ALBERT

Description: Decadal trends in the Antarctic sea ice extent ultimately controlled by ice-ocean feedback The Cryosphere Discussions, 7, 4585-4632, 2013 Author(s): H. Goosse and V. Zunz The large natural variability of the Antarctic sea ice is a key characteristic of the system that might be responsible for the small positive trend in sea ice extent observed since 1979. In order to gain insight in the processes responsible for this variability, we have analysed in a control simulation performed with a coupled climate model a strong positive ice-ocean feedback that amplifies sea ice variations. When sea ice concentration increases in a region, in particular close to the ice edge, the mixed layer depth tends to decrease. This can be caused by a net inflow of ice and thus of freshwater that stabilizes the water column. Another stabilizing mechanism at interannual time scales that appears more widespread in our simulation is associated with the downward salt transport due to the seasonal cycle of ice formation: brine is released in winter when ice is formed and mixed over a deep layer while the freshwater flux caused by ice melting is included in a shallow layer, resulting in a net vertical transport of salt. Because of this stronger stratification due to the presence of sea ice, more heat is stored at depth in the ocean and the vertical oceanic heat flux is reduced, which contributes to maintain a higher ice extent. This positive feedback is not associated with a particular spatial pattern. Consequently, the spatial distribution of the trend in ice concentration is largely imposed by the wind changes that can provide the initial perturbation. A positive freshwater flux could alternatively be the initial trigger but the amplitude of the final response of the sea ice extent is finally set up by the amplification related to ice-ocean feedback. Initial conditions have also an influence as the chance to have a large increase in ice extent is higher if starting from a state characterized by a low value.

Description: A data set of world-wide glacier length fluctuations The Cryosphere Discussions, 7, 4775-4811, 2013 Author(s): P. W. Leclercq, J. Oerlemans, H. J. Basagic, I. Bushueva, A. J. Cook, and R. Le Bris Glacier fluctuations contribute to variations in sea level and historical glacier length fluctuations are natural indicators of climate change. To study these subjects, long-term information of glacier change is needed. In this paper we present a~data set of global long-term glacier length fluctuations. The data set is a compilation of available information on changes in glacier length world-wide, including both measured and reconstructed glacier length fluctuations. All 471 length series start before 1950 and cover at least four decades. The longest record starts in 1534, but the majority of time series start after 1850. The number of available records decreases again after 1962. The data set has global coverage including records from all continents. However, the Canadian Arctic is not represented in the data set. The glacier length series show relatively small fluctuations until the mid-19th century followed by a global retreat that was strongest in the first half of the 20th century, although large variability in the length change of the different glaciers is observed. During the 20th century, calving glaciers retreated more than land terminating glaciers, but their relative length change was approximately equal. Besides calving, the glacier slope is the most important glacier property determining length change: steep glaciers have retreated less than glaciers with a gentle slope.

Description: Implementation and evaluation of prognostic representations of the optical diameter of snow in the detailed snowpack model SURFEX/ISBA-Crocus The Cryosphere Discussions, 7, 4443-4500, 2013 Author(s): C. M. Carmagnola, S. Morin, M. Lafaysse, F. Domine, B. Lesaffre, Y. Lejeune, G. Picard, and L. Arnaud In the SURFEX/ISBA-Crocus multi-layer snowpack model, the snow microstructure was up to now characterized by the grain size and by semi-empirical shape variables which cannot be measured easily in the field or linked to other relevant snow properties. In this work we introduce a new formulation of snow metamorphism directly based on equations describing the rate of change of the optical diameter ( d opt ). This variable is considered here to be equal to the equivalent sphere optical diameter, which is inversely proportional to the specific surface area (SSA). d opt thus represents quantitatively some of the geometric characteristics of a porous medium. Different prognostic rate equations of d opt , including a re-formulation of the original Crocus scheme and the parametrizations from Taillandier et al. (2007) and Flanner and Zender (2006), were evaluated by comparing their predictions to field measurements carried out at Summit Camp (Greenland) in May and June 2011 and at Col de Porte (French Alps) during the 2009/10 and 2011/12 winter seasons. We focused especially on results in terms of SSA. In addition, we tested the impact of the different formulations on the simulated density profile, the total snow height, the snow water equivalent (SWE) and the surface albedo. Results indicate that all formulations perform well, with median values of the RMSD between measured and simulated SSA lower than 10 m 2 kg −1 . Incorporating the optical diameter as a fully-fledged prognostic variable is an important step forward in the quantitative description of the snow microstructure within snowpack models, because it opens the way to data assimilation of various electromagnetic observations.

Description: Distribution and recent variations of supraglacial lakes on dendritic-type glaciers in the Khan Tengri-Tomur Mountains, Central Asia The Cryosphere Discussions, 7, 4545-4584, 2013 Author(s): Q. Liu, C. Mayer, and S. Liu Supraglacial lakes are widely distributed on glaciers in the Tomur-Khan Tengri Tianshan Mountains, Central Asia. The existence and development of supraglacial lakes play an important role in the ice melting processes and also in the storage and release of glacial melt water. Here we mapped the supraglacial lakes of eight typical debris-covered dendritic-type glaciers around the Tomur-Khan Tengri peaks based on 9 Landsat TM/ETM+ images acquired in the summers of 1990 until 2011. With a lower area limit of 3600 m 2 for a conservative identification of glacial lakes, we mapped 775 supraglacial lakes and 38 marginal glacial lakes in total. Our results indicate that supraglacial lakes (area 〉 3600 m 2 ) in the study region never develop beyond an elevation of about 3850 m a.s.l., 800 m lower than the maximum upper boundary of debris cover (4650 m a.s.l.). The area-elevation distribution shows that lakes are predominantly occured close to the altitude of 3250 m a.s.l., where the clean ice simultaneously disappears. The majority of the supraglacial lakes are found on the Tomur Glacier and the South Inylchek Glacier, two strongly debris-covered dendritic-type glaciers in the region. As for the multi-year variation of lake area, the summer total and mean areas of supraglacial lakes show some variability from 1990 and 2005 but increased noticeably between 2005 and 2011. The mean area of the mapped lakes reached a maximum in 2010. We found that the area of supraglacial lakes is positively correlated to the total precipitation in summer (July to September) but negatively correlated to the mean spring air temperature (April to June). Pre-summer air temperature fluctuations likely have a stronger impact on the different evolution processes of glacial drainage, evolving from unconnected to connected systems, which may lead to the drainage of larger supraglacial lakes and results in shrinkage of the total and mean lake area during the summer.

Description: Fracture-induced softening for large-scale ice dynamics The Cryosphere Discussions, 7, 4501-4544, 2013 Author(s): T. Albrecht and A. Levermann Floating ice shelves can exert a retentive and hence stabilizing force onto the inland ice sheet of Antarctica. However, this effect has been observed to diminish by fracture-coupled dynamic processes within the protective ice shelves leading to accelerated ice flow and hence to a sea-level contribution. In order to better understand the role of fractures in ice dynamics we apply a large-scale continuum representation of fractures and related fracture growth into the prognostic Parallel Ice Sheet Model (PISM). To this end we introduce a higher-order accuracy advection scheme for the transport of the two-dimensional fracture density across the regular computational grid. Dynamic coupling of fractures and ice flow is attained by a reduction of effective ice viscosity proportional to the inferred fracture density. This formulation implies the possibility of a non-linear threshold behavior due to self-amplified fracturing in shear regions triggered by small variations in damage threshold. As a result of prognostic flow simulations, flow patterns with realistically large across-flow velocity gradients in fracture-weakened regions as seen in observations are reproduced. This model framework is expandable to grounded ice streams and accounts for climate-induced effects on fracturing and hence on the ice-flow dynamics. It further allows for an enhanced fracture-based calving parameterization.

Description: Sea ice detection with space-based LIDAR The Cryosphere Discussions, 7, 4681-4701, 2013 Author(s): S. Rodier, Y. Hu, and M. Vaughan Monitoring long-term climate change in the Polar Regions relies on accurate, detailed and repeatable measurements of geophysical processes and states. These regions are among the Earth's most vulnerable ecosystems, and measurements there have shown rapid changes in the seasonality and the extent of snow and sea ice coverage. The authors have recently developed a promising new technique that uses lidar surface measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) mission to infer ocean surface ice-water phase. CALIPSO's 532 nm depolarization ratio measurements of the ocean surface are uniquely capable of providing information about the ever-changing sea surface state within the Polar Regions. With the finer resolution of the CALIPSO footprint (90 m diameter, spaced 335 m apart) and its ability to acquire measurements during both daytime and nighttime orbit segments and in the presence of clouds, the CALIPSO sea ice product provides fine-scale information on mixed phase scenes and can be used to assess/validate the estimates of sea-ice concentration currently provided by passive sensors. This paper describes the fundamentals of the CALIPSO sea-ice detection and classification technique. We present retrieval results from a six-year study, which are compared to existing data sets obtained by satellite-based passive remote sensors.

Description: Simulating the role of gravel on the dynamics of permafrost on the Qinghai-Tibetan Plateau The Cryosphere Discussions, 7, 4703-4740, 2013 Author(s): S. Yi, J. Chen, Q. Wu, and Y. Ding Gravel (particle size ≥ 2 mm) is common in soil profiles of the Qinghai-Tibetan Plateau (QTP). It has different thermal and hydrological properties than other fine mineral soils (particle size 〈 2 mm), which may have significant impacts on the thermal and hydrological processes of soil. However, few models have considered gravel. In this study, we implemented the thermal and hydraulic properties of gravel into the Dynamic Organic Soil-Terrestrial Ecosystem Model to develop new schemes to simulate the dynamics of permafrost on the QTP. Results showed that: (1) the widely used Farouki thermal scheme always simulated higher thermal conductivity of frozen soils than unfrozen soils with the same soil water content; therefore it tends to overestimate permafrost thickness strongly; (2) there exists a soil moisture threshold, below which the new set of schemes with gravel simulated smaller thermal conductivity of frozen soils than unfrozen soils; (3) soil with gravel has higher hydraulic conductivity and poorer water retention capability; and simulations with gravel were usually drier than those without gravel; and (4) the new schemes simulated faster upward degradation than downward degradation; and the simulated permafrost thicknesses were sensitive to the fraction of gravel, the gravel size, the thickness of soil with gravel, and the subsurface drainage. To reduce the uncertainties in the projection of permafrost degradation on the QTP, more effort should be made to: (1) developing robust relationships between soil thermal and hydraulic properties and gravel characteristics based on laboratory work; and (2) compiling spatial datasets of the vertical distribution of gravel content based on measurements during drilling or the digging of soil pits.

Description: Ice volume estimates for the Himalaya–Karakoram region: evaluating different methods The Cryosphere Discussions, 7, 4813-4854, 2013 Author(s): H. Frey, H. Machguth, M. Huss, C. Huggel, S. Bajracharya, T. Bolch, A. Kulkarni, A. Linsbauer, N. Salzmann, and M. Stoffel Ice volume estimates are crucial for assessing water reserves stored in glaciers. A variety of different methodologies exist but there is a lack of systematic comparative analysis thereof. Due to its large glacier coverage, such estimates are of particular interest for the Himalayan-Karakoram (HK) region. Here, three volume–area (V–A) relations, a slope-dependent estimation method, and two ice-thickness distribution models are applied to a complete glacier inventory of the HK region. An uncertainty and sensitivity assessment is performed to investigate the influence of the input glacier areas, and model approaches and parameters on the resulting total ice volumes. Results of the two ice-thickness distribution models are validated with local ice-thickness measurements at six glaciers. The resulting ice volumes for the entire HK region range from 2955 km 3 to 6455 km 3 , depending on the approach. Results from the ice thickness distribution models and the slope-dependent thickness estimations agree well with measured local ice thicknesses while V–A relations show stronger deviations. The study provides evidence on the significant effect of the selected method on results and underlines the importance of a careful and critical evaluation. More ice-thickness measurements are needed to improve models and results in the future.

Description: Black carbon concentrations from a Tibetan Plateau ice core spanning 1843–1982: recent increases due to emissions and glacier melt The Cryosphere Discussions, 7, 4855-4880, 2013 Author(s): M. Jenkins, S. Kaspari, S. Kang, B. Grigholm, and P. A. Mayewski Black carbon (BC) deposited on snow and glacier surfaces can reduce albedo and lead to accelerated melt. An ice core recovered from Guoqu glacier on Mt. Geladaindong and analyzed using a Single Particle Soot Photometer provides the first long-term (1843–1982) record of BC concentrations from the Central Tibetan Plateau. The highest concentrations are observed from 1975–1982, which corresponds to a 2.0-fold and 2.4-fold increase in average and median values, respectively, relative to 1843–1940. BC concentrations post-1940 are also elevated relative to the earlier portion of the record. Causes for the higher BC concentrations include increased regional BC emissions and subsequent deposition, and melt induced enrichment of BC, with the melt potentially accelerated due to the presence of BC at the glacier surface. A qualitative comparison of the BC and Fe (used as a dust proxy) records suggests that if changes in the concentrations of absorbing impurities at the glacier surface have influenced recent glacial melt, the melt may be due to the presence of BC rather than dust. Guoqu glacier has received no net ice accumulation since the 1980s, and is a potential example of a glacier where an increase in the equilibrium line altitude is exposing buried high impurity layers. That BC concentrations in the uppermost layers of the Geladaindong ice core are not substantially higher relative to deeper in the ice core suggests that some of the BC that must have been deposited on Guoqu glacier via wet or dry deposition between 1983 and 2005 has been removed from the surface of the glacier, potentially via supraglacial or englacial meltwater.

Description: 3D-VAR multilayer assimilation of X-band SAR data into a detailed snowpack model The Cryosphere Discussions, 7, 4881-4912, 2013 Author(s): X. V. Phan, L. Ferro-Famil, M. Gay, Y. Durand, M. Dumont, S. Morin, S. Allain, G. D'Urso, and A. Girard We introduce a variational data assimilation scheme to assimilate X-band Synthetic Aperture Radar (SAR) data into a snowpack evolution model. The structure properties of a snowpack, such as snow density and grain optical diameter of each layer, are simulated over a period of time by the snow metamorphism model Crocus, fed by the local reanalysis SAFRAN at a French alpine location. These parameters are used as inputs of an Electromagnetic Backscattering Model (EBM) based on Dense Media Radiative Transfer (DMRT) theory, which calculates the simulated total backscattering coefficient. Next, 3D-VAR data assimilation is implemented in order to minimize the discrepancies between model simulations and observations obtained from SAR acquisitions, by modifying the parameters of a multilayer snowpack calculated by Crocus. The algorithm then reinitializes Crocus with the optimized snowpack structure properties, and therefore allows it to continue the simulation of snowpack evolution where adjustments based on remote sensing data has been taken into account. Results obtained using TerraSAR-X acquisitions on Argentière Glacier (Mont-Blanc massif, French Alps) show the high potential of this method for improving snow cover simulation.

Description: The physical basis for gas transport through polar firn: a case study at Summit, Greenland The Cryosphere Discussions, 7, 2455-2487, 2013 Author(s): A. C. Adolph and M. R. Albert Compared to other natural porous materials, relatively little is known about the physical nature of polar firn. This intricate network of ice and pore space that comprises the top 60–100 m of the polar ice sheets is the framework that forms the natural archive of past climate information. Despite the many implications for ice core interpretation, direct measurements of physical properties throughout the firn column are limited. Models of gas transport through firn are used to interpret in-situ chemical data which is retrieved to analyze past atmospheric composition. These traditional models treat the firn as a "black box," with gas transport parameters tuned to match gas concentrations with depth to known atmospheric histories. Though this method has been largely successful and provided very useful insights, there are still many questions and uncertainties to be addressed. This work seeks to understand the impact of firn structure on gas transport in firn from a first principles standpoint through direct measurements of permeability, gas diffusivity and microstructure. The relationships between gas transport properties and microstructure will be characterized and compared to existing relationships for general porous media. Direct measurements of gas diffusivity are compared to diffusivities deduced from models based on firn air chemical sampling. Our comparison illuminates the primary importance of including microstructural parameters, beyond just porosity or density, in mass transport modeling, and it provides insights about the nature of gas transport throughout the firn column. Guidance is provided for development of next-generation firn air transport models.

Description: Solving Richards Equation for snow improves snowpack meltwater runoff estimations The Cryosphere Discussions, 7, 2373-2412, 2013 Author(s): N. Wever, C. Fierz, C. Mitterer, H. Hirashima, and M. Lehning The runoff from the snow cover during spring snow melt or rain-on-snow events is an important factor in the hydrological cycle. In this study, water transport schemes for a 1-dimensional physical based snowpack model are compared to 14 yr of lysimeter measurements at a high alpine site. The schemes include a simple bucket-type approach, an approximation of Richards Equation (RE), and the full RE. The results show that daily sums of runoff are strongly related to a positive energy balance of the snow cover and therefore, all water transport schemes show very similar performance in terms of Nash–Sutcliffe efficiency (NSE) coefficients (around 0.59) and r 2 values (around 0.77). Timing of the arrival of meltwater in spring at the bottom of the snowpack showed differences between the schemes, where especially in the bucket-type and approximated RE approach, meltwater release is slower than in the measurements. Overall, solving RE for the snow cover yields the best agreement between modelled and measured runoff. On sub-daily time scales, the water transport schemes behave very differently. Also here, solving RE provides the highest agreement between modelled and measured runoff in terms of NSE coefficient (0.48), where other water transport schemes loose any predictive power. This appears to be mainly due to bad timing of meltwater release during the day. Accordingly, solving RE for the snow cover improves several aspects of modelling snow cover runoff. The additional computational cost was found to be in the order of a factor of 1.5.

Description: An upper-bound estimate for the accuracy of volume-area scaling The Cryosphere Discussions, 7, 2293-2331, 2013 Author(s): D. Farinotti and M. Huss Volume-area scaling is the most popular method for estimating the ice volume of large glacier samples. Here, a series of resampling experiments based on different sets of synthetic data are presented in order to derive an upper-bound estimate (i.e. a level achieved only with ideal conditions) for the accuracy of its application. We also quantify the maximum accuracy expected when scaling is used for determining the glacier volume change, and area change of a given glacier population. A comprehensive set of measured glacier areas, volumes, area and volume changes is evaluated to investigate the impact of real-world data quality on the so assessed accuracies. For populations larger than a few thousand glaciers, the total ice volume can be recovered within 30% if all measurements available worldwide are used for estimating the scaling coefficients. Assuming no systematic biases in ice volume measurements, their uncertainty is of secondary importance. Knowing the individual areas of a glacier sample for two points in time allows recovering the corresponding ice volume change within 40% for populations larger than a few hundred glaciers, both for steady-state and transient geometries. If ice volume changes can be estimated without bias, glacier area changes derived from volume-area scaling show similar uncertainties as for the volume changes. This paper does not aim at making a final judgement about the suitability of volume-area scaling, but provides the means for assessing the accuracy expected from its application.

Description: Evaluation of the snow regime in dynamic vegetation land surface models using field measurements The Cryosphere Discussions, 7, 2333-2372, 2013 Author(s): E. Kantzas, M. Lomas, S. Quegan, and E. Zakharova An increasing number of studies have demonstrated the significant climatic and ecological changes occurring in the northern latitudes over the past decades. As coupled, earth-system models attempt to describe and simulate the dynamics and complex feedbacks of the Arctic environment, it is important to reduce their uncertainties in short-term predictions by improving the description of both the systems processes and its initial state. This study focuses on snow-related variables and extensively utilizes a historical data set (1966–1996) of field snow measurements acquired across the extend of the Former Soviet Union (FSU) to evaluate a range of simulated snow metrics produced by a variety of land surface models, most of them embedded in IPCC-standard climate models. We reveal model-specific issues in simulating snow dynamics such as magnitude and timings of SWE as well as evolution of snow density. We further employ the field snow measurements alongside novel and model-independent methodologies to extract for the first time (i) a fresh snow density value (57–117 kg m –3 ) for the region and (ii) mean monthly snowpack sublimation estimates across a grassland-dominated western (November–February) [9.2, 6.1, 9.15, 15.25] mm and forested eastern sub-sector (November–March) [1.53, 1.52, 3.05, 3.80, 12.20] mm; we subsequently use the retrieved values to assess relevant model outputs. The discussion session consists of two parts. The first describes a sensitivity study where field data of snow depth and snow density are forced directly into the surface heat exchange formulation of a land surface model to evaluate how inaccuracies in simulating snow metrics affect important modeled variables and carbon fluxes such as soil temperature, thaw depth and soil carbon decomposition. The second part showcases how the field data can be assimilated with ready-available optimization techniques to pinpoint model issues and improve their performance.

Description: Spatial debris-cover effect on the maritime glaciers of Mount Gongga, south-eastern Tibetan Plateau The Cryosphere Discussions, 7, 2413-2453, 2013 Author(s): Y. Zhang, Y. Hirabayashi, K. Fujita, S. Liu, and Q. Liu The Tibetan Plateau and surroundings contain a large number of debris-covered glaciers, on which debris cover affects glacier response to climate change by altering ice melting rates and spatial patterns of mass loss. Insufficient spatial distribution of debris thickness data makes it difficult to analyze regional debris-cover effects. Mount Gongga glaciers, maritime glaciers in the south-eastern Tibetan Plateau, are characterized by a substantial reduction in glacier length and ice mass in recent decades. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-derived thermal property of the debris layer reveals that 68% of the glaciers have extensive mantles of supraglacial debris in their ablation zones, in which the proportion of debris cover to total glacier area varies from 1.74% to 53.0%. Using a surface energy-mass balance model accounting for the debris-cover effect applied at a regional scale, we find that although the presence of supraglacial debris has a significant insulating effect on heavily debris-covered glaciers, it accelerates ice melting on ~ 10.2% of the total ablation area and produces rapid wastage of ~ 25% of the debris-covered glaciers, resulting in the similar mass losses between debris-covered and debris-free glaciers. Widespread debris cover also facilitates the development of active terminus regions. Regional differences in the debris-cover effect are apparent, highlighting the importance of debris cover for understanding glacier status and hydrology in both the Tibetan Plateau and other mountain ranges around the world.

Description: Decadal changes from a multi-temporal glacier inventory of Svalbard The Cryosphere Discussions, 7, 2489-2532, 2013 Author(s): C. Nuth, J. Kohler, M. König, A. von Deschwanden, J. O. Hagen, A. Kääb, G. Moholdt, and R. Pettersson We present a multi-temporal digital inventory of Svalbard glaciers with the most recent from the late 2000s containing 33 775 km 2 of glaciers, or 57% of the total land area of the archipelago. At present, 68% of the glaciated area of Svalbard drains through tidewater glaciers that have a summed terminus width of ~ 740 km. The glaciated area over the entire archipelago has decreased by an average of 80 km 2 a −1 over the past ~ 30 yr, representing a reduction of 7%. For a sample of ~ 400 glaciers (10 000 km 2 ) in the south and west of Spitsbergen, three digital inventories are available from 1930/60s, 1990 and 2007 from which we calculate average changes during 2 epochs. In the more recent epoch, the terminus retreat was larger than in the earlier epoch while area shrinkage was smaller. The contrasting pattern may be explained by the decreased lateral wastage of the glacier tongues. Temporal retreat rates for individual glaciers show a mix of accelerating and decelerating trends, reflecting the large spatial variability of glacier types and climatic/dynamic response times in Svalbard. Last, retreat rates estimated by dividing glacier area changes by the tongue width are larger than centerline retreat due to a more encompassing frontal change estimate with inclusion of lateral area loss.

Description: The effect of black carbon on reflectance of snow in the accumulation area of glaciers in the Baspa basin, Himachal Pradesh, India The Cryosphere Discussions, 7, 1359-1382, 2013 Author(s): A. V. Kulkarni, G. Vinay Kumar, H. S. Negi, J. Srinivasan, and S. K. Satheesh Himalayan glaciers are being extensively debated in scientific and public forums, as changes in their distribution can significantly affect the availability of water in many rivers originating in the region. The distribution of glaciers can be influenced by mass balance, and most of the glaciers located in the Pir Panjal and Greater Himalayan mountain ranges are losing mass at the rate of almost a meter per year. The Equilibrium Line Altitude (ELA) has also shifted upward by 400 m in the last two decades. This upward migration of ELA and the loss in mass could have been influenced by changes in temperature, precipitation and by the deposition of black carbon in the accumulation area of glaciers. The deposition of black carbon can reduce the albedo of snow in the accumulation area leading to faster melting of snow and causing more negative mass balance. In this investigation, a change in reflectance in the accumulation area of the Baspa basin is analysed for the year 2009, as the region has experienced extensive forest fires along with northern Indian biomass burning. The investigation has shown that: (1) The number of forest fires in the summer of 2009 was substantially higher than in any other year between 2001 and 2010; (2) the drop in reflectance in the visible region from April to May in the accumulation area was significantly higher in the year 2009 than in any other year from 2000 to 2012; (3) the temperature of the region was substantially lower than the freezing point during the active fire period of 2009, indicating the small influence of liquid water and grain size; (4) the drop in reflectance was observed only in the visible region, indicating role of contamination; (5) in the visible region, a mean drop in reflectance of 21± 5% was observed during the active fire period in the accumulation area. At some places, the drop was as high as 50 ± 5%. This can only be explained by the deposition of black carbon. The study suggests that a change in snow albedo in the accumulation area due to the deposition of black carbon from anthropogenic and natural causes can influence the mass balance of the glaciers in the Baspa basin, Himachal Pradesh, India.

Description: Supercooled interfacial water in fine grained soils probed by dielectric spectroscopy The Cryosphere Discussions, 7, 1441-1493, 2013 Author(s): A. Lorek and N. Wagner Water as thermodynamic state parameter affects nearly all physical, chemical and biological processes on the earth. Recent Mars observations as well as laboratory investigations suggest that water is also a key factor of current physical and chemical processes on the martian surface, e.g. rheological phenomena. Therefore it is of particular interest to get information about the liquid like state of water on martian analog soils in the temperature range below 0 °C. In this context, a parallel plate capacitor has been developed to obtain isothermal dielectric spectra of fine grained soils in the frequency range from 10 Hz to 1.1 MHz at martian like temperatures down to −70 °C. Two martian analogue soils have been investigated: a Ca-Bentonite (specific surface of 237 m 2 g −1 , up to 9.4% w/w gravimetric water content) and JSC Mars 1, a volcanic ash (specific surface of 146 m 2 g −1 , up to 7.4% w/w ). Three soil-specific relaxation processes are observed in the investigated frequency-temperature range: two weak high frequency processes (bound or hydrated water as well as ice) and a strong low frequency process due to counter ion relaxation and the Maxwell–Wagner effect. To characterize the dielectric relaxation behavior, a generalized fractional dielectric relaxation model is applied assuming three active relaxation processes with relaxation time of the i th process according to an Eyring equation. The real part of effective complex soil permittivity at 350 kHz was used to determine ice and liquid like water content by means of the Birchak or CRIM equation. There are evidence that Bentonite down to −70 °C has a liquid like water content of 1.17 monolayers and JSC Mars 1 a liquid like water content of 1.96 mono layers.

Description: Influence of snow depth distribution on surface roughness in alpine terrain: a multi-scale approach The Cryosphere Discussions, 7, 4633-4680, 2013 Author(s): J. Veitinger, B. Sovilla, and R. S. Purves In alpine terrain, the snow covered winter surface deviates from its underlying summer terrain due to the progressive smoothing caused by snow accumulation. Terrain smoothing is believed to be an important factor in avalanche formation, avalanche dynamics and affects surface heat transfer, energy balance as well as snow depth distribution. To characterize the effect of snow on terrain we use the concept of roughness. Roughness is calculated for several snow surfaces and its corresponding underlying terrain for three alpine basins in the Swiss Alps characterized by low medium and high terrain roughness. To this end, elevation models of winter and summer terrain are derived from high-resolution (1 m) measurements performed by airborne and terrestrial LIDAR. We showed that on basin scale terrain smoothing not only depends on mean snow depth in the basin but also on its variability. Terrain smoothing can be modelled in function of mean snow depth and its standard deviation using a power law. However, a relationship between terrain smoothing and snow depth does not exist on a pixel scale. Further we demonstrated the high persistence of snow surface roughness even in between winter seasons. Those persistent patterns might be very useful to improve the representation of a winter terrain without modelling of the snow cover distribution. This can potentially improve avalanche release area definition and in the long term natural hazard management strategies.

Description: High sensitivity of tidewater outlet glacier dynamics to shape The Cryosphere Discussions, 7, 551-572, 2013 Author(s): E. M. Enderlin, I. M. Howat, and A. Vieli Variability in tidewater outlet glacier behavior under similar external forcing has been attributed to differences in outlet shape (i.e. bed elevation and width), but this dependence has not been investigated in detail. Here we use a numerical ice flow model to show that the dynamics of tidewater outlet glaciers under external forcing are highly sensitive to width and bed topography. Our sensitivity tests indicate that for glaciers with similar discharge, the trunks of wider glaciers and those grounded over deeper basal depressions tend to be closer to flotation, so that less dynamically induced thinning results in rapid, unstable retreat following a perturbation. The lag time between the onset of the perturbation and unstable retreat varies with outlet shape, which may help explain intra-regional variability in tidewater outlet glacier behavior. Further, because the perturbation response is dependent on the thickness relative to flotation, varying the bed topography within the range of observational uncertainty can result in either stable or unstable retreat due to the same perturbation. Thus, extreme care must be taken when interpreting the future behavior of actual glacier systems using numerical ice flow models that are not accompanied by comprehensive sensitivity analyses.

Description: Effect of uncertainty in surface mass balance elevation feedback on projections of the future sea level contribution of the Greenland ice sheet – Part 2: Projections The Cryosphere Discussions, 7, 675-708, 2013 Author(s): T. L. Edwards, X. Fettweis, O. Gagliardini, F. Gillet-Chaulet, H. Goelzer, J. M. Gregory, M. Hoffman, P. Huybrechts, A. J. Payne, M. Perego, S. Price, A. Quiquet, and C. Ritz We apply a new parameterisation of the Greenland ice sheet (GrIS) feedback between surface mass balance (SMB: the sum of surface accumulation and surface ablation) and surface elevation in the MAR regional climate model (Edwards et al., 2013) to projections of future climate change using five ice sheet models (ISMs). The MAR climate projections are for 2000–2199, forced by the ECHAM5 and HadCM3 global climate models (GCMs) under the SRES A1B emissions scenario. The additional sea level contribution due to the SMB-elevation feedback averaged over five ISM projections for ECHAM5 and three for HadCM3 is 4.3% (best estimate; 95% credibility interval 1.8–6.9%) at 2100, and 9.6% (best estimate; 95% credibility interval 3.6–16.0%) at 2200. In all results the elevation feedback is significantly positive, amplifying the GrIS sea level contribution relative to the MAR projections in which the ice sheet topography is fixed: the lower bounds of our 95% credibility intervals (CIs) for sea level contributions are larger than the "no feedback" case for all ISMs and GCMs. Our method is novel in sea level projections because we propagate three types of modelling uncertainty – GCM and ISM structural uncertainties, and elevation feedback parameterisation uncertainty – along the causal chain, from SRES scenario to sea level, within a coherent experimental design and statistical framework. The relative contributions to uncertainty depend on the timescale of interest. At 2100, the GCM uncertainty is largest, but by 2200 both the ISM and parameterisation uncertainties are larger. We also perform a~perturbed parameter ensemble with one ISM to estimate the shape of the projected sea level probability distribution; our results indicates that the probability density is slightly skewed towards higher sea level contributions.

Description: Effect of uncertainty in surface mass balance–elevation feedback on projections of the future sea level contribution of the Greenland ice sheet – Part 1: Parameterisation The Cryosphere Discussions, 7, 635-674, 2013 Author(s): T. L. Edwards, X. Fettweis, O. Gagliardini, F. Gillet-Chaulet, H. Goelzer, J. M. Gregory, M. Hoffman, P. Huybrechts, A. J. Payne, M. Perego, S. Price, A. Quiquet, and C. Ritz We present a new parameterisation that relates surface mass balance (SMB: the sum of surface accumulation and surface ablation) to changes in surface elevation of the Greenland ice sheet (GrIS) for the MAR regional climate model. The motivation is to dynamically adjust SMB as the GrIS evolves, allowing us to force ice sheet models with SMB simulated by MAR while incorporating the SMB–elevation feedback, without the substantial technical challenges of coupling the two models. This also allows us to assess the effect of elevation feedback uncertainty on the GrIS contribution to sea level, using multiple global climate and ice sheet models, without the need for additional, expensive MAR simulations. We estimate this relationship separately below and above the equilibrium line altitude (ELA, separating negative and positive SMB) and for regions north and south of 77° N, from a set of MAR simulations in which we alter the ice sheet surface elevation. These give four "SMB lapse rates", gradients that relate SMB changes to elevation changes. We assess uncertainties within a Bayesian framework, estimating probability distributions for each gradient from which we present best estimates and credibility intervals (CIs) that bound 95% of the probability. Below the ELA our gradient estimates are mostly positive, because SMB usually increases with elevation: 0.54 (95% CI: −0.22 to 1.34) kg m −3 a −1 for the north, and 1.89 (1.03 to 2.61) kg m −3 a −1 for the south. Above the ELA the gradients are much smaller: 0.09 (−0.03 to 0.22) kg m −3 a −1 in the north, and 0.06 (−0.07 to 0.56) kg m −3 a −1 in the south, because SMB can either increase or decrease in response to increased elevation. Our statistically based approach allows us to make probabilistic assessments for the effect of elevation feedback uncertainty on sea level projections. In a companion paper we use the best estimates and upper and lower CI bounds in five ice sheet models, and the full probability distributions in another, to adjust simulated SMB from MAR forced by two global climate models for the SRES A1B scenario (Edwards et al., 2013).

Description: Snowdrift modelling for Vestfonna ice cap, north-eastern Svalbard The Cryosphere Discussions, 7, 709-741, 2013 Author(s): T. Sauter, M. Möller, R. Finkelnburg, M. Grabiec, D. Scherer, and C. Schneider The redistribution of snow by drifting and blowing snow frequently leads to an inhomogeneous snow mass distribution on larger ice caps. Together with the thermodynamic impact of drifting snow sublimation on the lower atmospheric boundary layer, these processes affect the glacier surface mass balance. This study provides a first quantification of snowdrift and sublimation of blowing and drifting snow on Vestfonna ice cap (Svalbard) by using the specifically designed "snow2blow" snowdrift model. The model is forced by atmospheric fields from the Weather Research and Forecasting model and resolves processes on a spatial resolution of 250 m. Comparison with radio-echo soudings and snow-pit measurements show that important local scale processes are resolved by the model and the overall snow accumulation pattern is reproduced. The findings indicate that there is a significant redistribution of snow mass from the interior of the ice cap to the surrounding areas and ice slopes. Drifting snow sublimation of suspended snow is found to be stronger during winter. It is concluded that both processes are strong enough to have a significant impact on glacier mass balance.

Description: Sensitivity of lake ice regimes to climate change in the nordic region The Cryosphere Discussions, 7, 743-788, 2013 Author(s): S. Gebre, T. Boissy, and K. Alfredsen A one-dimensional process-based multi-year lake ice model, MyLake, was used to simulate lake ice phenology and annual maximum lake ice thickness for the Nordic region comprising Fennoscandia and the Baltic countries. The model was first tested and validated using observational meteorological forcing on a candidate lake (Lake Atnsjøen) and using downscaled ERA-40 reanalysis data set. To simulate ice conditions for the contemporary period of 1961–2000, the model was driven by gridded meteorological forcings from ERA-40 global reanalysis data downscaled to a 25 km resolution using the Rossby Center Regional Climate Model (RCA). The model was then forced with two future climate scenarios from the RCA driven by two different GCMs based on the SRES A1B emissions scenario. The two climate scenarios correspond to two future time periods namely the 2050s (2041–2070) and the 2080s (2071–2100). To take into account the influence of lake morphometry, simulations were carried out for four different hypothetical lake depths (5 m, 10 m, 20 m, 40 m) placed at each of the 3708 grid cells. Based on a comparison of the mean predictions in the future 30 yr periods with the control (1961–1990) period, ice cover durations in the region will be shortened by 1 to 11 weeks in 2041–2070, and 3 to 14 weeks in 2071–2100. Annual maximum lake ice thickness, on the other hand, will be reduced by a margin of up to 60 cm by 2041–2070 and up to 70 cm by 2071–2100. The simulated changes in lake ice characteristics revealed that the changes are less dependent on lake depths though there are slight differences. The results of this study provide a~regional perspective of anticipated changes in lake ice regimes due to climate warming across the study area by the middle and end of this century.

Description: Uncertainties and re-analysis of glacier mass balance measurements The Cryosphere Discussions, 7, 789-839, 2013 Author(s): M. Zemp, E. Thibert, M. Huss, D. Stumm, C. Rolstad Denby, C. Nuth, S. U. Nussbaumer, G. Moholdt, A. Mercer, C. Mayer, P. C. Joerg, P. Jansson, B. Hynek, A. Fischer, H. Escher-Vetter, H. Elvehøy, and L. M. Andreassen Glacier-wide mass balance has been measured for more than sixty years and is widely used as an indicator of climate change and to assess the glacier contribution to runoff and sea level rise. Until present, comprehensive uncertainty assessments have rarely been carried out and mass balance data have often been applied using rough error estimation or without error considerations. In this study, we propose a framework for re-analyzing glacier mass balance series including conceptual and statistical toolsets for assessment of random and systematic errors as well as for validation and calibration (if necessary) of the glaciological with the geodetic balance results. We demonstrate the usefulness and limitations of the proposed scheme drawing on an analysis that comprises over 50 recording periods for a dozen glaciers and we make recommendations to investigators and users of glacier mass balance data. Reanalysis of glacier mass balance series needs to become a standard procedure for every monitoring programme to improve data quality and provide thorough uncertainty estimates.

Description: An iterative inverse method to estimate basal topography and initialize ice flow models The Cryosphere Discussions, 7, 873-920, 2013 Author(s): W. J. J. van Pelt, J. Oerlemans, C. H. Reijmer, R. Pettersson, V. A. Pohjola, E. Isaksson, and D. Divine We present and evaluate an inverse approach to reconstruct two-dimensional fields of bedrock topography and simultaneously initialize an ice flow model. The inverse method involves an iterative procedure in which an ice dynamical model (PISM) is run multiple times over a prescribed period, while being forced with space and time-dependent climate input. After every iteration bed heights are adjusted using information of the remaining misfit between observed and modeled surface topography. The inverse method is first applied in synthetic experiments with a constant climate forcing to verify convergence and robustness of the approach. In a next step, the inverse approach is applied to Nordenskiöldbreen, Svalbard, forced with height- and time-dependent climate input since 1300 AD. An L-curve stopping criterion is used to prevent overfitting. Validation against radar data reveals a high correlation (up to R = 0.89) between modeled and observed thicknesses. Remaining uncertainties can mainly be ascribed to inaccurate model physics, in particular uncertainty in the description of sliding. Results demonstrate the applicability of this inverse method to reconstruct the ice thickness distribution of glaciers and ice caps. In addition to reconstructing bedrock topography, the method provides a direct tool to initialize ice flow models for forecasting experiments. Application of the method is not constrained to a single model or glacier, indicating the potential to use the approach to compute the detailed thickness distribution of a single glacier, as well as the volume contained in a set of glaciers and ice caps.

Description: Cascading water underneath Wilkes Land, East Antarctic Ice Sheet, observed using altimetry and digital elevation models The Cryosphere Discussions, 7, 841-871, 2013 Author(s): T. Flament, E. Berthier, and F. Rémy We describe a major subglacial lake drainage close to the ice divide in Wilkes Land, East Antarctica, and the subsequent cascading of water underneath the ice sheet toward the coast. To analyze the event, we combined altimetry data from several sources and bedrock data. We estimated the total volume of water that drained from Lake Cook E2 by differencing digital elevation models (DEM) derived from ASTER and SPOT5 stereo-imagery. With 5.2 ± 0.5 km 3 , this is the largest single subglacial drainage event reported so far in Antarctica. Elevation differences between ICESat laser altimetry and the SPOT5 DEM indicate that the discharge lasted approximately 2 yr. A 13-m uplift of the surface, corresponding to a refilling of about 0.64 ± 0.32 km 3 , was observed between the end of the discharge in October 2008 and February 2012. Using Envisat radar altimetry, with its high 35-day temporal resolution, we monitored the subsequent filling and drainage of connected subglacial lakes located downstream. In particular, a transient temporal signal can be detected within the theoretical 500-km long flow paths computed with the BEDMAP2 data set. The volume of water traveling in this wave is in agreement with the volume that drained from Lake Cook E2 . These observations contribute to a better understanding of the water transport beneath the East Antarctic ice sheet.

Description: An inventory of glacier changes between 1973 and 2011 for the Geladandong Mountain area, China The Cryosphere Discussions, 7, 507-531, 2013 Author(s): J. Zhang, D. Braaten, X. Li, J. She, and F. Tao The snow and ice of the Geladangong Mountain area supply the headwaters of the Yangtze River, and long-term changes to glaciers and ice masses in this region due to a warming climate are of great concern. An inventory of glacier boundaries and changes over decades for the Geladandong Mountain area in China has been conducted using remote sensing imagery from Landsat (MSS, TM, ETM+), CERBES CCD, and GIS techniques. Variations in glacier extent has been measured using a~series of digital images since 1973, including Landsat MSS in 1973, Landsat TM in 1992, Landsat ETM+ in 2004, and CBERS CCD in 2011. All Landsat data are snow-free outside the glacier boundaries, allowing an unsupervised classification method to be used to extract glacier area. For the CBERS CCD data, some areas were covered by clouds and snow, requiring an initial unsupervised classification method to divide glacier, clouds and snow from other land types, followed by a supervised visual interpretation to extract glacier area. The results show a decrease in glacier ice cover in the study area during the past 38 yr. From 1973 to 2011, glacier area decreased from 107 105 hectares to 94 220 hectares, or a change of −12%. The speed at which ice cover is being lost has been decreasing during the past 38 yr. The rate of glacier area loss was 0.47% yr −1 from 1973–1992, 0.19% yr −1 from 1992–2004, and 0.14% yr −1 from 2004–2011. While most of the glaciers are shrinking, some are expanding. For the 1973 to 2004 period, retreating glaciers exposed 14 447 hectares of land, and advancing glaciers spread over 2682 hectares that were not covered by ice in 1973. The net glacier area decrease is 11 765 hectares from 1973–2004. For the 1973 to 2011 period, glaciers expanded over 3791 hectares, and retreated from 16 504 hectares.

Description: Characterizing supraglacial lake drainage and freezing on the Greenland Ice Sheet The Cryosphere Discussions, 7, 475-505, 2013 Author(s): N. Selmes, T. Murray, and T. D. James The behaviour of supraglacial lakes on the Greenland Ice Sheet has attracted a great deal of focus, specifically with regard to their fast drainage through hydrofracturing to the ice sheet base. However, a previous study has shown that this mode of drainage accounts for only 13% of the lakes on the Greenland Ice Sheet. No published work to date has studied what happens to those lakes that do not drain suddenly. We present here three possible modes by which lakes can disappear from the ice sheet, which will have strongly contrasting effects on glacial dynamics and the ice sheet water budget. Around half of all supraglacial lakes observed persisted through the melt season and froze at the end of summer. A third drained slowly, which we interpret to be a result of incision of the supraglacial lake exit-channel. The fate of 7% of lakes could not be observed due to cloud cover, and the remainder drained suddenly. Both fast and slow lake drainage types are absent at higher elevations where lakes tend to freeze despite having similar or longer life spans to lakes at lower elevations, suggesting the mechanisms of drainage are inhibited. Groups of neighbouring lakes were observed to drain suddenly on the same day suggesting a common trigger mechanism for drainage initiation. We find that great care must be taken when interpreting remotely sensed observations of lake drainage, as fast and slow lake drainage can easily be confused if the temporal resolution used is too coarse.

Description: Spectral reflectance of solar light from dirty snow: a simple theoretical model and its validation The Cryosphere Discussions, 7, 533-550, 2013 Author(s): A. Kokhanovsky A simple analytical equation for the snow albedo as the function of snow grain size, soot concentration, and soot mass absorption coefficient is presented. This simple equation can be used in climate models to assess the influence of snow pollution on snow albedo. It is shown that the squared logarithm of the albedo (in the visible) is directly proportional to the soot concentration. A new method of the determination of the soot mass absorption coefficient in snow is proposed. The equations derived are applied to a dusty snow layer as well.

Description: A combined approach of remote sensing and airborne electromagnetics to determine the volume of polynya sea ice in the Laptev Sea The Cryosphere Discussions, 7, 441-473, 2013 Author(s): L. Rabenstein, T. Krumpen, S. Hendricks, C. Koeberle, C. Haas, and J. A. Hoelemann A combined interpretation of synthetic aperture radar (SAR) satellite images and helicopter electromagnetic (HEM) sea-ice thickness data has provided an estimate of sea-ice volume formed in Laptev Sea polynyas during the winter of 2007/08. The evolution of the surveyed sea-ice areas, which were formed between late December 2007 and middle April 2008, was tracked using a series of SAR images with a sampling interval of 2–3 days. Approximately 160 km of HEM data recorded in April 2008 provided sea-ice thicknesses along profiles that transected sea-ice varying in age from 1–116 days. For the volume estimates, thickness information along the HEM profiles was extrapolated to zones of the same age. The error of areal mean thickness information was estimated to be between 0.2 m for younger ice and up to 1.55 m for older ice, with the primary error source being the spatially limited HEM coverage. Our results have demonstrated that the modal thicknesses and mean thicknesses of level ice correlated with the sea-ice age, but that varying dynamic and thermodynamic sea-ice growth conditions resulted in a rather heterogeneous sea-ice thickness distribution on scales of tens of kilometers. Taking all uncertainties into account, total sea-ice area and volume produced within the entire surveyed area were 52 650 km 2 and 93.6 ± 26.6 km 3 . The surveyed polynya contributed 2.0 ± 0.5% of the sea-ice produced throughout the Arctic during the 2007/08 winter. The SAR-HEM volume estimate compares well with the 112 km 3 ice production calculated with a high resolution ocean sea-ice model. Measured modal and mean-level ice thicknesses correlate with calculated freezing-degree-day thicknesses with a factor of 0.87–0.89, which was too low to justify the assumption of homogeneous thermodynamic growth conditions in the area, or indicates a strong dynamic thickening of level ice by rafting of even thicker ice.

Description: Weekly-gridded Aquarius L-band radiometer/scatterometer observations and salinity retrievals over the polar regions: applications for cryospheric studies The Cryosphere Discussions, 7, 5921-5970, 2013 Author(s): L. Brucker, E. Dinnat, and L. S. Koenig Passive and active observations at L band (frequency ~1.413 GHz) from the Aquarius/SAC-D mission offer new capabilities to study the polar regions. However, due to the lack of polar-gridded products, applications over the cryosphere are limited. To allow for an efficient use of the Aquarius data over the polar regions, and to move forward our understanding of the L-band observations of ice sheet, sea ice, permafrost, and polar oceans, we present three weekly-polar-gridded products. Aquarius data at latitudes higher than 50° were averaged and gridded into three weekly products of Brightness Temperature (TB), Normalized Radar Cross Section (NRCS), and Sea Surface Salinity (SSS). These products are suited for cryospheric studies, and each grid cell contains sea ice fraction and the standard deviation of TB, NRCS, and SSS along with the number of footprint observations collected during the seven-day cycle. The data sets are produced on the version 2.0 Equal-Area Scalable Earth (EASE2.0) grid, with a grid cell resolution of 36 km. This paper describes the products, and presents maps and time series of Aquarius weekly-gridded data over the Greenland and Antarctic ice sheets, sea ice in both hemispheres, subarctic land where seasonal snow and frozen soil may exist, and the polar oceans. A brief analysis of L-band observations is given to encourage future use of the products. They can be used for improving our understanding of low microwave frequency observations, and for the development of algorithms. The new weekly-polar-gridded datasets start in August 2011, with the first Aquarius observations, and it is anticipated that they will be updated on a monthly basis following the release schedule of the Level 2 data sets.

Description: Seasonal thaw settlement at drained thermokarst lake basins, Arctic Alaska The Cryosphere Discussions, 7, 5793-5822, 2013 Author(s): L. Liu, K. Schaefer, A. Gusmeroli, G. Grosse, B. M. Jones, T. Zhang, A. D. Parsekian, and H. A. Zebker Drained thermokarst lake basins (DTLBs) are ubiquitous landforms on arctic tundra lowlands, but their present-day dynamic states are seldom investigated. Here we report results based on high-resolution Interferometric Synthetic Aperture Radar (InSAR) measurements using space-borne data for a study area located near Prudhoe Bay, Alaska where we focus on the seasonal thaw settlement within DTLBs, averaged between 2006 and 2010. The majority (14) of the 18 DTLBs in the study area analyzed exhibited seasonal thaw settlement of 3–4 cm. However, four of the DTLBs analyzed exceeded 4 cm of thaw settlement, with one basin experiencing up to 12 cm. Combining the InSAR observations with the in situ active layer thickness measured using ground penetrating radar and mechanical probing, we calculated thaw strain, an index of thaw settlement strength along a transect across the basin that underwent large thaw settlement. We found thaw strains of 10–35% at the basin center, suggesting the seasonal melting of ground ice as a possible mechanism for the large settlement. These findings emphasize the dynamic nature of permafrost landforms, demonstrate the capability of the InSAR technique to remotely monitor surface deformation of individual DTLBs, and illustrate the combination of ground-based and remote sensing observations to estimate thaw strain. Our study highlights the need for better description of the spatial heterogeneity of landscape-scale processes for regional assessment of surface dynamics on arctic coastal lowlands.

Description: SMOS derived sea ice thickness: algorithm baseline, product specifications and initial verification The Cryosphere Discussions, 7, 5735-5792, 2013 Author(s): X. Tian-Kunze, L. Kaleschke, N. Maaß, M. Mäkynen, N. Serra, M. Drusch, and T. Krumpen Following the launch of ESA's Soil Moisture and Ocean salinity (SMOS) mission it has been shown that brightness temperatures at a low microwave frequency of 1.4 GHz (L-band) are sensitive to sea ice properties. In a first demonstration study, sea ice thickness has been derived using a semi-empirical algorithm with constant tie-points. Here we introduce a novel iterative retrieval algorithm that is based on a sea ice thermodynamic model and a three-layer radiative transfer model, which explicitly takes variations of ice temperature and ice salinity into account. In addition, ice thickness variations within a SMOS footprint are considered through a statistical thickness distribution function derived from high-resolution ice thickness measurements from NASA's Operation IceBridge campaign. This new algorithm has been used for the continuous operational production of a SMOS based sea ice thickness data set from 2010 on. This data set is compared and validated with estimates from assimilation systems, remote sensing data, and airborne electromagnetic sounding data. The comparisons show that the new retrieval algorithm has a considerably better agreement with the validation data and delivers a more realistic Arctic-wide ice thickness distribution than the algorithm used in the previous study.

Description: Mass change of Arctic ice caps and glaciers: implications of regionalizing elevation changes The Cryosphere Discussions, 7, 5889-5920, 2013 Author(s): J. Nilsson, L. Sandberg Sørensen, V. R. Barletta, and R. Forsberg Recent studies have determined mass changes of Arctic ice caps and glaciers from satellite altimetry. Determining regional mass balance of ice caps and glaciers using this technique is inherently difficult due to their size and geometry. Furthermore these studies have mostly relied on one method or the same types of methods to determine the regional mass balance, by extrapolating elevation changes using their relation to elevation. This makes the estimation of mass balance heavily dependent on the method used to regionalize the elevation changes. Left without consideration large discrepancies can arise in the mass change estimates and the interpretation of them. In this study we use Ice, Cloud, and land Elevation Satellite (ICESat) derived elevation changes from 2003–2009 and determine the impact of different regionalizing schemes on the mass change estimates of the Arctic ice caps and glaciers. Four different methods, based on interpolation and extrapolation of the elevation changes were used to quantify this effect on the regional mass changes. Secondly, a statistical criteria was developed to determine the optimum method for each region in order to derive robust mass changes and reduce the need of external validation data. In this study we found that the range or spread of the estimated mass changes, for the different regions, was highly correlated to the inter-annual variability of the elevation changes, driven by the different climatic conditions of the regions. Regions affected by a maritime climate show a large range in estimated values, on average 1.5–2 times larger than the predicted errors. For regions in a continental regime the opposite was observed, and the range of the values lies well inside the error estimates. We also found that the extrapolation methods tend on average to produce more negative values than the interpolation methods and that our four methods do not fully reproduce the original histogram. Instead, they produce more negative distributions than the original which may indicate that previous and these current estimates using ICESat observations might be overestimate by as much as 4–19%, depending on region. This should therefore be taken into account when deriving regional mass balance from satellite altimetry in regions which show high inter-annual variability of elevation changes. In these regions several different independent methods should be used to capture the elevation change pattern and then analyzed to determine the most suitable method. For regions in a continental climate regime, and with low variability of elevation changes, a single method may be sufficient to capture the regional elevation change pattern and hence mass balance.

Description: Large area land surface simulations in heterogeneous terrain driven by global datasets: application to mountain permafrost The Cryosphere Discussions, 7, 5853-5887, 2013 Author(s): J. Fiddes, S. Endrizzi, and S. Gruber Numerical simulations of land-surface processes are important in order to perform landscape-scale assessments of earth-systems. This task is problematic in complex terrain due to: (i) high resolution grids required to capture strong lateral variability, (ii) lack of meteorological forcing data where it is required. In this study we test a topography and climate processor, which is designed for use with large area land surface simulation, in complex and remote terrain. The scheme is driven entirely by globally available datasets. We simulate air temperature, ground surface temperature, snow depth and test the model with a large network of measurements in the Swiss Alps. We obtain RMSE values of 0.64 °C for air temperature, 0.67–1.34 °C for non-bedrock ground surface temperature, and 44.5 mm for snow depth, which is likely affected by poor input precipitation field. Due to this we trial a simple winter precipitation correction method based on melt-dates of the snow-pack. We present a test application of the scheme in the context of simulating mountain permafrost. The scheme produces a permafrost estimate of 2000 km 2 which compares well to published estimates. We suggest that this scheme represents a good first effort in application of numerical models over large areas in heterogeneous terrain.

Description: Transition of flow regime along a marine-terminating outlet glacier in East Antarctica The Cryosphere Discussions, 7, 4913-4936, 2013 Author(s): D. Callens, K. Matsuoka, D. Steinhage, B. Smith, and F. Pattyn We present results of a~multi-methodological approach to characterize the flow regime of West Ragnhild Glacier, the widest glacier in Dronning Maud Land, Antarctica. A new airborne radar survey points to substantially thicker ice (〉 2000 m) than previously thought. According to the new data, West Ragnhild Glacier discharges 13–14 Gt yr −1 . Therefore, it is one of the three major outlet glaciers in Dronning Maud Land. Glacier-bed topography is distinct between the upstream and downstream section. In the downstream section ( 〈 65 km upstream of the grounding line), the glacier overlies a wide and flat basin well below the sea level while the upstream region is more mountainous. Spectrum analysis of the bed topography reveals a clear contrast between these two regions, suggesting that the downstream area is sediment covered. The bed returned power varies by 30 dB within 20 km near the bed flatness transition, which suggests that water content at bed/ice interface increases over a short distance downstream, hence pointing to water-rich sediment. Ice flow speed observed in the downstream part of the glacier (~ 250 m yr −1 ) can only be explained if basal motion accounts for ~ 60% of the surface motion. All above lines of evidence (sediment bed, wetness and basal motion) and the relative flat grounding zone give the potential for West Ragnhild Glacier to be more sensitive to external forcing compared to other major outlet glaciers in this region which are more stable due to their bed geometry (e.g. Shirase Glacier).

Description: Assessment of heat sources on the control of fast flow of Vestfonna Ice Cap, Svalbard The Cryosphere Discussions, 7, 5097-5145, 2013 Author(s): M. Schäfer, F. Gillet-Chaulet, R. Gladstone, R. Pettersson, V. A. Pohjola, T. Strozzi, and T. Zwinger The dynamic regime of Svalbard's Nordaustlandet ice caps is dominated by fast flowing outlet glaciers, making assessment of their response to climate change challenging. A key element of the challenge lies in the fact that the motion of fast outlet glaciers occurs largely through basal sliding, and is governed by physical processes at the glacier bed, processes that are difficult both to observe and to simulate. Up to now, most of the sliding laws used in ice flow models were based on uniform parameters with a condition on temperature to identify regions of basal sliding. However these models are usually not able to reproduce observed velocities with sufficient accuracy. With the development of inverse methods, it is now common to infer a spatially varying field of sliding parameters from surface ice-velocity observations. These parameter distributions usually reflect a high spatial variability and represent valuable information to understand and test various hypotheses on physical processes involved in sliding. However, in these models, basal sliding is uncoupled from the thermal regime of basal ice and the evolution of the sliding parameters in prognostic simulations remains problematic. Here we explore the role of different heat sources (friction heating, strain heating and latent heat through percolation of melt water) on the development of sliding and fast flow through thermomechanical coupling on Nordaustlandet outlet glaciers. We focus on Vestfonna with a special emphasis on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate between 1995 and 2008 and possibly already prior to 1995. We try to reconcile the impacts of temperature and heat sources on basal friction coefficients inferred from observed surface velocities during these two periods. Our simulations reproduce a temperature profile from borehole measurements, allowing an interpretation of the vertical temperature structure in terms of temporal evolution of climate. We identify firn heating as a crucial heat source to explain Vestfonna's temperature distribution, especially in the thick areas in the center. However, friction heating is the dominant heat source at the bed of fast flowing outlet glaciers. Our findings do not support a purely temperature dependent sliding law for Vestfonna, implying that hydrology and/or sediment physics need to be represented in order to simulate fast flowing outlet glaciers.

Description: Mapping the bathymetry of supraglacial lakes and streams on the Greenland Ice Sheet using field measurements and high resolution satellite images The Cryosphere Discussions, 7, 4741-4773, 2013 Author(s): C. J. Legleiter, M. Tedesco, L. C. Smith, and B. T. Overstreet Recent melt events on the Greenland Ice Sheet (GrIS) accentuate the need to constrain estimates of sea level rise through improved characterization of meltwater pathways. This effort will require more precise estimates of the volume of water stored on the surface of the GrIS. We assessed the potential to obtain such information by mapping the bathymetry of supraglacial lakes and streams from WorldView2 (WV2) satellite images. Simultaneous {in situ} observations of depth and reflectance from two streams and a lake with measured depths up to 10.45 m were used to test a spectrally-based depth retrieval algorithm. We performed Optimal Band Ratio Analysis (OBRA) of continuous field spectra and spectra convolved to the bands of the WV2, Landsat, MODIS, and ASTER sensors. The field spectra yielded a strong relationship with depth ( R 2 = 0.94), and OBRA R 2 values were nearly as high (0.87–0.92) for convolved spectra, suggesting that these sensors' broader bands would be sufficient for depth retrieval. Our field measurements thus indicated that remote sensing of supraglacial bathymetry is not only feasible but potentially highly accurate. OBRA of spectra from 2 m-pixel WV2 images acquired within 3–72 h of our field observations produced an optimal R 2 value of 0.92 and unbiased, precise depth estimates, with mean and root-mean square errors 〈 1% and 10–25% of the mean depth. Bathymetric maps produced by applying OBRA relations revealed subtle features of lake and channel morphology. In addition to providing refined storage volume estimates for lakes of various sizes, this approach can help provide estimates of the transient flux of meltwater through streams.

Description: Morphology and distribution of liquid inclusions in young sea ice as imaged by magnetic resonance The Cryosphere Discussions, 7, 4977-5006, 2013 Author(s): R. J. Galley, B. G. T. Else, N.-X. Geilfus, A. A. Hare, D. Isleifson, L. Ryner, D. G. Barber, and S. Rysgaard In order to determine the morphology and distribution of liquid inclusions in young sea ice, magnetic resonance imaging of an 18 cm sea ice core was done using a Siemens 3T TIM TRIO human scanner. The sample was stored at about −20 °C until the beginning of a constructive interference steady state gradient echo sequence which lasted four and a half min. Here we present the first three-dimensional reconstruction of a brine drainage channel network in young sea ice using magnetic resonance imaging. The magnetic resonance image sequence data presented here clearly illustrate that brine drainage channels are established relatively quickly during ice formation, and indicates the amount and location of vertical and horizontal fluid permeability in young sea ice. A simple analysis of the image sequence reveals that magnetic resonance imaging is useful in describing the vertical profile of liquid fraction that compares well to volumes calculated for similar sea ice temperatures. Future work in this vein may include three-dimensional magnetic resonance scans of sea ice cores at in situ temperatures using different magnetic resonance sequences in order to improve the observation of inclusions, though this will necessitate both access to a scanner and the construction of a cooling system compatible with a magnetic resonance imager.

Description: Surge dynamics in the Nathorstbreen glacier system, Svalbard The Cryosphere Discussions, 7, 4937-4976, 2013 Author(s): M. Sund, T. R. Lauknes, and T. Eiken Nathorstbreen glacier system (NGS) recently experienced the largest surge in Svalbard since 1936, and is examined using spatial and temporal observations from DEM differencing, time-series of surface velocities from satellite synthetic aperture radar (SAR) and other sources. The upper basins with maximum accumulation during quiescence correspond to regions of initial lowering. Initial speed-up exceeds quiescent velocities by a factor of several tens of times. This suggests that polythermal glaciers surges are initiated in the temperate area before mass is displaced downglacier. Subsequent downglacier mass displacement coincides with areas of 100–200 times increased velocities (stage 2). After 〉 5 yr the joint NGS terminus advanced abruptly into tidewater during winter. The advance was followed by upglacier propagation of crevasses, indicating a re-action following from the already displaced mass and extending flow. NGS advanced ca. 15 km, while another ca. 3 km length was lost due to calving. Surface lowering of ca. 50 m was observed in some upglacier areas and during 5 yr the total area increased by 20%. Maximum measured flow rates were at least 25 m d −1 , 2500 times quiescence, while average velocities were about 10 m d −1 . The surges of Zawadzkibreen cycle with ca. 70 yr periods.

Description: Uncertainties in Arctic sea ice thickness and volume: new estimates and implications for trends The Cryosphere Discussions, 7, 5051-5095, 2013 Author(s): M. Zygmuntowska, P. Rampal, N. Ivanova, and L. H. Smedsrud Sea ice volume has been found to decrease in the last decades, evoked by changes in sea ice area and thickness. Estimates of sea ice area and thickness rely on a number of geophysical parameters which introduce large uncertainties. To quantify these uncertainties we use freeboard retrievals from ICESat and investigate different assumptions on snow depth, sea ice density and area. We find that uncertainties in ice area are of minor importance for the estimates of sea ice volume during the cold season in the Arctic basin. The choice of mean ice density used when converting sea ice freeboard into thickness mainly influences the resulting mean sea ice thickness, while snow depth on top of the ice is the main driver for the year-to-year variability, particularly in late winter. The absolute uncertainty in the mean sea ice thickness is 0.28 m in February/March and 0.21 m in October/November. The uncertainty in snow depth contributes up to 70% of the total uncertainty and the ice density 30–35%, with higher values in October/November. We find large uncertainties in the total sea ice volume and trend. The mean total sea ice volume is 10 120 ± 1278 km 3 in October/November and 13 254 ± 1858 km 3 in February/March for the time period 2005–2007. Based on these uncertainties we obtain trends in sea ice volume of −1445 ± 531 km^ 3 a −1 in October/November and −875 ± 257 km 3 a −1 in February/March over the ICESat period (2003–2008). Our results indicate that, taking into account the uncertainties, the decline in sea ice volume in the Arctic between the ICESat (2003–2008) and CryoSat-2 (2010–2012) periods may have been less dramatic than reported in previous studies.

Description: Modeling bulk density and snow water equivalent using daily snow depth observations. The Cryosphere Discussions, 7, 5007-5049, 2013 Author(s): J. L. McCreight and E. E. Small Bulk density is a fundamental property of snow relating its depth and mass. Previously, two simple models of bulk density (depending on snow depth, date, and location) have been developed to convert snow depth observations to snow water equivalent (SWE) estimates. However, these models were not intended for application at the daily time step. We develop a new model of bulk density for the daily timestep and demonstrate its improved skill over the existing models. Snow depth and density are negatively correlated at short (10 days) timescales while positively correlated at longer (90 days) timescales. We separate these scales of variability by modeling smoothed, daily snow depth (long time scales) and the observed positive and negative anomalies from the smoothed timeseries (short timescales) as separate terms. A climatology of fit is also included as a predictor variable. Over a half-million, daily observations of depth and SWE at 345 SNOTEL sites are used to fit models and evaluate their performance. For each location, we train the three models to the neighboring stations within 70 km, transfer the parameters to the location to be modeled, and evaluate modeled timeseries against the observations at that site. Our model exhibits improved statistics and qualitatively more-realistic behavior at the daily time step when sufficient local training data are available. We reduce density RMSE by 9.6% and 4.2% compared to previous models. Similarly, R 2 increases from 0.46 to 0.52 to 0.56 across models. Removing the challenge of parameter transfer increases R 2 scores for both the existing and new models, but the gain is greatest for the new model ( R 2 = 0.75). Our model shows general improvement over the existing models when data are more frequent than once every 5 days and at least 3 stations are available for training.

Description: Little Ice Age climate reconstruction from ensemble reanalysis of Alpine glacier fluctuations The Cryosphere Discussions, 7, 5147-5175, 2013 Author(s): M. P. Lüthi Mountain glaciers sample a combination of climate parameters – temperature, precipitation and radiation – by their rate of volume accumulation and loss. Flow dynamics acts as transfer function which maps volume changes to a length response of the glacier terminus. Long histories of terminus positions have been assembled for several glaciers in the Alps. Here I analyze terminus position histories from an ensemble of seven glaciers in the Alps with a macroscopic model of glacier dynamics to derive a history of glacier equilibrium line altitude (ELA) for the time span 400–2010 C.E. The resulting climatic reconstruction depends only on records of glacier variations. The reconstructed ELA history is similar to recent reconstructions of Alpine summer temperature and Atlantic Meridional Oscillation (AMO) index. Most reconstructed low-ELA periods coincide with large explosive volcano eruptions, hinting to mass balance reduction by volcanic radiative cooling. The glacier advances during the LIA, and the retreat after 1860 are thus explained by temperature and volcanic cooling alone.

Description: On the magnitude and frequency of Karakoram Glacier surges The Cryosphere Discussions, 7, 5177-5187, 2013 Author(s): D. J. Quincey and A. Luckman The return periods of Karakoram glacier surges are almost entirely unknown. Here, we present evidence of an historic surge of the Khurdopin Glacier that began in the mid-1970s and peaked in 1979. Measured surface displacements reached 〉 5 km yr –1 , two orders of magnitude faster than during quiescence and twice as large as any previously recorded velocity in the region. The Khurdopin Glacier next surged in the late-1990s, equating to a return period of 20 yr. Surge activity in the region needs to be better understood if accurate mass balance assessments of Hindu-Kush–Karakoram–Himalaya glaciers are to be made.

Description: Modeling the response of Lambert Glacier–Amery Ice Shelf system, East Antarctic, to uncertain climate forcing over the 21st and 22nd centuries The Cryosphere Discussions, 7, 5683-5709, 2013 Author(s): Y. Gong, S. L. Cornford, and A. J. Payne The interaction between the climate system and the large polar ice sheets regions is a key process in global environmental change. We carried out ice dynamic simulations of one of the largest drainage systems in East Antarctica: the Lambert Glacier–Amery Ice Shelf system, with an adaptive mesh ice sheet model. The ice sheet model is driven by surface accumulation and basal melt rates computed by two ocean and two atmosphere models. The change of the ice thickness and velocity in the ice shelf is mainly influenced by the basal melting distribution, but, although the ice shelf thins in the most of the simulations, there is little grounding line retreat. We find that the Lambert Glacier grounding line can retreat as much as 30 km if there is sufficient thinning of the ice shelf south of Clemence Massif, but none of the ocean models provide sufficiently high melt rates in that region. Overall, the increased accumulation computed by the atmosphere models outweighs ice stream acceleration so that the net contribution to sea level rise is negative.

Description: Importance of basal processes in simulations of a surging Svalbard outlet glacier The Cryosphere Discussions, 7, 5823-5851, 2013 Author(s): R. Gladstone, M. Schäfer, T. Zwinger, Y. Gong, T. Strozzi, J. Moore, R. Mottram, and F. Boberg The outlet glacier of Basin 3 (B3) of Austfonna icecap, Svalbard, is one of the fastest outlet glaciers in Svalbard, and shows dramatic changes since 1995. In addition to previously observed seasonal summer speed up associated with the melt season, the winter speed of B3 has accelerated approximately five fold since 1995. We use the Elmer/Ice full Stokes model for ice dynamics to infer spatial distributions of basal drag for the winter seasons of 1995, 2008 and 2011. This "inverse" method is based on minimising discrepancy between modelled and observed surface velocities, using satellite remotely sensed velocity fields. We generate steady state temperature distributions for the three time periods. Frictional heating caused by basal sliding contributes significantly to basal temperatures of the B3 outlet glacier, which exhibits a uniform steady state basal temperature at pressure melting point in all three cases. We present a sensitivity experiment consisting of transient simulations under present day forcing to demonstrate that using a temporally fixed basal drag field obtained through inversion can lead to thickness change errors of the order of 2 m per year. Hence it is essential to incorporate the evolution of basal processes in future projections of the basin. Informed by a combination of our inverse method results and previous studies, we hypothesize a system of processes and feedbacks involving till deformation and basal hydrology to explain both the seasonal accelerations and the ongoing inter-annual speed up, and speculate on the wider relevance of deformable till mechanics to non-surging glaciers.

Description: Response of ice cover on shallow lakes of the North Slope of Alaska to contemporary climate conditions (1950–2011): radar remote sensing and numerical modeling data analysis The Cryosphere Discussions, 7, 3783-3821, 2013 Author(s): C. M. Surdu, C. R. Duguay, L. C. Brown, and D. Fernández Prieto Air temperature and winter precipitation changes over the last five decades have impacted the timing, duration, and thickness of the ice cover on Arctic lakes as shown by recent studies. In the case of shallow tundra lakes, many of which are less than 3 m deep, warmer climate conditions could result in thinner ice covers and consequently, to a smaller fraction of lakes freezing to their bed in winter. However, these changes have not yet been comprehensively documented. The analysis of a 20 yr time series of ERS-1/2 synthetic aperture radar (SAR) data and a numerical lake ice model were employed to determine the response of ice cover (thickness, freezing to the bed, and phenology) on shallow lakes of the North Slope of Alaska (NSA) to climate conditions over the last six decades. Analysis of available SAR data from 1991–2011, from a sub-region of the NSA near Barrow, shows a reduction in the fraction of lakes that freeze to the bed in late winter. This finding is in good agreement with the decrease in ice thickness simulated with the Canadian Lake Ice Model (CLIMo), a lower fraction of lakes frozen to the bed corresponding to a thinner ice cover. Observed changes of the ice cover show a trend toward increasing floating ice fractions from 1991 to 2011, with the greatest change occurring in April, when the grounded ice fraction declined by 22% (α = 0.01). Model results indicate a trend toward thinner ice covers by 18–22 cm (no-snow and 53% snow depth scenarios, α = 0.01) during the 1991–2011 period and by 21–38 cm (α = 0.001) from 1950–2011. The longer trend analysis (1950–2011) also shows a decrease in the ice cover duration by ∼24 days consequent to later freeze-up dates by 5.9 days (α = 0.1) and earlier break-up dates by 17.7–18.6 days (α = 0.001).

Description: Albedo over snow and ice penitents The Cryosphere Discussions, 7, 3823-3851, 2013 Author(s): J. Abermann, C. Kinnard, and S. Lhermitte Both satellite and ground-based broadband albedo measurements over complex terrain show several limitations concerning feasibility and representativeness. In this study a series of experiments on Glaciar Tapado in the semi-arid Andes of Northern Chile is used to investigate the vertical dependence of albedo over a penitent-covered surface. The albedo–height relationship depends on the surface properties: over medium-sized snow penitents albedo increases with height, whereas over ice penitents little changes were found above the penitent tips and varying responses within the penitent troughs. The governing factor is the surface geometry and the ratio of penitent height to distance between their tips. Based on a model experiment it is shown that large parts of the variations above the tips can be explained geometrically, by varying influence of a confined albedo anomaly on a hemispherical sensor. Furthermore, the temporal evolution of broadband albedo over a penitent-covered surface is analyzed. In this context the albedo throughout two ablation seasons is discussed to place the experiments into a larger temporal context. Albedo measurements at an automated weather station show that broadband albedo over a penitents is low compared to expected values on a smooth surface. Albedo decreases early in the ablation season, and stabilizes from February onwards with variations being caused by fresh snow-fall events. The 2009/2010 and 2011/2012 seasons differ notably, where the latter shows lower albedo related to a different penitent evolution resulting in larger penitents. Finally, a comparison of the ground-based albedo observations with Landsat and MODIS-derived albedo showed that both satellite derived albedo products capture the albedo evolution with root mean square errors of 0.08 and 0.15, respectively, but also illustrate their shortcomings related to temporal resolution and spatial heterogeneity over mountain glaciers.

Description: A glacial systems model configured for large ensemble analysis of Antarctic deglaciation The Cryosphere Discussions, 7, 1533-1589, 2013 Author(s): R. Briggs, D. Pollard, and L. Tarasov This article describes the Memorial University of Newfoundland/Penn State University (MUN/PSU) glacial systems model (GSM) that has been developed specifically for large-ensemble data-constrained analysis of past Antarctic Ice Sheet evolution. Our approach emphasizes the introduction of a large set of model parameters to explicitly account for the uncertainties inherent in the modelling of such a complex system. At the core of the GSM is a 3-D thermo-mechanically coupled ice sheet model that solves both the shallow ice and shallow shelf approximations. This enables the different stress regimes of ice sheet, ice shelves, and ice streams to be represented. The grounding line is modelled through an analytical sub-grid flux parametrization. To this dynamical core the following have been added: a heavily parametrized basal drag component; a visco-elastic isostatic adjustment solver; a diverse set of climate forcings (to remove any reliance on any single method); tidewater and ice shelf calving functionality; and a new physically-motivated empirically-derived sub-shelf melt (SSM) component. To assess the accuracy of the latter, we compare predicted SSM values against a compilation of published observations. Within parametric and observational uncertainties, computed SSM for the present day ice sheet is in accord with observations for all but the Filchner ice shelf. The GSM has 31 ensemble parameters that are varied to account (in part) for the uncertainty in the ice-physics, the climate forcing, and the ice-ocean interaction. We document the parameters and parametric sensitivity of the model to motivate the choice of ensemble parameters in a quest to approximately bound reality (within the limits of 31 parameters).

Description: Changes in glacier Equilibrium-Line Altitude (ELA) in the western Alps over the 1984–2010 period: evaluation by remote sensing and modeling of the morpho-topographic and climate controls The Cryosphere Discussions, 7, 2247-2291, 2013 Author(s): A. Rabatel, A. Letréguilly, J.-P. Dedieu, and N. Eckert We present time series of equilibrium-line altitude (ELA) measured from the end-of-summer snowline altitude computed using satellite images, for 43 glaciers in the western Alps over the 1984–2010 period. More than 120 satellite images acquired by Landsat, SPOT and ASTER were used. In parallel, changes in climate parameters (summer cumulative positive degree days, CPDD, and winter precipitation) were analyzed over the same time period using 22 weather stations located inside and around the study area. Assuming a continuous linear trend over the study period: (1) the average ELA of the 43 glaciers increased by about 170 m; (2) summer CPDD increased by about 150 PDD at 3000 m a.s.l.; and (3) winter precipitation remained rather stationary. Summer CPDD showed homogeneous spatial and temporal variability; winter precipitation showed homogeneous temporal variability, but some stations showed a slightly different spatial pattern. Regarding ELAs, temporal variability between the 43 glaciers was also homogeneous, but spatially, glaciers in the southern part of the study area differed from glaciers in the northern part, mainly due to a different precipitation pattern. A sensitivity analysis of the ELAs to climate and morpho-topographic parameters (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morpho-topographic parameters; and (2) the interannual variability of the ELA is strongly controlled by climate parameters, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear low frequency fluctuations appear to be driven by winter precipitation anomalies. Finally, we used an expansion of Lliboutry's approach to reconstruct fluctuations in the ELA of any glacier of the study area with respect to morpho-topographic and climate parameters, by quantifying their respective weight and the related uncertainties in a consistent manner within a hierarchical Bayesian framework. This method was tested and validated using the ELA measured on the satellite images.

Description: A record of Antarctic sea ice extent in the Southern Indian Ocean for the past 300 yr and its relationship with global mean temperature The Cryosphere Discussions, 7, 3611-3625, 2013 Author(s): C. Xiao, R. Li, S. B. Sneed, T. Dou, and I. Allison The differing response of ice extent in the Arctic and Antarctic to global average temperature change, over approximately the last three decades, highlights the importance of reconstructing long-term sea ice history. Here, using high-resolution ice core records of methanesulfonate (MS − ) from the East Antarctic Ice Sheet in Princess Elizabeth Land, we reconstruct southern Indian Ocean sea ice extent (SIE) for the sector 70° E–100° E for the period 1708–2000 A.D. Annual MS − concentration positively correlates in this sector with satellite-derived SIE for the period 1973–2000 ( P 〈 0.05). The 293 yr MS − record of proxy SIE shows multi-decadal variations, with large decreases occurring in two warm intervals during the Little Ice Age, and during the 1940s. However, after the 1980s there is a change in phase between Antarctic SIE and global temperature change, with both increasing. This paradox is probably attributable to the strong anomaly in the Southern Annular Mode (SAM) in the recent three decades.

Description: Climate change implications for the glaciers of the Hindu-Kush, Karakoram and Himalayan region The Cryosphere Discussions, 7, 3717-3748, 2013 Author(s): A. J. Wiltshire The Hindu-Kush, Karakoram Himalaya (HKKH) region has a negative average glacial mass balance despite anomalous possible gains in the Karakoram. However, changes in climate may influence the mass balance across the HKKH. We use high resolution climate modelling to analyse the implications of unmitigated climate change on precipitation, snowfall, air temperature and accumulated degree days for the Hindu Kush, Karakoram, Jammu-Kashmir, Himachal Pradesh and West Nepal regions, and East Nepal and Bhutan. In our analysis we focus on the climate drivers of change rather than the glaciological response. We find a complex regional response to climate change, with possible increases in snowfall over the western HKKH and decreases in the east. Accumulated degree days are less spatially variable than precipitation and show an increase in potential ablation in all regions. Overall, the eastern Himalayan glaciers are expected to be most sensitive to climate change due to the decreases in snowfall and increased ablation associated with warming. The eastern glaciers are therefore projected to decline over the 21st century despite increasing precipitation. The western glaciers are expected to decline at a slower rate over the 21st century as a response to unmitigated climate compared to the glaciers of the east. Importantly, the glacier response depends on important glaciological factors, such as the extent of debris cover, which may be of critical importance in moderating the response to climatic change. Decadal variability has a large effect highlighting the need for long-term observation records to fully understand the impact of climate on the glaciers of the HKKH cryosphere. Spatial variability in projected snowfall patterns are likely to be a key driver of glacier mass balance over the 21st century. Importantly, the regional trends in snowfall do not necessarily follow the trends in precipitation. A key change in the HKKH cryosphere is a switch from snowfall to rainfall in the eastern Himalaya. Although glacial mass balance is likely to be sensitive to climate change, as overall precipitation is projected to increase this may lead to an overall increase in water resources. In the west, projections suggest that glacial mass balance could respond less to climate change than those in the east. However, projection uncertainty covers a small increase to a decrease in precipitation for the western HKKH and Indus basin and as a result the water resources of the highly populated Indus region may be more vulnerable to unmitigated climate change.

Description: Creep deformation and buttressing capacity of damaged ice shelves: theory and application to Larsen C ice shelf The Cryosphere Discussions, 7, 3567-3610, 2013 Author(s): C. P. Borstad, E. Rignot, J. Mouginot, and M. P. Schodlok Around the perimeter of Antarctica, much of the ice sheet discharges to the ocean through floating ice shelves. The buttressing provided by ice shelves is critical for modulating the flux of ice into the ocean, and the presently observed thinning of ice shelves is believed to be reducing their buttressing capacity and contributing to the acceleration and thinning of the grounded ice sheet. However, relatively little attention has been paid to the role that fractures play in the flow and stability of ice shelves and their capacity to buttress the flow of grounded ice. Here, we develop an analytical framework for describing the role that fractures play in the creep deformation and buttressing capacity of ice shelves. We apply principles of continuum damage mechanics to derive a new analytical relation for the creep of an ice shelf as a function of ice thickness, temperature, material properties, resistive backstress and damage. By combining this analytical theory with an inverse method solution for the spatial rheology of an ice shelf, both backstress and damage can be calculated. We demonstrate the applicability of this new theory using satellite remote sensing and Operation IceBridge data for the Larsen C ice shelf, finding damage associated with known crevasses and rifts. We find that increasing thickness of mélange between rift flanks correlates with decreasing damage, with some rifts deforming coherently with the ice shelf as if completely healed. We quantify the stabilizing backstress caused by ice rises and lateral confinement, finding high backstress associated with two ice rises that likely stabilize the ice front in its current configuration. Though overall the ice shelf appears stable at present, the ice in contact with the Bawden ice rise is weakened by fractures, and additional damage or thinning in this area could portend significant change for the shelf. Using this new approach, field and remote sensing data can be utilized to monitor the structural integrity of ice shelves, their ability to buttress the flow of ice at the grounding line, and thus their indirect contribution to ice sheet mass balance and global sea level.

Description: Results from the DAMOCLES ice-buoy campaigns in the transpolar drift stream 2007–2009 The Cryosphere Discussions, 7, 3749-3781, 2013 Author(s): M. Haller, B. Brümmer, and G. Müller During the EU research project DAMOCLES 18 ice buoys were deployed in the region of the Arctic transpolar drift (TPD). Sixteen of them formed a square with 400 km side-length. The measurements lasted from 2007 to 2009. The properties of the TPD and the impact of synoptic weather systems on the ice drift are analysed. Compared to Nansen's drift with the vessel Fram the measured speed of the TPD is here almost twice as fast. Within the TPD, the speed increases by a factor of almost three from the North Pole to the Fram Strait region. The hourly buoy position fixes show that the speed is underestimated by 10–20% if positions were taken at only 1–3 days intervals as it is usually done for satellite drift estimates. The geostrophic wind factor U i / U g , i.e. the ratio of ice speed U i and geostrophic wind speed U g , in the TPD amounts to 0.012 on average, but with regional and seasonal differences. The constant U i / U g relation breaks down for U g 〈 5 m s −1 . The impact of synoptic weather systems is studied applying a composite method. Cyclones (anticyclones) cause cyclonic (anticyclonic) vorticity and divergence (convergence) of the ice drift. The amplitudes are twice as large for cyclones as for anticyclones. The divergence caused by cyclones corresponds to a 0.1–0.5%/6 h open water area increase based on the composite averages, but reached almost 4% within one day during a strong August 2007 storm. This storm also caused a~long-lasting (over several weeks) rise of U i and U i / U g and changed the ice conditions in a way allowing ocean tidal motion to directly affect ice motion. The consequences of an increasing Arctic storm activity for the ice cover are discussed.

Description: Snow thickness retrieval over thick Arctic sea ice using SMOS satellite data The Cryosphere Discussions, 7, 3627-3674, 2013 Author(s): N. Maaß, L. Kaleschke, X. Tian-Kunze, and M. Drusch The microwave interferometric radiometer of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission measures at a frequency of 1.4 GHz in the L-band. In contrast to other microwave satellites, low frequency measurements in L-band have a large penetration depth in sea ice and thus contain information on the ice thickness. Previous ice thickness retrievals have neglected a snow layer on top of the ice. Here, we implement a snow layer in our emission model and investigate how snow influences L-band brightness temperatures and whether it is possible to retrieve snow thickness over thick Arctic sea ice from SMOS data. We find that the brightness temperatures above snow-covered sea ice are higher than above bare sea ice and that horizontal polarisation is more affected by the snow layer than vertical polarisation. In accordance with our theoretical investigations, the root mean square deviation between simulated and observed horizontally polarised brightness temperatures decreases from 20.0 K to 4.4 K, when we include the snow layer in the simulations. Under cold Arctic conditions we find brightness temperatures to increase with increasing snow thickness. Because dry snow is almost transparent in L-band, this brightness temperature's dependence on snow thickness origins from the thermal insulation of snow and its dependence on the snow layer thickness. This temperature effect allows us to retrieve snow thickness over thick sea ice. For the best simulation scenario and snow thicknesses up to 35 cm, the average snow thickness retrieved from horizontally polarised SMOS brightness temperatures agrees within 0.7 cm with the average snow thickness measured during the IceBridge flight campaign in the Arctic in spring 2012. The corresponding root mean square deviation is 6.3 cm, and the correlation coefficient is r 2 = 0.55.

Description: Influence of meter-scale wind-formed features on the variability of the microwave brightness temperature around Dome C in Antarctica The Cryosphere Discussions, 7, 3675-3716, 2013 Author(s): G. Picard, A. Royer, L. Arnaud, and M. Fily Space-borne passive microwave radiometers are widely used to retrieve information in snowy regions by exploiting the high sensitivity of microwave emission to snow properties. For the Antarctic Plateau, many studies presenting retrieval algorithms or numerical simulations have assumed, explicitly or not, that the subpixel-scale heterogeneity is negligible and that the retrieved properties were representative of whole pixels. In this paper, we investigate the spatial variations of brightness temperature over a range of a few kilometers in the Dome C area. Using ground-based radiometers towed by a vehicle allowing measurements with meter resolution, we collected brightness temperature transects at 11, 19 and 37 GHz at horizontal and vertical polarizations. The most remarkable observation was a series of regular undulations of the signal with a significant amplitude of up to 10 K at 37 GHz and a quasi-period of 30–50 m. In contrast, the variability at longer length scales seemed to be weak in the investigated area and the mean brightness temperature was close to AMSR-E and WindSat satellite observations for all the frequencies and polarisations. To establish a link between the snow characteristics and undulation-scale variations of microwave emission, we collected detailed snow grain size and density profiles to run the DMRT-ML microwave emission model at two points where opposite extrema of brightness temperature were observed. The numerical simulations revealed that the difference in density of the upper first meter of the snowpack explained most of the brightness temperature variations. In addition, we found in the field that these variations of density were linked to the hardness of the snowpack. Areas of hard snow – probably formed by the wind – were clearly visible and covered as much as 39% of the investigated area. Their brightness temperature was higher than in normal areas. This result implied that the microwave emission measured by satellites over Dome C is more complex than expected and very likely depends on the areal proportion of the two different types of areas having distinct snow properties.

Description: Updated cloud physics improve the modelled near surface climate of Antarctica of a regional atmospheric climate model The Cryosphere Discussions, 7, 3231-3260, 2013 Author(s): J. M. van Wessem, C. H. Reijmer, J. T. M. Lenaerts, W. J. van de Berg, M. R. van den Broeke, and E. van Meijgaard The physics package of the polar version of the regional atmospheric climate model RACMO2 has been updated from RACMO2.1 to RACMO2.3. The update constitutes, amongst others, the inclusion of a parameterization for cloud ice super-saturation, an improved turbulent and radiative flux scheme and a changed cloud scheme. In this study the effects of these changes on the modelled near-surface climate of Antarctica are presented. Significant biases remain, but overall RACMO2.3 better represents the near-surface climate in terms of the modelled surface energy balance, based on a comparison with 〉 750 months of data from nine automatic weather stations located in East Antarctica. Especially the representation of the sensible heat flux and net longwave radiative flux has improved with a decrease in biases of up to 40 %. These improvements are mainly caused by the inclusion of ice super-saturation, which has led to more moisture being transported onto the continent, resulting in more and optically thicker clouds and more downward longwave radiation. As a result, modelled surface temperatures have increased and the bias, when compared to 10 m snow temperatures from 64 ice core observations, has decreased from −2.3 K to −1.3 K. The weaker surface temperature inversion consequently improves the representation of the sensible heat flux, whereas wind speed remains unchanged.

Description: Estimation of volume changes of mountain glaciers from ICESat data: an example from the Aletsch Glacier, Swiss Alps The Cryosphere Discussions, 7, 3261-3291, 2013 Author(s): J. Kropáček, N. Neckel, and A. Bauder Worldwide estimation of recent changes in glacier volume is challenging, but becomes more feasible with the help of present and future remote sensing missions. NASA's Ice Cloud and Elevation Satellite (ICESat) mission provides accurate elevation estimates derived from the two way travel time of the emitted laser pulse. In this study two different methods were employed for derivation of surface elevation changes from ICESat records on example of the Aletsch Glacier. A statistical approach relies on elevation differences of ICESat points to a reference DEM while an analytical approach compares spatially similar ICESat tracks. Using the statistical approach, in the upper and lower parts of the ablation area, the surface lowering was found to be from −2.1 ± 0.15 m yr −1 to −2.6 ± 0.10 m yr −1 and from −3.3 ± 0.36 m yr −1 to −5.3 ± 0.39 m yr −1 , respectively, depending on the DEM used. Employing the analytical method, the surface lowering in the upper part of the ablation area was estimated as −2.5 ± 1.3 m yr −1 between 2006 and 2009. In the accumulation area both methods revealed no significant trend. The trend in surface lowering derived by the statistical method allows an estimation of the mean mass balance in the period 2003–2009 assuming constant ice density and a linear change of glacier surface lowering with altitude in the ablation area. The resulting mass balance was validated by a comparison to another geodetic approach based on the subtraction of two DEMs for the years 2000 and 2009. We conclude that ICESat data is a valid source of information on surface elevation changes and on mass balance of mountain glaciers.

Description: Snow density climatology across the former USSR The Cryosphere Discussions, 7, 3379-3415, 2013 Author(s): X. Zhong, T. Zhang, and K. Wang Snow density is one of the basic properties used to describe snow cover characteristics, and it is a key factor for retrieving snow depth and snow water equivalent, which are critical for water resources assessment and modeling inputs. In this study, we used long-term data from ground-based measurements to investigate snow density climatology and its spatiotemporal variations across the former Soviet Union (USSR) from 1966 to 2008. The results showed that the long-term monthly mean snow density was approximately 0.194 ± 0.046 g cm −3 over the study area. The maximum and minimum monthly mean snow density was ∼ 0.295 g cm −3 in June, and 0.135 g cm −3 in October, respectively. Maritime snow had the highest monthly mean snow density, while taiga snow had the lowest. The higher values of monthly snow density were mainly located in the European regions of the former USSR, in Arctic Russia, and in some regions of the Russian Far East, and the lower snow density occurred in central Siberia. Significant increasing trends of snow density from September through June of the next year were observed, however, the rate of the increase varied with different snow classes. The long-term (1966–2008) monthly and annual mean snow densities had significant decreasing trends, especially during the autumn months. Spatially, significant positive trends in monthly mean snow density lay in the southwestern areas of the former USSR in November and December and gradually expanded in Russia from February through April. Significant negative trends mainly lay in the European Russia and the southern Russia. Snow density decreased with elevation, at about 0.004 g cm −3 per 100 m increase in elevation. This same relationship existed for all snow classes except for maritime and ephemeral snow.

Description: Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change The Cryosphere Discussions, 7, 3497-3541, 2013 Author(s): B. C. Gunter, O. Didova, R. E. M. Riva, S. R. M. Ligtenberg, J. T. M. Lenaerts, M. A. King, M. R. van den Broeke, and T. Urban This study explores an approach that simultaneously estimates Antarctic mass balance and glacial isostatic adjustment (GIA) through the combination of satellite gravity and altimetry data sets. The results improve upon previous efforts by incorporating reprocessed data sets over a longer period of time, and now include a firn densification model to account for firn compaction and surface processes. A range of different GRACE gravity models were evaluated, as well as a new ICESat surface height trend map computed using an overlapping footprint approach. When the GIA models created from the combination approach were compared to in-situ GPS ground station displacements, the vertical rates estimated showed consistently better agreement than existing GIA models. In addition, the new empirically derived GIA rates suggest the presence of strong uplift in the Amundsen Sea and Philippi/Denman sectors, as well as subsidence in large parts of East Antarctica. The total GIA mass change estimates for the entire Antarctic ice sheet ranged from 53 to 100 Gt yr −1 , depending on the GRACE solution used, and with an estimated uncertainty of ±40 Gt yr −1 . Over the time frame February 2003–October 2009, the corresponding ice mass change showed an average value of −100 ± 44 Gt yr −1 (EA: 5 ± 38, WA: −105 ± 22), consistent with other recent estimates in the literature, with the mass loss mostly concentrated in West Antarctica. The refined approach presented in this study shows the contribution that such data combinations can make towards improving estimates of present day GIA and ice mass change, particularly with respect to determining more reliable uncertainties.

Description: Simulation of wind-induced snow transport in alpine terrain using a fully coupled snowpack/atmosphere model The Cryosphere Discussions, 7, 2191-2245, 2013 Author(s): V. Vionnet, E. Martin, V. Masson, G. Guyomarc'h, F. Naaim-Bouvet, A. Prokop, Y. Durand, and C. Lac In alpine regions, wind-induced snow transport strongly influences the spatio-temporal evolution of the snow cover throughout the winter season. To gain understanding on the complex processes that drive the redistribution of snow, a new numerical model is developed. It couples directly the detailed snowpack model Crocus with the atmospheric model Meso-NH. Meso-NH/Crocus simulates snow transport in saltation and in turbulent suspension and includes the sublimation of suspended snow particles. A detailed representation of the first meters of the atmosphere allows a fine reproduction of the erosion and deposition process. The coupled model is evaluated against data collected around the experimental site of Col du Lac Blanc (2720 m a.s.l., French Alps). For this purpose, a blowing snow event without concurrent snowfall has been selected and simulated. Results show that the model captures the main structures of atmospheric flow in alpine terrain, the vertical profile of wind speed and the snow particles fluxes near the surface. However, the horizontal resolution of 50 m is found to be insufficient to simulate the location of areas of snow erosion and deposition observed by terrestrial laser scanning. When activated, the sublimation of suspended snow particles causes a reduction in deposition of 5.3%. Total sublimation (surface + blowing snow) is three times higher than surface sublimation in a simulation neglecting blowing snow sublimation.

Description: A particle based simulation model for glacier dynamics The Cryosphere Discussions, 7, 921-941, 2013 Author(s): J. A. Åström, T. I. Riikilä, T. Tallinen, T. Zwinger, D. Benn, J. C. Moore, and J. Timonen A particle-based computer simulation model was developed for investigating the dynamics of glaciers. In the current model, large ice bodies are made of discrete elastic particles which are bound together by massless and elastic beams. The beams can break which induces brittle behaviour. At loads below fracture, beams may also break and reform with small probabilities in order to incorporate slowly deforming viscous behaviour in the model. This model has the advantage that it can simulate important physical processes such as ice calving and fracturing in a more realistic way than traditional continuum models. Two simulations were performed: (1) calving of an ice block partially supported in water, which could represent a grounded marine glacier terminus, and (2) fracturing of an ice block on an inclined plane of varying basal friction, which could represent transition to fast flow or surging. For benchmarking purposes the deformation of an ice block on a slip-free surface was compared to that of a similar block simulated with a Finite Element full-Stokes continuum model. In spite of several simplifications, which include restriction to two-dimenions and simplified rheology for water, the model introduced was able to reproduce the size distributions of the icebergs and the debris observed in calving. The size distributions we produce may be approximated by universal scaling laws. On a moderate slope, a large ice block was stable as long as there was enough of friction against the substrate. This was a quiescent state. For a critical length of frictional contact global sliding began, and the model block disintegrated in a manner suggestive of a surging glacier. In this case the fragment size distribution produced was typical of a grinding process.

Description: The effects of additional black carbon on Arctic sea ice surface albedo: variation with sea ice type and snow cover The Cryosphere Discussions, 7, 943-973, 2013 Author(s): A. A. Marks and M. D. King Black carbon in sea ice will decrease sea ice surface albedo through increased absorption of incident solar radiation, exacerbating sea ice melting. Previous literature has reported different albedo responses to additions of black carbon in sea ice and has not considered how a snow cover may mitigate the effect of black carbon in sea ice. Sea ice is predominately snow covered. Visible light absorption and light scattering coefficients are calculated for a typical first year and multi-year sea ice and "dry" and "wet" snow types that suggest black carbon is the dominating absorbing impurity. The albedo response of first year and multi-year sea ice to increasing black carbon, from 1–1024 ng g −1 , in a top 5 cm layer of a 155 cm thick sea ice was calculated using the radiative transfer model: TUV-snow. Sea ice albedo is surprisingly unresponsive to black carbon additions up to 100 ng g −1 with a decrease in albedo to 98.7% of the original albedo value due to an addition of 8 ng g −1 of black carbon in first year sea ice compared to an albedo decrease to 99.6% for the same black carbon mass ratio increase in multi-year sea ice. The first year sea ice proved more responsive to black carbon additions than the multi-year ice. Comparison with previous modelling of black carbon in sea ice suggests a more scattering sea ice environment will be less responsive to black carbon additions. Snow layers on sea ice may mitigate the effects of black carbon in sea ice. "Wet" and "dry" snow layers of 0.5, 1, 2, 5 and 10 cm were added onto the sea ice surface and the snow surface albedo calculated with the same increase in black carbon in the underlying sea ice. Just a 0.5 cm layer of snow greatly diminishes the effect of black carbon on surface albedo, and a 2–5 cm layer (less than half the e -folding depth of snow) is enough to "mask" any change in surface albedo owing to additional black carbon in sea ice, but not thick enough to ignore the underlying sea ice.

Description: Sea ice dynamics influence halogen deposition to Svalbard The Cryosphere Discussions, 7, 1075-1100, 2013 Author(s): A. Spolaor, J. Gabrieli, T. Martma, J. Kohler, M. Björkman, E. Isaksson, C. Varin, P. Vallelonga, J. M. C. Plane, and C. Barbante Sea ice is an important parameter in the climate system and its changes impact upon the polar albedo and the atmospheric and oceanic circulation. Iodine (I) and bromine (Br) have been measured in a shallow ice core drilled at the summit of the Holtedahlfonna glacier (Northwest Spitsbergen, Svalbard). Changing I concentrations can be linked to the spring maximum sea ice extension. Bromine enrichment, indexed to the Br/Na sea water mass ratio, appears to be influenced by changes in the seasonal sea ice area. I is emitted from marine biota and so the retreat of spring sea ice coincides with enlargement of the open ocean surface which enhances marine primary production and consequent I emission. The observed Br enrichment can be explained by greater Br emissions during the Br explosion that have been observed to occur above first year sea ice during the early springtime. In this work we present the first comparison between halogens in surface snow and Arctic sea ice extension. Although further investigation is required to characterize potential depositional and post-depositional processes, these preliminary findings suggest that I and Br can be linked to variability in the spring maximum sea ice extension and seasonal sea ice surface area.

Description: Data assimilation and prognostic whole ice-sheet modelling with the variationally derived, higher-order, open source, and fully parallel ice sheet model VarGlaS The Cryosphere Discussions, 7, 1029-1074, 2013 Author(s): D. J. Brinkerhoff and J. V. Johnson We introduce a novel, higher order, finite element ice sheet model called VarGlaS (Variational Glacier Simulator). Contrary to standard procedure in ice sheet modelling, VarGlaS formulates ice sheet motion as the minimization of an energy functional, conferring advantages such as a consistent platform for making numerical approximations, a coherent relationship between motion and heat generation, and implicit boundary treatment. VarGlaS also solves the equations of enthalpy rather than temperature, avoiding the solution of a contact problem. Rather than include a lengthy model spin-up procedure, VarGlaS possesses an automated framework for model inversion. These capabilities are brought to bear on several benchmark problems in ice sheet modelling, as well as a 500 yr simulation of the Greenland ice sheet at high resolution. VarGlaS performs well in benchmarking experiments, and given a constant climate, predicts an overall mass evolution of the Greenland ice sheet that matches well with observational data.

Description: Southwest-facing slopes control the formation of debris-covered glaciers in the Bhutan Himalaya The Cryosphere Discussions, 7, 1673-1705, 2013 Author(s): H. Nagai, K. Fujita, T. Nuimura, and A. Sakai To understand the formation conditions of debris-covered glaciers, we examined the dimension and shape of debris-covered areas and potential debris-supply (PDS) slopes of 208 glaciers in the Bhutan Himalaya. This was undertaken using satellite images with 2.5 m spatial resolution for manual delineation of debris-covered areas and PDS slopes. The most significant correlation exists between surface area of southwest-facing PDS slopes and debris-covered area. This result suggests that the southwest-facing PDS slopes supply the largest quantity of debris mantle. The shape of debris-covered areas is also an important variable quantitatively defined using a geometric index. Elongate or stripe-like debris-covered areas on north-flowing glaciers are common throughout the Bhutan Himalaya, associated with the small quantities of debris from north-facing PDS slopes. In contrast, south-flowing glaciers have large ablation zones, entirely covered by debris. Our findings suggest that this difference is caused by effective diurnal freeze–thaw cycles rather than seasonal freeze–thaw cycles, permafrost degradation, or snow avalanches. In terms of geographic setting, local topography also contributes to glacier debris supply and the proportion of debris cover on the studied glaciers is suppressed by the arid Tibetan climate, whereas the north-to-south asymmetric topography of the Bhutan Himalaya has less influence on the proportion of debris cover.

Description: Desert dust deposition on Mt. Elbrus, Caucasus Mountains, Russia in 2009–2012 as recorded in snow and shallow ice core: high-resolution "provenancing", transport patterns, physical properties and soluble ionic composition The Cryosphere Discussions, 7, 1621-1672, 2013 Author(s): S. Kutuzov, M. Shahgedanova, V. Mikhalenko, I. Lavrentiev, and S. Kemp A record of dust deposition events between 2009 and 2012 on Mt. Elbrus, Caucasus Mountains derived from a snow pit and a shallow ice core is presented for the first time for this region. A combination of isotopic analysis, SEVIRI red-green-blue composite imagery, MODIS atmospheric optical depth fields derived using the Deep Blue algorithm, air mass trajectories derived using the HYSPLIT model and analysis of meteorological data enabled identification of dust source regions with high temporal (hours) and spatial (cf. 20–100 km) resolution. Seventeen dust deposition events were detected; fourteen occurred in March–June, one in February and two in October. Four events originated in the Sahara, predominantly in north-eastern Libya and eastern Algeria. Thirteen events originated in the Middle East, in the Syrian Desert and northern Mesopotamia, from a mixture of natural and anthropogenic sources. Dust transportation from Sahara was associated with vigorous Saharan depressions, strong surface winds in the source region and mid-tropospheric south-westerly flow with daily winds speeds of 20–30 m s −1 at 700 hPa level and, although these events were less frequent, they resulted in higher dust concentrations in snow. Dust transportation from the Middle East was associated with weaker depressions forming over the source region, high pressure centered over or extending towards the Caspian Sea and a weaker southerly or south-easterly flow towards the Caucasus Mountains with daily wind speeds of 12–18 m s −1 at 700 hPa level. Higher concentrations of nitrates and ammonium characterise dust from the Middle East deposited on Mt. Elbrus in 2009 indicating contribution of anthropogenic sources. The modal values of particle size distributions ranged between 1.98 μm and 4.16 μm. Most samples were characterised by modal values of 2.0–2.8 μm with an average of 2.6 μm and there was no significant difference between dust from the Sahara and the Middle East.

Description: Influence of high-order mechanics on simulation of glacier response to climate change: insights from Haig Glacier, Canadian Rocky Mountains The Cryosphere Discussions, 7, 1707-1748, 2013 Author(s): S. Adhikari and S. J. Marshall Evolution of glaciers in response to climate change has mostly been simulated using simplified dynamical models. Because these models do not account for the influence of high-order physics, corresponding results may exhibit some biases. For Haig Glacier in the Canadian Rocky Mountains, we test this hypothesis by comparing simulation results obtained from 3-D numerical models that deal with different assumptions concerning ice-flow physics, ranging from simple shear-deformation to comprehensive Stokes flow. In glacier retreat scenarios, we find a minimal role of high-order mechanics in glacier evolution, as geometric effects at our site (the presence of an overdeepened bed) result in limited horizontal movement of ice (flow speed on the order of a few meters per year). Consequently, high-order and reduced models all predict that Haig Glacier ceases to exist by ca. 2080 under ongoing climate warming. The influence of high-order mechanics is evident, however, in glacier advance scenarios, where ice speeds are greater and ice dynamical effects become more important. To generalize these findings for other glacier applications, we advise that high-order mechanics are important and therefore should be considered while modelling the evolution of active glaciers. Reduced model predictions may, however, be adequate for other glaciologic and topographic settings, particularly where flow speeds are low.

Description: Brief communication "The 2013 Erebus Glacier tongue calving event" The Cryosphere Discussions, 7, 1749-1760, 2013 Author(s): C. L. Stevens, P. Sirguey, G. H. Leonard, and T. G. Haskell The Erebus Glacier Tongue, a~small floating glacier in southern McMurdo Sound, is one of the best-studied ice tongues in Antarctica. Despite this, its calving on the 27 February 2013 (UTC) was around 10 yr earlier than previously predicted. The calving was likely a result of ocean currents and the absence of fast ice. The subsequent trajectory of the newly-created iceberg supports previous descriptions of the surface ocean circulation in southern McMurdo Sound.

Description: Tidally-induced velocity variations of the Beardmore Glacier, Antarctica, and their representation in satellite measurements of ice velocity The Cryosphere Discussions, 7, 1761-1785, 2013 Author(s): O. J. Marsh, W. Rack, D. Floricioiu, N. R. Golledge, and W. Lawson Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, in both linear and non-linear patterns. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, shown here to be present in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of 〉50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This decreases rapidly to

Description: The snowdrift effect on snow deposition: insights from a comparison of a snow pit profile and meteorological observations The Cryosphere Discussions, 7, 1415-1439, 2013 Author(s): M. Ding, C. Xiao, R. Zhang, D. Qin, B. Jin, B. Sun, L. Bian, J. Ming, C. Li, A. Xie, W. Yang, and Y. Ma A high-frequency and precise ultrasonic sounder was used to record precipitated/deposited snow and drift events over a 3 yr period (17 January 2005 to 4 January 2008) at the Eagle automatic weather station (AWS) site. Through a comparison of the meteorological data with snow pit chemical/isotopic dating results, the snowdrift process effect during snow accumulation was assessed. We believe that ice/firn cores are the most important proxies of climate and the environment because of their high resolution and their preservation of historical greenhouse gas levels, although their limitations and measurement uncertainties must be taken into account, due to the event-driven snow dominates the snow deposition. This study found a difference between two dating results of up to 12 months for a ~ 95 cm snow pit, where the annual snow accumulation rate is 30.3 cm. A weakness is also indicated when simulating the surface mass balance in Antarctica.

Description: A decade of supraglacial lake volume estimates across a land-terminating margin of the Greenland Ice Sheet The Cryosphere Discussions, 7, 1383-1414, 2013 Author(s): A. A. W. Fitzpatrick, A. L. Hubbard, J. E. Box, D. J. Quincey, D. van As, A. P. B. Mikkelsen, S. H. Doyle, C. F. Dow, B. Hasholt, and G. A. Jones Supraglacial lakes represent an ephemeral storage buffer for runoff and lead to significant, yet short-lived, episodes of ice-flow acceleration by decanting large fluxes of meltwater and energy into the ice sheet's hydrological system. Here, a field-validated methodology for calculating lake volume is used to quantify storage and drainage across Russell Glacier catchment, west Greenland, from 2002 onwards. Using 502 optical satellite images, water volume at ~200 seasonally occurring lakes was derived from a depth-reflectance relationship, independently calibrated and field-validated against lake bathymetry. Inland expansion of lakes is strongly correlated with air temperature: during the record melt years of 2010 and 2012, lakes formed and drained earlier, attaining their maximum volume 38 and 20 days before the 11 yr mean, as well as occupying a greater area and forming at higher elevations (〉1800 m) than previously. Although lakes occupy only 2% of the catchment surface area, they temporarily store up to 13% of the bulk meltwater discharged. Across Russell Glacier, 28% of supraglacial lakes drain rapidly and clustering of such events in space and time suggests a synoptic trigger-mechanism. Furthermore, we find no evidence to support a unifying critical size or depth-dependent drainage threshold hypothesis.

Description: Influence of supraglacial lakes and ice-sheet geometry on seasonal ice-flow variability The Cryosphere Discussions, 7, 1101-1118, 2013 Author(s): I. Joughin, S. B. Das, G. E. Flowers, M. D. Behn, R. B. Alley, M. A. King, B. E. Smith, J. Bamber, M. R. van den Broeke, and J. H. van Angelen Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal melt water to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

Description: LiDAR snow cover studies on glacier surface: significance of snow- and ice dynamical processes The Cryosphere Discussions, 7, 1787-1832, 2013 Author(s): K. Helfricht, M. Kuhn, M. Keuschnig, and A. Heilig The storage of water within the seasonal snow cover is a substantial source for runoff in high mountain catchments. Information about the spatial distribution of snow accumulation is necessary for calibration and validation of hydro-meteorological models. Generally only a small number of precipitation measurements deliver precipitation input for modeling in remote mountain areas. The spatial interpolation and extrapolation of measurements of precipitation is still difficult. Multi-temporal application of Light Detecting And Ranging (LiDAR) techniques from aircraft, so-called airborne laser scanning (ALS), enables to derive surface elevations changes even in inaccessible terrain. Within one snow accumulation season these surface elevation changes can be interpreted as snow depths as a first assumption for snow hydrological studies. However, dynamical processes in snow, firn and ice are contributing to surface elevation changes on glaciers. To evaluate the magnitude and significance of these processes on alpine glaciers in the present state, ALS derived surface elevation changes were compared to converted snow depths from 35.4 km of ground penetrating radar (GPR) profiles on four glaciers in the high alpine region of Ötztal Alps. LANDSAT data were used to distinguish between firn and ice areas of the glaciers. In firn areas submerging ice flow and densification of firn and snow are contributing to a mean relative deviation of ALS surface elevation changes from actually observed snow depths of −20.0% with a mean standard deviation of 17.1%. Deviations between ALS surface elevation changes and GPR snow depth are small along the profiles on the glacier tongues. At these areas mean absolute deviation of ALS surface elevation changes and GPR snow depth is 0.004 m with a mean standard deviation of 0.27 m. Emergence flow leads to distinct positive deviations only at the very front of the glacier tongues. Snow depths derived from ALS deviate less from actually measured snow depths than expected errors of in-situ measurements of solid precipitation. Hence, ALS derived snow depths are an important data source for both, spatial distribution and total sum of the snow cover volume stored on the investigated glaciers and in the corresponding high mountain catchments at the end of an accumulation season.

Description: Meteorological drivers of ablation processes on a cold glacier in the semiarid Andes of Chile The Cryosphere Discussions, 7, 1833-1870, 2013 Author(s): S. MacDonell, C. Kinnard, T. Mölg, L. Nicholson, and J. Abermann Meteorological and surface change measurements collected during a 2.5 yr period are used to calculate surface mass and energy balances at 5324 m a.s.l. on Guanaco Glacier, a cold-based glacier in the semi-arid Andes of Chile. Meteorological conditions are marked by extremely low vapour pressures (annual mean of 1.1 hPa), strong winds (annual mean of 10 m s −1 ), high shortwave radiation receipt (mean annual 295 W m −2 ) and low precipitation rates (mean annual 45 mm w.e.). Net shortwave radiation provides the greatest source of energy to the glacier surface, and net longwave radiation dominates energy losses. The turbulent latent heat flux is always negative, which means that the surface is always losing mass via sublimation, which is the main form of ablation at the site. Sublimation rates are most strongly correlated with net shortwave radiation, incoming shortwave radiation, albedo and vapour pressure. Low glacier surface temperatures restrict melting for much of the period, however episodic melting occurs during the austral summer, when warm, humid, calm and high pressure conditions restrict sublimation and make more energy available for melting. Low accumulation (131 mm w.e. over the period) and relatively high ablation (1435 mm w.e.) means that mass change over the period was negative (−1304 mm w.e.), which continued the negative trend recorded in the region over the last few decades.

Description: Influence of regional precipitation patterns on stable isotopes in ice cores from the central Himalayas The Cryosphere Discussions, 7, 1871-1905, 2013 Author(s): H. Pang, S. Hou, S. Kaspari, and P. A. Mayewski Several ice cores have been recovered from the Dasuopu Glacier and the East Rongbuk (ER) Glacier in the central Himalayas since the 1990s. Although the distance between the ER and the Dasuopu ice core drilling sites is only ∼125 km, the stable isotopic record (δ 18 O or δD) of the ER core is interpreted as a precipitation proxy while the Dasuopu core as a temperature proxy. Thus, the climatological significance of the stable isotopic records of these Himalayan ice cores remains a subject of debate. Based on analysis of regional precipitation patterns over the region, we find that the different interpretations of the Dasuopu and Everest isotopic records may not be contradictive. The north–south and west–east seesaws of the Indian Summer Monsoon (ISM) precipitation are primarily responsible for precipitation falling at the ER site, which results in a negative correlation between the ER δ 18 O or δD record and precipitation amount along the southern slope of the central Himalayas, corresponding to the "amount effect". In addition to the ISM precipitation, non-summer monsoonal precipitation associated with winter westerlies also significantly contributes to precipitation falling at the Dasuopu site, which may cause a positive correlation between the Dasuopu stable isotopic record and temperature, in response to the "temperature effect". Our results have important implications for interpreting the stable isotopic ice core records recovered from different climatological regimes of the Himalayas.

Description: Spatial-temporal dynamics of chemical composition of surface snow in East Antarctic along the transect Station Progress-Station Vostok The Cryosphere Discussions, 7, 2007-2028, 2013 Author(s): T. V. Khodzher, L. P. Golobokova, Y. A. Shibaev, V. Y. Lipenkov, and J. R. Petit This paper presents data on chemical composition of the Antarctic snow sampled during the 53rd Russian Antarctic Expedition (RAE, 2008) along the first tractor traverse (TT) from Station Progress to Station Vostok (East Antarctica). Snow samples were obtained from the cores drilled at 55.3, 253, 337, 369, 403, 441, 480, 519, 560, 618, 819, and 1276 km from Station Progress. Data on horizontal and deep distribution of chemical components in the snow provide evidence of spatial and temporal variations of conditions for the snow cover formation along the transect under study. Sea salt was the main source for chemical composition of snow cover near the ice edge. Concentrations of marine-derived components decreased further inland. A hypothesis was put forward that some ions in the snow cover of the central part of East Antarctica were likely to be of continental origin. Elevated concentrations of sulphate ions of continental origin were recorded in some profiles of the transect at a depth of 130–150 cm which was attributed to buried signals of the Pinatubo volcano eruption (1991).

Description: Diffusive equilibration of N 2 , O 2 and CO 2 mixing ratios in a 1.5 million years old ice core The Cryosphere Discussions, 7, 2029-2060, 2013 Author(s): B. Bereiter, H. Fischer, J. Schwander, and T. F. Stocker In the framework of the International Partnerships in Ice Core Sciences, one of the most important target is to retrieve an Antarctic ice core that extends over the last 1.5 million years, i.e. an ice core that enters the climate era when glacial-interglacial cycles followed the obliquity cycles of the sun. In such an ice core the annual layers of the oldest ice would be thinned by a factor of about 100 and the climatic information of a 10 000 yr interval would be contained in less than 1m of ice. The gas record in such an Antarctic ice core can potentially reveal the role of greenhouse gas forcing on these 40 000 yr cycles. However, besides the extreme thinning of the annual layers, also the long residence time of the trapped air in the ice and the relatively high ice temperatures near the bedrock favour diffusive exchanges. To investigate the changes in the O 2 /N 2 ratio, as well as the trapped CO 2 concentrations, we modelled the diffusive exchange of the trapped gases O 2 , N 2 and CO 2 along the vertical axis. Even though the boundary conditions of a potential drilling site are not yet well constrained and the uncertainties in the permeation coefficients of the air constituents in the ice are large, the results suggest that in the oldest ice the precessional variations in the O 2 /N 2 ratio will be damped by 50–100%, whereas CO 2 concentration changes associated with glacial-interglacial variations will experience a damping of only 5%. This significant attenuation of the precessional O 2 /N 2 signal in the ice older than 1 Myr will limit the possibility to use the O 2 /N 2 ratio for orbital tuning of the ice core age scale.

Description: Characterization of L-band synthetic aperture radar (SAR) backscatter from floating and grounded lake ice in arctic Alaska The Cryosphere Discussions, 7, 2061-2088, 2013 Author(s): M. Engram, K. W. Anthony, F. J. Meyer, and G. Grosse Synthetic aperture radar (SAR) backscatter from floating lake ice is high, in contrast to low backscatter values from lake ice that is frozen completely to the lake bed (grounded ice). Knowledge of floating vs. grounded lake ice is useful for determining winter water supply, fish habitat, heat transfer to permafrost, and to observe changes in perennial lake ice status that could correlate with variations in local climate. Here, we compare calibrated L-band (23.6 cm wavelength) single- and L-band quadrature-polarized SAR return to the backscatter intensity of C-band (5.6 cm wavelength) SAR from floating and grounded lake ice over two regions in Alaska. Our primary goal was to determine if C or L-band is more useful to distinguish floating from grounded lake ice. C-band SAR showed far greater contrast between floating and grounded lake ice, making it the preferred wavelength for identifying lake ice regimes. L-band SAR backscatter was much lower from floating ice than C-band and it was different for our two study regions. Furthermore, since L-band is sensitive to ebullition bubbles trapped by lake ice (bubbles increase backscatter), this study helps to elucidate potential confounding factors of bubbles in efforts to detect floating vs. grounded ice using L-band SAR.

Description: The sensitivity of flowline models of tidewater glaciers to parameter uncertainty The Cryosphere Discussions, 7, 2567-2593, 2013 Author(s): E. M. Enderlin, I. M. Howat, and A. Vieli Depth-integrated (1-D) flowline models have been widely used to simulate fast-flowing tidewater glaciers and predict future change because their computational efficiency allows for continuous grounding line tracking, high horizontal resolution, and a physically-based calving criterion, which are all essential to realistic modeling of tidewater glaciers. As with all models, the values for parameters describing ice rheology and basal friction must be assumed and/or tuned based on observations. For prognostic studies, these parameters are typically tuned so that the glacier matches observed thickness and speeds at an initial state, to which a perturbation is applied. While it is well know that ice flow models are sensitive to these parameters, the sensitivity of tidewater glacier models has not been systematically investigated. Here we investigate the sensitivity of such flowline models of outlet glacier dynamics to uncertainty in three key parameters that influence a glacier's resistive stress components. We find that, within typical observational uncertainty, similar initial (i.e. steady-state) glacier configurations can be produced with substantially different combinations of parameter values, leading to differing transient responses after a perturbation is applied. In cases where the glacier is initially grounded near flotation across a basal overdeepening, as typically observed for rapidly changing glaciers, these differences can be dramatic owing to the threshold of stability imposed by the flotation criterion. The simulated transient response is particularly sensitive to the parameterization of ice rheology: differences in ice temperature of ∼ 2 °C can determine whether the glaciers thin to flotation and retreat unstably or remain grounded on a marine shoal. Due the highly non-linear dependence of tidewater glaciers on model parameters, we recommend that their predictions are accompanied by sensitivity tests that take parameter uncertainty into account.

Description: Comparison of automatic segmentation of full polarimetric SAR sea ice images with manually drawn ice charts The Cryosphere Discussions, 7, 2595-2634, 2013 Author(s): M.-A. N. Moen, A. P. Doulgeris, S. N. Anfinsen, A. H. H. Renner, N. Hughes, S. Gerland, and T. Eltoft In this paper we investigate the performance of an algorithm for automatic segmentation of full polarimetric, synthetic aperture radar (SAR) sea ice scenes. The algorithm uses statistical and polarimetric properties of the backscattered radar signals to segment the SAR image into a specified number of classes. This number was determined in advance from visual inspection of the SAR image and by available in-situ measurements. The segmentation result was then compared to ice charts drawn by ice service analysts. The comparison revealed big discrepancies between the charts of the analysts, and between the manual and the automatic segmentations. In the succeeding analysis, the automatic segmentation chart was labeled into ice types by sea ice experts, and the SAR features used in the segmentation were interpreted in terms of physical sea ice properties. Studies of automatic and robust estimation of the number of ice classes in SAR sea ice scenes will be highly relevant for future work.

Description: Impact of physical properties and accumulation rate on pore close-off in layered firn The Cryosphere Discussions, 7, 2533-2566, 2013 Author(s): S. A. Gregory, M. R. Albert, and I. Baker Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric concentrations. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age-gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from WAIS Divide in West Antarctica and Megadunes in East Antarctica. We found that the open pore structure plays a more important role than density in predicting gas transport properties, through the porous firn matrix. For both WAIS Divide and Megadunes, fine grained layers experience close-off shallower in the firn column than do coarse grained layers, regardless of which grain sized layer is the more dense layer at depth. Pore close-off occurs at an open porosity that is accumulation rate dependent. Low accumulation sites, with coarse grains, close-off at lower open porosities ( 〈 10%) than the open porosity (〉 10%) of high accumulation sites with finer grains. The depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.

Description: Modeling surface response of the Greenland Ice Sheet to interglacial climate The Cryosphere Discussions, 7, 2703-2723, 2013 Author(s): D. Rau and I. Rogozhina This study presents a~new approach to parameterizing surface mass balance (SMB) of the Greenland Ice Sheet (GIS) under interglacial climate validated against recent satellite observations and the results of a high-resolution model on a regional scale. Based on detailed analysis of the modeled SMB, we conclude that existing parameterizations fail to capture either spatial pattern or amplitude of the observed surface responses of the GIS. This is due to multiple simplifying assumptions adopted by the majority of modeling studies within the framework of a positive degree-day method. Modeled surface melting is found to be highly sensitive to a choice of daily temperature standard deviation (SD), which is generally assumed to have uniform distribution across Greenland. The range of commonly used SD values does not however receive support from climate datasets available. In this region, SD distribution is highly inhomogeneous and characterized by low values during summer months in areas where most surface melting occurs. Our approach is to make use of spatially variable SD and here we show that this leads to significant improvements in the modeled SMB over the instrumental record. Our findings necessitate evaluating potential consequences of the simplified SMB treatment for assessment of the history and future of glaciation on Earth.

Description: A note on the water budget of temperate glaciers The Cryosphere Discussions, 7, 2679-2702, 2013 Author(s): J. Oerlemans In this note the total dissipative melting in temperate glaciers is studied. The analysis is based on the notion that the dissipation is determined by the loss of potential energy, due to the downward motion of mass (ice, snow, meltwater and rain). A mathematical formulation of the dissipation is developed and applied to a simple glacier geometry. In a next step, meltwater production resulting from enhanced ice motion during a glacier surge is calculated. The amount of melt energy available follows directly from the lowering of the centre of gravity of the glacier. To illustrate the concept, schematic calculations are presented for a number of glaciers with different geometric characteristics. Typical dissipative melt rates, expressed as water-layer depth averaged over the glacier, range from a few cm per year for smaller glaciers to half a meter per year for Franz-Josef Glacier, one of the most active glaciers in the world (in terms of mass turnover). The total generation of meltwater during a surge is typically half a meter. For Variegated Glacier a value of 70 cm is found, for Kongsvegen 20 cm. These values refer to water layer depth averaged over the entire glacier. The melt rate depends on the duration of the surge. It is generally an order of magnitude larger than the water production by "normal" dissipation. On the other hand, the additional basal melt rate during a surge is comparable in magnitude to the water input from meltwater and precipitation. This suggests that enhanced melting during a surge does not grossly change the total water budget of a glacier. Basal water generated by enhanced sliding is an important ingredient of many theories of glacier surges. It provides a positive feedback mechanism that actually makes the surge happen. The results found here suggest that this can only work if water generated by enhanced sliding is accumulating in a part of the glacier base where surface meltwater and rain has no or very limited access. This finding seems compatible with the fact that on many glaciers surges are initiated in the lower accumulation zone.

Description: Parameter and state estimation with a time-dependent adjoint marine ice sheet model The Cryosphere Discussions, 7, 2845-2890, 2013 Author(s): D. N. Goldberg and P. Heimbach To date, assimilation of observations into large-scale ice models has consisted predominantly of time-independent inversions of surface velocities for basal traction, bed elevation, or ice stiffness, and has relied primarily on analytically-derived adjoints of diagnostic ice velocity models. To overcome limitations of such "snapshot" inversions, i.e. their inability to assimilate time-dependent data, or to produce initial states with minimum artificial drift and suitable for time-dependent simulations, we have developed an adjoint of a time-dependent parallel glaciological flow model. The model implements a hybrid shallow shelf-shallow ice stress balance, involves a prognostic equation for ice thickness evolution, and can represent the floating, fast-sliding, and frozen bed regimes of a marine ice sheet. The adjoint is generated by a combination of analytic methods and the use of algorithmic differentiation (AD) software. Several experiments are carried out with idealized geometries and synthetic observations, including inversion of time-dependent surface elevations for past thicknesses, and simultaneous retrieval of basal traction and topography from surface data. Flexible generation of the adjoint for a range of independent uncertain variables is exemplified through sensitivity calculations of grounded ice volume to changes in basal melting of floating and basal sliding of grounded ice. The results are encouraging and suggest the feasibility, using real observations, of improved ice sheet state estimation and comprehensive transient sensitivity assessments.

Description: Grain shape influence on light extinction in snow The Cryosphere Discussions, 7, 2801-2843, 2013 Author(s): Q. Libois, G. Picard, J. L. France, L. Arnaud, M. Dumont, C. M. Carmagnola, and M. D. King The energy budget and the photochemistry of a snowpack greatly depend on the penetration of solar radiation into the snowpack. While representing snow by a collection of spherical particles has been a successful option in the numerical computation of the albedo, such models poorly reproduce light extinction measurements. Here, we explore the limits of the spherical representation by using numerical tools and experimental data. For this, we investigate the influence of grain shape on light extinction in the visible and near-infrared (NIR) ranges. To compute light extinction, we developed a multi-layer radiative transfer model based on the δ-Eddington approximation and analytical expressions of the albedo, α, and the asymptotic flux extinction coefficient (AFEC), k e . The snowpack is characterized by the profiles of density, specific surface area (SSA) and two parameters ( B and g G ) depending only on the grain shape. The aim of the paper is to estimate the values of B and g G and to understand how they impact macroscopic optical properties of snow. First, the values of B and g G are deduced from simulations with ray tracing models for a variety of simple geometric shapes. The results show that spherical grains propagate light deeper into snow than the other shapes we have investigated, in agreement with theoretical and experimental studies from the literature. Then we present an experimental method to retrieve B for natural snow using optical measurements. Analytical expressions of albedo and AFEC demonstrate that B can be retrieved from simultaneous measurements of albedo and AFEC of a snow layer, or similarly from vertical profiles of reflectance and light intensity in a snowpack. Such measurements were performed in Antarctica and in the Alps and led to values of B between 0.8 and 2.0, which significantly differs from the theoretical value for spherical grains: B = 1.25. In addition, values of B were estimated from data in the literature. This led to a wider range of values (1.0–9.9) which may be partially explained by the accuracy of the data. We demonstrate that grain shape has a significant influence on AFEC in natural snow. It highlights the large variety of natural snow microstructure and the importance of considering grain shape for snow optics questions. It experimentally demonstrates that spherical grains are inappropriate to model light extinction in snow, an important result that should be considered in further studies dedicated to subsurface absorption of shortwave radiation and snow photochemistry.

Description: Feedbacks and mechanisms affecting the global sensitivity of glaciers to climate change The Cryosphere Discussions, 7, 2761-2800, 2013 Author(s): B. Marzeion, A. H. Jarosch, and J. M. Gregory Mass loss by glaciers has been an important contributor to sea level rise in the past and is projected to contribute a substantial fraction of total sea level rise during the 21st century. Here, we use a model of the world's glaciers in order to quantify equilibrium sensitivities of global glacier mass to climate change, and to investigate the role of changes in glacier hypsometry for long term mass changes. We find that 21st century glacier mass loss to a~large degree is governed by the glaciers responding to 20th century climate change. This limits the influence of 21st century climate change on glacier mass loss, and explains why there are relatively small differences in glacier mass loss under greatly different scenarios of climate change. Because of the geographic distribution of glaciers, both temperature and precipitation anomalies experienced by glaciers are vastly stronger than on global average. The projected increase in precipitation partly compensates for the mass loss caused by warming, but this compensation is negligible at higher temperature anomalies since an increasing fraction of precipitation at the glacier sites it liquid. Loss of low-lying glacier area, and more importantly, eventual complete disappearance of glaciers, strongly limit the projected sea level contribution from glaciers in coming centuries. The adjustment of glacier hypsometry to changes in the forcing reduces the sensitivity of global glacier mass to changes in global mean temperature by a factor of two to three. This result is a second reason for the relatively weak dependence of glacier mass loss on future climate scenario, and helps explain why glacier mass loss in the first half of the 20th century was of the same order of magnitude as in the second half of the 20th century, even though the rate of warming was considerably smaller.

Description: Seasonal evolution of snow permeability under equi-temperature and temperature-gradient conditions The Cryosphere Discussions, 7, 2725-2759, 2013 Author(s): F. Domine, S. Morin, E. Brun, and M. Lafaysse The permeability K of snow to air flow affects the transfer of energy, water vapor and chemical species between the snow and the atmosphere. Yet today little is known of the temporal evolution of snow permeability as a function of metamorphic regime. Furthermore, our ability to simulate snow permeability over the seasonal evolution of a snowpack has not been tested. Here we have measured the evolution of snow permeability in a subarctic snowpack subject to high temperature-gradient (TG) metamorphism. We have also measured the evolution of the same snowpack deposited over tables so that it evolved in the equi-temperature (ET) regime. Permeability varies in the range 31 × 10 –10 (ET regime) to 650 × 10 –10 m 2 (TG regime). Permeability increases over time in TG conditions and decreases under ET conditions. Using measurements of density ρ and of specific surface area (SSA), from which the equivalent sphere radius r is determined, we show that the equation linking SSA, density ρ and permeability, K = 3.0 r 2 e (–0.013 ρ ) (with K in m 2 , r in m and ρ in kg m −3 ) obtained in a previous study adequately predicts permeability values. The detailed snowpack model Crocus is used to simulate the physical properties of the TG and ET snowpacks. For the most part, all variables are well reproduced. Simulated permeabilities are up to a factor of two greater than measurements for depth hoar layers, which we attribute to snow microstructure, as the aerodynamic properties of hollow depth hoar crystals are different from those of spheres. Finally, the large difference in permeabilities between ET and TG metamorphic regimes will impact atmosphere-snow energy and mass exchanges and these effects deserve consideration in predicting the effect of climate change on snow properties and snow-atmosphere interactions.

Description: 2001–2010 glacier changes in the Central Karakoram National Park: a contribution to evaluate the magnitude and rate of the "Karakoram anomaly" The Cryosphere Discussions, 7, 2891-2941, 2013 Author(s): U. Minora, D. Bocchiola, C. D'Agata, D. Maragno, C. Mayer, A. Lambrecht, B. Mosconi, E. Vuillermoz, A. Senese, C. Compostella, C. Smiraglia, and G. Diolaiuti Karakoram is one of the most glacierized region worldwide, and glaciers therein are the main water resource of Pakistan. The attention paid to this area is increasing, because the evolution of its glaciers recently depicted a situation of general stability, known as "Karakoram Anomaly", in contrast to glacier retreat worldwide. Here we focused our attention upon the glacier evolution within the Central Karakoram National Park (CKNP, a newborn park of this region, ca. 12 162 km 2 in area) to assess the magnitude and rate of such anomaly. By means of Remote Sensing data (i.e.: Landsat images), we analyzed a sample of more than 700 glaciers, and we found out their area change between 2001 and 2010 is not significant (+27 km 2 ± 42 km 2 ), thus confirming their stationarity. We analyzed climate data, snow coverage from MODIS, and supraglacial debris presence, as well as potential (con-) causes. We found a slight decrease of summer temperatures (down to −1.5 °C during 1980–2009) and an increase of wet days during winter (up +3.3 days yr −1 during 1980–2009), possibly increasing snow cover duration, consistently with MODIS data. We further detected considerable supra-glacial debris coverage (ca. 20% of the glacier area which rose up to 31% considering only the ablation area), which could have reduced buried ice melting during the last decade. These results provide further ground to uphold the existence of the Karakoram Anomaly, and present an useful template for assessment of water availability within the glaciers of the CKNP.

Description: What drives basin scale spatial variability of snow water equivalent during two extreme years? The Cryosphere Discussions, 7, 2943-2977, 2013 Author(s): G. A. Sexstone and S. R. Fassnacht This study uses a combination of field measurements and Natural Resource Conservation Service (NRCS) operational snow data to understand the drivers of snow water equivalent (SWE) spatial variability at the basin scale. Historic snow course snowpack density observations were analyzed within a multiple linear regression snow density model to estimate SWE directly from snow depth measurements. Snow surveys were completed on or about 1 April 2011 and 2012 and combined with NRCS operational measurements to investigate the spatial variability of SWE. Bivariate relations and multiple linear regression models were developed to understand the relation of SWE with terrain and canopy variables (derived using a geographic information system (GIS)). Calculation of SWE directly from snow depth measurement using the snow density model has strong statistical performance and model validation suggests the model is transferable to independent data within the bounds of the original dataset. This pathway of estimating SWE directly from snow depth measurement is useful when evaluating snowpack properties at the basin scale, where many time consuming measurements of SWE are often not feasible. During both water year (WY) 2011 and 2012, elevation and location (UTM Easting and UTM Northing) were the most important model variables, suggesting that orographic precipitation and storm track patterns are likely consistent drivers of basin scale SWE variability. Terrain characteristics, such as slope, aspect, and curvature, were also shown to be important variables, but to a lesser extent at the scale of interest.

Description: Evidence for spring mountain snowpack retreat from a Landsat-derived snow cover climate data record The Cryosphere Discussions, 7, 2089-2117, 2013 Author(s): C. J. Crawford A Landsat snow cover climate data record (CDR) of visible mountain snow-covered area (SCA) across interior northwestern USA during spring was compared with ground-based snow telemetry (SNOTEL) snow-water-equivalent (SWE) measurements and mean surface temperature and total precipitation observations. Landsat spring SCA on 1 June was positively correlated with 15 May and 1 June SWE, negatively correlated with spring temperatures (April–June), and positively correlated with March precipitation. Using linear regression with predicted residual error sum-of-squares (PRESS) cross-validation, spring SCA was reconstructed (1901–2009) for the mountains of central Idaho and southwestern Montana using instrumental spring surface temperature records. The spring SCA reconstruction shows natural internal variability at interannual to decadal timescales including above average SCA in the 1900s, 1910s, 1940s-1970s, and below average SCA in the 1920s, 1930s, and since the mid 1980s. The reconstruction also reveals a~centennial trend towards decreasing spring SCA with estimated losses of −36.2 % since 1901. Based on the inferred thermal relationship between temperature and snow, strong evidence emerges for mountain snowpack retreat triggered by spring warming, a signal that includes both feedback and response mechanisms. Expanding snow cover CDRs to include additional operational satellite retrievals will add temporal SCA estimates during other snow accumulation and melt intervals for improved satellite-instrumental climate model calibration. Merging Landsat snow cover CDRs with instrumental climate records is a formidable method to monitor climate-driven changes in western US snowpack extent over 20th and 21st centuries.

Description: Monitoring water accumulation in a glacier using magnetic resonance imaging The Cryosphere Discussions, 7, 2119-2151, 2013 Author(s): A. Legchenko, C. Vincent, J. M. Baltassat, J. F. Girard, E. Thibert, O. Gagliardini, M. Descloitres, A. Gilbert, S. Garambois, A. Chevalier, and H. Guyard Tête Rousse is a small polythermal glacier located in the Mont Blanc area (French Alps) at an altitude of 3100 to 3300 m. Recent accumulation of melt water in the glacier was assumed to occur, but such accumulation had yet to be confirmed. Using Surface Nuclear Magnetic Resonance imaging (3-D-SNMR), we showed that the temperate part of the Tête Rousse glacier contains two separate water-filled caverns (central and upper caverns). In 2009, the central cavern contained about 55 000 m 3 of water. Since 2010, the cavern is drained every year. Using 3-D-SNMR, we monitored the changes caused by this pumping in the water distribution within the glacier body. Twice a year, we carried out magnetic resonance imaging of the entire glacier and estimated the volume of water accumulated in the central cavern. Our results show the changes in cavern geometry and recharge rate: in two years, the central cavern lost about 73% of its initial volume, but 65% were lost in one year after the first pumping. We also observed that, after being drained, the cavern was recharged at an average rate of 20 to 25 m 3 d −1 over the winter months and 120 to 180 m 3 d −1 in summer. These observations illustrate how ice and water may refill englacial volume being emptied by artificial draining. Comparison of the 3-D-SNMR results with those obtained by drilling and pumping showed a very good correspondence, confirming the high reliability of 3-D-SNMR imaging.

Description: Technical Note: On the use of the mushy-layer Rayleigh number for the interpretation of sea-ice-core data The Cryosphere Discussions, 7, 3209-3230, 2013 Author(s): M. Vancoppenolle, D. Notz, F. Vivier, J. Tison, B. Delille, G. Carnat, J. Zhou, F. Jardon, P. Griewank, A. Lourenço, and T. Haskell We examine some practical aspects of using a mushy-layer Rayleigh number for the interpretation of sea-ice-core data. In principle, such analysis should allow one to determine convectively active regions within the ice core by identifying those regions in which the mush-Rayleigh number is super-critical. In practice, however, a quantitative analysis is complicated by uncertainties regarding the specific formulation of both the mush-Rayleigh number itself and of the sea-ice permeability that is crucial for quantifying the Rayleigh number. Additionally, brine loss from highly permeable sections of the ice core, in particular close to the ice–ocean interface, and typically weekly ice core sampling, limit the practical applicability of the Rayleigh number for ice-core interpretation. We here quantify these uncertainties, suggest a standard method for the computation of the Rayleigh number for sea ice and discuss possibilities and limitations of ice-core interpretation based on the Rayleigh number.

Description: Decay of a long-term monitored glacier: the Careser glacier (Ortles-Cevedale, European Alps) The Cryosphere Discussions, 7, 3293-3335, 2013 Author(s): L. Carturan, C. Baroni, M. Becker, A. Bellin, O. Cainelli, A. Carton, C. Casarotto, G. Dalla Fontana, A. Godio, T. Martinelli, M. C. Salvatore, and R. Seppi The continuation of valuable, long-term glacier observation series is threatened by the accelerated mass loss which currently affects a large portion of so-called "benchmark" glaciers. In this work we present the evolution of the Careser glacier, from the beginning of systematic observation at the end of the nineteenth century to its current condition in 2012. In addition to having one of the longest and richest observation record among the Italian glaciers, Careser is unique in the Italian Alps for its 45 yr mass balance series started in 1967. In the present study, variations in the length, area and volume of the glacier since 1897 are examined, updating the series of direct mass balance observations and extending it into the past using the geodetic method. The glacier is currently strongly out of balance and in rapid decay; its average mass loss rate over the last three decades was −1.5 m water equivalent per year, increasing to −2.0 m water equivalent per year in the last decade. If mass loss continues at this pace, the glacier will disappear within a few decades, putting an end to this unique observation series.

Description: Seasonal and annual mass balances of Mera and Pokalde glaciers (Nepal Himalaya) since 2007 The Cryosphere Discussions, 7, 3337-3378, 2013 Author(s): P. Wagnon, C. Vincent, Y. Arnaud, E. Berthier, E. Vuillermoz, S. Gruber, M. Ménégoz, A. Gilbert, M. Dumont, J. M. Shea, D. Stumm, and B. K. Pokhrel In the Everest region, Nepal, ground-based monitoring programs were started on the debris-free Mera Glacier (27.7° N, 86.9° E; 5.1 km 2 , 6420 to 4940 m a.s.l.) in 2007 and on the small Pokalde Glacier (27.9° N, 86.8° E; 0.1 km 2 , 5690 to 5430 m a.s.l., ∼ 25 km North of Mera Glacier) in 2009. These glaciers lie on the southern flank of the central Himalaya under the direct influence of the Indian monsoon and receive more than 80% of their annual precipitation in summer (June to September). Despite a large inter-annual variability with glacier-wide mass balances ranging from −0.77± 0.40 m w.e. in 2011–2012 (Equilibrium-line altitude (ELA) at ∼ 6055 m a.s.l.) to + 0.46 ± 0.40 m w.e. in 2010–2011 (ELA at ∼ 5340 m a.s.l.), Mera Glacier has been shrinking at a moderate mass balance rate of −0.10± 0.40 m w.e. yr −1 since 2007. Ice fluxes measured at two distinct transverse cross sections at ∼ 5350 m a.s.l. and ∼ 5520 m a.s.l. confirm that the mean state of this glacier over the last one or two decades corresponds to a limited mass loss, in agreement with remotely-sensed region-wide mass balances of the Everest area. Seasonal mass balance measurements show that ablation and accumulation are concomitant in summer which in turn is the key season controlling the annual glacier-wide mass balance. Unexpectedly, ablation occurs at all elevations in winter due to wind erosion and sublimation, with remobilized snow likely being sublimated in the atmosphere. Between 2009 and 2012, the small Pokalde Glacier lost mass more rapidly than Mera Glacier with respective mean glacier-wide mass balances of −0.72 and −0.26 ± 0.40 m w.e. yr −1 . Low-elevation glaciers, such as Pokalde Glacier, have been usually preferred for in-situ observations in Nepal and more generally in the Himalayas, which may explain why compilations of ground-based mass balances are biased toward negative values compared with the regional mean under the present-day climate.

Description: Constraining GRACE-derived cryosphere-attributed signal to irregularly shaped ice-covered areas The Cryosphere Discussions, 7, 3417-3447, 2013 Author(s): W. Colgan, S. Luthcke, W. Abdalati, and M. Citterio We use a Monte Carlo approach to invert a spherical harmonic representation of cryosphere-attributed mass change in order to infer the most likely underlying mass changes within irregularly shaped ice-covered areas at nominal 26 km resolution. By inverting a spherical harmonic representation through the incorporation of additional fractional ice coverage information, this approach seeks to eliminate signal leakage between non- and ice-covered areas. The spherical harmonic representation suggests a Greenland mass loss of 251 ± 25 Gt yr −1 over the December 2003 to December 2010 period. The inversion suggests 218 ± 20 Gt yr −1 was due to the ice sheet proper, and 34 ± 5 Gt yr −1 (or ~ 14%) was due to Greenland peripheral glaciers and ice caps (GrPGIC). This mass loss from GrPGIC exceeds that inferred from all ice masses on both Ellesmere and Devon Islands combined. This partition therefore highlights that GRACE-derived "Greenland" mass loss cannot be taken as synonymous with "Greenland ice sheet" mass loss when making comparisons with estimates of ice sheet mass balance derived from techniques that only sample the ice sheet proper.

Description: A subglacial hydrological model dedicated to glacier sliding The Cryosphere Discussions, 7, 3449-3496, 2013 Author(s): B. de Fleurian, O. Gagliardini, T. Zwinger, G. Durand, E. Le Meur, D. Mair, and P. Råback The flow of glaciers and ice-streams is strongly influenced by the presence of water at the interface between ice and bedrock. In this paper, a hydrological model evaluating the subglacial water pressure is developed with the final aim of estimating the sliding velocities of glaciers. The global model fully couples the subglacial hydrology and the ice dynamics through a water-dependent friction law. The hydrological part of the model follows a double continuum approach which relies on the use of porous layers to compute water heads in inefficient and efficient drainage systems. This method has the advantage of a relatively low computational cost that would allow its application to large ice bodies such as Greenland or Antarctica ice-streams. The hydrological model has been implemented in the finite element code Elmer/Ice, which simultaneously computes the ice flow. Herein, we present an application to the Haut Glacier d'Arolla for which we have a large number of observations, making it well suited to the purpose of validating both the hydrology and ice flow model components. The selection of hydrological, under-determined parameters from a wide range of values is guided by comparison of the model results with available glacier observations. Once this selection has been performed, the coupling between subglacial hydrology and ice dynamics is undertaken throughout a melt season. Results indicate that this new modelling approach for subglacial hydrology is able to reproduce the broad temporal and spatial patterns of the observed subglacial hydrological system. Furthermore, the coupling with the ice dynamics shows good agreement with the observed spring speed-up.

Description: Review article: the false–bottom ice The Cryosphere Discussions, 7, 5659-5682, 2013 Author(s): D. V. Alexandrov, J. Jouzel, I. Nizovtseva, and L. B. Ryashko Nansen from his observations in the Beaufort Sea published in 1897 noted that heat transfer from the fresh water (with a~temperature of 0 °C) to the arctic salt water (with a temperature of −1.6 °C) is the only source of ice accretion during the polar summer. This transfer mechanism, unusual at first sight, is responsible for the initiation and evolution of a false bottom ice, changing ice properties to a great extent and affecting various processes while interacting with the ocean and the atmosphere. The processes of false bottom ice growth from below (i.e. from the ocean to the atmosphere) become of prime importance in the era of global warming and climate change. In this review, we summarize the theoretical approaches, field and laboratory observations, conducted during more than 100 yr, in order to address the problem of false bottoms to a broad community of readers. We also discuss the recent modeling advances to which we have contributed. A "false bottom" is a thin layer of ice which forms in summer underneath the floe, where fresh water lies between the salt water and the ice. Such false bottoms represent the only significant source of ice growth in the Arctic during the spring-summer period. Their evolution influences the mass balance of the Arctic sea-ice cover, which is recognized as an indicator of climate change. However, the quantity, aerial extent and other properties of false bottoms are difficult to measure because coring under the surface melt ponds leads to direct mixing of surface and under-ice water. This explains why their aerial extent and overall volume is still not known despite the fact that the upper limit of the present-day estimate of the false bottom ice coverage is approximately half of the sea ice surface. The growth of false bottoms also leads to other important consequences for various physical, chemical and biological processes associated with their dynamics.

Description: ESA's Ice Sheets CCI: validation and inter-comparison of surface elevation changes derived from laser and radar altimetry over Jakobshavn Isbræ, Greenland – Round Robin results The Cryosphere Discussions, 7, 5433-5460, 2013 Author(s): J. F. Levinsen, K. Khvorostovsky, F. Ticconi, A. Shepherd, R. Forsberg, L. S. Sørensen, A. Muir, N. Pie, D. Felikson, T. Flament, R. Hurkmans, G. Moholdt, B. Gunter, R. C. Lindenbergh, and M. Kleinherenbrink In order to increase the understanding of the changing climate, the European Space Agency has launched the Climate Change Initiative (ESA CCI), a program which joins scientists and space agencies into 13 projects either affecting or affected by the concurrent changes. This work is part of the Ice Sheets CCI and four parameters are to be determined for the Greenland Ice Sheet (GrIS), each resulting in a dataset made available to the public: Surface Elevation Changes (SEC), surface velocities, grounding line locations, and calving front locations. All CCI projects have completed a so-called Round Robin exercise in which the scientific community was asked to provide their best estimate of the sought parameters as well as a feedback sheet describing their work. By inter-comparing and validating the results, obtained from research institutions world-wide, it is possible to develop the most optimal method for determining each parameter. This work describes the SEC Round Robin and the subsequent conclusions leading to the creation of a method for determining GrIS SEC values. The participants used either Envisat radar or ICESat laser altimetry over Jakobshavn Isbræ drainage basin, and the submissions led to inter-comparisons of radar vs. altimetry as well as cross-over vs. repeat-track analyses. Due to the high accuracy of the former and the high spatial resolution of the latter, a method, which combines the two techniques will provide the most accurate SEC estimates. The data supporting the final GrIS analysis stem from the radar altimeters on-board Envisat, ERS-1 and ERS-2. The accuracy of laser data exceeds that of radar altimetry; the Round Robin analysis has, however, proven the latter equally capable of dealing with surface topography thereby making such data applicable in SEC analyses extending all the way from the interior ice sheet to margin regions. This shows good potential for a~future inclusion of ESA CryoSat-2 and Sentinel-3 radar data in the analysis, and thus for obtaining reliable SEC estimates throughout the entire GrIS.