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: Air temperature variability over three glaciers in the Ortles-Cevedale (Italian Alps): effects of glacier disintegration, intercomparison of calculation methods, and impacts on mass balance modeling The Cryosphere Discussions, 8, 6147-6192, 2014 Author(s): L. Carturan, F. Cazorzi, F. De Blasi, and G. Dalla Fontana Glacier mass balance models rely on accurate spatial calculation of input data, in particular air temperature. Lower temperatures (the so-called glacier cooling effect), and lower temperature variability (the so-called glacier damping effect) generally occur over glaciers, compared to ambient conditions. These effects, which depend on the geometric characteristics of glaciers and display a high spatial and temporal variability, have been mostly investigated on medium- to large-size glaciers so far, while observations on smaller ice bodies are scarce. Using a dataset from 8 on-glacier and 4 off-glacier weather stations, collected in summer 2010 and 2011, we analyzed the air temperature variability and wind regime over three different glaciers in the Ortles-Cevedale. The magnitude of the cooling effect and the occurrence of katabatic boundary layer (KBL) processes showed remarkable differences among the three ice bodies, suggesting the likely existence of important reinforcing mechanisms during glacier decay and disintegration. None of the methods proposed in the literature for calculating on-glacier temperature from off-glacier data fully reproduced our observations. Among them, the more physically-based procedure of Greuell and Böhm (1998) provided the best overall results where the KBL prevail, but it was not effective elsewhere (i.e. on smaller ice bodies and close to the glacier margins). The accuracy of air temperature estimations strongly impacted the results from a mass balance model which was applied to the three investigated glaciers. Most importantly, even small temperature deviations caused distortions in parameter calibration, thus compromising the model generalizability.

Description: Strategy of valid 14 C dates choice in syngenetic permafrost The Cryosphere Discussions, 8, 5589-5621, 2014 Author(s): Y. K. Vasil'chuk and A. C. Vasil'chuk The main problem of radiocarbon dating within permafrost is the uncertain reliability of the 14 C dates. Syngenetic sediments contain allochthonous organic deposit that originated at a distance from its present position. Due to the very good preservation of organic materials in permafrost conditions and numerous re-burials of the fossils from ancient deposits into younger ones the dates could be both younger and older than the true age of dated material. The strategy for the most authentic radiocarbon date selection for dating of syncryogenic sediments is considered taking into account the fluvial origin of the syngenetic sediments. The re-deposition of organic material is discussed in terms of cyclic syncryogenic sedimentation and also the possible re-deposition of organic material in subaerial-subaqueous conditions. The advantages and the complications of dating organic micro-inclusions from ice wedges by the accelerator mass spectrometry (AMS) method are discussed applying to true age of dated material search. Radiocarbon dates of different organic materials from the same samples are compared. The younger age of the yedoma from cross-sections of Duvanny Yar in Kolyma River and Mamontova Khayata in the mouth of Lena River is substantiated due to the principle of the choice of the youngest 14 C date from the set.

Description: Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project The Cryosphere Discussions, 8, 5539-5588, 2014 Author(s): B. de Boer, A. M. Dolan, J. Bernales, E. Gasson, H. Goelzer, N. R. Golledge, J. Sutter, P. Huybrechts, G. Lohmann, I. Rogozhina, A. Abe-Ouchi, F. Saito, and R. S. W. van de Wal In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere–ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently published Bedmap2 bedrock topography indicate a significantly larger contribution to Pliocene sea-level rise from the East Antarctic ice sheet for all six models relative to the simulations with Bedmap1.

Description: Snow mass decrease in the Northern Hemisphere (1979/80–2010/11) The Cryosphere Discussions, 8, 5623-5644, 2014 Author(s): Z. Li, J. Liu, L. Huang, N. Wang, B. Tian, J. Zhou, Q. Chen, and P. Zhang Snow cover has a key effect on climate change and hydrological cycling, as well as water supply to a sixth of the world's population across the Northern Hemisphere. However, reliable data on trends in snow cover in the Northern Hemisphere is lacking. Snow water equivalent (SWE) is a common measure of the amount of equivalent water of the snow pack. Here we verify the accuracy of three existing global SWE products and merge the most accurate aspects of them to generate a new SWE product covering the last 32 years (1979/80–2010/11). Using this new SWE product, we show that there has been a significant decreasing trend in the total mass of snow in the Northern Hemisphere. The most notable changes in total snow mass are −16.45 ± 6.68 and −13.55 ± 7.80 Gt year −1 in January and February, respectively. These are followed by March and December, which have trends of −12.58 ± 6.88 and −10.70 ± 5.62 Gt year −1 , respectively, from 1979/80 to 2010/11. During the same period, the temperature in the study area raised 0.17 °C decade −1 , which is thought to be the main reason of SWE decline.

Description: Factors controlling Slope Environmental Lapse Rate (SELR) of temperature in the monsoon and cold-arid glacio-hydrological regimes of the Himalaya The Cryosphere Discussions, 8, 5645-5686, 2014 Author(s): R. J. Thayyen and A. P. Dimri Moisture, temperature and precipitation interplay forced through the orographic processes sustains the Himalayan cryospheric system. However, factors controlling the Slope Environmental Lapse Rate (SELR) of temperature along the higher Himalayan mountain slopes across various glacio-hydrologic regimes remain as a key knowledge gap. Present study dwells on the orographic processes driving the moisture–temperature interplay in the monsoon and cold-arid glacio-hydrological regimes of the Himalaya. Systematic data collection at three altitudes between 2540 and 3763 m a.s.l. in the Garhwal Himalaya (hereafter called monsoon regime) and between 3500 and 5600 m a.s.l. in the Ladakh Himalaya (herefater called cold-arid regime) revealed moistrue control on temperature distribution at temporal and spatial scales. Observed daily SELR of temperature ranges between 9.0 to 1.9 °C km −1 and 17.0 to 2.8 °C km −1 in the monsoon and cold-arid regimes respectively highlighting strong regional variability. Moisture influx to the region, either from Indian summer monsoon (ISM) or from Indian winter monsoon (IWM) forced lowering of SELR. This phenophena of "monsoon lowering" of SELR is due to the release latent heat of condensation from orographically focred lifted air parcel. Seasonal response of SELR in the monsoon regime is found to be closly linked with the variations in the local lifting condensation levels (LCL). Contrary to this, cold-arid system is characterised by the extremely high values of daily SELR upto 17 °C km −1 signifying the extremely arid conditions prevailing in summer. Distinctly lower SELR devoid of monsoon lowering at higher altitude sections of monsoon and cold-arid regimes suggests sustained wetter high altitude regimes. We have proposed a SELR model for both glacio-hydrological regimes demostrating with two sections each using a derivative of the Clausius–Clapeyron relationship by deriving monthly SELR indices. It has been proposed that the manifestations of presence or absence of moisture is the single most important factor determining the temperature distribution along the higher Himalayan slopes driven by the orographic forcings. This work also suggests that the arbitary use of temperature lapse rate to extrapolate temperature to the higher Himalaya is extremely untenable.

Description: A model study of Abrahamsenbreen, a surging glacier in northern Spitsbergen The Cryosphere Discussions, 8, 5687-5726, 2014 Author(s): J. Oerlemans and W. J. J. van Pelt The climate sensitivity of Abrahamsenbreen, a 20 km long surge-type glacier in northern Spitsbergen, is studied with a simple glacier model. A scheme to describe the surges is included, which makes it possible to account for the effect of surges on the total mass budget of the glacier. A climate reconstruction back to AD 1300, based on ice-core data from Lomonosovfonna and climate records from Longyearbyen, is used to drive the model. The model is calibrated by requesting that it produces the correct Little Ice Age maximum glacier length and simulates the observed magnitude of the 1978-surge. Abrahamsenbreen is strongly out of balance with the current climate. If climatic conditions will remain as they were for the period 1989–2010, the glacier will ultimately shrink to a length of about 4 km (but this will take hundreds of years). For a climate change scenario involving a 2 m yr −1 rise of the equilibrium line from now onwards, we predict that in the year 2100 Abrahamsenbreen will be about 12 km long. The main effect of a surge is to lower the mean surface elevation and to increase the ablation area, thereby causing a negative perturbation of the mass budget. We found that the occurrence of surges leads to a somewhat stronger retreat of the glacier in a warming climate. Because of the very small bed slope, Abrahamsenbreen is sensitive to small perturbations in the equilibrium-line altitude E . For a decrease of E of only 160 m, the glacier would steadily grow into the Woodfjorddalen until after 2000 years it would reach the Woodfjord and calving could slow down the advance.

Description: Brief communication "Snow profile associated measurements (SPAM) – a new instrument for quick snow profile measurements" The Cryosphere Discussions, 5, 1737-1748, 2011 Author(s): P. Lahtinen A new instrument concept (SPAM) for snow profile associated measurements is presented. The potential of the concept is demonstrated by presenting preliminary results obtained with the prototype instrument. With this concept it is possible to retrieve rapid snow profiles of e.g. light extinction, reflectance, temperature and snow layer structure with high vertical resolution. As a side-product, also snow depth is retrieved.

Description: Modeling the impact of wintertime rain events on the thermal regime of permafrost The Cryosphere Discussions, 5, 1697-1736, 2011 Author(s): S. Westermann, J. Boike, M. Langer, T. V. Schuler, and B. Etzelmüller In this study, we present field measurements and numerical process modeling from Western Svalbard showing that the ground surface temperature below the snow is impacted by strong wintertime rain events. During such events, rain water percolates to the bottom of the snow pack, where it freezes and releases latent heat. In the winter season 2005/2006, on the order of 20 to 50 % of the wintertime precipitation fell as rain, thus confining the surface temperature to close to 0 °C for several weeks. The measured average ground surface temperature during the snow-covered period is −0.6 °C, despite of a snow surface temperature of on average −8.5 °C. For the considered period, the temperature threshold below which permafrost is sustainable on long timescales is exceeded. We present a simplified model of rain water infiltration in the snow coupled to a transient permafrost model. While small amounts of rain have only minor impact on the ground surface temperature, strong rain events have a long-lasting impact. We show that consecutively applying the conditions encountered in the winter season 2005/2006 results in the formation of an unfrozen zone in the soil after three to five years, depending on the prescribed soil properties. If water infiltration in the snow is disabled in the model, more time is required for the permafrost to reach a similar state of degradation.

Description: Investigating changes in basal conditions of Variegated Glacier prior and during its 1982–1983 surge The Cryosphere Discussions, 5, 1461-1494, 2011 Author(s): M. Jay-Allemand, F. Gillet-Chaulet, O. Gagliardini, and M. Nodet The Variegated Glacier (Alaska) is known to surge periodically after a sufficient amount of cumulative mass balance is reached, but this observation is difficult to link with changes in the basal conditions. Here, using a 10-year dataset, consisting in surface topography and surface velocity observations along a flow line for 25 dates, we have reconstructed the evolution of the basal conditions prior and during the 1982–1983 surge. The model solves the full-Stokes problem along the central flow line using the finite element method. For the 25 dates of the dataset, the basal friction parameter distribution is inferred using the inverse method proposed by Arthern and Gudmundson (2010). This method is here slightly modified by incorporating a regularisation term in the cost function to avoid short wave length changes in the friction parameter. Our results indicate that dramatic changes in the basal conditions occurred between 1973 to 1983. Prior to the surge, periodical changes can be observed between winter and summer, with a regular increase of the sliding from 1973 to 1982. During the surge, the basal friction decreased dramatically and an area of very low friction moved from the upper part of the glacier to its terminus. Using a more complex friction law, these changes in basal sliding are then interpreted in terms of basal water pressure. It confirms that dramatic changes took place in the subglacial drainage system of Variegated Glacier, moving from a relatively efficient drainage system prior to the surge to an inefficient one during the surge. By reconstructing the water pressure evolution at the base of the glacier it is possible to infer realistic scenarios for the hydrological history leading to the occurrence of a surge.

Description: Comparison of aeolian snow transport events and snow mass fluxes between observations and simulations made by the regional climate model MAR in Adélie Land, East Antarctica The Cryosphere Discussions, 8, 6007-6032, 2014 Author(s): A. Trouvilliez, H. Gallée, F. Naaim-Bouvet, C. Genthon, C. Amory, V. Favier, C. Agosta, L. Piard, and H. Bellot The regional climate model MAR including a coupled snow pack/aeolian snow transport parameterisation is compared with aeolian snow mass fluxes at a fine spatial resolution (5 km horizontally and 2 m vertically) and at a fine temporal resolution (30 min) over 1 month in Antarctica. Numerous feedbacks are taken into account in the MAR including the drag partitioning caused by the roughness elements. Wind speed is correctly simulated with a positive value of the Nash test (0.60 and 0.37) but the wind speeds above 10 m s −1 are underestimated. The aeolian snow transport events are correctly reproduced with a good temporal resolution except for the aeolian snow transport events with a particles' maximum height below 1 m. The simulated threshold friction velocity, calculated without snowfall, is overestimated. The simulated aeolian snow mass fluxes between 0 to 2 m have the same variations but are underestimated compared to the second-generation FlowCapt values and so is the simulated relative humidity at 2 m. This underestimation is not entirely due to the underestimation of the simulated wind speed. The MAR underestimates the aeolian snow quantity that pass through the first two meters by a factor ten compared to the second-generation FlowCapt value (13 990 kg m −1 and 151 509 kg m −1 respectively). It will conduct the MAR, with this parametrisation, to underestimate the effect of the aeolian snow transport on the Antarctic surface mass balance.

Description: Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area The Cryosphere Discussions, 8, 6033-6057, 2014 Author(s): J. Gaume, G. Chambon, N. Eckert, M. Naaim, and J. Schweizer Dry-snow slab avalanches are generally caused by a sequence of fracture processes including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually improved our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically-based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by elasticity of the slab and the slab possible tensile failure. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and to trigger a tensile failure within the slab, (2) partial-slope release if fracture arrest and slab tensile failure occurs due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and of the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as it is often claimed in the literature.

Description: Climatic signals from 76 shallow firn cores in Dronning Maud Land, East Antarctica The Cryosphere Discussions, 8, 5961-6005, 2014 Author(s): S. Altnau, E. Schlosser, E. Isaksson, and D. Divine The spatial and temporal distribution of surface mass balance (SMB) and δ 18 O were investigated in the first comprehensive study of a set of 76 firn cores retrieved by various expeditions during the past three decades in Dronning Maud Land, East Antarctica. The large number of cores was used to calculate stacked records of SMB and δ 18 O, which considerably increased the signal-to-noise ratio compared to earlier studies and facilitated the detection of climatic signals. Considerable differences between cores from the interior plateau and the coastal cores were found. The δ 18 O of both the plateau and the ice shelf cores exhibit a slight positive trend over the second half of the 20th century. In the corresponding period, the SMB has a negative trend in the ice shelf cores, but increases on the plateau. Comparison with meteorological data from Neumayer Station revealed that for the ice shelf regions atmospheric dynamic effects are more important than thermodynamics, while on the plateau, the temporal variations of SMB and δ 18 O occur mostly in parallel, thus can be explained by thermodynamic effects. The Southern Annular Mode (SAM) exhibits a positive trend since the mid-1960s, which is assumed to lead to a cooling of East Antarctica. This is not confirmed by the firn core data in our data set. Changes in the atmospheric circulation that result in a changed seasonal distribution of precipitation/accumulation could partly explain the observed features in the ice shelf cores.

Description: Warming of waters in an East Greenland fjord prior to glacier retreat: mechanisms and connection to large-scale atmospheric conditions The Cryosphere Discussions, 5, 1335-1364, 2011 Author(s): P. Christoffersen, R. I. Mugford, K. J. Heywood, I. Joughin, J. A. Dowdeswell, J. P. M. Syvitski, A. Luckman, and T. J. Benham Hydrographic data acquired in Kangerlugssuaq Fjord and adjacent seas in 1993 and 2004 are used together with ocean reanalysis to elucidate water mass change and ice-ocean-atmosphere interactions in East Greenland. The hydrographic data show substantial warming of fjord waters between 1993 and 2004 and warm subsurface conditions coincide with the rapid retreat of Kangerlugssuaq Glacier in 2004–2005. The ocean reanalysis shows that the warm properties of fjord waters in 2004 are related to a major peak in oceanic shoreward heat flux into a cross-shelf trough on the outer continental shelf. The heat flux into this trough varies according to seasonal exchanges with the atmosphere as well as from deep seasonal intrusions of subtropical waters. Both mechanisms contribute to high (low) shoreward heat flux when winds from the northeast are weak (strong). The combined effect of surface heating and inflow of subtropical waters is seen in the hydrographic data, which were collected after periods when along-shore coastal winds from the north were strong (1993) and weak (2004). We show that coastal winds vary according to the pressure gradient defined by a semi-permanent atmospheric pressure system over Greenland and a persistent atmospheric low situated near Iceland. The magnitude of this pressure gradient is controlled by longitudinal variability in the position of the Icelandic Low.

Description: Recent wind driven high sea ice export in the Fram Strait contributes to Arctic sea ice decline The Cryosphere Discussions, 5, 1311-1334, 2011 Author(s): L. H. Smedsrud, A. Sirevaag, K. Kloster, A. Sorteberg, and S. Sandven Arctic sea ice area decrease has been visible for two decades, and continues at a steady rate. Apart from melting, the southward drift through Fram Strait is the main loss. We present high resolution sea ice drift across 79° N from 2004 to 2010. The ice drift is based on radar satellite data and correspond well with variability in local geostrophic wind. The underlying current contributes with a constant southward speed close to 5 cm s −1 , and drives about 33 % of the ice export. We use geostrophic winds derived from reanalysis data to calculate the Fram Strait ice area export back to 1957, finding that the sea ice area export recently is about 25 % larger than during the 1960's. The increase in ice export occurred mostly during winter and is directly connected to higher southward ice drift velocities, due to stronger geostrophic winds. The increase in ice drift is large enough to counteract a decrease in ice concentration of the exported sea ice. Using storm tracking we link changes in geostrophic winds to more intense Nordic Sea low pressure systems. Annual sea ice export likely has a significant influence on the summer sea ice variability and we find low values in the 60's, the late 80's and 90's, and particularly high values during 2005–2008. The study highlight the possible role of variability in ice export as an explanatory factor for understanding the dramatic loss of Arctic sea ice the last decades.

Description: Spatio-temporal measurements and analysis of snow depth in a rock face The Cryosphere Discussions, 5, 1383-1418, 2011 Author(s): V. Wirz, M. Schirmer, S. Gruber, and M. Lehning Snow in rock faces plays a key role in the alpine environment for permafrost distribution, snow water storage or run off in spring. However, a detailed assessment of snow depths in steep rock walls has never been attempted. To understand snow distribution in rock walls a high-resolution terrestrial laser scanner (TLS), including a digital camera, was used to obtain snow depth (HS) data with a resolution of one metre. The mean HS, the snow covered area and their evolution in the rock face were compared to a neighbouring smoother catchment and a flat field station at similar elevation. Further we analyzed the patterns of HS distribution in the rock face after different periods and investigated the main factors contributing to them. In a first step we could show that with TLS reliable information on surface data of a steep rocky surface can be obtained. In comparison to the flatter sites in the vicinity, mean HS in the rock face was lower during the entire winter, but trends of snow depth changes were similar. We observed repeating accumulation and ablation patterns in the rock face, while maximum snow depth loss always occurred at those places with maximum snow depth gain. Further analysis of the main factors contributing to the snow depth distribution in the rock face revealed terrain-wind-interaction processes to be dominant. Processes related to slope angle seem to play a role, but no linear function of slope angle and snow depth was found. Further analyses should involve measurements in rock faces with other characteristics and higher temporal resolutions to be able to distinguish individual processes better. Additionally the relation of spatial and temporal distribution of snow depth to terrain-wind interactions should be tested.

Description: Utility of late summer transient snowline migration rate on Taku Glacier, Alaska The Cryosphere Discussions, 5, 1365-1382, 2011 Author(s): M. Pelto On Taku Glacier, Alaska a combination of field observations of snow water equivalent (SWE) from snowpits and probing in the vicinity of the transient snowline (TSL) are used to quantify the mass balance gradient. The balance gradient is determined from the difference in elevation and SWE from the TSL to snowpits at 1000 m from 1998–2010 and ranges from 2.6–3.8 mm m −1 . Probing transects from 950 m–1100 m directly measure SWE and yield a slightly higher balance gradient of 3.3–3.8 mm m −1 . TSL is identified in MODIS and Landsat 4 and 7 Thematic Mapper imagery for 31 dates during the 2004–2010 period on Taku Glacier to assess the consistency of its rate of rise and usefulness in assessing mass balance. In 2010, the TSL rose from 750 m on 28 July, 800 m on 5 August, 875 m on 14 August, 925 m on 30 August, and to 975 m on 20 September. The mean observed probing balance gradient was 3.3 mm m −1 and TSL rise was 3.7 m day −1 , yielding an ablation rate of 12.2 mm day −1 on Taku Glacier from mid-July to mid-September. A comparison of the TSL rise in the region from 750–1100 m on Taku Glacier during eleven different periods of more than 14 days during the ablation season with repeat imagery indicates a mean TSL rise of 3.7 m day −1 on Taku Glacier, the rate of rise is relatively consistent ranging from 3.0 to 4.8 m day −1 . This is useful for ascertaining the final ELA if imagery or observations are not available within a week or two of the end of the ablation season. From mid-July-mid-September the mean ablation from 750–1100 m determined from the TSL rise and the observed balance gradient varied from 11 to 18 mm day −1 on Taku Glacier during the 2004–2010 period.

Description: Spatial and temporal variability of snow accumulation in Dronning Maud Land, East Antarctica, including two deep ice coring sites at Dome Fuji and EPICA DML The Cryosphere Discussions, 5, 2061-2114, 2011 Author(s): S. Fujita, P. Holmlund, I. Andersson, I. Brown, H. Enomoto, Y. Fujii, K. Fujita, K. Fukui, T. Furukawa, M. Hansson, K. Hara, Y. Hoshina, M. Igarashi, Y. Iizuka, S. Imura, S. Ingvander, T. Karlin, H. Motoyama, F. Nakazawa, H. Oerter, L. E. Sjöberg, S. Sugiyama, S. Surdyk, J. Ström, R. Uemura, and F. Wilhelms To better understand the spatio-temporal variability of the glaciological environment in Dronning Maud Land (DML), East Antarctica, investigations were carried out along the 2800-km-long Japanese-Swedish IPY 2007/2008 traverse. The route covers ice sheet ridges and two deep ice coring sites at Dome Fuji and EPICA DML. The surface mass balance (SMB) distribution was derived based on analysis of isochrones within snow pits, firn cores and subsurface radar signals. The SMB averaged over various time scales in the Holocene was determined. This was then compared with various glaciological data. We find that the large-scale distribution of the SMB depends on the surface elevation, continentality and interactions between ice sheet ridges and the prevailing counterclockwise windfield in DML. A different SMB is found for the windward and leeward sides of the ridges. Local-scale variability in the SMB is essentially governed by bedrock topography which determines the local surface topography. In the eastern part of DML, the accumulation rate in the second half of the 20th century is found to be higher by 15 % compared to averages over longer periods of 722 a or 7.9 ka before AD 2008. A similar trend has been reported for many inland plateau sites in East Antarctica.

Description: Relative effect of slope and equilibrium line altitude on the retreat of Himalayan glaciers The Cryosphere Discussions, 5, 2571-2604, 2011 Author(s): T. N. Venkatesh, A. V. Kulkarni, and J. Srinivasan A majority of glaciers in the Himalayas have been retreating. In this paper, we show that there are two major factors which control the advance/retreat of the Himalayan glaciers. They are the slope of the glacier and changes in the equilibrium line altitude. While it is well known, that these factors are important, we propose a new way of combining them and use it to predict retreat. Our model has been applied to the movement of eight Himalayan glaciers during the past 25 years. The model explains why the Gangotri glacier is retreating while Zemu of nearly the same length is stationary, even though they are subject to similar environmental changes. The model has also been applied to a larger set of glaciers in the Parbati basin, for which retreat based on satellite data is available, though over a shorter time period.

Description: Brief Communication: Can recent ice discharges following the Larsen-B ice-shelf collapse be used to infer the driving mechanisms of millennial-scale variations of the Laurentide ice sheet? The Cryosphere Discussions, 5, 3113-3127, 2011 Author(s): J. Alvarez-Solas, A. Robinson, and C. Ritz The effects of an ice-shelf collapse on inland glacier dynamics have recently been widely studied, especially since the breakup of Antarctic Peninsula's Larsen-B ice shelf in 2002. Several studies have documented acceleration of the ice streams that were flowing into the former ice shelf. The mechanism responsible for such a speed-up lies with the removal of the ice-shelf backforce. Independently, it is also well documented that during the last glacial period, the Northern Hemisphere ice sheets experienced large discharges into the ocean, likely reflecting ice flow acceleration episodes on the millennial time scale. The classic interpretation of the latter is based on the existence of an internal thermo-mechanical feedback with the potential to generate oscillatory behavior in the ice sheets. Here we would like to widen the debate by considering that Larsen-B-like glacial analog episodes could have contributed significantly to the registered millennial-scale variablity.

Description: Optimisation of quasi-3D electrical resistivity imaging – application and inversion for investigating heterogeneous mountain permafrost The Cryosphere Discussions, 5, 3383-3421, 2011 Author(s): D. Schwindt and C. Kneisel This study aimed to optimise the application, efficiency and interpretability of quasi-3D resistivity imaging for investigating the heterogeneous permafrost distribution at mountain sites by a systematic forward modelling approach. A three dimensional geocryologic model, representative for most mountain permafrost settings, was developed. Based on this geocryologic model quasi-3D models were generated by collating synthetic orthogonal 2D arrays, demonstrating the effects of array types and electrode spacing on resolution and interpretability of the inversion results. The effects of minimising the number of 2D arrays per quasi-3D grid were tested by enlarging the spacing between adjacent lines and by reducing the number of perpendicular tie lines with regard to model resolution and loss of information value. Synthetic and measured quasi-3D models were investigated with regard to the lateral and vertical resolution, reliability of inverted resistivity values, the possibility of a quantitative interpretation of resistivities and the response of the inversion process on the validity of quasi-3D models. Results show that setups using orthogonal 2D arrays with electrode spacings of 2 m and 3 m are capable of delineating lateral heterogeneity with high accuracy and also deliver reliable data on active layer thickness. Detection of permafrost thickness, especially if the permafrost base is close to the penetration depth of the setups, and the reliability of absolute resistivity values emerged to be a weakness of the method. Quasi-3D imaging has proven to be a promising tool for investigating permafrost in mountain environments especially for delineating the often small-scale permafrost heterogeneity, and therefore provides an enhanced possibility for aligning permafrost distribution with site specific surface properties and morphological settings.

Description: Seasonal variations of glacier dynamics at Kronebreen, Svalbard revealed by calving related seismicity The Cryosphere Discussions, 5, 3291-3321, 2011 Author(s): A. Köhler, A. Chapuis, C. Nuth, J. Kohler, and C. Weidle We detect and cluster waveforms of seismic signals recorded close to the calving front of Kronebreen, Svalbard, to identify glacier-induced seismic events and to investigate their relation to calving processes. Single-channel geophone data recorded over several months in 2009 and 2010 are combined with eleven days of direct visual observations of the glacier front. We apply a processing scheme which combines conventional seismic event detection using a sensitive trigger algorithm and unsupervised clustering of all detected signals based on their waveform characteristics by means of Self-Organizing Maps (SOMs). We are able to distinguish between false detections, instrumental artifacts, and three classes of signals which are, with different degrees of uncertainty, emitted by calving activity. About 10 % of the directly observed calving events close to the geophone (

Description: Recent acceleration of ice loss in the Northern Patagonia Icefield based on an updated decennial evolution The Cryosphere Discussions, 5, 3323-3381, 2011 Author(s): P. López and G. Casassa Ice elevation changes of the Northern Patagonia Icefield (NPI) were analyzed by comparing three Digital Elevation Models (DEM) corresponding to 1975 (constructed based on topographic maps), the SRTM DEM of 2000 yr and a SPOT 5 DEM of 2005. In addition, the glacier length fluctuations and the surface area evolution between 2001 and 2011 of 25 glaciers of the NPI were studied: the information extracted from the Landsat ETM+ satellite image of 11 March 2001 was compared to the measurements performed based on the Landsat ETM+ satellite image of 19 February 2011. From a global point of view, the majority of the studied glaciers thinned, retreated and lost surface between 2001 and 2011, only few glaciers (Leones, Nef, Pared Sur and Soler) located on the eastern side of the NPI have been stable. Glaciers located on the western side of the NPI suffered a stronger wasting compared to the glaciers located on the eastern side. Overall, over the ablation areas of the NPI (below 1150 m a.s.l.) a more rapid thinning of 2.6 m yr −1 occurred between 2000 and 2005 yr compared to the period 1975–2000, in which a mean thinning of 1.7 m yr −1 was measured for the same zones of the NPI. For the whole period (1975–2005) the most important thinning of the ablation areas has been estimated for HPN-1 Glacier (4.4 m yr −1 ) followed by Benito (3.4 m yr −1 ), Fraenkel (2.4 m yr −1 ), Gualas (2.1 m yr −1 ) and Acodado glaciers, all of them located on the western side of the NPI. Between 2001 and 2011, a noteworthy retreat of 1.9 km was experienced by Gualas Glacier and by Reichert Glacier with 1.6 km, both located on the north-western side of the NPI. On the south-western side of the NPI, during the same decennia, Steffen Glacier experienced a remarkable retreat of 1.6 km as well. During the 2001–2011 period, Steffen Glacier more than doubled its rate of retreat (compared to the 1979–2001 period) and experienced the disintegration of its main front as well as a lateral tongue that retreated 3.1 km. The most significant retreat observed on the eastern side was experienced by Colonia Glacier (1 km). Area loss was also relevant during the period 2001–2011. Overall, the icefield experienced a reduction of 50.6 km 2 which represents a 1.3 % relative to the surface area calculated for 2001 yr. The most remarkable surface reduction was observed for HPN-1 Glacier that lost 3.2 % of its surface estimated in 2001, followed by Steffen Glacier (2.8 %). We suggest that the glacier shrinking observed in the NPI is controlled firstly by atmospheric warming, as it has been reported in this area. Nevertheless, updated climatic studies are needed in order to confirm this suggestion. If the detected past climate trends persist, in the future, glaciers of the NPI will continuous or even increase their rate of shrinking generating important consequences for this region like the production of Glacier Lake Outburst Flood events or the decrease of the melt-water runoff in the long-term future.

Description: Arctic sea ice variability and trends, 1979–2010 The Cryosphere Discussions, 6, 957-979, 2012 Author(s): D. J. Cavalieri and C. L. Parkinson Analyses of 32 yr (1979–2010) of Arctic sea ice extents and areas derived from satellite passive microwave radiometers are presented for the Northern Hemisphere as a whole and for nine Arctic regions. There is an overall negative yearly trend of −51.5 ± 4.1 × 10 3 km 2 yr −1 (−4.1 ± 0.3% decade −1 ) in sea ice extent for the hemisphere. The sea ice extent trends for the individual Arctic regions are all negative except for the Bering Sea: −3.9 ± 1.1 × 10 3 km 2 yr −1 (−8.7 ± 2.5% decade −1 ) for the Seas of Okhotsk and Japan, +0.3 ± 0.8 × 10 3 km 2 yr −1 (+1.2 ± 2.7% decade −1 ) for the Bering Sea, −4.4 ± 0.7 × 10 3 km 2 yr −1 (−5.1 ± 0.9% decade −1 ) for Hudson Bay, −7.6 ± 1.6 × 10 3 km 2 yr −1 (−8.5 ± 1.8% decade −1 ) for Baffin Bay/Labrador Sea, −0.5 ± 0.3 × 10 3 km 2 yr −1 (−5.9 ± 3.5% decade −1 ) for the Gulf of St. Lawrence, −6.5 ± 1.1 × 10 3 km 2 yr −1 (−8.6 ± 1.5% decade −1 ) for the Greenland Sea, −13.5 ± 2.3 × 10 3 km 2 yr −1 (−9.2 ± 1.6% decade −1 ) for the Kara and Barents Seas, −14.6 ± 2.3 × 10 3 km 2 yr −1 (−2.1 ± 0.3% decade −1 ) for the Arctic Ocean, and −0.9 ± 0.4 × 10 3 km 2 yr −1 (−1.3 ± 0.5% decade −1 ) for the Canadian Archipelago. Similarly, the yearly trends for sea ice areas are all negative except for the Bering Sea. On a seasonal basis for both sea ice extents and areas, the largest negative trend is observed for summer with the next largest negative trend being for autumn.

Description: Ikaite crystals in melting sea ice – implications for p CO 2 and pH levels in Arctic surface waters The Cryosphere Discussions, 6, 1015-1035, 2012 Author(s): S. Rysgaard, R. N. Glud, K. Lennert, M. Cooper, N. Halden, R. J. G. Leakey, F. C. Hawthorne, and D. Barber A major issue of Arctic marine science is to understand whether the Arctic Ocean is, or will be, a source or sink for air-sea CO 2 exchange. This has been complicated by the recent discoveries of ikaite (CaCO 3 ·6H 2 O) in Arctic and Antarctic sea ice, which indicate that multiple chemical transformations occur in sea ice with a possible effect on CO 2 and pH conditions in surface waters. Here we report on biogeochemical conditions, microscopic examinations and x-ray diffraction analysis of single crystals from an actively melting 1.7 km 2 (0.5–1 m thick) drifting ice floe in the Fram Strait during summer. Our findings show that ikaite crystals are present throughout the sea ice but with larger crystals appearing in the upper ice layers. Ikaite crystals placed at elevated temperatures gradually disintegrated into smaller crystallites and dissolved. During our field campaign in late June, melt reduced the ice flow thickness by ca. 0.2 m per week and resulted in an estimated 1.6 ppm decrease of p CO 2 in the ocean surface mixed layer. This corresponds to an air-sea CO 2 uptake of 11 mmol m −2 sea ice d −1 or to 3.5 ton km −2 ice floe week −1 .

Description: Inferring snow pack ripening and melt out from distributed ground surface temperature measurements The Cryosphere Discussions, 6, 563-591, 2012 Author(s): M.-O. Schmid, S. Gubler, J. Fiddes, and S. Gruber The seasonal snow cover and its melting are heterogeneous both in space and time. Describing and modelling this variability are important because it affects divers phenomena such as runoff, ground temperatures or slope movements. This study investigates the derivation of melting characteristics based on spatial clusters of temperature measurements. Results are based on data from Switzerland where ground surface temperatures were measured with miniature loggers (iButtons) at 40 locations, referred to as footprints. At each footprint, ten iButtons have been distributed randomly few cm below the ground surface over an area of 10 m × 10 m. Footprints span elevations of 2100–3300 m a.s.l. and slope angles of 0–55°, as well as diverse slope expositions and types of surface cover and ground material. Based on two years of temperature data, the basal ripening date and the melt-out date are determined for each iButton, aggregated to the footprint level and further analysed. The date of melt out could be derived for nearly all iButtons, the ripening date could be extracted for only approximately half of them because it requires ground freezing below the snow pack. The variability within a footprint is often considerable and one to three weeks difference between melting or ripening of the points in one footprint is not uncommon. The correlation of mean annual ground surface temperatures, ripening date and melt-out date is moderate, making them useful intuitive complementary measured for model evaluation.

Description: Impact of spatial resolution on the modelling of the Greenland ice sheet surface mass balance between 1990–2010, using the regional climate model MAR The Cryosphere Discussions, 6, 635-672, 2012 Author(s): B. Franco, X. Fettweis, C. Lang, and M. Erpicum With the aim to force an ice dynamical model, the Greenland ice sheet (GrIS) surface mass balance (SMB) was modelled at different spatial resolutions (15–50 km) for the period 1990–2010, using the regional climate model MAR (Modèle Atmosphérique Régional) forced by the ERA-INTERIM reanalysis. This comparison revealed that (i) the inter-annual variability of the SMB components is consistent within the different spatial resolutions investigated, (ii) the MAR model simulates heavier precipitation on average over the GrIS with diminishing spatial resolution, and (iii) the SMB components (except precipitation) can be derived from a simulation at lower resolution with an ''intelligent'' interpolation. This interpolation can also be used to approximate the SMB components over another topography/ice sheet mask of the GrIS. These results are important for the forcing of an ice dynamical model, needed to enable future projections of the GrIS contribution to sea level rise over the coming centuries.

Description: Uncertainty in future solid ice discharge from Antarctica The Cryosphere Discussions, 6, 673-714, 2012 Author(s): R. Winkelmann, A. Levermann, K. Frieler, and M. A. Martin Future solid ice discharge from Antarctica under climate scenarios based on the Extended Concentration Pathways is investigated with the Potsdam Parallel Ice Sheet Model (PISM-PIK), a shallow model with a consistent representation of the ice flow in sheet, shelves and the transition zone. Both the uncertainty in the climate forcing as well as the intra-model uncertainty are combined into a probability distribution for solid ice discharge from Antarctica until the year 2500 under the ECP scenarios: All simulations are performed for a 81-member perturbed-physics ensemble and the likely ranges of surface and ocean warming under the emission pathways derived from the results of 20 CMIP3-AOGCMS. The effects of surface warming, ocean warming and increased precipitation on solid ice discharge are separately considered. We find that solid ice discharge caused by enhanced sub-shelf melting exceeds that caused by surface warming. Increasing precipitation leads to a change from net sea-level rise to sea-level drop. Our results suggest that the history of the ice-sheet plays an important role with respect to projections of solid ice discharge. Although all climate-change-forced simulations begin with the year 1850, the ice discharge around 2000 is significantly smaller than observed. Observed changes in ice discharge are reached around 2077 under the ECP-8.5 scenario. During the subsequent century, ice discharge reaches up to 0.24 m.

Description: Snow specific surface area simulation using the one-layer snow model in the Canadian LAnd Surface Scheme (CLASS) The Cryosphere Discussions, 6, 5255-5289, 2012 Author(s): A. Roy, A. Royer, B. Montpetit, P. A. Bartlett, and A. Langlois Snow grain size is a key parameter for modeling microwave snow emission properties and the surface energy balance because of its influence on the snow albedo, thermal conductivity and diffusivity. A model of the specific surface area (SSA) of snow was implemented in the one-layer snow model in the Canadian LAnd Surface Scheme (CLASS) version 3.4. This offline multilayer model (CLASS-SSA) simulates the decrease of SSA based on snow age, snow temperature and the temperature gradient under dry snow conditions, whereas it considers the liquid water content for wet snow metamorphism. We compare the model with ground-based measurements from several sites (alpine, Arctic and sub-Arctic) with different types of snow. The model provides simulated SSA in good agreement with measurements with an overall point-to-point comparison RMSE of 8.1 m 2 kg −1 , and a RMSE of 4.9 m 2 kg −1 for the snowpack average SSA. The model, however, is limited under wet conditions due to the single-layer nature of the CLASS model, leading to a single liquid water content value for the whole snowpack. The SSA simulations are of great interest for satellite passive microwave brightness temperature assimilations, snow mass balance retrievals and surface energy balance calculations with associated climate feedbacks.

Description: Net accumulation rates derived from ice core stable isotope records of Pío XI glacier, Southern Patagonia Icefield The Cryosphere Discussions, 6, 5291-5316, 2012 Author(s): M. Schwikowski, M. Schläppi, P. Santibañez, A. Rivera, and G. Casassa Pío XI, the largest glacier of the Southern Patagonia Icefield, reached its neoglacial maximum extent in 1994 and is one of the few glaciers in that area which is not retreating. In view of the recent warming it is important to understand glacier responses to climate changes. Due to its remoteness and the harsh conditions in Patagonia, no systematic mass balance studies have been performed. In this study we derived net accumulation rates for the period 2000 to 2006 from a 50 m (33.2 4 m weq) ice core collected in the accumulation area of Pío XI (2600 m a.s.l., 49°16´40´´ S, 73°21´14´´ W). Borehole temperatures indicate near temperate ice, but the average melt percent is only 16% ± 14%. Records of stable isotopes are well preserved and were used for identification of annual layers. Net accumulation rates range from 3.4 to 7.1 water equivalent (m weq) with an average of 5.8 m weq, comparable to precipitation amounts at the Chilean coast, but not as high as expected for the Icefield. Ice core stable isotope data correlate well with upper air temperatures and may be used as temperature proxy.

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: A general treatment of snow microstructure exemplified by an improved relation for the thermal conductivity The Cryosphere Discussions, 6, 4673-4693, 2012 Author(s): H. Löwe, F. Riche, and M. Schneebeli Finding relevant microstructural parameters beyond the density is a longstanding problem which hinders the formulation of accurate parametrizations of physical properties of snow. Towards a remedy we address the effective thermal conductivity tensor of snow via known anisotropic, second-order bounds. The bound provides an explicit expression for the thermal conductivity and predicts the relevance of a microstructural anisotropy parameter Q which is given by an integral over the two-point correlation function and unambiguously defined for arbitrary snow structures. For validation we compiled a comprehensive data set of 167 snow samples. The set comprises individual samples of various snow types and entire time series of metamorphism experiments under isothermal and temperature gradient conditions. All samples were digitally reconstructed by micro-computed tomography to perform microstructure-based simulations of heat transport. The incorporation of anisotropy via Q considerably reduces the root mean square error over the usual density-based parametrization. The systematic quantification of anisotropy via the two-point correlation function suggests a generalizable route to incorporate microstructure into snowpack models. We indicate the inter-relation of the conductivity to other properties and outline a potential impact of Q on dielectric constant, permeability and adsorption rate of diffusing species in the pore space.

Description: Radio-frequency probes of Antarctic ice birefringence at South Pole vs. East Antarctica; evidence for a changing ice fabric The Cryosphere Discussions, 6, 4695-4731, 2012 Author(s): D. Besson, N. Doolin, M. Stockham, and I. Kravchenko Following pioneering efforts in East Antarctica, we herein report on the amplitude and temporal characteristics of polarized surface radar echo data collected in South Polar ice using radio sounding equipment with 0.5-ns echo-time precision. We observe strong echoes at 6, 9.6, 13.9, 17, and 19 μs following vertical pulse emission from the surface, in the upper half of the ice sheet. The synchronicity of those echoes for all broadcast azimuthal polarizations affirms the lack of observable birefringence over the upper half of the ice sheet, in contrast to East Antarctica measurements in the vicinity of Dome Fuji, and signifies a dramatic difference in the character of the ice sheet in the intervening 1400 km. Of the five strongest echoes, three exhibit an evident correlation with the local surface ice flow direction, qualitatively consistent with measurements in East Antarctica. Our radio sounding measurements also permit the most precise determination to date of the ice thickness at South Pole.

Description: Boreal snow cover variations induced by aerosol emissions in the middle of the 21st century The Cryosphere Discussions, 6, 4733-4769, 2012 Author(s): M. Ménégoz, G. Krinner, Y. Balkanski, A. Cozic, O. Boucher, and P. Ciais We used a coupled climate-chemistry model to quantify the impacts of aerosols on snow cover both for the present-day and for the middle of the 21st century. Black carbon (BC) deposition over continents induces a reduction in the Mean Number of Days With Snow at the Surface (MNDWS) that ranges from 0 to 10 days over large areas of Eurasia and Northern America for the present-day relative to the pre-industrial period. This is mainly due to BC deposition during the spring, a period of the year when the remaining of snow accumulated during the winter is exposed to both strong solar radiation and large amount of aerosol deposition induced themselves by a high level of transport of particles from polluted areas. North of 30° N, this deposition flux represents 222 Gg BC month −1 on average from April to June in our simulation. A large reduction in BC emissions is expected in the future in the Radiative Concentration Pathway (RCP) scenarios. Considering this scenario in our simulation leads to a decrease in the spring BC deposition down to 110 Gg month −1 in the 2050s in the RCP8.5 scenario. However, despite the reduction of the aerosol impact on snow, the MNDWS is strongly reduced by 2050, with a decrease ranging from 10 to 100 days from pre-industrial values over large parts of the Northern Hemisphere. This reduction is essentially due to temperature increase, which is quite strong in the RCP8.5 scenario in the absence of climate mitigation policies. Moreover, the projected sea-ice retreat in the next decades will open new routes for shipping in the Arctic. However, a large increase in shipping emissions in the Arctic by the mid 21st century does not lead to significant changes of BC deposition over snow-covered areas in our simulation. Therefore, the MNDWS is clearly not affected through snow darkening effects associated to these Arctic ship emissions. In an experiment without nudging toward atmospheric reanalyses, we simulated however some changes of the MNDWS considering such aerosol ship emissions. These changes are generally not statistically significant in boreal continents, except in the Quebec and in the West Siberian plains, where they range between −5 and −10 days. They are induced both by radiative forcings of the aerosols when they are in the atmosphere, and by all the atmospheric feedbacks. Climate change by the mid 21st century could also cause biomass burning activity (forest fires) to become more intense and occur earlier in the season. In an idealized scenario in which forest fires are 50% stronger and occur 2 weeks earlier than at present, we simulated an increase in spring BC deposition of 21 Gg BC month −1 over continents located north of 30° N. This BC deposition does not impact directly the snow cover through snow darkening effects. However, in an experiment considering all the aerosol forcings and atmospheric feedbacks, enhanced fire activity induces a significant decrease of the MNDWS reaching a dozen of days in Quebec and in Eastern Siberia.

Description: Ikaite crystal distribution in Arctic winter sea ice and implications for CO 2 system dynamics The Cryosphere Discussions, 6, 5037-5068, 2012 Author(s): S. Rysgaard, D. H. Søgaard, M. Cooper, M. Pućko, K. Lennert, T. N. Papakyriakou, F. Wang, N. X. Geilfus, R. N. Glud, J. Ehn, D. F. McGinnnis, K. Attard, J. Sievers, J. W. Deming, and D. Barber The precipitation of ikaite (CaCO 3 ·6H 2 O) in polar sea ice is critical to the efficiency of the sea ice-driven carbon pump and potentially important to the global carbon cycle, yet the spatial and temporal occurrence of ikaite within the ice is poorly known. We report unique observations of ikaite in unmelted ice and vertical profiles of ikaite abundance and concentration in sea ice for the crucial season of winter. Ice was examined from two locations: a 1 m thick land-fast ice site and a 0.3 m thick polynya site, both in the Young Sound area (74° N, 20° W) of NE Greenland. Ikaite crystals, ranging in size from a few µm to 700 µm were observed to concentrate in the interstices between the ice platelets in both granular and columnar sea ice. In vertical sea-ice profiles from both locations, ikaite concentration determined from image analysis, decreased with depth from surfaceice values of 700–900 µmol kg −1 ice (~ 25 × 10 6 crystals kg −1 ) to bottom-layer values of 100–200 µmol kg −1 ice (1–7 × 10 6 kg −1 ), all of which are much higher (4–10 times) than those reported in the few previous studies. Direct measurements of total alkalinity (TA) in surface layers fell within the same range as ikaite concentration whereas TA concentrations in bottom layers were twice as high. This depth-related discrepancy suggests interior ice processes where ikaite crystals form in surface sea ice layers and partly dissolved in bottom layers. From these findings and model calculations we relate sea ice formation and melt to observed p CO 2 conditions in polar surface waters, and hence, the air-sea CO 2 flux.

Description: Mass balance, runoff and surges of the Bering Glacier, Alaska The Cryosphere Discussions, 6, 5095-5117, 2012 Author(s): W. Tangborn The historical net, ablation and accumulation daily balances and runoff of the Bering Glacier, Alaska are determined for the 1951–2011 period with the PTAA (precipitation-temperature-area-altitude) model, using daily precipitation and temperature observations collected at the Cordova and Yakutat weather stations, together with the area-altitude distribution of the glacier. The mean annual balance for this 61-yr period is −0.6 mwe, the accumulation balance is +1.4 and the ablation balance is −2.0 mwe. Periodic surges of this glacier transport large volumes of ice to lower elevations where the ablation rate is higher, producing more negative balances and increasing runoff. During the 1993–1995 surge the average ablation balance is −3.3 mwe, over a meter greater than the 1951–2011 average. Runoff from the Bering Glacier (derived from simulated ablation and precipitation as rain) is highly correlated with the four glacier surges that have been observed since 1951. Ice volume loss for the 1972–2003 period measured with the PTAA model is 2.3 km 3 we a −1 and closely agrees with losses for the same period measured with the geodetic method.

Description: Global glacier volumes and sea level – effects of ice below the surface of the ocean and of new local lakes on land The Cryosphere Discussions, 6, 5169-5179, 2012 Author(s): W. Haeberli and A. Linsbauer The potential contribution of glaciers and ice caps to sea level rise is usually calculated by comparing the estimated total ice volume with the surface area of the ocean. Part of this total ice volume, however, does not contribute to sea-level rise, because it is below the surface of the ocean or below the levels of future lakes on land. The present communication points to this so far overlooked phenomenon and provides a first order-of-magnitude estimate. It is shown that the effect is small (most likely 1 to 5 cm sea-level equivalent) but systematic, could primarily affect earlier stages of global glacier vanishing and should therefore be adequately considered. Now-available techniques of slope-related high-resolution glacier-bed modelling have the potential to provide more detailed assessments in the future.

Description: Micrometeorological conditions and surface mass and energy fluxes on Lewis glacier, Mt Kenya, in relation to other tropical glaciers The Cryosphere Discussions, 6, 5181-5224, 2012 Author(s): L. Nicholson, R. Prinz, T. Mölg, and G. Kaser The Lewis Glacier on Mt Kenya is one of the best-studied tropical glaciers, but full understanding of the interaction of the glacier mass balance and climate forcing has been hampered by a lack of long term meteorological data. Here we present 2.5 yr of meteorological data collected from the glacier surface from October 2009–February 2012, which indicate that mean meteorological conditions in the upper zone of Lewis Glacier are comparable to those experienced in the ablation zones of South American tropical glaciers. In the context of other glaciated mountains of equatorial east Africa, the summit zone of Mt Kenya shows strong diurnal cycles of convective cloud development as opposed to the Rwenzoris where cloud cover persists throughout the diurnal cycle and Kilimanjaro where clear skies prevail. Surface energy fluxes were calculated for the meteorological station site using a physical mass- and energy-balance model driven by hourly measured meteorological data and additional input parameters that were determined by Monte Carlo optimization. Sublimation rate was lower than those reported on other tropical glaciers and melt rate was high throughout the year, with the glacier surface reaching the melting point on an almost daily basis. Surface mass balance is influenced by both solid precipitation and air temperature, with radiation providing the greatest net source of energy to the surface. Cloud cover typically reduces the net radiation balance compared to clear sky conditions, and thus the more frequent formation of convective clouds over the summit of Mt Kenya, and the associated higher rate of snow accumulation are important in limiting the rate of mass loss from the glacier surface. The analyses shown here are the basis for glacier-wide mass and energy balance modeling to determine the climate proxy offered by the glaciers of Mt Kenya.

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: A century of ice retreat on Kilimanjaro: the mapping reloaded The Cryosphere Discussions, 6, 4233-4265, 2012 Author(s): N. J. Cullen, P. Sirguey, T. Mölg, G. Kaser, M. Winkler, and S. J. Fitzsimons A new and consistent time series of glacier retreat on Kilimanjaro over the last century has been established by re-interpreting two historical maps and processing nine satellite images, which removes uncertainty about the location and extent of past and present ice bodies. Three-dimensional visualization techniques were used in conjunction with aerial and ground-based photography to facilitate the interpretation of ice boundaries over eight epochs between 1912 and 2011. The glaciers have retreated from their former extent of 11.40 km 2 in 1912 to 1.76 km 2 in 2011, which represents a total loss of about 85% of the ice cover over the last 100 yr. The total loss of ice cover is in broad agreement with previous estimates but to further characterize the spatial and temporal variability of glacier retreat a cluster analysis using topographical information (elevation, slope and aspect) was performed to segment the ice cover as observed in 1912, which resulted in three glacier zones being identified. Linear extrapolation of the retreat in each of the three identified glacier assemblages imply the ice cover on the western slopes of Kilimanjaro will be gone before 2020, while the remaining ice bodies on the plateau and southern slopes will most likely disappear by 2040. It is highly unlikely that any body of ice will be present on Kilimanjaro after 2060 if present-day climatological conditions are maintained. Importantly, the geo-statistical approach developed in this study provides us with an additional tool to characterize the physical processes governing glacier retreat on Kilimanjaro. It remains clear that to use glacier response to unravel past climatic conditions on Kilimanjaro the transition from growth to decay of the plateau glaciers must be further resolved, in particular the mechanisms responsible for vertical cliff development.

Description: Inhomogeneous snow distribution and depletion patterns at grid scale in a shallow snowpack region The Cryosphere Discussions, 6, 4171-4203, 2012 Author(s): H. Li, J. Wang, Z. Tang, and J. Wang Understanding inhomogeneous snow processes at the grid scale is crucial for distributed snow hydrology research. Many studies on inhomogeneous snow processes focus on the annual similarity of snow distribution and depletion and the roles of topography and other environmental conditions. In contrast, this study examines the snow distribution and depletion patterns at a small grid scale in a shallow snowpack region and analyzes how meteorological factors influence these patterns by using the SNOWPACK model for scenario simulations. These simulations enable quantification of the role of three main meteorological factors: shortwave radiation, longwave radiation, and air temperature. The study region is located in the Northeastern Qinghai-Tibet plateau. The results of the study indicate the following two points. (1) During different snowmelt periods, spatial similarity exists between the periodical cumulative snow distributions, and the relationships between snow cover fraction and mean snow water equivalent are similar. However, this similarity is not applicable to the period before snowmelt. (2) Shortwave radiation has a~major impact on the snow distribution and depletion patterns at the small grid scale. Increasing shortwave radiation can greatly promote the heterogeneity of the snow distribution. The contributions of longwave radiation and air temperature to the heterogeneity of snow distribution are minor. Moreover, there are similarities between the simulated snow distributions when considering the scenarios of increases in longwave radiation or in air temperature.

Description: Thinning and slowdown of Greenland's Mittivakkat Gletscher The Cryosphere Discussions, 6, 4387-4415, 2012 Author(s): S. H. Mernild, N. T. Knudsen, M. J. Hoffman, J. C. Yde, W. H. Lipscomb, E. Hanna, J. K. Malmros, and R. S. Fausto Here, we document changes for the Mittivakkat Gletscher, the glacier in Greenland (disconnected to the Greenland Ice Sheet, GrIS) having the longest observed mass balance and surface velocity time series (since 1995). Between 1986 and 2011, this glacier decreased by 15% in mean ice thickness and 30% in volume. We attribute these changes to summer warming and less winter snowfall. The vertical strain was able to compensate about 60% of the elevation change due to surface mass balance (SMB) in the lower part, and about 25% in the upper part. The annual mean ice surface velocity decreased by 30%, likely as a dynamic effect of ice thinning. Mittivakkat Gletscher summer surface velocities were on average 50–60% above winter background values, and up to 160% higher during peak velocity events.

Description: Variability of light transmission through Arctic land-fast sea ice during spring The Cryosphere Discussions, 6, 4363-4385, 2012 Author(s): M. Nicolaus, C. Petrich, S. R. Hudson, and M. A. Granskog The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along repeated (March, May, June 2010) transects under un-deformed land-fast sea ice at Barrow, Alaska. The objective was to quantify seasonal evolution and spatial variability of light transmittance through snow and sea ice. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for Chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt may cause as much spatial variability of relative light transmittance as the contrast of ponded and white ice during summer. In both instances, a spatial variability of up to three times above and below the mean was measured. In addition, we found a thirtyfold increase of light transmittance as a result of partial snowmelt. Hence, the seasonal evolution of transmittance through sea ice exceeded the spatial variability. Nevertheless, more comprehensive under-ice radiation measurements are needed for a more generalized and large-scale understanding of the under-ice energy budget for physical, biological, and geochemical applications.

Description: Brief communication "Important role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet" The Cryosphere Discussions, 6, 4101-4122, 2012 Author(s): X. Fettweis, E. Hanna, C. Lang, A. Belleflamme, M. Erpicum, and H. Gallée Since 2007, there has been a succession of surface melt records over the Greenland Ice Sheet (GrIS) in continuity of the trend observed since the end of the 1990s towards increasing melt. But, these last two decades are characterized by an increase of negative phases of the North-Atlantic Oscillation (NAO) favouring warmer and drier summers than normal over GrIS. In this context, we use a circulation type classification based on the daily 500 hPa geopotential height to evaluate the role of the atmospheric dynamics in this surface melt acceleration since 20 yr. Due to the lack of direct observations, the interannual melt variability is gauged here by the summer (June-July-August) mean temperature at 700 hPa over Greenland; analogous atmospheric circulations in the past show that ~70% of the 1992–2011 warming at 700 hPa over Greenland has been driven by changes in the atmospheric flow frequencies. Indeed, the occurrence of anticyclones in surface and at 500 hPa centred over the GrIS has doubled since the end of 1990s which induces southerly warm air advection along the Western Greenland coast and over the neighbouring Canadian islands. These changes in the NAO modes explain also why no significant warming has been observed these last five summers over Svalbard, where northerly atmospheric flows are more frequent than before. Therefore, the recent warmer summers over Greenland, Ellesmere and Baffin Islands can not be considered as a long term climate warming but are more rather a consequence of the NAO variability impacting the atmospheric heat transport. While no global model from the CMIP5 database projects consequent changes in NAO through this century, we can not exclude that these changes in NAO are due to global warming.

Description: Surface deformation detected by the space-observed small baseline SAR interferometry over permafrost environment in Tibet Plateau, China The Cryosphere Discussions, 6, 4071-4099, 2012 Author(s): F. Chen and H. Lin The evolution of permafrost and the active layer is highly related to climate change because of its feedback effects involving water and carbon storage. In this study, we firstly examined the relationship of regional water balance, geomorphological process and anthropogenic activities by means of Small Baseline Synthetic Aperture Radar Interferometry (SB-InSAR) to monitor the surface movements overlaid on the permafrost of Tibet Plateau (TP), China, using 3.5-yr observation span of L-band ALOS PALSAR data (June, 2007 to December, 2010). The estimated displacements (primarily in the range of −30 mm yr −1 to 30 mm yr −1 ) and time-series implied evolutions of the active layer and permafrost beneath. The motion trend along slopes was complicated, and thus interdisciplinary interpretations were required. Water level variations of inland lakes were then detected, although further investigations were required for validation. Anthropogenic influences on this frail permafrost environment were significant, proved by the remarkable surface settlement along the embankment of Qinghai-Tibet Railway. Consequently, it is crucial and necessary to monitor this arid and cold plateau owing to the combination of climate change, geo-hazards prediction as well as the regional sustainable development.

Description: Mechanical effect of mélange-induced buttressing on embayment-terminating glacier dynamics The Cryosphere Discussions, 6, 4123-4136, 2012 Author(s): D. Seneca Lindsey and T. K. Dupont Embayment terminating glaciers interact dynamically with seasonal sea ice and icebergs, a mixture we refer to as mélange. For certain glaciers, mélange prevents calved bergs from rotating away from the front, thus allowing the ice front to advance into the embayment. Here we demonstrate that mélange can, if rigid enough, provide sufficient buttressing to reduce the calving rate, while leaving the ice-front velocity largely unaffected. The net result is additional ice-front advance. Observations indicate a seasonal advance/retreat cycle has occurred at Jakobshavn Isbræ since the 1950s. We model an idealized Jakobshavn Isbræ-like scenario and find that mélange may be responsible for a seasonal ice-front advance of up to 0.6 km. These results come from a model that incorporates mélange into the interior of the domain, includes relevant stresses, and models drag via a kinematic boundary condition. A weakening or loss of mélange due to increasing temperatures would lead to further mass loss from glaciers such as Jakobshavn Isbræ.

Description: Heterogeneity in Glacier response from 1973 to 2011 in the Shyok valley, Karakoram, India The Cryosphere Discussions, 6, 3049-3078, 2012 Author(s): R. Bhambri, T. Bolch, P. Kawishwar, D. P. Dobhal, D. Srivastava, and B. Pratap A glacier inventory for the Shyok and Chang Chenmo basins was generated for the year 2002 using semi-automated methods based on Landsat ETM+ and SRTM3 DEM data. Glacier change analysis was carried out for 134 glaciers based on Hexagon KH-9 (years 1973, 1974) and Landsat TM/ETM+ (1989, 2002 and 2011) images. The 2002 inventory contains 2123 glaciers with an area of 2977.9±92.2 km 2 in the entire study area including Shyok (1605 glaciers; area 2499±77.4 km 2 ) and Chang Chenmo basins (518 glaciers; area 478.7±14.8 km 2 ). Out of 2123 glaciers, only eight glaciers have higher elevation ranges than 2000 m. On average, the glacier area in Chang Chenmo basin exhibited no changes during the study period. However, individual absolute glacier area changes varied from −0.7±0.03 km 2 to +0.2±0.01 km 2 between 1973 and 2011. 10 glaciers exhibited an area increase of 1.7±0.07 km 2 in total while 36 glaciers lost about total 1.8±0.07 km 2 . The glacier area decreased by 11±0.47 km 2 from 1973 to 1989 in the Shyok basin whereas an increase in area of 8.2±0.33 km 2 was observed during 1989–2002. The area has further increased by 5.6±0.21 km 2 from 2002 to 2011 in the respective basin. This individual glacier response heterogeneity can be attributed to surging and possibly due to decreased temperature in last decades. However, further detailed studies are needed to understand glacier surge mechanism and the possible mass gain.

Description: The impact of heterogeneous surface temperatures on the 2-m air temperature over the Arctic Ocean in spring The Cryosphere Discussions, 6, 3011-3048, 2012 Author(s): A. Tetzlaff, L. Kaleschke, C. Lüpkes, F. Ament, and T. Vihma The influence of spatial surface temperature changes over the Arctic Ocean on the 2-m air temperature variability is estimated using backward trajectories based on ERA-Interim and the JRA25 wind fields. They are initiated at Alert, Barrow and at the Tara drifting station. Three different methods are used. The first one compares mean ice surface temperatures along the trajectories to the observed 2-m air temperatures at the stations. The second one correlates the observed temperatures to air temperatures obtained using a simple Lagrangian box model which only includes the effect of sensible heat fluxes. For the third method, mean sensible heat fluxes from the model are correlated with the difference of the air temperatures at the model starting point and the observed temperatures at the stations. The calculations are based on MODIS ice surface temperatures and four different sets of ice concentration derived from SSM/I and AMSR-E data. Under nearly cloud free conditions, up to 90% of the 2-m air temperature variance can be explained for Alert, and 60% for Barrow using these methods. The differences are attributed to the different ice conditions, which are characterized by high ice concentration around Alert and lower ice concentration near Barrow. These results are robust for the different sets of reanalyses and ice concentration data. Near-surface winds of both reanalyses show a large inconsistency in the Central Arctic, which leads to a large difference in the correlations between modeled and observed 2-m air temperatures at Tara. Explained variances amount to 70% using JRA and only 45% using ERA. The results also suggest that near-surface temperatures at a given site are influenced by the variability of surface temperatures in a domain of about 150 to 350 km radius around the site.

Description: Uncertainties in the global temperature change caused by carbon release from permafrost thawing The Cryosphere Discussions, 6, 1367-1404, 2012 Author(s): E. J. Burke, I. P. Hartley, and C. D. Jones Under climate change thawing permafrost will cause old carbon which is currently frozen and inert to become vulnerable to decomposition and release into the climate system. This paper develops a simple framework for estimating the impact of this permafrost carbon release on the global mean temperature (P-GMT). The analysis is based on simulations made with the Hadley Centre climate model (HadGEM2-ES) for a range of representative CO 2 concentration pathways. Results using the high concentration pathway (RCP 8.5) suggest that by 2100 the annual methane (CH 4 ) emission rate is 2–59 Tg CH 4 yr −1 and 50–270 Pg C has been released as CO 2 with an associated P-GMT of 0.08–0.36 °C (all 5th–95th percentile ranges). P-GMT is considerably lower – between 0.02 and 0.11 °C – for the low concentration pathway (RCP2.6). The uncertainty in climate model scenario causes about 50% of the spread in P-GMT by the end of the 21st century, indicating that the effect of permafrost thaw on global mean temperature is currently controllable by mitigation measures. The distribution of soil carbon, in particular how it varies with depth, contributes to about half of the remaining spread in P-GMT by 2100 with quality of soil carbon and decomposition processes contributing a further quarter each. These latter uncertainties could be reduced through additional observations. Over the next 20–30 yr, whilst scenario uncertainty is small, improving our knowledge of the quality of soil carbon will contribute significantly to reducing the spread in the, albeit relatively small, P-GMT.

Description: A simple inverse method for the distribution of basal sliding coefficients under ice sheets, applied to Antarctica The Cryosphere Discussions, 6, 1405-1444, 2012 Author(s): D. Pollard and R. M. DeConto Variations in intrinsic bed conditions that affect basal sliding, such as the distribution of deformable sediment versus hard bedrock, are important boundary conditions for large-scale ice-sheet models, but are hard to observe and remain largely uncertain below the modern Greenland and Antarctic ice sheets. Here a very simple model-based method is described for deducing the modern spatial distribution of basal sliding coefficients. The model is run forward in time, and the basal sliding coefficient at each grid point is periodically increased or decreased depending on whether the local ice surface elevation is too high or too low compared to observed, in areas of unfrozen bed. The method considerably reduces large-scale errors in Antarctic ice elevation, from several 100's to a few 10 m in most regions. Remaining ice elevation errors over mountain ranges such as the Transantarctics are further improved by parameterizing the possible effect of sub-grid topography in the basal sliding law, representing sliding in deep valleys. Results are briefly compared with previous work using relatively sophisticated control methods, and the method is applied to alternate topographies of the Recovery Glacier basin.

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: Brief communication: Historical glacier length changes in West Greenland The Cryosphere Discussions, 6, 3491-3501, 2012 Author(s): P. W. Leclercq, A. Weidick, F. Paul, T. Bolch, M. Citterio, and J. Oerlemans Past glacier fluctuations provide insight into glacier dynamics, climate change, and the contribution of glaciers to sea-level rise. Here, the length fluctuations since the 19th century of 18 local glaciers in West and South Greenland are presented, extending and updating the study by Weidick (1968). The studied glaciers all show an overall retreat with an average of 1.2 ± 0.2 km over the 20th century, indicating a general rise of the equilibrium line along the west coast of Greenland during the last century. The rate of retreat was largest in the first half of the 20th century.

Description: Satellite-Derived Volume Loss Rates and Glacier Speeds for the Cordillera Darwin Icefield, Chile The Cryosphere Discussions, 6, 3503-3538, 2012 Author(s): A. K. Melkonian, M. J. Willis, M. E. Pritchard, A. Rivera, F. Bown, and S. A. Bernstein We produce the first icefield-wide volume change rate and glacier velocity estimates for the Cordillera Darwin Icefield (CDI), a 2605 km 2 temperate icefield in Southern Chile (69.6° W, 54.6° S). Velocities are measured from optical and radar imagery between 2001–2011. Thirty-seven digital elevation models (DEMs) from ASTER and the SRTM are stacked and a weighted linear regression is applied to elevations on a pixel-by-pixel basis to estimate volume change rates. The CDI lost mass at an average rate of 3.9 ± 0.3 Gt yr −1 between 2000 and 2011, equivalent to a sea level rise (SLR) of 0.01 ± 0.001 mm yr −1 . Thinning is widespread, with concentrations near the front of two northern glaciers (Marinelli, Darwin) and one western (CDI-08) glacier. Thickening is apparent in the south, most notably over the advancing Garibaldi Glacier. We attribute this thinning pattern to warmer temperatures, particularly in the north, which triggered rapid retreat at Marinelli Glacier (~4 km from 2001–2011). Velocities are obtained over many of the swiftly flowing glaciers for the first time. We provide a repeat speed timeseries at the Marinelli Glacier. Maximum front speeds there accelerated from 7.5 m day −1 in 2001 to 9.5 m day −1 in 2003, to a peak of 10 m day −1 in 2011.

Description: An updated and quality controlled surface mass balance dataset for Antarctica The Cryosphere Discussions, 6, 3667-3702, 2012 Author(s): V. Favier, C. Agosta, S. Parouty, G. Durand, G. Delaygue, H. Gallée, A.-S. Drouet, A. Trouvilliez, and G. Krinner We present an updated and quality controlled surface mass balance (SMB) database for the Antarctic ice sheet. We retrieved a total of 5284 SMB data documented with important meta-data, to which a filter was applied to discard data with limited spatial and temporal representativeness, too small measurement accuracy, or lack of quality control. A total of 3438 reliable data was obtained, which is about four times more than by applying the same data filtering process to previously available databases. New important data with high spatial resolution are now available over long traverses, and at low elevation in some areas. However, the quality control led to a considerable reduction in the spatial density of data in several regions, particularly over West Antarctica. Over interior plateaus, where the SMB is low, the spatial density of measurements remained high. This quality controlled dataset was compared to results from ERA-Interim reanalysis to assess model representativeness over Antarctica, and also to identify large areas where data gaps impede model validation. Except for very few areas (e.g. Adelie Land), the elevation range between 200 m and 1000 m a.s.l. is not correctly sampled in the field, and measurements do not allow a thorough validation of models in regions with complex topography, where the highest scattering of SMB values is reported. Clearly, increasing the spatial density of field measurements at low elevations, in the Antarctic Peninsula and in West Antarctica remains a scientific priority.

Description: Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models The Cryosphere Discussions, 6, 3447-3489, 2012 Author(s): A. Levermann, R. Winkelmann, S. Nowicki, J. L. Fastook, K. Frieler, R. Greve, H. H. Hellmer, M. A. Martin, M. Mengel, A. J. Payne, D. Pollard, T. Sato, R. Timmermann, W. L. Wang, and R. A. Bindschadler The largest uncertainty in projections of future sea-level change still results from the potentially changing dynamical ice discharge from Antarctica. While ice discharge can alter through a number of processes, basal ice-shelf melting induced by a warming ocean has been identified as a major if not the major cause for possible additional ice flow across the grounding line. Here we derive dynamic ice-sheet response functions for basal ice-shelf melting using experiments carried out within the Sea-level Response to Ice Sheet Evolution (SeaRISE) intercomparison project with five different Antarctic ice-sheet models. As used here these response functions provide separate contributions for four different Antarctic drainage regions. Under the assumptions of linear-response theory we project future ice-discharge for each model, each region and each of the four Representative Concentration Pathways (RCP) using oceanic temperatures from 19 comprehensive climate models of the Coupled Model Intercomparison Project, CMIP-5, and two ocean models from the EU-project Ice2Sea. Uncertainty in the climatic forcing, the oceanic response and the ice-model differences is combined into an uncertainty range of future Antarctic ice-discharge induced from basal ice-shelf melt. The additional ice-loss (Table 6) is clearly scenario-dependent and results in a median of 0.07 m (66%-range: 0.04–0.10 m; 90%-range: −0.01–0.26 m) of global sea-level equivalent for the low-emission RCP-2.6 scenario and yields 0.1 m (66%-range: 0.06–0.14 m; 90%-range: −0.01–0.45 m) for the strongest RCP-8.5. If only models with an explicit representation of ice-shelves are taken into account the scenario dependence remains and the values change to: 0.05 m (66%-range: 0.03–0.08 m) for RCP-2.6 and 0.07 m (66%-range: 0.04–0.11 m) for RCP-8.5. These results were obtained using a time delay between the surface warming signal and the subsurface oceanic warming as observed in the CMIP-5 models. Without this time delay the ranges for all ice-models changes to 0.10 m (66%-range: 0.07–0.12 m; 90%-range: 0.01–0.28 m) for RCP-2.6 and 0.15 m (66%-range: 0.10–0.21 m; 90%-range: 0.02–0.53 m) for RCP-8.5. All probability distributions as provided in Fig. 10 are highly skewed towards high values.

Description: Event-driven deposition: a new paradigm for snow-cover modelling in Antarctica based on surface measurements The Cryosphere Discussions, 6, 3575-3612, 2012 Author(s): C. D. Groot Zwaaftink, A. Cagnati, A. Crepaz, C. Fierz, M. Lehning, G. Macelloni, and M. Valt Antarctic surface snow is studied by means of continuous measurements and observations over a period of 3 yr at Dome C. Snow observations include precipitation, daily records of deposition and erosion, snow temperatures at several depths, and snow profiles. Together with meteorological data from automatic weather stations, this forms a unique and complete dataset of snow conditions on the Antarctic Plateau. Large differences in snow amounts and density exist between precipitation measured 1 m above the surface and deposition on the surface. We then used the snow-cover model SNOWPACK to simulate the snow-cover evolution for different deposition parameterizations. The main adaptation of the model described here is a new event-driven accumulation scheme. The scheme assumes that snow is added to the snow cover permanently only for periods of strong winds. This assumption followed from the comparison between precipitation observations and daily records of changes in snow height, which showed that over a period of 235 days there was precipitation on 40% and deposition on 25% of the days, but precipitation accompanied by deposition on 14% of the days only. This confirms that precipitation is not necessarily the driving force behind snow height changes. A comparison of simulated snow height to stake farm measurements over 3 yr showed that we underestimate the total accumulation by about 64%, when the total snow deposition is constrained by the precipitation measurements. This is because the precipitation measured above the surface and used to drive the model, even though comparable to ECMWF forecasts in its total magnitude, should be seen as a lower boundary of accumulation. As a result of the new deposition mechanism, we found a good agreement between model results and measurements of snow temperatures and recorded snow profiles. In spite of the underestimated accumulation, the results strongly suggest that we can obtain quite realistic simulations of the Antarctic snow cover by the introduction of event-driven snow accumulation.

Description: How does internal variability influence the ability of CMIP5 models to reproduce the recent trend in Southern Ocean sea ice extent? The Cryosphere Discussions, 6, 3539-3573, 2012 Author(s): V. Zunz, H. Goosse, and F. Massonnet Observations over the last 30 yr have shown that the sea ice extent in the Southern Ocean has slightly increased since 1979. Mechanisms responsible for this positive trend have not been well established yet and climate models are generally unable to simulate correctly this expansion. In this study, we focus on two related hypotheses that could explain the misrepresentation of the positive trend in sea ice extent by climate models: an unrealistic internal variability and an inadequate initialization of the system. For that purpose, we analyze the evolution of sea ice around the Antarctic simulated by 24 different general circulation models involved in the 5th Coupled Model Intercomparison Project (CMIP5). On the one hand, historical simulations, driven by external forcing and initialized without observations, are examined. They provide information about the mean state, the variability and the trend in sea ice extent simulated by each model. On the other hand, decadal prediction experiments, driven by external forcing and initialized with some observed fields, allow us to assess the impact of the representation of the observed initial state on the quality of model predictions. Our analyses show that CMIP5 models respond to the forcing, including the one induced by stratospheric ozone depletion, by reducing the sea ice cover in the Southern Ocean. Some simulations display an increase in sea ice extent. However, models strongly overestimate the variability of sea ice extent and the initialization methods currently used in models do not improve systematically the simulated trends in sea ice extent. On the basis of those results, a critical role of the internal variability in the observed increase in the sea ice extent in the Southern Ocean could not be ruled out but current models results appear inadequate to test more precisely this hypothesis.

Description: An estimate of global glacier volume The Cryosphere Discussions, 6, 3647-3666, 2012 Author(s): A. Grinsted I asses the feasibility of multi-variate scaling relationships to estimate glacier volume from glacier inventory data. I calibrate scaling laws against volume observations of optimized towards the purpose of estimating the total global ice volume. This is applied individually to each record in the Randolph Glacier Inventory which is the first globally complete inventory of glaciers and ice caps. I estimate that the total volume of all glaciers in the world is 0.35 ± 0.07 m sea level equivalent. This is substantially less than a recent state-of-the-art estimate. Area volume scaling bias issues for large ice masses, and incomplete inventory data are offered as explanations for the difference.

Description: Mapping radiation transfer through sea ice using a remotely operated vehicle (ROV) The Cryosphere Discussions, 6, 3613-3646, 2012 Author(s): M. Nicolaus and C. Katlein Light (solar short-wave radiation) transmission into and through sea ice is of high importance for various processes in Polar Regions. The amount of energy transferred through the ice determines formation and melt of sea ice and finally contributes to warming of the uppermost ocean. At the same time the amount and distribution of light, as the primary source of energy, is of critical importance for sea-ice associated organisms and bio-geochemical processes. However, our current understanding of these processes and their interdisciplinary interactions is still sparse. The main reason is that the under-ice environment is difficult to access and measurements require large logistical and instrumental efforts. Particularly, it was not possible to map light conditions under sea ice over larger areas. Here we present a detailed methodical description of operating spectral radiometers on a remotely operated vehicle (ROV) in the Central Arctic under sea ice. This new measurement concept resulted in a~most comprehensive data set of spectral radiance and irradiance under and above sea ice, complemented through various additional in-situ measurements of sea-ice, snow, and surface properties. Finally, such data sets allow quantifying the spatial variability of light under sea ice, especially highlighting differences between ponded and white ice as well as different ice types.

Description: Variability of mass changes at basin scale for Greenland and Antarctica The Cryosphere Discussions, 6, 3397-3446, 2012 Author(s): V. R. Barletta, L. S. Sørensen, and R. Forsberg During the last decade, the GRACE mission has provided valuable data for determining the mass changes of the Greenland and Antarctic ice sheets. Yet, discrepancies still exist in the published mass balance results, and analyses on the sources of errors and discrepancies are lacking. Here, we present monthly mass changes together with trends derived from GRACE data at basin scale for both the Greenland and Antarctica ice sheets and we assess, for the first time systematically, the variability and errors for each of the possible sources of discrepancies: mass inference methods, data sets and background models. We find a very good agreement between the monthly mass change results derived from two independent methods, which represents a cross validation. For the monthly solutions, we find that most of the variability is caused by the use of different data sets rather than different methods. Besides the well-known GIA trend uncertainty, we find that the degree-1 variability and the recent de-aliasing corrections have significant impact on monthly time series and trends respectively. We also show the remarkable differences between the use of release RL04 and the new RL05, and how the latter results in smaller mass trends for the majority of the basins. The overall variability of the solutions well exceeds the uncertainties propagated from the data errors and the leakage (as done in the past), hence we calculate new sound total errors for the monthly solutions and the trends. For the whole GRACE period our trend estimate for Greenland is −234 ± 20 Gt yr −1 and −83 ± 36 Gt yr −1 for Antarctica (−111 ± 15 Gt yr −1 in the western part). These trends show a clear (with respect to our errors) increase of mass loss in the last four years.

Description: The influence of climate and hydrological variables on opposite anomaly in active layer thickness between Eurasian and North American watersheds The Cryosphere Discussions, 6, 2537-2574, 2012 Author(s): H. Park, J. Walsh, A. N. Fedorov, A. B. Sherstiukov, Y. Iijima, and T. Ohata This study not only examined the spatiotemporal variations of permafrost active layer thickness (ALT) during 1948–2006 over the terrestrial Arctic regions experiencing climate changes, but also identified the associated drivers based on observational data and a simulation conducted by a land surface model (CHANGE). The focus on the ALT extends previous studies that have emphasized ground temperatures in permafrost regions. The Ob, Yenisey, Lena, Yukon, and Mackenzie watersheds are foci of the study. Time series of ALT in Eurasian watersheds showed generally increasing trends, while ALT in North American watersheds showed decreases. An opposition of ALT variations implicated with climate and hydrological variables was most significant when the Arctic air temperature entered into a warming phase. The warming temperatures were not simply expressed to increases in ALT. Since 1990 when the warming increased, the forcing of the ALT by the higher Annual Thawing Index in the Mackenzie and Yukon Basins was offset by the combined effects of less insulation caused by thinner snow depth and drier soil during summer. In contrast, the increasing Annual Thawing Index together with thicker snow depth and higher summer soil moisture in the Lena contributed to the increase in ALT. The results imply that the soil thermal and moisture regimes formed in the pre-thaw season(s) provide memory that manifests itself during the summer. While it is widely believed that ALT will increase with global warming, this hypothesis may need modification because the ALT also shows responses to variations in snow depth and soil moisture that can over-ride the effect of air temperature. The dependence of the hydrological variables driven by the atmosphere further increases the uncertainty in future changes of the permafrost active layer.

Description: A recent bifurcation in Arctic sea-ice cover The Cryosphere Discussions, 6, 2621-2651, 2012 Author(s): V. N. Livina and T. M. Lenton There is ongoing debate over whether Arctic sea-ice has already passed a "tipping point", or whether it will do so in future, with several recent studies arguing that the loss of summer sea ice does not involve a bifurcation because it is highly reversible in models. Recently developed methods can detect and sometimes forewarn of bifurcations in time-series data, hence we applied them to satellite data for Arctic sea-ice cover. Here we show that a new low ice cover state has appeared from 2007 onwards, which is distinct from the normal state of seasonal sea ice variation, suggesting a bifurcation has occurred from one attractor to two. There was no robust early warning signal of critical slowing down prior to this bifurcation, consistent with it representing the appearance of a new ice cover state rather than the loss of stability of the existing state. The new low ice cover state has been sampled predominantly in summer-autumn and seasonal forcing combined with internal climate variability are likely responsible for triggering recent transitions between the two ice cover states. However, all early warning indicators show destabilization of the summer-autumn sea-ice since 2007. This suggests the new low ice cover state may be a transient feature and further abrupt changes in summer-autumn Arctic sea-ice cover could lie ahead; either reversion to the normal state or a yet larger ice loss.

Description: A simple approach to providing a more consistent Arctic sea ice extent timeseries from the 1950s to present The Cryosphere Discussions, 6, 2827-2853, 2012 Author(s): W. N. Meier, J. Stroeve, A. Barrett, and F. Fetterer Observations for passive microwave satellite sensors have provided a continuous and consistent record of sea ice extent since late 1978. Earlier records, compiled from ice charts and other sources exist, but are not consistent with the satellite record. Here, a method is presented to adjust a compilation of pre-satellite sources to remove discontinuities between the two periods and create a more consistent combined 59-yr timeseries spanning 1953–2011. This adjusted combined timeseries shows more realistic behavior across the transition between the two individual timeseries and thus provides higher confidence in trend estimates from 1953 through 2011. The long-term timeseries is used to calculate linear trend estimates and compare them with trend estimates from the satellite period. The results indicate that trends through the 1960s were largely positive (though not statistically significant) and then turned negative by the mid-1970s and have been consistently negative since, reaching statistical significance (at the 95% confidence level) by the late 1980s. The trend for September (when Arctic extent reaches its seasonal minimum) for the satellite period, 1979–2011 is –12.9% decade –1 , nearly double the 1953–2011 trend of –6.8% decade –1 (relative to the 1981–2010 mean). The recent decade (2002–2011) stands out as a period of persistent decline in ice extent. The combined 59-yr timeseries puts the strong observed decline in the Arctic sea ice cover during 1979–2011 in a longer-term context and provides a useful resource for comparisons with historical model estimates.

Description: Greenland ice sheet surface mass balance: evaluating simulations and making projections with regional climate models The Cryosphere Discussions, 6, 2059-2113, 2012 Author(s): J. G. L. Rae, G. Aðalgeirsdóttir, T. L. Edwards, X. Fettweis, J. M. Gregory, H. T. Hewitt, J. A. Lowe, P. Lucas-Picher, R. H. Mottram, A. J. Payne, J. K. Ridley, S. R. Shannon, W. J. van de Berg, R. S. W. van de Wal, and M. R. van den Broeke Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its contribution to sea level rise, with greater accuracy than is possible from coarser-resolution general circulation models (GCMs). This is the first time an intercomparison has been carried out of RCM results for Greenland climate and SMB. Output from RCM simulations for the recent past with the four RCMs is evaluated against available observations. The evaluation highlights the importance of using a detailed snow physics scheme, especially regarding the representations of albedo and meltwater refreezing. Simulations with three of the RCMs for the 21st century using SRES scenario A1B from two GCMs produce trends of between −5.5 and −1.1 Gt yr −2 in SMB (equivalent to +0.015 and +0.003 mm sea level equivalent yr −2 ), with trends of smaller magnitude for scenario E1, in which emissions are mitigated. Results from one of the RCMs whose present-day simulation is most realistic indicate that an annual-mean near-surface air temperature increase over Greenland of ~2 ○ C would be required for the mass loss to increase such that it exceeds accumulation, thereby causing the SMB to become negative, which has been suggested as a threshold beyond which the ice-sheet would eventually be eliminated.

Description: Review article of the current state of glaciers in the tropical Andes: a multi-century perspective on glacier evolution and climate change The Cryosphere Discussions, 6, 2477-2536, 2012 Author(s): A. Rabatel, B. Francou, A. Soruco, J. Gomez, B. Cáceres, J. L. Ceballos, R. Basantes, M. Vuille, J.-E. Sicart, C. Huggel, M. Scheel, Y. Lejeune, Y. Arnaud, M. Collet, T. Condom, G. Consoli, V. Favier, V. Jomelli, R. Galarraga, P. Ginot, L. Maisincho, J. Mendoza, M. Ménégoz, E. Ramirez, P. Ribstein, W. Suarez, M. Villacis, and P. Wagnon The aim of this paper is to provide the community with a comprehensive overview of the studies of glaciers in the tropical Andes conducted in recent decades leading to the current status of the glaciers in the context of climate change. In terms of changes in surface area and length, we show that the glacier retreat in the tropical Andes over the last three decades is unprecedented since the maximum extension of the LIA (mid 17th–early 18th century). In terms of changes in mass balance, although there have been some sporadic gains on several glaciers, we show that the trend has been quite negative over the past 50 yr, with a mean mass balance deficit for glaciers in the tropical Andes that is slightly more negative than the computed global average. A break point in the trend appeared in the late 1970s with mean annual mass balance per year decreasing from −0.2 m w.e. in the period 1964–1975 to −0.76 m w.e. in the period 1976–2010. In addition, even if glaciers are currently retreating everywhere in the tropical Andes, it should be noted that as a percentage, this is much more pronounced on small glaciers at low altitudes that do not have a permanent accumulation zone, and which could disappear in the coming years/decades. Monthly mass balance measurements performed in Bolivia, Ecuador and Colombia showed that variability of the surface temperature of the Pacific Ocean is the main factor governing variability of the mass balance variability at the interannual to decadal time scale. Precipitation did not display a significant trend in the tropical Andes in the 20th century, and consequently cannot explain the glacier recession. On the other hand, temperature increased at a significant rate of 0.10 °C decade −1 in the last 70 yr. The higher frequency of El Niño events and changes in its spatial and temporal occurrence since the late 1970s together with a warming troposphere over the tropical Andes may thus explain much of the recent dramatic shrinkage of glaciers in this part of the world.

Description: Mechanisms causing reduced Arctic sea ice loss in a coupled climate model The Cryosphere Discussions, 6, 2653-2687, 2012 Author(s): A. E. West, A. B. Keen, and H. T. Hewitt The fully-coupled climate model HadGEM1 produces one of the most accurate simulations of the historical record of Arctic sea ice seen in the IPCC AR4 multi-model ensemble. In this study, we examine projections of sea ice decline out to 2030, produced by two ensembles of HadGEM1 with natural and anthropogenic forcings included. These ensembles project a significant slowing of the rate of ice loss to occur after 2010, with some integrations even simulating a small increase in ice area. We use an energy budget of the Arctic to examine the causes of this slowdown. A negative feedback effect by which rapid reductions in ice thickness north of Greenland reduce ice export is found to play a major role. A slight reduction in ocean-to-ice heat flux in the relevant period, caused by changes in the MOC and subpolar gyre in some integrations, is also found to play a part. Finally, we assess the likelihood of a slowdown occurring in the real world due to these causes.

Description: A computationally efficient model for the Greenland ice sheet The Cryosphere Discussions, 6, 2751-2788, 2012 Author(s): J. Haqq-Misra, P. Applegate, B. Tuttle, R. Nicholas, and K. Keller We present a one-dimensional model of the Greenland Ice Sheet (GIS) for use in analysis of future sea level rise. Simulations using complex three-dimensional models suggest that the GIS may respond in a nonlinear manner to anthropogenic climate forcing and cause potentially nontrivial sea level rise. These GIS projections are, however, deeply uncertain. Analyzing these uncertainties is complicated by the substantial computational demand of the current generation of complex three-dimensional GIS models. As a result, it is typically computationally infeasible to perform the large number of model evaluations required to carefully explore a multi-dimensional parameter space, to fuse models with observational constraints, or to assess risk-management strategies in Integrated Assessment Models (IAMs) of climate change. Here we introduce GLISTEN (GreenLand Ice Sheet ENhanced), a computationally efficient, mechanistically based, one-dimensional flow-line model of GIS mass balance capable of reproducing key instrumental and paleo-observations as well as emulating more complex models. GLISTEN is based on a simple model developed by Pattyn (2006). We have updated and extended this original model by improving its computational functionality and representation of physical processes such as precipitation, ablation, and basal sliding. The computational efficiency of GLISTEN enables a systematic and extensive analysis of the GIS behavior across a wide range of relevant parameters and can be used to represent a potential GIS threshold response in IAMs. We demonstrate the utility of GLISTEN by performing a pre-calibration and analysis. We find that the added representation of processes in GLISTEN, along with pre-calibration of the model, considerably improves the hindcast skill of paleo-observations.

Description: Snow cover thickness estimation by using radial basis function networks The Cryosphere Discussions, 6, 2437-2475, 2012 Author(s): A. Guidali, E. Binaghi, V. Pedoia, and M. Guglielmin This work investigates learning and generalisation capabilities of radial basis function networks (RBFN) used to solve snow cover thickness estimation model as regression and classification. The model is based on a minimal set of climatic and topographic data collected from a limited number of stations located in the Italian Central Alps. Several experiments have been conceived and conducted adopting different evaluation indexes in both regression and classification tasks. The snow cover thickness estimation by RBFN has been proved a valuable tool able to deal with several critical aspects arising from the specific experimental context.

Description: The first complete glacier inventory for the whole of Greenland The Cryosphere Discussions, 6, 2399-2436, 2012 Author(s): P. Rastner, T. Bolch, N. Mölg, H. Machguth, and F. Paul Glacier inventories provide important baseline information for the determination of water resources, glacier-specific changes in area and volume, climate change impacts, and the past, potential and future contribution of glaciers to sea-level rise. Though heavily glacierized and thus highly relevant for all of the above points, such an inventory of all local glaciers and icecaps (GIC) was not available so far for Greenland. Here we present the details and results of our inventory, that has been compiled from more than 70 Landsat scenes mostly acquired between 1999 and 2002 using semi-automated multispectral mapping techniques. A digital elevation model (DEM) was used to derive drainage divides from watershed analysis and topographic parameters for each glacier entity. We assigned to each entity one of three connectivity levels (CL0, CL1, CL2; i.e. no, weak, and strong connection) with the ice sheet to distinguish the local GIC from the ice sheet and its outlet glaciers and to serve the specific needs of different user communities. All GIC larger 0.05 km 2 include ~20 300 entities (of which 900 are marine terminating), covering an area of 129 983 ± 4029 km 2 , or 89 273 ± 2767 km 2 without the CL2 GIC. The latter is about 50% more than according to all previous estimates. Glaciers smaller 0.5 km 2 contribute only 1.5% to the total area but more than 50% (11 000) to the total number. In contrast, the 25 largest GIC (〉500 km 2 ) contribute 28% to the total area, but only 0.1% to the total number. Most of the ice was located at elevations around 1000 m, except in the eastern sector with elevation arround 1700 m. In addition, a strong dependence of the median elevation to the distance from the ocean was found, but only a weak dependence on aspect. All data will be made available in the Global Land Ice Measurement from Space (GLIMS) glacier database.

Description: Borehole temperatures reveal a changed energy budget at Mill Island, East Antarctica over recent decades The Cryosphere Discussions, 6, 2575-2595, 2012 Author(s): J. L. Roberts, A. D. Moy, T. D. van Ommen, M. A. J. Curran, A. P. Worby, I. D. Goodwin, and M. Inoue A borehole temperature record from the Mill Island (East Antarctic) icecap reveals a large surface warming signal manifested as a 0.75 K temperature difference over the approximate 100 m depth below the seasonally varying zone. The temperature profile shows a break in gradient between 49 and 69 m depth, which we model with inverse numerical simulations, indicating that surface warming started around the austral summer of 1980/1981 AD ± 5 yr. This warming of approximately 0.37 K per decade is large by Antarctic standards and is only exceeded in regions of the Antarctic Peninsula. While this warming may reflect regional scale air temperature increases, the lack of comparable trends for other East Antarctic sites suggests local influences are largely responsible for the observed trend. Alteration of the surface energy budget arising from changes in radiation balances due to local cloud, the amount of liquid deposition and local air temperatures associated with altered air/sea exchanges potentially play a key role at this location due to the proximity of the Shackleton Ice Shelf and sea-ice zone.

Description: Analysis of the snow-atmosphere energy balance during wet-snow instabilities and implications for avalanche prediction The Cryosphere Discussions, 6, 2715-2749, 2012 Author(s): C. Mitterer and J. Schweizer Wet-snow avalanches are notoriously difficult to predict, as their formation mechanism is poorly understood and in-situ measurements closely related to instability are inexistent. Instead, air temperature is commonly used as predictor variable for days with high wet-snow avalanche danger – with limited success. As melt water is a major driver of wet-snow instability and snow melt depends on the energy input into the snow cover, we computed the energy balance and study whether it is a better proxy than meteorological parameters such as air temperature for predicting periods with high wet-snow avalanche activity. The energy balance was partly measured and partly modelled for virtual slopes at different elevations for the aspects south and north using the 1-D snow cover model SNOWPACK. We used measured meteorological variables and computed energy balance and its components to compare wet-snow avalanche days to non-avalanche days for four consecutive winter seasons in the surroundings of Davos, Switzerland. Air temperature, the net shortwave radiation and the energy input integrated over 3 or 5 days showed best results in discriminating event from non-event days. Multivariate statistics, however, revealed that for better predicting avalanche days, information on the cold content of the snowpack is necessary. Wet-snow avalanche activity was closely related to periods when large parts of the snowpack reached an isothermal state (0 °C) and energy input exceeded a maximum value of 200 kJ m −2 in one day, or the 3-day sum of positive energy input was larger than 1.2 MJ m −2 . Prediction accuracy with measured meteorological variables was as good as with computed energy balance parameters, but simulated energy balance variables accounted better for different aspects, slopes and elevations than meteorological data.

Description: Greenland Ice Sheet contribution to sea-level rise from a new-generation ice-sheet model The Cryosphere Discussions, 6, 2789-2826, 2012 Author(s): F. Gillet-Chaulet, O. Gagliardini, H. Seddik, M. Nodet, G. Durand, C. Ritz, T. Zwinger, R. Greve, and D. G. Vaughan Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise. The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. The most usual ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; an unstructured mesh to usefully resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. By conducting perturbation experiments, we investigate how current ice loss will endure over the next century. Although we find that increasing ablation tends to reduce outflow and on its own has a stabilising effect, if destabilisation processes maintain themselves over time, current increases in the rate of ice loss are likely to continue.

Description: Snow accumulation variability in Adelie Land (East Antarctica) derived from radar and firn core data. A 600 km transect from Dome C The Cryosphere Discussions, 6, 2855-2889, 2012 Author(s): D. Verfaillie, M. Fily, E. Le Meur, O. Magand, B. Jourdain, L. Arnaud, and V. Favier Polar ice sheets mass balance is a timely topic intensively studied in the context of global change and sea-level rise. However, obtaining mass balance estimates in Antarctica in particular, remains difficult due to various logistical problems. In the framework of the TASTE-IDEA program, labeled as an International Polar Year project, continuous Ground Penetrating Radar (GPR) measurements were carried out during a traverse realised in Adelie Land (East Antarctica) during the 2008–2009 austral summer between the Italo-French Dome C (DC) polar plateau site and French Dumont D'Urville (DdU) coastal station. The aim of this study was to process and interpret GPR data in terms of snow accumulation, to analyse its spatial and temporal variability along the DC-DdU traverse and compare it with historical data and modeling. The emphasis has been put on the last 300 yr, from the pre-industrial to recent time period. Beta-radioactivity counting and gamma spectrometry were studied in cores at LGGE laboratory, providing a depth-age calibration for radar measurements. Over the 600 km of usable GPR data, depth and snow accumulation were determined with the help of three distinct layers visible on the radargrams (≈1730, 1799 and 1941 AD). Preliminary results reveal a gradual accumulation increase towards the coast and the occurrence of previously undocumented undulating structures between 300 and 600 km from DC. Results agree fairly well with data from previous studies and modeling. Concluding on temporal variations is difficult because of the margin of error introduced by density estimation. This study should have various applications such as for model validation.