance of glaciers The Cryosphere, 6, 1295-1322, 2012 Author(s): B. Marzeion, A. H. Jarosch, and M. Hofer We present estimates of sea-level change caused by the global surface mass balance of glaciers, based on the reconstruction and projection of the surface mass balance of all the individual glaciers of the world, excluding the ice sheets in Greenland and Antarctica. The model is validated using a leave-one-glacier-out cross-validation scheme against 3997 observed surface mass balances of 255 glaciers, and against 756 geodetically observed, temporally integrated volume and surface area changes of 341 glaciers. When forced with observed monthly precipitation and temperature data, the glaciers of the world are reconstructed to have lost mass corresponding to 114 ± 5 mm sea-level equivalent (SLE) between 1902 and 2009. Using projected temperature and precipitation anomalies from 15 coupled general circulation models from the Coupled Model Intercomparison Project phase 5 (CMIP5) ensemble, they are projected to lose an additional 148 ± 35 mm SLE (scenario RCP26), 166 ± 42 mm SLE (scenario RCP45), 175 ± 40 mm SLE (scenario RCP60), or 217 ± 47 mm SLE (scenario RCP85) during the 21st century. Based on the extended RCP scenarios, glaciers are projected to approach a new equilibrium towards the end of the 23rd century, after having lost either 248 ± 66 mm SLE (scenario RCP26), 313 ± 50 mm SLE (scenario RCP45), or 424 ± 46 mm SLE (scenario RCP85). Up until approximately 2100, ensemble uncertainty within each scenario is the biggest source of uncertainty for the future glacier mass loss; after that, the difference between the scenarios takes over as the biggest source of uncertainty. Ice mass loss rates are projected to peak 2040 ∼ 2050 (RCP26), 2050 ∼ 2060 (RCP45), 2070 ∼ 2090 (RCP60), or 2070 ∼ 2100 (RCP85).

Description: Calibration of a surface mass balance model for global-scale applications The Cryosphere, 6, 1463-1481, 2012 Author(s): R. H. Giesen and J. Oerlemans Global applications of surface mass balance models have large uncertainties, as a result of poor climate input data and limited availability of mass balance measurements. This study addresses several possible consequences of these limitations for the modelled mass balance. This is done by applying a simple surface mass balance model that only requires air temperature and precipitation as input data, to glaciers in different regions. In contrast to other models used in global applications, this model separately calculates the contributions of net solar radiation and the temperature-dependent fluxes to the energy balance. We derive a relation for these temperature-dependent fluxes using automatic weather station (AWS) measurements from glaciers in different climates. With local, hourly input data, the model is well able to simulate the observed seasonal variations in the surface energy and mass balance at the AWS sites. Replacing the hourly local data by monthly gridded climate data removes summer snowfall and winter melt events and, hence, influences the modelled mass balance most on locations with a small seasonal temperature cycle. Modelled winter mass balance profiles are fitted to observations on 82 glaciers in different regions to determine representative values for the multiplication factor and vertical gradient of precipitation. For 75 of the 82 glaciers, the precipitation provided by the climate dataset has to be multiplied with a factor above unity; the median factor is 2.5. The vertical precipitation gradient ranges from negative to positive values, with more positive values for maritime glaciers and a median value of 1.5 mm a −1 m −1 . With calibrated precipitation, the modelled annual mass balance gradient closely resembles the observations on the 82 glaciers, the absolute values are matched by adjusting either the incoming solar radiation, the temperature-dependent flux or the air temperature. The mass balance sensitivity to changes in temperature is particularly sensitive to the chosen calibration method. We additionally calculate the mass balance sensitivity to changes in incoming solar radiation, revealing that widely observed variations in irradiance can affect the mass balance by a magnitude comparable to a 1 °C change in temperature or a 10% change in precipitation.

Description: The stability of grounding lines on retrograde slopes The Cryosphere, 6, 1497-1505, 2012 Author(s): G. H. Gudmundsson, J. Krug, G. Durand, L. Favier, and O. Gagliardini The stability of marine ice sheets grounded on beds that slope upwards in the overall direction of flow is investigated numerically in two horizontal dimensions. We give examples of stable grounding lines on such retrograde slopes illustrating that marine ice sheets are not unconditionally unstable in two horizontal dimensions. Retrograde bed slopes at the grounding lines of marine ice sheets, such as the West Antarctic Ice Sheet (WAIS), do not per se imply an instability, nor do they imply that these regions are close to a threshold of instability. We therefore question those estimates of the potential near-future contribution of WAIS to global sea level change based solely on the notion that WAIS, resting on a retrograde slope, must be inherently unstable.

Description: Observations of enhanced thinning in the upper reaches of Svalbard glaciers The Cryosphere, 6, 1369-1381, 2012 Author(s): T. D. James, T. Murray, N. E. Barrand, H. J. Sykes, A. J. Fox, and M. A. King Changes in the volume and extent of land ice of the Svalbard archipelago have been the subject of considerable research since their sensitivity to changes in climate was first noted. However, the measurement of these changes is often necessarily based on point or profile measurements which may not be representative if extrapolated to a whole catchment or region. Combining high-resolution elevation data from contemporary laser-altimetry surveys and archived aerial photography makes it possible to measure historical changes across a glacier's surface without the need for extrapolation. Here we present a high spatial resolution time-series for six Arctic glaciers in the Svalbard archipelago spanning 1961 to 2005. We find high variability in thinning rates between sites with prevalent elevation changes at all sites averaging −0.59 ± 0.04 m a −1 between 1961–2005. Prior to 1990, ice surface elevation was changing at an average rate of −0.52 ± 0.09 m a −1 which decreased to −0.76 ± 0.10 m a −1 after 1990. Setting the elevation changes against the glaciers' altitude distribution reveals that significant increases in thinning rates are occurring most notably in the glaciers' upper reaches. We find that these changes are coincident with a decrease in winter precipitation at the Longyearbyen meteorological station and could reflect a decrease in albedo or dynamic response to lower accumulation. Further work is required to understand fully the causes of this increase in thinning rates in the glaciers' upper reaches. If on-going and occurring elsewhere in the archipelago, these changes will have a significant effect on the region's future mass balance. Our results highlight the importance of understanding the climatological context of geodetic mass balance measurements and demonstrate the difficulty of using index glaciers to represent regional changes in areas of strong climatological gradients.

Description: Brief communication "Importance of slope-induced error correction in volume change estimates from radar altimetry" The Cryosphere, 6, 447-451, 2012 Author(s): R. T. W. L. Hurkmans, J. L. Bamber, and J. A. Griggs In deriving elevation change rates (d H /d t ) from radar altimetry, the slope-induced error is usually assumed to cancel out in repeat measurements. These measurements, however, represent a location that can be significantly further upslope than assumed, causing an underestimate of the basin-integrated volume change. In a case-study for the fast-flowing part of Jakobshavn Isbræ, we show that a relatively straightforward correction for slope-induced error increases elevation change rates by up to several metres per year and significantly reduces the volume change error with respect to laser altimetry for the area of interest.

Description: Melt ponds on Arctic sea ice determined from MODIS satellite data using an artificial neural network The Cryosphere, 6, 431-446, 2012 Author(s): A. Rösel, L. Kaleschke, and G. Birnbaum Melt ponds on sea ice strongly reduce the surface albedo and accelerate the decay of Arctic sea ice. Due to different spectral properties of snow, ice, and water, the fractional coverage of these distinct surface types can be derived from multispectral sensors like the Moderate Resolution Image Spectroradiometer (MODIS) using a spectral unmixing algorithm. The unmixing was implemented using a multilayer perceptron to reduce computational costs. Arctic-wide melt pond fractions and sea ice concentrations are derived from the level 3 MODIS surface reflectance product. The validation of the MODIS melt pond data set was conducted with aerial photos from the MELTEX campaign 2008 in the Beaufort Sea, data sets from the National Snow and Ice Data Center (NSIDC) for 2000 and 2001 from four sites spread over the entire Arctic, and with ship observations from the trans-Arctic HOTRAX cruise in 2005. The root-mean-square errors range from 3.8 % for the comparison with HOTRAX data, over 10.7 % for the comparison with NSIDC data, to 10.3 % and 11.4 % for the comparison with MELTEX data, with coefficient of determination ranging from R 2 =0.28 to R 2 =0.45. The mean annual cycle of the melt pond fraction per grid cell for the entire Arctic shows a strong increase in June, reaching a maximum of 15 % by the end of June. The zonal mean of melt pond fractions indicates a dependence of the temporal development of melt ponds on the geographical latitude, and has its maximum in mid-July at latitudes between 80° and 88° N. Furthermore, the MODIS results are used to estimate the influence of melt ponds on retrievals of sea ice concentrations from passive microwave data. Results from a case study comparing sea ice concentrations from ARTIST Sea Ice-, NASA Team 2-, and Bootstrap-algorithms with MODIS sea ice concentrations indicate an underestimation of around 40 % for sea ice concentrations retrieved with microwave algorithms.

Description: Modelling borehole temperatures in Southern Norway – insights into permafrost dynamics during the 20th and 21st century The Cryosphere, 6, 553-571, 2012 Author(s): T. Hipp, B. Etzelmüller, H. Farbrot, T. V. Schuler, and S. Westermann This study aims at quantifying the thermal response of mountain permafrost in southern Norway to changes in climate since 1860 and until 2100. A transient one-dimensional heat flow model was used to simulate ground temperatures and associated active layer thicknesses for nine borehole locations, which are located at different elevations and in substrates with different thermal properties. The model was forced by reconstructed air temperatures starting from 1860, which approximately coincides with the end of the Little Ice Age in the region. The impact of climate warming on mountain permafrost to 2100 is assessed by using downscaled air temperatures from a multi-model ensemble for the A1B scenario. Borehole records over three consecutive years of ground temperatures, air temperatures and snow cover data served for model calibration and validation. With an increase of air temperature of ~1.5 °C over 1860–2010 and an additional warming of ~2.8 °C until 2100, we simulate the evolution of ground temperatures for each borehole location. In 1860 the lower limit of permafrost was estimated to be ca. 200 m lower than observed today. According to the model, since the approximate end of the Little Ice Age, the active-layer thickness has increased by 0.5–5 m and 〉10 m for the sites Juvvasshøe and Tron, respectively. The most pronounced increases in active layer thickness were modelled for the last two decades since 1990 with increase rates of +2 cm yr −1 to +87 cm yr −1 (20–430%). According to the A1B climate scenario, degradation of mountain permafrost is suggested to occur throughout the 21st century at most of the sites below ca. 1800 m a.s.l. At the highest locations at 1900 m a.s.l., permafrost degradation is likely to occur with a probability of 55–75% by 2100. This implies that mountain permafrost in southern Norway is likely to be confined to the highest peaks in the western part of the country.

Description: Sensitivity of a Greenland ice sheet model to atmospheric forcing fields The Cryosphere, 6, 999-1018, 2012 Author(s): A. Quiquet, H. J. Punge, C. Ritz, X. Fettweis, H. Gallée, M. Kageyama, G. Krinner, D. Salas y Mélia, and J. Sjolte Predicting the climate for the future and how it will impact ice sheet evolution requires coupling ice sheet models with climate models. However, before we attempt to develop a realistic coupled setup, we propose, in this study, to first analyse the impact of a model simulated climate on an ice sheet. We undertake this exercise for a set of regional and global climate models. Modelled near surface air temperature and precipitation are provided as upper boundary conditions to the GRISLI (GRenoble Ice Shelf and Land Ice model) hybrid ice sheet model (ISM) in its Greenland configuration. After 20 kyrs of simulation, the resulting ice sheets highlight the differences between the climate models. While modelled ice sheet sizes are generally comparable to the observed one, there are considerable deviations among the ice sheets on regional scales. These deviations can be explained by biases in temperature and precipitation near the coast. This is especially true in the case of global models. But the deviations between the climate models are also due to the differences in the atmospheric general circulation. To account for these differences in the context of coupling ice sheet models with climate models, we conclude that appropriate downscaling methods will be needed. In some cases, systematic corrections of the climatic variables at the interface may be required to obtain realistic results for the Greenland ice sheet (GIS).

Description: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers The Cryosphere, 6, 821-839, 2012 Author(s): J. E. Box, X. Fettweis, J. C. Stroeve, M. Tedesco, D. K. Hall, and K. Steffen Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite-derived surface albedo with calibrated regional climate modeled surface air temperature and surface downward solar irradiance, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback over 12 summer periods beginning in 2000. We find that, while albedo feedback defined by the change in net solar shortwave flux and temperature over time is positive over 97% of the ice sheet, when defined using paired annual anomalies, a second-order negative feedback is evident over 63% of the accumulation area. This negative feedback damps the accumulation area response to warming due to a positive correlation between snowfall and surface air temperature anomalies. Positive anomaly-gauged feedback concentrated in the ablation area accounts for more than half of the overall increase in melting when satellite-derived melt duration is used to define the timing when net shortwave flux is sunk into melting. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007, enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.

Description: Surface and snowdrift sublimation at Princess Elisabeth station, East Antarctica The Cryosphere, 6, 841-857, 2012 Author(s): W. Thiery, I. V. Gorodetskaya, R. Bintanja, N. P. M. Van Lipzig, M. R. Van den Broeke, C. H. Reijmer, and P. Kuipers Munneke In the near-coastal regions of Antarctica, a significant fraction of the snow precipitating onto the surface is removed again through sublimation – either directly from the surface or from drifting snow particles. Meteorological observations from an Automatic Weather Station (AWS) near the Belgian research station Princess Elisabeth in Dronning Maud Land, East-Antarctica, are used to study surface and snowdrift sublimation and to assess their impacts on both the surface mass balance and the surface energy balance during 2009 and 2010. Comparison to three other AWSs in Dronning Maud Land with 11 to 13 yr of observations shows that sublimation has a significant influence on the surface mass balance at katabatic locations by removing 10–23% of their total precipitation, but at the same time reveals anomalously low surface and snowdrift sublimation rates at Princess Elisabeth (17 mm w.e. yr −1 compared to 42 mm w.e. yr −1 at Svea Cross and 52 mm w.e. yr −1 at Wasa/Aboa). This anomaly is attributed to local topography, which shields the station from strong katabatic influence, and, therefore, on the one hand allows for a strong surface inversion to persist throughout most of the year and on the other hand causes a lower probability of occurrence of intermediately strong winds. This wind speed class turns out to contribute most to the total snowdrift sublimation mass flux, given its ability to lift a high number of particles while still allowing for considerable undersaturation.

Description: Variability of sea ice deformation rates in the Arctic and their relationship with basin-scale wind forcing The Cryosphere, 6, 1553-1559, 2012 Author(s): A. Herman and O. Glowacki The temporal variability of the moments of probability distribution functions (pdfs) of total sea ice deformation rates in the Arctic is analyzed in the context of the basin-scale wind forcing acting on the ice. The pdfs are estimated for 594 satellite-derived sea ice deformation maps from 11 winter seasons between 1996/1997 and 2007/2008, provided by the RADARSAT Geophysical Processor System. The temporal scale analyzed equals 3 days. The moments of the pdfs, calculated for a range of spatial scales (12.5–900 km), have two dominating components of variability: a seasonal cycle, with deformation rates decreasing throughout winter towards a minimum in March; and a short-term, synoptic variability, strongly correlated with the area-averaged magnitude of the wind stress over the Arctic, estimated based on the NCEP-DOE Reanalysis-2 data (correlation coefficient of 0.71 for the mean deformation rate). Due to scaling properties of the moments, logarithms of higher moments are strongly correlated with the wind stress as well. Exceptions are observed only at small spatial scales, as a result of extreme deformation events, not directly associated with large-scale wind forcing. By repeating the analysis within regions of different sizes and locations, we show that the wind–ice deformation correlation is largest at the basin scale and decreases with decreasing size of the area of study. Finally, we suggest that a positive trend in seasonally averaged correlation between sea ice deformation rates and the wind forcing, present in the analyzed data, may be related to an observed decrease in the multi-year ice area in the Arctic, indicating possibly even stronger correlations in the future.

Description: Area change of glaciers in the Canadian Rocky Mountains, 1919 to 2006 The Cryosphere, 6, 1541-1552, 2012 Author(s): C. Tennant, B. Menounos, R. Wheate, and J. J. Clague Glaciers in the Canadian Rocky Mountains constitute an important freshwater resource. To enhance our understanding of the influence climate and local topography have on glacier area, large numbers of glaciers of different sizes and attributes need to be monitored over periods of many decades. We used Interprovincial Boundary Commission Survey (IBCS) maps of the Alberta–British Columbia (BC) border (1903–1924), BC Terrain Resource Information Management (TRIM) data (1982–1987), and Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) imagery (2000–2002 and 2006) to document planimetric changes in glacier cover in the central and southern Canadian Rocky Mountains between 1919 and 2006. Over this period, glacier cover in the study area decreased by 590 ± 70 km 2 (40 ± 5%), 17 of 523 glaciers disappeared and 124 glaciers fragmented into multiple ice masses. Glaciers smaller than 1.0 km 2 experienced the greatest relative area loss (64 ± 8%), and relative area loss is more variable with small glaciers, suggesting that the local topographic setting controls the response of these glaciers to climate change. Small glaciers with low slopes, low mean/median elevations, south to west aspects, and high insolation experienced the largest reduction in area. Similar rates of area change characterize the periods 1919–1985 and 1985–2001; −6.3 ± 0.6 km 2 yr −1 (−0.4 ± 0.1% yr −1 ) and −5.0 ± 0.5 km 2 yr −1 (−0.5 ± 0.1% yr −1 ), respectively. The rate of area loss, however, increased over the period 2001–2006; −19.3 ± 2.4 km 2 yr −1 (−2.0 ± 0.2% yr −1 ). Applying size class-specific scaling factors, we estimate a total reduction in glacier cover in the central and southern Canadian Rocky Mountains for the period 1919–2006 of 750 km 2 (30%).

Description: P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model The Cryosphere, 6, 1163-1174, 2012 Author(s): D. Draebing and M. Krautblatter P-wave refraction seismics is a key method in permafrost research but its applicability to low-porosity rocks, which constitute alpine rock walls, has been denied in prior studies. These studies explain p-wave velocity changes in freezing rocks exclusively due to changing velocities of pore infill, i.e. water, air and ice. In existing models, no significant velocity increase is expected for low-porosity bedrock. We postulate, that mixing laws apply for high-porosity rocks, but freezing in confined space in low-porosity bedrock also alters physical rock matrix properties. In the laboratory, we measured p-wave velocities of 22 decimetre-large low-porosity ( 〈 10%) metamorphic, magmatic and sedimentary rock samples from permafrost sites with a natural texture (〉 100 micro-fissures) from 25 °C to −15 °C in 0.3 °C increments close to the freezing point. When freezing, p-wave velocity increases by 11–166% perpendicular to cleavage/bedding and equivalent to a matrix velocity increase from 11–200% coincident to an anisotropy decrease in most samples. The expansion of rigid bedrock upon freezing is restricted and ice pressure will increase matrix velocity and decrease anisotropy while changing velocities of the pore infill are insignificant. Here, we present a modified Timur's two-phase-equation implementing changes in matrix velocity dependent on lithology and demonstrate the general applicability of refraction seismics to differentiate frozen and unfrozen low-porosity bedrock.

Description: Sensitivity of Greenland Ice Sheet surface mass balance to surface albedo parameterization: a study with a regional climate model The Cryosphere, 6, 1175-1186, 2012 Author(s): J. H. van Angelen, J. T. M. Lenaerts, S. Lhermitte, X. Fettweis, P. Kuipers Munneke, M. R. van den Broeke, E. van Meijgaard, and C. J. P. P. Smeets We present a sensitivity study of the surface mass balance (SMB) of the Greenland Ice Sheet, as modeled using a regional atmospheric climate model, to various parameter settings in the albedo scheme. The snow albedo scheme uses grain size as a prognostic variable and further depends on cloud cover, solar zenith angle and black carbon concentration. For the control experiment the overestimation of absorbed shortwave radiation (+6%) at the K-transect (west Greenland) for the period 2004–2009 is considerably reduced compared to the previous density-dependent albedo scheme (+22%). To simulate realistic snow albedo values, a small concentration of black carbon is needed, which has strongest impact on melt in the accumulation area. A background ice albedo field derived from MODIS imagery improves the agreement between the modeled and observed SMB gradient along the K-transect. The effect of enhanced meltwater retention and refreezing is a decrease of the albedo due to an increase in snow grain size. As a secondary effect of refreezing the snowpack is heated, enhancing melt and further lowering the albedo. Especially in a warmer climate this process is important, since it reduces the refreezing potential of the firn layer that covers the Greenland Ice Sheet.

Description: Variable glacier response to atmospheric warming, northern Antarctic Peninsula, 1988–2009 The Cryosphere, 6, 1031-1048, 2012 Author(s): B. J. Davies, J. L. Carrivick, N. F. Glasser, M. J. Hambrey, and J. L. Smellie The northern Antarctic Peninsula has recently exhibited ice-shelf disintegration, glacier recession and acceleration. However, the dynamic response of land-terminating, ice-shelf tributary and tidewater glaciers has not yet been quantified or assessed for variability, and there are sparse data for glacier classification, morphology, area, length or altitude. This paper firstly classifies the area, length, altitude, slope, aspect, geomorphology, type and hypsometry of 194 glaciers on Trinity Peninsula, Vega Island and James Ross Island in 2009 AD. Secondly, this paper documents glacier change 1988–2009. In 2009, the glacierised area was 8140±262 km 2 . From 1988–2001, 90% of glaciers receded, and from 2001–2009, 79% receded. This equates to an area change of −4.4% for Trinity Peninsula eastern coast glaciers, −0.6% for western coast glaciers, and −35.0% for ice-shelf tributary glaciers from 1988–2001. Tidewater glaciers on the drier, cooler eastern Trinity Peninsula experienced fastest shrinkage from 1988–2001, with limited frontal change after 2001. Glaciers on the western Trinity Peninsula shrank less than those on the east. Land-terminating glaciers on James Ross Island shrank fastest in the period 1988–2001. This east-west difference is largely a result of orographic temperature and precipitation gradients across the Antarctic Peninsula, with warming temperatures affecting the precipitation-starved glaciers on the eastern coast more than on the western coast. Reduced shrinkage on the western Peninsula may be a result of higher snowfall, perhaps in conjunction with the fact that these glaciers are mostly grounded. Rates of area loss on the eastern side of Trinity Peninsula are slowing, which we attribute to the floating ice tongues receding into the fjords and reaching a new dynamic equilibrium. The rapid shrinkage of tidewater glaciers on James Ross Island is likely to continue because of their low elevations and flat profiles. In contrast, the higher and steeper tidewater glaciers on the eastern Antarctic Peninsula will attain more stable frontal positions after low-lying ablation areas are removed, reaching equilibrium more quickly.

Description: A minimal, statistical model for the surface albedo of Vestfonna ice cap, Svalbard The Cryosphere, 6, 1049-1061, 2012 Author(s): M. Möller The ice cap Vestfonna is located in northeastern Svalbard and forms one of the largest ice bodies of the Eurasian Arctic. Its surface albedo plays a key role in the understanding and modelling of its energy and mass balance. The principle governing factors for albedo evolution, i.e. precipitation and air temperature and therewith snow depth and melt duration, were found to vary almost exclusively with terrain elevation throughout the ice cap. Hence, surface albedo can be expected to develop a comparable pattern. A new statistical model is presented that estimates this mean altitudinal albedo profile of the ice cap on the basis of a minimal set of meteorological variables on a monthly resolution. Model calculations are based on a sigmoid function of the artificial quantity rain-snow ratio and a linear function of cumulative snowfall and cumulative positive degree days. Surface albedo fields of the MODIS snow product MOD10A1 from the period March to October in the years 2001–2008 serve as a basis for both calibration and cross-validation of the model. The meteorological model input covers the period September 2000 until October 2008 and is based on ERA-Interim data of a grid point located close to the ice cap. The albedo model shows a good performance. The root mean square error between observed and modelled albedo values along the altitudinal profile is 0.057±0.028 (mean ± one standard deviation). The area weighted mean even reduces to a value of 0.054. Distinctly higher deviations (0.07–0.09) are only present throughout the very lowest and uppermost parts of the ice cap that are either small in area or hardly affected by surface melt. Thus, the new, minimal, statistical albedo model presented in this study is found to reproduce the albedo evolution on Vestfonna ice cap on a high level of accuracy and is thus suggested to be fully suitable for further application in broader energy or mass-balance studies of the ice cap.

Description: Inferring snowpack ripening and melt-out from distributed measurements of near-surface ground temperatures The Cryosphere, 6, 1127-1139, 2012 Author(s): M.-O. Schmid, S. Gubler, J. Fiddes, and S. Gruber Seasonal snow cover and its melt regime are heterogeneous both in time and space. Describing and modelling this variability is important because it affects diverse phenomena such as runoff, ground temperatures or slope movements. This study presents the derivation of melting characteristics based on spatial clusters of ground surface temperature (GST) measurements. Results are based on data from Switzerland where ground surface temperatures were measured with miniature loggers (iButtons) at 40 locations referred to as footprints. At each footprint, up to ten iButtons have been distributed randomly over an area of 10 m × 10 m, placed a few cm below the ground surface. Footprints span elevations of 2100–3300 m a.s.l. and slope angles of 0–55°, as well as diverse slope expositions and types of surface cover and ground material. Based on two years of temperature data, the basal ripening date and the melt-out date are determined for each iButton, aggregated to the footprint level and further analysed. The melt-out date could be derived for nearly all iButtons; the ripening date could be extracted for only approximately half of them because its detection based on GST requires ground freezing below the snowpack. The variability within a footprint is often considerable and one to three weeks difference between melting or ripening of the points in one footprint is not uncommon. The correlation of mean annual ground surface temperatures, ripening date and melt-out date is moderate, suggesting that these metrics are useful for model evaluation.