ALBERT

Beschreibung: 〈b〉Simulated single-layer forest canopies delay Northern Hemisphere snowmelt〈/b〉〈br〉 Markus Todt, Nick Rutter, Christopher G. Fletcher, and Leanne M. Wake〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-270,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Vegetation is often represented by a single layer in global land models. Studies have found deficient simulation of thermal radiation beneath forest canopies when represented by single-layer vegetation. This study corrects thermal radiation in forests for a global land model using single-layer vegetation in order to assess the effect of deficient thermal radiation on snow cover and snowmelt. Results indicate that single-layer vegetation causes snow in forests being too cold and melting too late.

Beschreibung: 〈b〉Brief Communication: Early season snowpack loss and implications for oversnow vehicle recreation travel planning〈/b〉〈br〉 Benjamin J. Hatchett and Hilary G. Eisen〈br〉 The Cryosphere, 13, 21-28, https://doi.org/10.5194/tc-13-21-2019, 2019〈br〉 We examine the timing of early season snowpack relevant to oversnow vehicle (OSV) recreation over the past 3 decades in the Lake Tahoe region (USA). Data from two independent data sources suggest that the timing of achieving sufficient snowpack has shifted later by 2 weeks. Increasing rainfall and more dry days play a role in the later onset. Adaptation strategies are provided for winter travel management planning to address negative impacts of loss of early season snowpack for OSV usage.

Beschreibung: 〈b〉Greenland Ice Sheet late-season melt: Investigating multi-scale drivers of K-transect events〈/b〉〈br〉 Thomas J. Ballinger, Thomas L. Mote, Kyle Mattingly, Angela C. Bliss, Edward Hanna, Dirk van As, Melissa Prieto, Saeideh Gharehchahi, Xavier Fettweis, Brice Noël, Paul C. J. P. Smeets, Mads H. Ribergaard, and John Cappelen〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-285,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Studies have questioned links between Arctic marginal sea open water duration and Greenland Ice Sheet (GrIS) surface balance changes, namely melt events. Through analyses involving observations and climate models, we show that late summer through autumn “unseasonal” melt events are primarily driven by the northward movement of warm, moist air masses across the western ice sheet edge, while near-surface, off-ice winds block heat transfer off nearby Baffin Bay.

Beschreibung: 〈b〉Distributed Temperature Profiling System Provides Spatially Dense Measurements and Insights about Permafrost Distribution in an Arctic Watershed〈/b〉〈br〉 Emmanuel Léger, Baptiste Dafflon, Yves Robert, Craig Ulrich, John E. Peterson, Sébastien Biraud, Vladimir E. Romanovsky, and Susan S. Hubbard〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-264,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Soil temperature has been recognized as a property that strongly influences a myriad of hydro-biogeochemical processes, as well as containing important information on the properties modulating the soil thermal flux. In spite of its importance, our ability to acquire soil temperature data with high spatial and temporal resolution and coverage is limited, because of the high cost of equipment, the difficulties of deployment, and the complexities of data management. Here we propose a new strategy that we call Distributed Temperature Profiling (DTP), which consists of cheap, low-impact, low-power, vertically resolved temperature probes that independently and autonomously record soil temperature. We developed a prototype DTP system for characterizing and monitoring near-surface thermal properties, using an unprecedented number of laterally and vertically distributed temperature measurements. The DTP system was tested in an Arctic ecosystem near Nome, AK, to identify near-surface permafrost distribution and various thermal regimes in a discontinuous permafrost environment during the summer time. Results show that the DTP system enabled successful acquisition of vertically resolved profiles of summer soil temperature over the top 0.8 m at numerous locations. DTP also enabled high resolution identification and lateral delineation of near-surface permafrost locations from surrounding zones with no permafrost or deep permafrost table locations overlain by a perennially thawed layer. The DTP strategy overcomes some of the limitations associated with – and complements the strengths of – borehole-based soil temperature sensing as well as Fiber-Optic Distributed Temperature Sensing (FO-DTS) approaches. Combining DTP data with co-located topographic and vegetation maps obtained using Unmanned Aerial Vehicle (UAV) and Electrical Resistivity Tomography (ERT) data allowed us to identify correspondences between surface and subsurface property distribution, and in particular between topography, vegetation, shallow soil properties, and near-surface permafrost. Finally, the results highlight the considerable value of the newly developed DTP strategy for investigating the significant variability and complexity of subsurface thermal and hydrological regimes in discontinuous permafrost regions.〈/p〉

Beschreibung: 〈b〉On the timescales and length scales of the Arctic sea ice thickness anomalies: a study based on 14 reanalyses〈/b〉〈br〉 Leandro Ponsoni, François Massonnet, Thierry Fichefet, Matthieu Chevallier, and David Docquier〈br〉 The Cryosphere, 13, 521-543, https://doi.org/10.5194/tc-13-521-2019, 2019〈br〉 The Arctic is a main component of the Earth's climate system. It is fundamental to understand the behavior of Arctic sea ice coverage over time and in space due to many factors, e.g., shipping lanes, the travel and tourism industry, hunting and fishing activities, mineral resource extraction, and the potential impact on the weather in midlatitude regions. In this work we use observations and results from models to understand how variations in the sea ice thickness change over time and in space.

Beschreibung: 〈b〉Brief communication: Evaluation and inter-comparisons of Qinghai–Tibet Plateau permafrost maps based on a new inventory of field evidence〈/b〉〈br〉 Bin Cao, Tingjun Zhang, Qingbai Wu, Yu Sheng, Lin Zhao, and Defu Zou〈br〉 The Cryosphere, 13, 511-519, https://doi.org/10.5194/tc-13-511-2019, 2019〈br〉 Many maps have been produced to estimate permafrost distribution over the Qinghai–Tibet Plateau. However the evaluation and inter-comparisons of them are poorly understood due to limited in situ measurements. We provided an in situ inventory of evidence of permafrost presence or absence, with 1475 sites over the Qinghai–Tibet Plateau. Based on the in situ measurements, our evaluation results showed a wide range of map performance, and the estimated permafrost region and area are extremely large.

Beschreibung: 〈b〉Impact of assimilating sea ice concentration, sea ice thickness and snow depth in a coupled ocean–sea ice modelling system〈/b〉〈br〉 Sindre Fritzner, Rune Graversen, Kai H. Christensen, Philip Rostosky, and Keguang Wang〈br〉 The Cryosphere, 13, 491-509, https://doi.org/10.5194/tc-13-491-2019, 2019〈br〉 In this work, a coupled ocean and sea-ice ensemble-based assimilation system is used to assess the impact of different observations on the assimilation system. The focus of this study is on sea-ice observations, including the use of satellite observations of sea-ice concentration, sea-ice thickness and snow depth for assimilation. The study showed that assimilation of sea-ice thickness in addition to sea-ice concentration has a large positive impact on the coupled model.

Beschreibung: 〈b〉Hydrologic Diversity in Glacier Bay Alaska: Spatial Patterns and Temporal Change〈/b〉〈br〉 Ryan L. Crumley, David F. Hill, Jordan P. Beamer, and Elizabeth Holzenthal〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-1,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 In this study we investigate the historical (1980–2015) and forecast (2070–2099) components of freshwater runoff to Glacier Bay, Alaska using a modeling approach. We find that many of the historically snow-dominated watersheds in Glacier Bay National Park and Preserve may transition towards rainfall-dominated hydrographs in a future scenario in which CO〈sub〉2〈/sub〉 emissions are not mitigated. The changes in timing and volume of freshwater entering Glacier Bay will affect bay ecology and hydrochemistry.

Beschreibung: 〈b〉Robust uncertainty assessment of the spatio-temporal transferability of glacier mass and energy balance models〈/b〉〈br〉 Tobias Zolles, Fabien Maussion, Stephan Peter Galos, Wolfgang Gurgiser, and Lindsey Nicholson〈br〉 The Cryosphere, 13, 469-489, https://doi.org/10.5194/tc-13-469-2019, 2019〈br〉 A mass and energy balance model was subjected to sensitivity and uncertainty analysis on two different Alpine glaciers. The global sensitivity analysis allowed for a mass balance measurement independent assessment of the model sensitivity and functioned as a reduction of the model free parameter space. A novel approach of a multi-objective optimization estimates the uncertainty of the simulated mass balance and the energy fluxes. The final model uncertainty is up to 1300 kg m〈sup〉−3〈/sup〉 per year.

Beschreibung: 〈b〉Four decades of Antarctic surface elevation changes from multi-mission satellite altimetry〈/b〉〈br〉 Ludwig Schröder, Martin Horwath, Reinhard Dietrich, Veit Helm, Michiel R. van den Broeke, and Stefan R. M. Ligtenberg〈br〉 The Cryosphere, 13, 427-449, https://doi.org/10.5194/tc-13-427-2019, 2019〈br〉 We developed an approach to combine measurements of seven satellite altimetry missions over the Antarctic Ice Sheet. Our resulting monthly grids of elevation changes between 1978 and 2017 provide unprecedented details of the long-term and interannual variation. Derived mass changes agree well with contemporaneous data of surface mass balance and satellite gravimetry and show which regions were responsible for the significant accelerations of mass loss in recent years.

Beschreibung: 〈b〉Two-dimensional Inversion of wideband spectral data from the Capacitively Coupled Resistivity method – First Applications in periglacial environments〈/b〉〈br〉 Jan Mudler, Andreas Hördt, Anita Przyklenk, Gianluca Fiandaca, Pradip Kumar Maurya, and Christian Hauck〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-288,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The Capacitively Coupled Resistivity method (CCR) enables the determination of frequency dependent electrical parameters of the subsurface. CCR is well suited for the application in cryospheric areas because it provides logistical advantages regarding the coupling on hard surfaces and highly resistive grounds. With our new spectral two-dimensional inversion, we can identify subsurface structures based on full spectral information. We show first results of the Inversion method on the field scale.

Beschreibung: 〈b〉Brief communication: Analysis of organic matter in surface snow by PTR-MS – implications for dry deposition dynamics in the Alps〈/b〉〈br〉 Dušan Materić, Elke Ludewig, Kangming Xu, Thomas Röckmann, and Rupert Holzinger〈br〉 The Cryosphere, 13, 297-307, https://doi.org/10.5194/tc-13-297-2019, 2019〈br〉 〈p〉The exchange of organic matter (OM) between the atmosphere and snow is poorly understood due to the complex nature of OM and the convoluted processes of deposition, re-volatilisation, and chemical and biological processing. OM that is finally retained in glaciers potentially holds a valuable historical record of past atmospheric conditions; however, our understanding of the processes involved is insufficient to translate the measurements into an interpretation of the past atmosphere. This study examines the dynamic processes of post-precipitation OM change at the alpine snow surface with the goal of interpreting the processes involved in surface snow OM.〈/p〉

Beschreibung: 〈b〉Modelling the Antarctic Ice Sheet across the Mid Pleistocene Transition – Implications for Oldest Ice〈/b〉〈br〉 Johannes Sutter, Hubertus Fischer, Klaus Grosfeld, Nanna B. Karlsson, Thomas Kleiner, Brice Van Liefferinge, and Olaf Eisen〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-24,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The Antarctic Ice Sheet may have played an important role in moderating the transition between warm and cold climate epochs over the last million of years. We find that the Antarctic Ice Sheet grew considerably about 0.9 Myr ago, a time when ice age/warm age cycles changed from a 40 000 to a 100 000 yr periodicity. Our findings also suggest that ice as old as 1.5 Myr still exists at the bottom of the East Antarctic ice sheet despite the major climate reorganizations in the past.

Beschreibung: 〈b〉The vertical structure of precipitation at two stations in East Antarctica derived from micro rain radars〈/b〉〈br〉 Claudio Durán-Alarcón, Brice Boudevillain, Christophe Genthon, Jacopo Grazioli, Niels Souverijns, Nicole P. M. van Lipzig, Irina V. Gorodetskaya, and Alexis Berne〈br〉 The Cryosphere, 13, 247-264, https://doi.org/10.5194/tc-13-247-2019, 2019〈br〉 Precipitation is the main input in the surface mass balance of the Antarctic ice sheet, but it is still poorly understood due to a lack of observations in this region. We analyzed the vertical structure of the precipitation using multiyear observation of vertically pointing micro rain radars (MRRs) at two stations located in East Antarctica. The use of MRRs showed the potential to study the effect of climatology and hydrometeor microphysics on the vertical structure of Antarctic precipitation.

Beschreibung: 〈b〉Recent changes in pan-Antarctic surface snowmelt detected by AMSR-E and AMSR2〈/b〉〈br〉 Lei Zheng, Chunxia Zhou, Tingjun Zhang, Qi Liang, and Kang Wang〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-279,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Snowmelt plays a key role in mass and energy balance in polar regions. In this study, we report on the spatial and temporal variations of the surface snowmelt over the Antarctic sea ice and ice sheet (pan-Antarctic) based on AMSR-E and AMSR2. Overall, the pan-Antarctic surface snowmelt showed a trend toward later melt onset during the 2002–2017 period. The decreasing surface snowmelt on the Antarctic ice sheet was very likely linked with the enhancing summer Southern Annular Mode.

Beschreibung: 〈b〉Water tracks intensify surface energy and mass exchange in the Antarctic McMurdo Dry Valleys〈/b〉〈br〉 Tobias Linhardt, Joseph S. Levy, and Christoph K. Thomas〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-8,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 We measured sensible, latent and soil heat fluxes along with radiation in the McMurdo Dry Valleys, Antarctica, at a water track and at predominant dry soils, serving as reference. We found a distinct impact of water tracks on surface energy and mass exchange which suggests that water tracks may serve as an indicator of change in the McMurdo Dry Valleys.

Beschreibung: 〈b〉Brief communication: Rapid machine-learning-based extraction and measurement of ice wedge polygons in high-resolution digital elevation models〈/b〉〈br〉 Charles J. Abolt, Michael H. Young, Adam L. Atchley, and Cathy J. Wilson〈br〉 The Cryosphere, 13, 237-245, https://doi.org/10.5194/tc-13-237-2019, 2019〈br〉 We present a workflow that uses a machine-learning algorithm known as a convolutional neural network (CNN) to rapidly delineate ice wedge polygons in high-resolution topographic datasets. Our workflow permits thorough assessments of polygonal microtopography at the kilometer scale or greater, which can improve understanding of landscape hydrology and carbon budgets. We demonstrate that a single CNN can be trained to delineate polygons with high accuracy in diverse tundra settings.

Beschreibung: 〈b〉Estimation of turbulent heat flux over leads using satellite thermal images〈/b〉〈br〉 Meng Qu, Xiaoping Pang, Xi Zhao, Jinlun Zhang, Qing Ji, and Pei Fan〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-262,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Can we ignore the contribution of small ice leads when estimating the turbulent heat flux? Combining bulk formulae and a fetch-limited model with surface temperature from MODIS and Landsat-8 Thermal Infrared Sensor (TIRS) images, we found small leads account for 25 % of the turbulent heat flux, due to its large total area. Estimated turbulent heat flux is larger from TIRS than that from MODIS with a coarser resolution, and larger using fetch-limited model than that using bulk formulae.

Beschreibung: 〈b〉Automatically delineating the calving front of Jakobshavn Isbræ from multi-temporal TerraSAR-X images: a deep learning approach〈/b〉〈br〉 Enze Zhang, Lin Liu, and Lingcao Huang〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-14,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Conventionally, calving front positions have been manually delineated from remote sensing images. We design a novel method to automatically delineate the calving front positions of Jakobshavn Isbræ based on deep learning, the first of this kind for Greenland outlet glaciers. We generate high temporal resolution (about two measurements every month) calving fronts. Demonstrating through this successful case study on Jakobshavn Isbræ, our methodology can be applied to many other tidewater glaciers.

Beschreibung: 〈b〉IcePAC – a probabilistic tool to study sea ice spatio-temporal dynamics: application to the Hudson Bay area〈/b〉〈br〉 Charles Gignac, Monique Bernier, and Karem Chokmani〈br〉 The Cryosphere, 13, 451-468, https://doi.org/10.5194/tc-13-451-2019, 2019〈br〉 The IcePAC tool is made to estimate the probabilities of specific sea ice conditions based on historical sea ice concentration time series from the EUMETSAT OSI-409 product (12.5 km grid), modelled using the beta distribution and used to build event probability maps, which have been unavailable until now. Compared to the Canadian ice service atlas, IcePAC showed promising results in the Hudson Bay, paving the way for its usage in other regions of the cryosphere to inform stakeholders' decisions.

Beschreibung: 〈b〉Spatio-temporal variability and decadal trends of snowmelt processes on Antarctic sea ice observed by satellite scatterometers〈/b〉〈br〉 Stefanie Arndt and Christian Haas〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-27,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉The timing and intensity of snowmelt processes on sea ice are key drivers determining the seasonal sea-ice energy and mass budgets. In the Arctic, satellite passive microwave and radar observations have revealed a trend towards an earlier snowmelt onset during the last decades, which is an important aspect of Arctic amplification and sea ice decline. Around Antarctica, snowmelt on perennial ice is weak and very different than in the Arctic, with most snow surviving the summer.〈/p〉 〈p〉Here we compile time series of snowmelt-onset dates on seasonal and perennial Antarctic sea ice from 1992 to 2014/15 using active microwave observations from European Remote Sensing Satellite (ERS-1/2), Quick Scatterometer (QSCAT) and Advanced Scatterometer (ASCAT) radar scatterometers. We define two snowmelt transition stages: A weak backscatter rise indicating the initial warming and metamorphism of the snowpack (pre-melt), followed by a rapid backscatter rise indicating the onset of thaw-freeze cycles (snowmelt).〈/p〉 〈p〉Results show large interannual variability with an average pre-melt onset date of 29 November and melt onset of 10 December, respectively, on perennial ice, without any significant trends over the study period, consistent with the small trends of Antarctic sea ice extent. There was a latitudinal gradient from early snowmelt onsets in mid-November in the northern Weddell Sea to late (end-December) or even absent snowmelt conditions in the southern Weddell Sea.〈/p〉 〈p〉We show that QSCAT Ku-band (13.4 GHz signal frequency) derived pre-melt and snowmelt onset dates are earlier by 25 and 11 days, respectively, than ERS and ASCAT C-band (5.6 GHz) derived dates. This offset has been considered when constructing the time series. Snowmelt onset dates from passive microwave observations (37 GHz) are later by 13 and 5 days than those from the scatterometers, respectively.〈/p〉 〈p〉Based on these characteristic differences between melt onset dates observed by different microwave wavelengths, we developed a conceptual model which illustrates how the evolution of seasonal snow temperature profiles affects different microwave bands with different penetration depths. These suggest that future multi-frequency active/passive microwave satellite missions could be used to resolve melt processes throughout the vertical snow column.〈/p〉

Beschreibung: 〈b〉Moisture transport in observations and reanalyses as a proxy for snow accumulation in East Antarctica〈/b〉〈br〉 Ambroise Dufour, Claudine Charrondière, and Olga Zolina〈br〉 The Cryosphere, 13, 413-425, https://doi.org/10.5194/tc-13-413-2019, 2019〈br〉 The East Antarctic Ice Sheet is thicker and larger than its western counterpart. Whether it gains or loses mass depends in part on the snowfall but this is difficult to measure and model inside the continent. Fortunately, the weather balloons launched from a network of stations along the coast provide an indirect estimate. Indeed, they track the water vapour that will eventually precipitate inland. It turns out there has been no consistent change in moisture transport from 1980 to 2017.

Beschreibung: 〈b〉Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures〈/b〉〈br〉 Chao Qi, David J. Prior, Lisa Craw, Sheng Fan, Maria-Gema Llorens, Albert Griera, Marianne Negrini, Paul D. Bons, and David L. Goldsby〈br〉 The Cryosphere, 13, 351-371, https://doi.org/10.5194/tc-13-351-2019, 2019〈br〉 Ice deformed in nature develops crystallographic preferred orientations, CPOs, which induce an anisotropy in ice viscosity. Shear experiments of ice revealed a transition in CPO with changing temperature/strain, which is due to the change of dominant CPO-formation mechanism: strain-induced grain boundary migration dominates at higher temperatures and lower strains, while lattice rotation dominates at other conditions. Understanding these mechanisms aids the interpretation of CPOs in natural ice.

Beschreibung: 〈b〉Assessment of the Greenland ice sheet–atmosphere feedbacks for the next century with a regional atmospheric model coupled to an ice sheet model〈/b〉〈br〉 Sébastien Le clec'h, Sylvie Charbit, Aurélien Quiquet, Xavier Fettweis, Christophe Dumas, Masa Kageyama, Coraline Wyard, and Catherine Ritz〈br〉 The Cryosphere, 13, 373-395, https://doi.org/10.5194/tc-13-373-2019, 2019〈br〉 Quantifying the future contribution of the Greenland ice sheet (GrIS) to sea-level rise in response to atmospheric changes is important but remains challenging. For the first time a full representation of the feedbacks between a GrIS model and a regional atmospheric model was implemented. The authors highlight the fundamental need for representing the GrIS topography change feedbacks with respect to the atmospheric component face to the strong impact on the projected sea-level rise.

Beschreibung: 〈b〉Change detection of bare-ice albedo in the Swiss Alps 〈/b〉〈br〉 Kathrin Naegeli, Matthias Huss, and Martin Hoelzle〈br〉 The Cryosphere, 13, 397-412, https://doi.org/10.5194/tc-13-397-2019, 2019〈br〉 The paper investigates the temporal changes of bare-ice glacier surface albedo in the Swiss Alps between 1999 and 2016 from a regional to local scale using satellite data. Significant negative trends were found in the lowermost elevations and margins of the ablation zones. Although significant changes of glacier ice albedo are only present over a limited area, we emphasize that albedo feedback will considerably enhance the rate of glacier mass loss in the Swiss Alps in the near future.

Beschreibung: 〈b〉Validation of the sea ice surface albedo scheme of the regional climate model HIRHAM–NAOSIM using aircraft measurements during the ACLOUD/PASCAL campaigns〈/b〉〈br〉 Evelyn Jäkel, Johannes Stapf, Manfred Wendisch, Marcel Nicolaus, Wolfgang Dorn, and Annette Rinke〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-266,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The sea ice surface albedo parameterization of a coupled regional climate model was validated against aircraft measurements performed in May/June 2017 north of Svalbard. The albedo parameterization was run offline from the model using the measured parameters surface temperature and snow depth to calculate the surface albedo and the individual fractions of the ice surface subtypes. An adjustment of the variables and additionally accounting for cloud cover reduced the root mean squared error.

Beschreibung: 〈b〉Global glacier volume projections under high-end climate change scenarios〈/b〉〈br〉 Sarah Shannon, Robin Smith, Andy Wiltshire, Tony Payne, Matthias Huss, Richard Betts, John Caesar, Aris Koutroulis, Darren Jones, and Stephan Harrison〈br〉 The Cryosphere, 13, 325-350, https://doi.org/10.5194/tc-13-325-2019, 2019〈br〉 We present global glacier volume projections for the end of this century, under a range of high-end climate change scenarios, defined as exceeding 2 °C global average warming. The ice loss contribution to sea level rise for all glaciers excluding those on the peripheral of the Antarctic ice sheet is 215.2 ± 21.3 mm. Such large ice losses will have consequences for sea level rise and for water supply in glacier-fed river systems.

Beschreibung: 〈b〉Quantifying the light absorption and source attribution of insoluble light-absorbing particles on Tibetan Plateau glaciers between 2013 and 2015〈/b〉〈br〉 Xin Wang, Hailun Wei, Jun Liu, Baiqing Xu, Mo Wang, Mingxia Ji, and Hongchun Jin〈br〉 The Cryosphere, 13, 309-324, https://doi.org/10.5194/tc-13-309-2019, 2019〈br〉 A large survey on measuring optical and chemical properties of insoluble light-absorbing impurities (ILAPs) from seven glaciers was conducted on the Tibetan Plateau (TP) during 2013–2015. The results indicated that the mixing ratios of black carbon (BC), organic carbon (OC), and iron (Fe) all showed a tendency to decrease from north to south, and the industrial pollution (33.1 %), biomass and biofuel burning (29.4 %), and soil dust (37.5 %) were the major sources of the ILAPs on the TP.

Beschreibung: 〈b〉Subglacial hydrological control on flow of an Antarctic Peninsula palaeo-ice stream〈/b〉〈br〉 Robert D. Larter, Kelly A. Hogan, Claus-Dieter Hillenbrand, James A. Smith, Christine L. Batchelor, Matthieu Cartigny, Alex J. Tate, James D. Kirkham, Zoë A. Roseby, Gerhard Kuhn, Alastair G. C. Graham, and Julian A. Dowdeswell〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-273,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 We present high-resolution bathymetry data that provide the most complete and detailed imagery of any Antarctic paleo-ice stream bed. These data show how subglacial water was delivered to and influenced the dynamic behavior of the ice stream. Our observations provide insights relevant to understanding the behavior of modern ice streams and forecasting the contributions that they will make to future sea-level rise.

Beschreibung: 〈b〉Estimation of the Antarctic surface mass balance using the regional climate model MAR (1979–2015) and identification of dominant processes〈/b〉〈br〉 Cécile Agosta, Charles Amory, Christoph Kittel, Anais Orsi, Vincent Favier, Hubert Gallée, Michiel R. van den Broeke, Jan T. M. Lenaerts, Jan Melchior van Wessem, Willem Jan van de Berg, and Xavier Fettweis〈br〉 The Cryosphere, 13, 281-296, https://doi.org/10.5194/tc-13-281-2019, 2019〈br〉 Antarctic surface mass balance (ASMB), a component of the sea level budget, is commonly estimated through modelling as observations are scarce. The polar-oriented regional climate model MAR performs well in simulating the observed ASMB. MAR and RACMO2 share common biases we relate to drifting snow transport, with a 3 times larger magnitude than in previous estimates. Sublimation of precipitation in the katabatic layer modelled by MAR is of a magnitude similar to an observation-based estimate.

Beschreibung: 〈b〉Simulated retreat of Jakobshavn Isbræ during the 21st century〈/b〉〈br〉 Xiaoran Guo, Liyun Zhao, Rupert Gladstone, Sainan Sun, and John C. Moore〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-7,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉The early in the 21st century retreat of Jakobshavn Isbræ, one of Greenland's largest outlet glaciers, into its over-deepened bedrock trough was accompanied by acceleration to unprecedented ice-stream speeds. Such dramatic changes suggested the possibility of substantial mass loss over the rest of this century. Using a three-dimensional ice-sheet model with parameterizations to represent the effects of ice mélange buttressing, crevasse-depth-based calving and submarine melting, we can reproduce its recent evolution. The model can accurately replicate its inter-annual variations in grounding line and terminus position, including new modes of seasonal fluctuations that emerged after arriving at the over-deepened basin and the disappearance of a persistent floating ice shelf. The shear margin induced decreases in ice viscosity we simulate are particularly important in reproducing the large observed inter-annual changes in terminus velocity. We use this model to project Jakobshavn's evolution over this century when forced by the IPCC RCP4.5 climate scenario and simulated by ocean temperatures from 7 Earth System Models along with surface runoff derived from RACMO. In our simulations, Jakobshavn's grounding line continues to retreat ~ 18.5 km by the end of this century with total mass loss of ~ 2030 Gt (5.6 mm sea-level-rise equivalent). Despite the relative success of the model in simulating the recent behavior of the glacier, the model does not simulate winter calving events that have become relatively more important.〈/p〉

Beschreibung: 〈b〉Responses of sub-ice platelet layer thickening rate and frazil-ice concentration to variations in ice-shelf water supercooling in McMurdo Sound, Antarctica〈/b〉〈br〉 Chen Cheng, Adrian Jenkins, Paul R. Holland, Zhaomin Wang, Chengyan Liu, and Ruibin Xia〈br〉 The Cryosphere, 13, 265-280, https://doi.org/10.5194/tc-13-265-2019, 2019〈br〉 The sub-ice platelet layer (SIPL) under fast ice is most prevalent in McMurdo Sound, Antarctica. Using a modified plume model, we investigated the responses of SIPL thickening rate and frazil concentration to variations in ice shelf water supercooling in McMurdo Sound. It would be key to parameterizing the relevant process in more complex three-dimensional, primitive equation ocean models, which relies on the knowledge of the suspended frazil size spectrum within the ice–ocean boundary layer.

Beschreibung: 〈b〉Past and future dynamics of the Brunt Ice Shelf from seabed bathymetry and ice shelf geometry〈/b〉〈br〉 Dominic A. Hodgson, Tom A. Jordan, Jan De Rydt, Peter T. Fretwell, Samuel A. Seddon, David Becker, Kelly A. Hogan, Andrew M. Smith, and David G. Vaughan〈br〉 The Cryosphere, 13, 545-556, https://doi.org/10.5194/tc-13-545-2019, 2019〈br〉 The Brunt Ice Shelf in Antarctica is home to Halley VIa, the latest in a series of six British research stations that have occupied the ice shelf since 1956. A recent rapid growth of rifts in the Brunt Ice Shelf signals the onset of its largest calving event since records began. Here we consider whether this calving event will lead to a new steady state for the ice shelf or an unpinning from the bed, which could predispose it to accelerated flow or collapse.

Beschreibung: 〈b〉Uncertainty in predicting the Eurasian snow: Intercomparison of land surface models coupled to a regional climate model〈/b〉〈br〉 Da-Eun Kim and Seon Ki Park〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-15,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 An accurate prediction of the Eurasian snow is essentially important in predicting the climate and weather phenomena in Asia. Regional climate models are mostly coupled with several land surface models (LSMs) in which the land surface process parameters are calculated under their own physical principles and parameterization schemes. We show that prediction of the Eurasian snow cover is sensitive to the choice of LSMs coupled to regional climate models, and hence the future climate projections.

Beschreibung: 〈b〉Apparent discrepancy of Tibetan ice core δ〈sup〉18〈/sup〉O records may be attributed to misinterpretation of chronology〈/b〉〈br〉 Shugui Hou, Wangbin Zhang, Hongxi Pang, Shuangye Wu, Theo M. Jenk, and Margit Schwikowski〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-295,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The apparent discrepancy between the Holocene δ〈sup〉18〈/sup〉O records of the Guliya and the Chongce ice cores may be attributed to a possible misinterpretation of the Guliya ice core chronology.

Beschreibung: 〈b〉Development of physically based liquid water schemes for Greenland firn-densification models〈/b〉〈br〉 Vincent Verjans, Amber Leeson, C. Max Stevens, Michael MacFerrin, Brice Noël, and Michiel R. van den Broeke〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-21,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Firn models rely on empirical approaches for representing the percolation and refreezing of meltwater through the firn column. We develop liquid water schemes of different levels of complexities for firn models and compare their performances with respect to observations of density profiles from Greenland. Our results demonstrate that physically advanced water schemes do not lead to better agreement with density observations. Uncertainties in other processes contribute more to model discrepancy.

Beschreibung: 〈b〉Shear failure of weak snow layers in the first hours after burial〈/b〉〈br〉 Benjamin Reuter, Neige Calonne, and Ed Adams〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-268,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Storm snow instabilities often crest during storms which hampers field experiements. Yet, layers of nature-like snow can be created in the lab. We shear tested samples containing typical storm snow and other weak layers. Failure was consistently located in the weak layer and ocurred after linear elastic-perfectly plastic deformation. Measurements of shear modulus and fracture toughness indicate that surface hoar and precipitation particles are equally fragile in the first hours after burial.

Beschreibung: 〈b〉Definition differences and internal variability affect the simulated Arctic sea ice melt season〈/b〉〈br〉 Abigail Smith and Alexandra Jahn〈br〉 The Cryosphere, 13, 1-20, https://doi.org/10.5194/tc-13-1-2019, 2019〈br〉 Here we assessed how natural climate variations and different definitions impact the diagnosed and projected Arctic sea ice melt season length using model simulations. Irrespective of the definition or natural variability, the sea ice melt season is projected to lengthen, potentially by as much as 4–5 months by 2100 under the business as usual scenario. We also find that different definitions have a bigger impact on melt onset, while natural variations have a bigger impact on freeze onset.

Beschreibung: 〈b〉Warming of SE Greenland shelf waters in 2016 primes large glacier for runaway retreat〈/b〉〈br〉 Suzanne L. Bevan, Adrian J. Luckman, Douglas I. Benn, Tom Cowton, and Joe Todd〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-260,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Kangerdlugssuaq Glacier in Greenland retreated significantly in the early 2000s and typified the response of calving glaciers to climate change. Satellite images show that it has recently retreated even further. The current retreat follows the appearance of extremely warm surface waters on the continental shelf during the summer of 2016 which likely entered the fjord and caused the rigid mass of sea-ice and icebergs, that normally inhibits calving, to melt and break up.

Beschreibung: 〈b〉Arctic sea-ice-free season projected to extend into autumn〈/b〉〈br〉 Marion Lebrun, Martin Vancoppenolle, Gurvan Madec, and François Massonnet〈br〉 The Cryosphere, 13, 79-96, https://doi.org/10.5194/tc-13-79-2019, 2019〈br〉 The present analysis shows that the increase in the Arctic ice-free season duration will be asymmetrical, with later autumn freeze-up contributing about twice as much as earlier spring retreat. This feature is robustly found in a hierarchy of climate models and is consistent with a simple mechanism: solar energy is absorbed more efficiently than it can be released in non-solar form and should emerge out of variability within the next few decades.

Beschreibung: 〈b〉Brief communication: Supraglacial debris-cover changes in the Caucasus Mountains〈/b〉〈br〉 Levan G. Tielidze, Tobias Bolch, Roger D. Wheate, Stanislav S. Kutuzov, Ivan I. Lavrentiev, and Michael Zemp〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-259,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Debris cover on glaciers can significantly alter melt, and hence, glacier mass balance and runoff. Debris coverage typically increases with shrinking glaciers. Here, we present data on debris cover and its changes for 559 glaciers located in different regions of the Greater Caucasus mountains based on 1986, 2000 and 2014 Landsat and SPOT images. Over this time period, the total glacier area decreased from 691.5 km〈sup〉2〈/sup〉 to 590.0 km〈sup〉2〈/sup〉 (0.52 % yr〈sup〉−1〈/sup〉). Thereby, the debris covered area increased from ~ 11 to ~ 24 % on the northern, and from ~ 4 to 10 % on the southern macro-slope between 1986 and 2014. Overall, we found 18 % debris cover for the year 2014. With the glacier shrinkage, debris-covered area and the number of debris-covered glaciers increased as a function of elevation, slope, aspect, glacier morphological type, Little Ice Age moraines, and lithology.

Beschreibung: 〈b〉Brief communication: Pancake ice floe size distribution during the winter expansion of the Antarctic marginal ice zone〈/b〉〈br〉 Alberto Alberello, Miguel Onorato, Luke Bennetts, Marcello Vichi, Clare Eayrs, Keith MacHutchon, and Alessandro Toffoli〈br〉 The Cryosphere, 13, 41-48, https://doi.org/10.5194/tc-13-41-2019, 2019〈br〉 Existing observations do not provide quantitative descriptions of the floe size distribution for pancake ice floes. This is important during the Antarctic winter sea ice expansion, when hundreds of kilometres of ice cover around the Antarctic continent are composed of pancake floes (〈i〉D〈/i〉 = 0.3–3 m). Here, a new set of images from the Antarctic marginal ice zone is used to measure the shape of individual pancakes for the first time and to infer their size distribution.

Beschreibung: 〈b〉Version 2 of the EUMETSAT OSI SAF and ESA CCI sea-ice concentration climate data records〈/b〉〈br〉 Thomas Lavergne, Atle Macdonald Sørensen, Stefan Kern, Rasmus Tonboe, Dirk Notz, Signe Aaboe, Louisa Bell, Gorm Dybkjær, Steinar Eastwood, Carolina Gabarro, Georg Heygster, Mari Anne Killie, Matilde Brandt Kreiner, John Lavelle, Roberto Saldo, Stein Sandven, and Leif Toudal Pedersen〈br〉 The Cryosphere, 13, 49-78, https://doi.org/10.5194/tc-13-49-2019, 2019〈br〉 The loss of polar sea ice is an iconic indicator of Earth’s climate change. Many satellite-based algorithms and resulting data exist but they differ widely in specific sea-ice conditions. This spread hinders a robust estimate of the future evolution of sea-ice cover. In this study, we document three new climate data records of sea-ice concentration generated using satellite data available over the last 40 years. We introduce the novel algorithms, the data records, and their uncertainties.

Beschreibung: 〈b〉Distinguishing ice-rich and ice-poor permafrost to map ground temperatures and -ice content in the Swiss Alps〈/b〉〈br〉 Robert Kenner, Jeannette Noetzli, Martin Hoelzle, Hugo Raetzo, and Marcia Phillips〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-235,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 A new countrywide permafrost distribution map of Switzerland is presented. Zone 1 indicates ground temperatures and zone 2 excess ground ice which is located outside of zone 1. By distinguishing ice-poor and ice-rich permafrost we achieved a clear improvement in the cartographic representation of commonly occurring permafrost-free elevational belts which are bordered above and below by permafrost. The reproduction of such permafrost gaps allowed a higher accuracy and unambiguity of the map.

Beschreibung: 〈b〉The optical characteristics and sources of chromophoric dissolved organic matter (CDOM) in seasonal snow of northwestern China〈/b〉〈br〉 Yue Zhou, Hui Wen, Jun Liu, Wei Pu, Qingcai Chen, and Xin Wang〈br〉 The Cryosphere, 13, 157-175, https://doi.org/10.5194/tc-13-157-2019, 2019〈br〉 We first investigated the optical characteristics and potential sources of chromophoric dissolved organic matter (CDOM) in seasonal snow over northwestern China. The abundance of CDOM showed regional variation. At some sites strongly influenced by local soil, the absorption of CDOM cannot be neglected compared to black carbon. We found two humic-like and one protein-like fluorophores in snow. The major sources of snow CDOM were soil, biomass burning, and anthropogenic pollution.

Beschreibung: 〈b〉Thermal conductivity of firn at Lomonosovfonna, Svalbard, derived from subsurface temperature measurements〈/b〉〈br〉 Sergey Marchenko, Gong Cheng, Per Lötstedt, Veijo Pohjola, Rickard Pettersson, Ward van Pelt, and Carleen Reijmer〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-294,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Thermal conductivity (〈i〉k〈/i〉) of firn at Lomonosovfonna, Svalbard, is estimated using measured temperature evolution and density. The optimized 〈i〉k〈/i〉 values (0.2–1.6 W/(m K)) increase downwards and over time and are most sensitive to systematic errors in measured temperature values and their depths, particularly in the lower part of the profile. Compared to density-based parameterizations our 〈i〉k〈/i〉 values are consistently larger, suggesting a faster conductive heat exchange in firn.

Beschreibung: 〈b〉Evaluating the destabilization susceptibility of active rock glaciers in the French Alps〈/b〉〈br〉 Marco Marcer, Charlie Serrano, Alexander Brenning, Xavier Bodin, Jason Goetz, and Philippe Schoeneich〈br〉 The Cryosphere, 13, 141-155, https://doi.org/10.5194/tc-13-141-2019, 2019〈br〉 This study aims to assess the occurrence of rock glacier destabilization in the French Alps, a process that causes a landslide-like behaviour of permafrost debris slopes. A significant number of the landforms in the region were found to be experiencing destabilization. Multivariate analysis suggested a link between destabilization occurrence and permafrost thaw induced by climate warming. These results call for a regional characterization of permafrost hazards in the context of climate change.

Beschreibung: 〈b〉Rapid decline of Arctic sea ice volume: Causes and consequences〈/b〉〈br〉 Jean-Claude Gascard, Jinlun Zhang, and Mehrad Rafizadeh〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-2,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 4 comments)〈br〉 From ERA Interim surface air temperature reanalysis, we estimated Freezing Degrees Days (FDD) over the whole Arctic Ocean during the freezing period each year for the past 40 years. We deduced sea ice growth from FDD that we compared with model (PIOMAS) and satellite (Cryosat-2) estimations. The warming of the Atmosphere and the vertical heat fluxes from the Ocean are contributing to the Arctic sea ice rapid decline. A disappearance of Arctic sea ice in summer is predictable within 15 years.

Beschreibung: 〈b〉Scaling of instability time-scales of Antarctic outlet glaciers based on one-dimensional similitude analysis〈/b〉〈br〉 Anders Levermann and Johannes Feldmann〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-252,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 1 comment)〈br〉 Using scaling analysis we propose that all potential marine ice-sheet instabilities in Antarctica will be slower than the currently observed instability in Amundsen Sea Sector.

Beschreibung: 〈b〉Characterizing the behaviour of surge- and non-surge-type glaciers in the Kingata Mountains, eastern Pamir, from 1999 to 2016〈/b〉〈br〉 Mingyang Lv, Huadong Guo, Xiancai Lu, Guang Liu, Shiyong Yan, Zhixing Ruan, Yixing Ding, and Duncan J. Quincey〈br〉 The Cryosphere, 13, 219-236, https://doi.org/10.5194/tc-13-219-2019, 2019〈br〉 We highlight 28 glaciers in the Kingata Mountains, among which 17 have changed markedly over the last decade. We identify four advancing and 13 surge-type glaciers. The dynamic evolution of the surges is similar to that of Karakoram, suggesting that both hydrological and thermal controls are important for surge initiation and recession. Topography seems to be a dominant control on non-surge glacier behaviour. Most glaciers experienced a significant and diverse change in their motion patterns.

Beschreibung: 〈b〉Converting Snow Depth to Snow Water Equivalent Using Climatological Variables〈/b〉〈br〉 David F. Hill, Elizabeth A. Burakowski, Ryan L. Crumley, Julia Keon, J. Michelle Hu, Anthony A. Arendt, Katreen Wikstrom Jones, and Gabriel J. Wolken〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-286,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 We present a new statistical model for converting snow depths to water equivalent. The only variables required are snow depth, day of year, and location. We use the location to look up climatological parameters such as mean annual precipitation and temperature characteristics. The model is simple by design so that it can applied to depth measurements anywhere, anytime. The model is shown to perform better than other widely used approaches.

Beschreibung: 〈b〉Sensitivity of active-layer freezing process to snow cover in Arctic Alaska〈/b〉〈br〉 Yonghong Yi, John S. Kimball, Richard H. Chen, Mahta Moghaddam, and Charles E. Miller〈br〉 The Cryosphere, 13, 197-218, https://doi.org/10.5194/tc-13-197-2019, 2019〈br〉 To better understand active-layer freezing process and its climate sensitivity, we developed a new 1 km snow data set for permafrost modeling and used the model simulations with multiple new in situ and P-band radar data sets to characterize the soil freeze onset and duration of zero curtain in Arctic Alaska. Results show that zero curtains of upper soils are primarily affected by early snow cover accumulation, while zero curtains of deeper soils are more closely related to maximum thaw depth.

Beschreibung: 〈b〉Brief communication: Full-field deformation measurement for uniaxial compression of sea ice by using the digital image correlation method〈/b〉〈br〉 Anliang Wang, Zhijun Wei, Xiaodong Chen, Shunying Ji, Yu Liu, and Longbang Qing〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-263,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 We applied digital images to measuring the full-field deformation of sea ice in the mechanical experiment. The characteristics of failure mode, nonlinear behavior and crack propagation are all captured by the strain field of specimens. To our knowledge, this is the first attempt to experimentally capture sequential full-field deformations in the mechanical properties of sea ice. This achievement will extend the ability to further explore the complex mechanical behaviors of sea ice.

Beschreibung: 〈b〉Multisensor validation of tidewater glacier flow fields derived from SAR intensity tracking〈/b〉〈br〉 Christoph Rohner, David Small, Daniel Henke, Martin P. Lüthi, and Andreas Vieli〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-278,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The recent increase in ice flow and calving rates of ocean-terminating glaciers contributes substantially to the mass loss of the Greenland ice sheet. Using in situ reference observations, we validate the satellite-based method of iterative offset tracking of Sentinel-1A data for deriving flow-speeds. Our investigations highlight the importance of spatial resolution near the fast-flowing calving front, resulting in significantly higher ice velocities compared to large scale operational products.

Beschreibung: 〈b〉Permafrost distribution and conditions at the headwalls of two receding glaciers (Schladminger and Hallstadt glaciers) in the Dachstein Massif, Northern Calcareous Alps, Austria〈/b〉〈br〉 Matthias Rode, Harald Schnepfleitner, Oliver Sass, Andreas Kellerer-Pirklbauer, and Christoph Gitschthaler〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-281,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Permafrost distribution in rockwalls surrounding receding glaciers is an important factor for rock slope failure and rockwall retreat. The Northern Calcareous Alps of the Eastern European Alps form a geological and climatological transition zone between the Alpine Foreland and the Central Alps. Some of highest summits of this area are located in the Dachstein Massif (47°28'32'' N, 13°36'23'' E) in Austria reaching up to 2995 m a.s.l. Occurrence, thickness and thermal regime of permafrost at this partly glaciated mountain massif are scarcely known and related knowledge is primarily based on regional modeling approaches. We applied a multi method approach with continuous ground surface and near-surface temperature monitoring, measurement of bottom temperature of the winter snow cover, electrical resistivity tomography/ERT, airborne photogrammetry, topographic maps, visual observations and field mapping for permafrost assessment. Our research focused on steep rockwalls consisting of massive limestone above several receding glaciers exposed to different slope aspects at elevations between c.2600–2700 m a.s.l. We aimed to quantify distribution and conditions of bedrock permafrost particularly at the transition zone between the present glacier surface and the adjacent rockwalls. 〈/p〉 〈p〉 Low ground temperature data suggest that permafrost is mainly found at cold, north exposed rockwalls. At southeast exposed rockwalls permafrost is only expected in very favourable cold conditions at shadowed higher elevations (2700 m a.s.l.). ERT measurements reveal high resistivities (〉 30.000 ohm.m) at ≥ 1.5 m depth at north-exposed slopes (highest measured resistivity values 〉 100 kohm.m). Based on laboratory studies and additional measurements with small scale ERT, these values indicate permafrost existence. Such permafrost bodies were found in the rockwalls at all measurement sites independent of investigated slope orientation. ERT data indicate large permafrost bodies at north exposed sites whereas discontinuous permafrost bodies prevail at northwest and northeast facing rockwalls. In summary, permafrost distribution and conditions around the headwalls of the glaciers of the Dachstein Massif is primarily restricted to the north exposed sector, whereas at the south exposed sector permafrost is restricted to the summit region.〈/p〉

Beschreibung: 〈b〉initMIP-Antarctica: An ice sheet model initialization experiment of ISMIP6〈/b〉〈br〉 Hélène Seroussi, Sophie Nowicki, Erika Simon, Ayako Abe Ouchi, Torsten Albrecht, Julien Brondex, Stephen Cornford, Christophe Dumas, Fabien Gillet-Chaulet, Heiko Goelzer, Nicholas R. Golledge, Jonathan M. Gregory, Ralf Greve, Matthew J. Hoffman, Angelika Humbert, Philippe Huybrechts, Thomas Kleiner, Eric Larour, Gunter Leguy, William H. Lipscomb, Daniel Lowry, Matthias Mengel, Mathieu Morlighem, Frank Pattyn, Anthony J. Payne, David Pollard, Stephen Price, Aurélien Quiquet, Thomas Reerink, Ronja Reese, Christian B. Rodehacke, Nicole-Jeanne Schlegel, Andrew Shepherd, Sainan Sun, Johannes Sutter, Jonas Van Breedam, Roderik S. W. van de Wal, Ricarda Winkelmann, and Tong Zhang〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-271,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 〈p〉Ice sheet numerical modeling is the best approach to estimate the dynamic contribution of Antarctica to sea level rise over the coming centuries. The influence of initial conditions on ice sheet model simulations, however, is still unclear. To better understand this influence, an initial state intercomparison exercise (initMIP) has been developed to compare, evaluate, and improve initialization procedures and estimate their impact on century-scale simulations. initMIP is the first set of experiments of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), which is the primary Coupled Model Intercomparison Project Phase 6 (CMIP6) activity focusing on the Greenland and Antarctic Ice Sheets. Following initMIP-Greenland, initMIP-Antarctica has been designed to explore uncertainties associated with model initialization and spin-up and to evaluate the impact of changes in external forcings. Starting from the state of the Antarctic ice sheet at the end of the initialization procedure, three forward experiments are each run for 100 years: a control run, a run with a surface mass balance (SMB) anomaly, and a run with a basal melting anomaly beneath floating ice. This study presents the results of initMIP-Antarctica from 25 simulations performed by 16 international modeling groups. The submitted results use different initial conditions and initialization methods, as well as ice flow model parameters and reference external forcings. We find a good agreement among model responses to the SMB anomaly, but large variations in responses to the basal melting anomaly. These variations can be attributed to differences in the extent of ice shelves and their upstream tributaries, the numerical treatment of grounding line, as well as the initial ocean conditions applied, suggesting that ongoing efforts to better represent ice shelves in continental-scale models should continue.〈/p〉

Beschreibung: 〈b〉New insight from CryoSat-2 sea ice thickness for sea ice modelling〈/b〉〈br〉 David Schröder, Danny L. Feltham, Michel Tsamados, Andy Ridout, and Rachel Tilling〈br〉 The Cryosphere, 13, 125-139, https://doi.org/10.5194/tc-13-125-2019, 2019〈br〉 This paper uses sea ice thickness data (CryoSat-2) to identify and correct shortcomings in simulating winter ice growth in the widely used sea ice model CICE. Adding a model of snow drift and using a different scheme for calculating the ice conductivity improve model results. Sensitivity studies demonstrate that atmospheric winter conditions have little impact on winter ice growth, and the fate of Arctic summer sea ice is largely controlled by atmospheric conditions during the melting season.

Beschreibung: 〈b〉Contributions of advection and melting processes to the decline in sea ice in the Pacific sector of the Arctic Ocean〈/b〉〈br〉 Haibo Bi, Qinghua Yang, Xi Liang, and Haijun Huang〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-11,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 The Arctic sea ice extent is diminishing, which is deemed as an immediate response to a warmer earth. However, quantitative estimate about the contribution due to transport and melt to the sea ice loss is still in vague. This study mainly utilizes satellite observations to quantify the dynamic and thermodynamic aspects of ice loss for a nearly 40 years (1979–2016). In addition, the potential impacts on ice reduction due to different atmospheric circulation pattern is highlighted.

Beschreibung: 〈b〉Sensitivity of centennial mass loss projections of the Amundsen basin to the friction law〈/b〉〈br〉 Julien Brondex, Fabien Gillet-Chaulet, and Olivier Gagliardini〈br〉 The Cryosphere, 13, 177-195, https://doi.org/10.5194/tc-13-177-2019, 2019〈br〉 Here, we apply a synthetic perturbation to the most active drainage basin of Antarctica and show that centennial mass loss projections obtained through ice flow models depend strongly on the implemented friction law, i.e. the mathematical relationship between basal drag and sliding velocities. In particular, the commonly used Weertman law considerably underestimates the sea-level contribution of this basin in comparison to two water pressure-dependent laws which rely on stronger physical bases.

Beschreibung: 〈b〉On the multi-fractal scaling properties of sea ice deformation〈/b〉〈br〉 Pierre Rampal, Véronique Dansereau, Einar Olason, Sylvain Bouillon, Timothy Williams, and Abdoulaye Samaké〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-290,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 In this article, we look at how the Arctic sea ice cover, as a solid body, behaves on different temporal and spatial scales.We show that the numerical model neXtSIM that uses a new approach to simulate the mechanics of sea ice is reproducing the characteristics of how sea ice deforms as observed by satellite.We discuss the importance of this model performance in the context of simulating climate processes taking place in polar regions,like the exchange of energy between the ocean and atmosphere.

Beschreibung: 〈b〉The role of debris cover in the evolution of Zmuttgletscher, Switzerland, since the end of the Little Ice Age〈/b〉〈br〉 Nico Mölg, Tobias Bolch, Andrea Walter, and Andreas Vieli〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-292,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Debris can partly protect glaciers from melting. But many debris-covered glaciers change similar to debris-free glaciers. To better understand the debris influence we investigated 150 years of development of Zmuttgletscher in Switzerland. We found an increase of debris extent over time and a link to glacier flow velocity changes. We also found an influence of debris on the melt locally, but only a small volume change reduction over the whole glacier, also because of the influence of ice cliffs.

Beschreibung: 〈b〉Differential InSAR for tide modelling in Antarctic ice-shelf grounding zones〈/b〉〈br〉 Christian T. Wild, Oliver J. Marsh, and Wolfgang Rack〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-269,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 In Antarctica, ocean tides control the motion of ice sheets near the coastline as well as melt rates underneath the floating ice. By combining the spatial advantage of rare but highly accurate satellite images with the temporal advantage of tide-prediction models, vertical displacement of floating ice due to ocean tides can now be predicted accurately. This allows the detailed study of ice-flow dynamics in areas that matter the most to the stability of Antarctica's ice sheets.

Beschreibung: 〈b〉Arctic freshwater fluxes: sources, tracer budgets and inconsistencies〈/b〉〈br〉 Alexander Forryan, Sheldon Bacon, Takamasa Tsubouchi, Sinhué Torres-Valdés, and Alberto C. Naveira Garabato〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-247,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 We compare contol-volume and geochemical tracer-based methods of estimating the Arctic Ocean freshwater budget and find both methods in good agreement. Inconsistencies arise from the distinction between "Atlantic" and "Pacific" waters in the geochemical calculations. The definition of Pacific waters is particularly problematic, due to the non-conservative nature of the nutrients underpinning the definition and the low salinity characterising waters entering the Arctic through Bering Strait.

Beschreibung: 〈b〉Observation of the process of snow accumulation on the Antarctic Plateau by time lapse laserscanning〈/b〉〈br〉 Ghislain Picard, Laurent Arnaud, Romain Caneil, Eric Lefebvre, and Maxim Lamare〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2019-4,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 To study how snow accumulates in Antarctica, we use daily surface elevation recorded by an automatic laserscanner. We show that new snow often accumulates in thick patches covering a small fraction of the surface. Most patches are removed by erosion within weeks, implying that only a few contribute to the snowpack. The result is a great heterogeneity on the surface and in the snowpack. These findings are important for surface mass and energy balance, photochemistry and ice core interpretation.

Beschreibung: 〈b〉The evolution of snow bedforms in the Colorado Front Range and the processes that shape them〈/b〉〈br〉 Kelly Kochanski, Robert S. Anderson, and Gregory E. Tucker〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-293,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 0 comments)〈br〉 Wind-blown snow does not lie flat. It forms dunes, ripples, and anvil-shaped sastrugi. These features ornament much of the snow on Earth, and change the snow's effects on polar climates, but they have rarely been studied. We spent three winters watching snow move through the Colorado Front Range. We present our findings, including the first time-lapse videos of snow dune and sastrugi growth, here.

Beschreibung: 〈b〉Past and future interannual variability in Arctic sea ice in coupled climate models〈/b〉〈br〉 John R. Mioduszewski, Stephen Vavrus, Muyin Wang, Marika Holland, and Laura Landrum〈br〉 The Cryosphere, 13, 113-124, https://doi.org/10.5194/tc-13-113-2019, 2019〈br〉 Arctic sea ice is projected to thin substantially in every season by the end of the 21st century with a corresponding increase in its interannual variability as the rate of ice loss peaks. This typically occurs when the mean ice thickness falls between 0.2 and 0.6 m. The high variability in both growth and melt processes is the primary factor resulting in increased ice variability. This study emphasizes the importance of short-term variations in ice cover within the mean downward trend.

Beschreibung: 〈b〉Origin, burial and preservation of late Pleistocene-age glacier ice in Arctic permafrost (Bylot Island, NU, Canada)〈/b〉〈br〉 Stephanie Coulombe, Daniel Fortier, Denis Lacelle, Mikhail Kanevskiy, and Yuri Shur〈br〉 The Cryosphere, 13, 97-111, https://doi.org/10.5194/tc-13-97-2019, 2019〈br〉 This study provides a detailed description of relict glacier ice preserved in the permafrost of Bylot Island (Nunavut). We demonstrate that the 18O composition (-34.0 0.4 ‰) of the ice is consistent with the late Pleistocene age ice in the Barnes Ice Cap. As most of the glaciated Arctic landscapes are still strongly determined by their glacial legacy, the melting of these large ice bodies could have significant impacts on permafrost geosystem landscape dynamics and ecosystems.

Beschreibung: 〈b〉Impacts of topographic shading on direct solar radiation for valley glaciers in complex topography〈/b〉〈br〉 Matthew Olson and Summer Rupper〈br〉 The Cryosphere, 13, 29-40, https://doi.org/10.5194/tc-13-29-2019, 2019〈br〉 Solar radiation is the largest energy input for most alpine glaciers. However, many models oversimplify the influence of topographic shading. Also, no systematic studies have explored the variable impact of shading on glacier ice. We find that shading can significantly impact modeled solar radiation, particularly at low elevations, at high latitudes, and for glaciers with a north/south orientation. Excluding the effects of shading will overestimate modeled solar radiation for alpine glaciers.

Beschreibung: 〈b〉Spatiotemporal Distributions of Icebergs in a Temperate Fjord: Columbia Fjord, Alaska〈/b〉〈br〉 Sarah U. Neuhaus, Slawek M. Tulaczyk, and Carolyn Branecky Begeman〈br〉 The Cryosphere Discuss., https//doi.org/10.5194/tc-2018-230,2019〈br〉 〈b〉Manuscript under review for TC〈/b〉 (discussion: open, 1 comment)〈br〉 Relatively few studies have been done on icebergs inside fjords, despite the fact that the majority of recent sea level rise has resulted from glaciers terminating in fjords. We examine the size and spatial distribution of icebergs in Columbia Fjord, Alaska to determine their influence on fjord dynamics and conclude that in such a warm fjord, the cooling effects of iceberg melt are negligible.

Beschreibung: The quantity and quality of river discharge in Arctic regions is influenced by many processes including climate, watershed attributes and, increasingly, hydrological cycle intensification and permafrost thaw. We used a hydrological model to quantify baseline conditions and investigate the changing character of hydrological elements for Arctic watersheds between Utqiagvik (formerly known as Barrow)) and just west of Mackenzie River over the period 1981–2010. A synthesis of measurements and model simulations shows that the region exports 31.9 km3 yr−1 of freshwater via river discharge, with 55.5 % (17.7 km3 yr−1) coming collectively from the Colville, Kuparuk, and Sagavanirktok rivers. The simulations point to significant (p

Beschreibung: Sublimation of snow particles during transport has been recognized as an important ablation process on the Antarctic ice sheet. The resulting increase in moisture content and cooling of the ambient air are thermodynamic negative feedbacks that both contribute to increase the relative humidity of the air, inhibiting further sublimation when saturation is reached. This self-limiting effect and the associated development of saturated near-surface air layers in drifting snow conditions have mainly been described through modelling studies and a few field observations. A set of meteorological data, including drifting snow mass fluxes and vertical profiles of relative humidity, collected at site D17 in coastal Adélie Land (East Antarctica) during 2013 is used to study the relationship between saturation of the near-surface atmosphere and the occurrence of drifting snow in a katabatic wind region that is among the most prone to snow transport by wind. Atmospheric moistening by the sublimation of the windborne snow particles generally results in a strong increase in relative humidity with the magnitude of drifting snow and a decrease in its vertical gradient, suggesting that windborne-snow sublimation can be an important contributor to the local near-surface moisture budget. Despite a high incidence of drifting snow at the measurement location (60.1 % of the time), saturation, when attained, is however most often limited to a thin air layer below 1 m above ground. The development of a near-surface saturated air layer up to the highest measurement level of 5.5 m is observed in only 8.2 % of the drifting snow occurrences or 6.3 % of the time and mainly occurs in strong wind speed and drift conditions. This relatively rare occurrence of ambient saturation is explained by the likely existence of moisture-removal mechanisms inherent to the katabatic and turbulent nature of the boundary-layer flow that weaken the negative feedback of windborne-snow sublimation. Such mechanisms, potentially quite active in katabatic-generated windborne-snow layers all over Antarctica, may be very important in understanding the surface mass and atmospheric moisture budgets of the ice sheet by enhancing windborne-snow sublimation.

Beschreibung: We study a firn and ice core drilled at the new “Lock-In” site in East Antarctica, located 136 km away from Concordia station towards Dumont d'Urville. High-resolution chemical and physical measurements were performed on the core, with a particular focus on the trapping zone of the firn where air bubbles are formed. We measured the air content in the ice, closed and open porous volumes in the firn, firn density, firn liquid conductivity, major ion concentrations, and methane concentrations in the ice. The closed and open porosity volumes of firn samples were obtained using the two independent methods of pycnometry and tomography, which yield similar results. The measured increase in the closed porosity with density is used to estimate the air content trapped in the ice with the aid of a simple gas-trapping model. Results show a discrepancy, with the model trapping too much air. Experimental errors have been considered but do not explain the discrepancy between the model and the observations. The model and data can be reconciled with the introduction of a reduced compression of the closed porosity compared to the open porosity. Yet, it is not clear if this limited compression of closed pores is the actual mechanism responsible for the low amount of air in the ice. High-resolution density measurements reveal the presence of strong layering, manifesting itself as centimeter-scale variations. Despite this heterogeneous stratification, all layers, including the ones that are especially dense or less dense compared to their surroundings, display similar pore morphology and closed porosity as a function of density. This implies that all layers close in a similar way, even though some close in advance or later compared to the bulk firn. Investigation of the chemistry data suggests that in the trapping zone, the observed stratification is partly related to the presence of chemical impurities.

Beschreibung: Climate change, including warmer winter temperatures, a shortened snowfall season, and more rain-on-snow events, threatens nordic skiing as a sport. In response, over-summer snow storage, attempted primarily using woodchips as a cover material, has been successfully employed as a climate change adaptation strategy by high-elevation and/or high-latitude ski centers in Europe and Canada. Such storage has never been attempted at a site that is both low elevation and midlatitude, and few studies have quantified storage losses repeatedly through the summer. Such data, along with tests of different cover strategies, are prerequisites to optimizing snow storage strategies. Here, we assess the rate at which the volume of two woodchip-covered snow piles (each ∼200 m3), emplaced during spring 2018 in Craftsbury, Vermont (45∘ N and 360 m a.s.l.), changed. We used these data to develop an optimized snow storage strategy. In 2019, we tested that strategy on a much larger, 9300 m3 pile. In 2018, we continually logged air-to-snow temperature gradients under different cover layers including rigid foam, open-cell foam, and woodchips both with and without an underlying insulating blanket and an overlying reflective cover. We also measured ground temperatures to a meter depth adjacent to the snow piles and used a snow tube to measure snow density. During both years, we monitored volume change over the melt season using terrestrial laser scanning every 10–14 d from spring to fall. In 2018, snow volume loss ranged from 0.29 to 2.81 m3 d−1, with the highest rates in midsummer and lowest rates in the fall; mean rates of volumetric change were 1.24 and 1.50 m3 d−1, 0.55 % to 0.72 % of initial pile volume per day. Snow density did increase over time, but most volume loss was the result of melting. Wet woodchips underlain by an insulating blanket and covered with a reflective sheet were the most effective cover combination for minimizing melt, likely because the aluminized surface reflected incoming short-wave radiation while the wet woodchips provided significant thermal mass, allowing much of the energy absorbed during the day to be lost by long-wave emission at night. The importance of the pile surface-area-to-volume ratio is demonstrated by 4-fold lower rates of volumetric change for the 9300 m3 pile emplaced in 2019; it lost

Beschreibung: To provide estimates of past glacier mass changes over the course of the 20th century, an adequate initial state is required. However, empirical evidence about past glacier states at regional or global scales is largely incomplete, both spatially and temporally, calling for the use of automated numerical methods. This study presents a new way to initialize the Open Global Glacier Model from past climate information and present-day glacier states. We use synthetic experiments to show that even with these perfectly known but incomplete boundary conditions, the problem of model initialization is an ill-posed inverse problem leading to nonunique solutions, and we propose an ensemble approach as a way forward. The method works as follows: we generate a large set of physically plausible glacier candidates for a given year in the past (e.g., 1850 in the Alps), all of which are then modeled forward to the date of the observed glacier outline and evaluated by comparing the results of the forward runs to the present-day states. We test the approach on 2660 Alpine glaciers and determine error estimates of the method from the synthetic experiments. The results show that the solution is often nonunique, as many of the reconstructed initial states converge towards the observed state in the year of observation. We find that the median state of the best 5 % of all acceptable states is a reasonable best estimate. The accuracy of the method depends on the type of the considered observation for the evaluation (glacier length, area, or geometry). Trying to find past states from only present-day length instead of the full geometry leads to a sharp increase in uncertainty. Our study thus also provides quantitative information on how well the reconstructed initial glacier states are constrained through the limited information available to us. We analyze which glacier characteristics influence the reconstructability of a glacier, and we discuss ways to develop the method further for real-world applications.

Beschreibung: Observed snow stratigraphy and snow stability are of key importance for avalanche forecasting. Such observations are rare and snow cover models can improve the spatial and temporal resolution. To evaluate snow stability, failure initiation and crack propagation have to be considered. Recently, a new stability criterion relating to crack propagation, namely the critical crack length, was implemented into the snow cover model SNOWPACK. The critical crack length can also be measured in the field with a propagation saw test, which allows for an unambiguous comparison. To validate and improve the parameterization for the critical crack length, we used data from 3 years of field experiments performed close to two automatic weather stations above Davos, Switzerland. We monitored seven distinct weak layers and performed in total 157 propagation saw tests on a weekly basis. Comparing modeled to measured critical crack length showed some discrepancies stemming from model assumption. Hence, we replaced two variables of the original parameterization, namely the weak layer shear modulus and thickness, with a fit factor depending on weak layer density and grain size. With these adjustments, the normalized root-mean-square error between modeled and observed critical crack lengths decreased from 1.80 to 0.28. As the improved parameterization accounts for grain size, values of critical crack lengths for snow layers consisting of small grains, which in general are not weak layers, become larger. In turn, critical weak layers appear more prominently in the vertical profile of critical crack length simulated with SNOWPACK. Hence, minimal values in modeled critical crack length better match observed weak layers. The improved parameterization of critical crack length may be useful for both weak layer detection in simulated snow stratigraphy and also providing more realistic snow stability information – and hence may improve avalanche forecasting.

Beschreibung: Snow water equivalent (SWE) measurements of seasonal snowpack are crucial in many research fields. Yet accurate measurements at a high temporal resolution are difficult to obtain in high mountain regions. With a cosmic ray sensor (CRS), SWE can be inferred from neutron counts. We present the analyses of temporally continuous SWE measurements by a CRS on an alpine glacier in Switzerland (Glacier de la Plaine Morte) over two winter seasons (2016/17 and 2017/18), which differed markedly in the amount and timing of snow accumulation. By combining SWE with snow depth measurements, we calculate the daily mean density of the snowpack. Compared to manual field observations from snow pits, the autonomous measurements overestimate SWE by +2 % ± 13 %. Snow depth and the bulk snow density deviate from the manual measurements by ±6 % and ±9 %, respectively. The CRS measured with high reliability over two winter seasons and is thus considered a promising method to observe SWE at remote alpine sites. We use the daily observations to classify winter season days into those dominated by accumulation (solid precipitation, snow drift), ablation (snow drift, snowmelt) or snow densification. For each of these process-dominated days the prevailing meteorological conditions are distinct. The continuous SWE measurements were also used to define a scaling factor for precipitation amounts from nearby meteorological stations. With this analysis, we show that a best-possible constant scaling factor results in cumulative precipitation amounts that differ by a mean absolute error of less than 80 mm w.e. from snow accumulation at this site.

Beschreibung: The meltwater scavenging coefficient (MSC) of black carbon (BC) is a crucial parameter in snow and sea ice models, as it determines the BC enrichment in the surface layer of melting snow over sea ice and therefore modulates the BC–snow–albedo feedbacks. We present a new method for MSC estimation by sampling the melt–refreeze ice layer that is produced from refreezing of the meltwater within snowpack and its overlying snow and measuring their physical characteristics in Elson Lagoon northeast of Utqiaġvik (formerly Barrow), Alaska, during the melting season. The bias of estimated MSC ranges from −5.4 % to 7.3 %, which is not exactly dependent on the degree of ablation. The average MSC value calculated with this proposed method is slightly lower than that derived from the repeating sampling (RS) method in Elson Lagoon while still being within its best estimate range. Further estimation demonstrates that the MSC in the Canada Basin (23.6 %±2.1 %) is close to that in Greenland (23.0 %±12.5 %) and larger than that in the Chukchi Sea (17.9 %±5.0 %) in the northwest of Utqiaġvik. Elson Lagoon has the lowest MSC (14.5 %±2.6 %) in the study areas. The method suggested in this study provides a possible approach for large-scale measurements of MSC over the sea ice area in the Arctic. Of course, this method depends on the presence of a melt–refreeze ice layer in the observation area.

Beschreibung: We report on results of a systematic inter-comparison of 10 global sea-ice concentration (SIC) data products at 12.5 to 50.0 km grid resolution for both the Arctic and the Antarctic. The products are compared with each other with respect to differences in SIC, sea-ice area (SIA), and sea-ice extent (SIE), and they are compared against a global wintertime near-100 % reference SIC data set for closed pack ice conditions and against global year-round ship-based visual observations of the sea-ice cover. We can group the products based on the concept of their SIC retrieval algorithms. Group I consists of data sets using the self-optimizing EUMETSAT OSI SAF and ESA CCI algorithms. Group II includes data using the Comiso bootstrap algorithm and the NOAA NSIDC sea-ice concentration climate data record (CDR). The standard NASA Team and the ARTIST Sea Ice (ASI) algorithms are put into group III, and NASA Team 2 is the only element of group IV. The three CDRs of group I (SICCI-25km, SICCI-50km, and OSI-450) are biased low compared to a 100 % reference SIC data set with biases of −0.4 % to −1.0 % (Arctic) and −0.3 % to −1.1 % (Antarctic). Products of group II appear to be mostly biased high in the Arctic by between +1.0 % and +3.5 %, while their biases in the Antarctic range from −0.2 % to +0.9 %. Group III product biases are different for the Arctic, +0.9 % (NASA Team) and −3.7 % (ASI), but similar for the Antarctic, −5.4 % and −5.6 %, respectively. The standard deviation is smaller in the Arctic for the quoted group I products (1.9 % to 2.9 %) and Antarctic (2.5 % to 3.1 %) than for group II and III products: 3.6 % to 5.0 % for the Arctic and 4.0 % to 6.5 % for the Antarctic. We refer to the paper to understand why we could not give values for group IV here. We discuss the impact of truncating the SIC distribution, as naturally retrieved by the algorithms around the 100 % sea-ice concentration end. We show that evaluation studies of such truncated SIC products can result in misleading statistics and favour data sets that systematically overestimate SIC. We describe a method to reconstruct the non-truncated distribution of SIC before the evaluation is performed. On the basis of this evaluation, we open a discussion about the overestimation of SIC in data products, with far-reaching consequences for surface heat flux estimations in winter. We also document inconsistencies in the behaviour of the weather filters used in products of group II, and we suggest advancing studies about the influence of these weather filters on SIA and SIE time series and their trends.

Beschreibung: A survey of airborne radioactive isotopes (137Cs, 238Pu, 239+240Pu, 241Am, and 210Pb) and trace metals (Pb, Cu, Zn, Cd, Fe, Al) in tundra soils and cryoconite hole material sampled from several locations in the Kaffiøyra region of Spitsbergen revealed significant variability in spatial concentration. Lithogenic radionuclides (230Th, 232Th, 234U, 238U) show less variability than the airborne radionuclides because their activity concentrations are controlled by mixing of local material derived from different types of bedrock. Activity ratios of the artificial radionuclides in most cryoconite samples differ from global fallout signatures. The contribution of radionuclides from additional and more specific sources might be enhanced by non-continuous exposure of cryoconite to atmospheric deposition. We assumed that the main source of Pu, which was detected only in cryoconite samples, is derived from nuclear tests and non-exploded weapons-grade material. Approximately one-third of the total observed Pu activity concentration is 238Pu, most likely originating from the SNAP9A satellite re-entry, which was powered by a 238Pu thermoelectric generator. In samples from Waldemarbreen the influence of glacial local morphology on the capability of cryoconite for trapping and accumulating airborne radionuclides is apparent. Local glacial morphology plays an important role in determining the accumulation of airborne pollutants. Trace metal concentrations in soils were typical or slightly higher than concentrations characteristic for the natural background; the 206Pb∕207Pb signature was also close to the natural ratio of the parent rocks. Conversely, trace metal concentrations in cryoconite samples (Pb and Cd) were higher than in soil samples and exceeded natural values. Cryoconite is an effective monitor of the spread of artificial radionuclides and heavy metals in their surrounding environment.

Beschreibung: Thinning rates of ice shelves vary widely around Antarctica, and basal melting is a major component of ice shelf mass loss. In this study, we present records of basal melting at a unique spatial and temporal resolution for East Antarctica, derived from autonomous phase-sensitive radars. These records show spatial and temporal variations of basal melting in 2017 and 2018 at Nivlisen, an ice shelf in central Dronning Maud Land. The annually averaged basal melt rates are in general moderate (∼0.8 m yr−1). Radar profiling of the ice shelf shows variable ice thickness from smooth beds to basal crevasses and channels. The highest basal melt rates (3.9 m yr−1) were observed close to a grounded feature near the ice shelf front. Daily time-varying measurements reveal a seasonal melt signal 4 km from the ice shelf front, at an ice draft of 130 m, where the highest daily basal melt rates occurred in summer (up to 5.6 m yr−1). In comparison with wind, air temperatures, and sea ice cover from reanalysis and satellite data, the seasonality in basal melt rates indicates that summer-warmed ocean surface water was pushed by wind beneath the ice shelf front. We observed a different melt regime 35 km into the ice shelf cavity, at an ice draft of 280 m, with considerably lower basal melt rates (annual average of 0.4 m yr−1) and no seasonality. We conclude that warm deep-ocean water at present has a limited effect on the basal melting of Nivlisen. On the other hand, a warming in surface waters, as a result of diminishing sea ice cover, has the potential to increase basal melting near the ice shelf front. Continuous in situ monitoring of Antarctic ice shelves is needed to understand the complex mechanisms involved in ice shelf–ocean interactions.

Beschreibung: Glaciers in tropical regions are very sensitive to climatic variations and thus strongly affected by climate change. The majority of the tropical glaciers worldwide are located in the Peruvian Andes, which have shown significant ice loss in the last century. Here, we present the first multi-temporal, region-wide survey of geodetic mass balances and glacier area fluctuations throughout Peru covering the period 2000–2016. Glacier extents are derived from Landsat imagery by performing automatic glacier delineation based on a combination of the NDSI and band ratio method and final manual inspection and correction. The mapping of debris-covered glacier extents is supported by synthetic aperture radar (SAR) coherence information. A total glacier area loss of -548.5±65.7 km2 (−29 %, −34.3 km2 a−1) is obtained for the study period. Using interferometric satellite SAR acquisitions, bi-temporal geodetic mass balances are derived. An average specific mass balance of -296±41 kg m−2 a−1 is found throughout Peru for the period 2000–2016. However, there are strong regional and temporal differences in the mass budgets ranging from 45±97 to -752±452 kg m−2 a−1. The ice loss increased towards the end of the observation period. Between 2013 and 2016, a retreat of the glacierized area of -203.8±65.7 km2 (−16 %, −101.9 km2 a−1) is mapped and the average mass budget amounts to -660±178 kg m−2 a−1. The glacier changes revealed can be attributed to changes in the climatic settings in the study region, derived from ERA-Interim reanalysis data and the Oceanic Nino Index. The intense El Niño activities in 2015/16 are most likely the trigger for the increased change rates in the time interval 2013–2016. Our observations provide fundamental information on the current dramatic glacier changes for local authorities and for the calibration and validation of glacier change projections.

Beschreibung: The effect of the North Atlantic Ocean on the Greenland Ice Sheet through submarine melting of Greenland's tidewater glacier calving fronts is thought to be a key driver of widespread glacier retreat, dynamic mass loss and sea level contribution from the ice sheet. Despite its critical importance, problems of process complexity and scale hinder efforts to represent the influence of submarine melting in ice-sheet-scale models. Here we propose parameterizing tidewater glacier terminus position as a simple linear function of submarine melting, with submarine melting in turn estimated as a function of subglacial discharge and ocean temperature. The relationship is tested, calibrated and validated using datasets of terminus position, subglacial discharge and ocean temperature covering the full ice sheet and surrounding ocean from the period 1960–2018. We demonstrate a statistically significant link between multi-decadal tidewater glacier terminus position change and submarine melting and show that the proposed parameterization has predictive power when considering a population of glaciers. An illustrative 21st century projection is considered, suggesting that tidewater glaciers in Greenland will undergo little further retreat in a low-emission RCP2.6 scenario. In contrast, a high-emission RCP8.5 scenario results in a median retreat of 4.2 km, with a quarter of tidewater glaciers experiencing retreat exceeding 10 km. Our study provides a long-term and ice-sheet-wide assessment of the sensitivity of tidewater glaciers to submarine melting and proposes a practical and empirically validated means of incorporating ocean forcing into models of the Greenland ice sheet.

Beschreibung: Sea ice volume export through the Fram Strait plays an important role in the Arctic freshwater and energy redistribution. The combined model and satellite sea ice thickness (CMST) data set assimilates CryoSat-2 and soil moisture and ocean salinity (SMOS) thickness products together with satellite sea ice concentration. The CMST data set closes the gap of stand-alone satellite-derived sea ice thickness in summer and therefore allows us to estimate sea ice volume export during the melt season. In this study, we first validate the CMST data set using field observations, and then we estimate the continuous seasonal and interannual variations in Arctic sea ice volume flux through the Fram Strait from September 2010 to December 2016. The results show that seasonal and interannual sea ice volume export vary from about -240(±40) to -970(±60) km3 and -1970(±290) to -2490(±280) km3, respectively. The sea ice volume export reaches its maximum in spring and about one-third of the yearly total volume export occurs in the melt season. The minimum monthly sea ice export is −11 km3 in August 2015, and the maximum (−442 km3) appears in March 2011. The seasonal relative frequencies of sea ice thickness and drift suggest that the Fram Strait outlet in summer is dominated by sea ice that is thicker than 2 m with relatively slow seasonal mean drift of about 3 km d−1.

Beschreibung: Accurate and timely information on avalanche occurrence is key for avalanche warning, crisis management and avalanche documentation. Today such information is mainly available at isolated locations provided by observers in the field. The achieved reliability, considering accuracy, completeness and reliability of the reported avalanche events, is limited. In this study we present the spatially continuous mapping of a large avalanche period in January 2018 covering the majority of the Swiss Alps (12 500 km2). We tested different satellite sensors available for rapid mapping during the first avalanche period. Based on these experiences, we tasked SPOT6 and SPOT7 for data acquisition to cover the second, much larger avalanche period. We manually mapped the outlines of 18 737 individual avalanche events, applying image enhancement techniques to analyze regions in the shade as well as in brightly illuminated ones. The resulting dataset of mapped avalanche outlines, having unique completeness and reliability, is evaluated to produce maps of avalanche occurrence and avalanche size. We validated the mapping of the avalanche outlines using photographs acquired from helicopters just after the avalanche period. This study demonstrates the applicability of optical, very high spatial resolution satellite data to map an exceptional avalanche period with very high completeness, accuracy and reliability over a large region. The generated avalanche data are of great value in validating avalanche bulletins, in completing existing avalanche databases and for research applications by enabling meaningful statistics on important avalanche parameters.

Beschreibung: The modeling of ice sheets in Earth system models (ESMs) is an active area of research with applications to future sea level rise projections and paleoclimate studies. A major challenge for surface mass balance (SMB) modeling with ESMs arises from their coarse resolution. This paper evaluates the elevation class (EC) method as an SMB downscaling alternative to the dynamical downscaling of regional climate models. To this end, we compare EC-simulated elevation-dependent surface energy and mass balance gradients from the Community Earth System Model 1.0 (CESM1.0) with those from the regional climate model RACMO2.3. The EC implementation in CESM1.0 combines prognostic snow albedo, a multilayer snow model, and elevation corrections for two atmospheric forcing variables: temperature and humidity. Despite making no corrections for incoming radiation and precipitation, we find that the EC method in CESM1.0 yields similar SMB gradients to RACMO2.3, in part due to compensating biases in snowfall, surface melt, and refreezing gradients. We discuss the sensitivity of the results to the lapse rate used for the temperature correction. We also evaluate the impact of the EC method on the climate simulated by the ESM and find minor cooling over the Greenland ice sheet and Barents and Greenland seas, which compensates for a warm bias in the ESM due to topographic smoothing. Based on our diagnostic procedure to evaluate the EC method, we make several recommendations for future implementations.

Beschreibung: The mass balance of the Greenland Ice Sheet (GrIS) in a warming climate is of critical interest in the context of future sea level rise. Increased melting in the GrIS percolation zone due to atmospheric warming over the past several decades has led to increased mass loss at lower elevations. Previous studies have hypothesized that this warming is accompanied by a precipitation increase, as would be expected from the Clausius–Clapeyron relationship, compensating for some of the melt-induced mass loss throughout the western GrIS. This study tests that hypothesis by calculating snow accumulation rates and trends across the western GrIS percolation zone, providing new accumulation rate estimates in regions with sparse in situ data or data that do not span the recent accelerating surface melt. We present accumulation records from sixteen 22–32 m long firn cores and 4436 km of ground-penetrating radar, covering the past 20–60 years of accumulation, collected across the western GrIS percolation zone as part of the Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) project. Trends from both radar and firn cores, as well as commonly used regional climate models, show decreasing accumulation rates of 2.4±1.5 % a−1 over the 1996–2016 period, which we attribute to shifting storm tracks related to stronger atmospheric summer blocking over Greenland. Changes in atmospheric circulation over the past 20 years, specifically anomalously strong summertime blocking, have reduced GrIS surface mass balance through both an increase in surface melting and a decrease in accumulation rates.

Beschreibung: Smith, Pope, and Kohler glaciers and the corresponding Crosson and Dotson ice shelves have undergone speedup, thinning, and rapid grounding-line retreat in recent years, leaving them in a state likely conducive to future retreat. We conducted a suite of numerical model simulations of these glaciers and compared the results to observations to determine the processes controlling their recent evolution. The model simulations indicate that the state of these glaciers in the 1990s was not inherently unstable, i.e., that small perturbations to the grounding line would not necessarily have caused the large retreat that has been observed. Instead, sustained, elevated melt at the grounding line was needed to cause the observed retreat. Weakening of the margins of Crosson Ice Shelf may have hastened the onset of grounding-line retreat but is unlikely to have initiated these rapid changes without an accompanying increase in melt. In the simulations that most closely match the observed thinning, speedup, and retreat, modeled grounding-line retreat and ice loss continue unabated throughout the 21st century, and subsequent retreat along Smith Glacier's trough appears likely. Given the rapid progression of grounding-line retreat in the model simulations, thinning associated with the retreat of Smith Glacier may reach the ice divide and undermine a portion of the Thwaites catchment as quickly as changes initiated at the Thwaites terminus.

Beschreibung: Differential interferometric synthetic aperture radar (DInSAR) is an essential tool for detecting ice-sheet motion near Antarctica's oceanic margin. These space-borne measurements have been used extensively in the past to map the location and retreat of ice-shelf grounding lines as an indicator for the onset of marine ice-sheet instability and to calculate the mass balance of ice sheets and individual catchments. The main difficulty in interpreting DInSAR is that images originate from a combination of several SAR images and do not indicate instantaneous ice deflection at the times of satellite data acquisitions. Here, we combine the sub-centimetre accuracy and spatial benefits of DInSAR with the temporal benefits of tide models to infer the spatio-temporal dynamics of ice–ocean interaction during the times of satellite overpasses. We demonstrate the potential of this synergy with TerraSAR-X data from the almost-stagnant southern McMurdo Ice Shelf (SMIS). We then validate our algorithm with GPS data from the fast-flowing Darwin Glacier, draining the Antarctic Plateau through the Transantarctic Mountains into the Ross Sea. We are able to reconstruct DInSAR-derived vertical displacements to 7 mm mean absolute residual error and generally improve traditional tide-model output by up to 39 % from 10.8 to 6.7 cm RMSE against GPS data from areas where ice is in local hydrostatic equilibrium with the ocean and by up to 74 % from 21.4 to 5.6 cm RMSE against GPS data in feature-rich coastal areas where tide models have not been applicable before. Numerical modelling then reveals Young's modulus of E=1.0±0.56 GPa and an ice viscosity of ν=10±3.65 TPa s when finite-element simulations of tidal flexure are matched to 16 d of tiltmeter data, supporting the hypothesis that strain-dependent anisotropy may significantly decrease effective viscosity compared to isotropic polycrystalline ice on large spatial scales. Applications of our method include the following: refining coarsely gridded tide models to resolve small-scale features at the spatial resolution and vertical accuracy of SAR imagery, separating elastic and viscoelastic contributions in the satellite-derived flexure measurement, and gaining information about large-scale ice heterogeneity in Antarctic ice-shelf grounding zones, the missing key to improving current ice-sheet flow models. The reconstruction of the individual components forming DInSAR images has the potential to become a standard remote-sensing method in polar tide modelling. Unlocking the algorithm's full potential to answer multi-disciplinary research questions is desired and demands collaboration within the scientific community.

Beschreibung: Pingos are common features in permafrost regions that form by subsurface massive-ice aggradation and create hill-like landforms. Pingos on Spitsbergen have been previously studied to explore their structure, formation timing and connection to springs as well as their role in postglacial landform evolution. However, detailed hydrochemical and stable-isotope studies of massive-ice samples recovered by drilling have yet to be used to study the origin and freezing conditions in pingos. Our core record of 20.7 m thick massive pingo ice from Grøndalen is differentiated into four units: two characterised by decreasing δ18O and δD and increasing d (units I and III) and two others showing the opposite trend (units II and IV). These delineate changes between episodes of closed-system freezing with only slight recharge inversions of the water reservoir and more complicated episodes of groundwater freezing under semi-closed conditions when the reservoir was recharged. The water source for pingo formation shows similarity to spring water data from the valley with prevalent Na+ and HCO3- ions. The sub-permafrost groundwater originates from subglacial meltwater that most probably followed the fault structures of Grøndalen and Bøhmdalen. The presence of permafrost below the pingo ice body suggests that the talik is frozen, and the water supply and pingo growth are terminated. The maximum thaw depth of the active layer reaching the top of the massive ice leads to its successive melt with crater development and makes the pingo extremely sensitive to further warming.

Beschreibung: Frontal ablation contributes significantly to the mass balance of tidewater glaciers in Svalbard and can be recovered with high temporal resolution using continuous seismic records. Determination of the relative contribution of dynamic ice loss through calving to frontal ablation requires precise estimates of calving volumes at the same temporal resolution. We combine seismic and hydroacoustic observations close to the calving front of Kronebreen, a marine-terminating glacier in Svalbard, with repeat lidar scanning of the glacier front. Simultaneous time-lapse photography is used to assign volumes measured from lidar scans to seismically detected calving events. Empirical models derived from signal properties such as integrated amplitude are able to replicate volumes of individual calving events and cumulative subaerial ice loss over different lidar scan intervals from seismic and hydroacoustic data alone. This enables quantification of the contribution of calving to frontal ablation, which we estimate for Kronebreen to be about 18 %–30 %, slightly below the subaerially exposed area of the glacier front. We further develop a model calibrated for the permanent seismic Kings Bay station (KBS) at about 15 km distance from the glacier front, where 15 %–60 % of calving events can be detected under variable noise conditions due to reduced signal amplitudes at distance. Between 2007 and 2017, we find a 5 %–30 % contribution of calving ice blocks to frontal ablation, which emphasizes the importance of underwater melting (roughly 4–9 m d−1). This study shows the feasibility to seismically monitor not only frontal ablation rates but also the dynamic ice loss contribution continuously and at high temporal resolution.

Beschreibung: The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

Beschreibung: Single-layer vegetation schemes in modern land surface models have been found to overestimate diurnal cycles in longwave radiation beneath forest canopies. This study introduces an empirical correction, based on forest-stand-scale simulations, which reduces diurnal cycles of sub-canopy longwave radiation. The correction is subsequently implemented in land-only simulations of the Community Land Model version 4.5 (CLM4.5) in order to assess the impact on snow cover. Nighttime underestimations of sub-canopy longwave radiation outweigh daytime overestimations, which leads to underestimated averages over the snow cover season. As a result, snow temperatures are underestimated and snowmelt is delayed in CLM4.5 across evergreen boreal forests. Comparison with global observations confirms this delay and its reduction by correction of sub-canopy longwave radiation. Increasing insolation and day length change the impact of overestimated diurnal cycles on daily average sub-canopy longwave radiation throughout the snowmelt season. Consequently, delay of snowmelt in land-only simulations is more substantial where snowmelt occurs early.

Beschreibung: Cellular-automata-based modelling for simulating snow bedforms and snow deposition is introduced in this study. The well-known ReSCAL model, previously used for sand bedforms, is adapted for this purpose by implementing a simple sintering mechanism. The effect of sintering is first explored for solitary barchan dunes of different sizes and flow conditions. Three types of behaviour are observed: small barchans continue their motion without any perceptible difference while large barchans sinter immediately. Barchans of intermediate size split, leaving behind a sintered core and a smaller barchan is formed. It is found that sintering introduces an upper limit to the size of bedforms that can remain mobile. The concept of “maximum streamwise length” (MSL) is introduced and MSL is identified for different wind speeds using the solitary dune scenario. Simulations of the full evolution from an initially flat snow layer to a complex dune field are performed next. It is found that the largest bedforms lie below the MSL threshold. Additionally, it is found that shallow snow layers are most susceptible to mechanical destabilization by the wind.

Beschreibung: Spatial variability in snowpack properties negatively impacts our capacity to make direct measurements of snow water equivalent (SWE) using satellites. A comprehensive data set of snow microstructure (94 profiles at 36 sites) and snow layer thickness (9000 vertical profiles across nine trenches) collected over two winters at Trail Valley Creek, NWT, Canada, was applied in synthetic radiative transfer experiments. This allowed for robust assessment of the impact of estimation accuracy of unknown snow microstructural characteristics on the viability of SWE retrievals. Depth hoar layer thickness varied over the shortest horizontal distances, controlled by subnivean vegetation and topography, while variability in total snowpack thickness approximated that of wind slab layers. Mean horizontal correlation lengths of layer thickness were less than a metre for all layers. Depth hoar was consistently ∼30 % of total depth, and with increasing total depth the proportion of wind slab increased at the expense of the decreasing surface snow layer. Distinct differences were evident between distributions of layer properties; a single median value represented density and specific surface area (SSA) of each layer well. Spatial variability in microstructure of depth hoar layers dominated SWE retrieval errors. A depth hoar SSA estimate of around 7 % under the median value was needed to accurately retrieve SWE. In shallow snowpacks

Beschreibung: Besides its negative trend, the interannual and the interdecadal changes in the Arctic sea ice have also been pronounced in recent decades. The three leading modes in the sea level pressure (SLP) variability in the Arctic (70–90∘ N) – the Arctic Oscillation (AO), the Arctic Dipole (AD), and the third mode (A3) – are analyzed to understand the linkage between sea ice variability and large-scale atmospheric circulation in boreal summer (June–August). This study also compares the decadal changes of the modes between the early (1982–1997) and the recent (1998–2017) periods and their influences on the Arctic sea ice extent (SIE). Only the AD mode shows a significant correlation increase with SIE in summer (JJA) from −0.05 in the early period to 0.57 in the recent period. The AO and the A3 modes show a less significant relationship with SIE for the two periods. The AD is characterized by a dipole pattern of SLP, which modulates the strength of meridional surface winds and the Transpolar Drift Stream (TDS). The major circulation change in the late 1990s is that the direction of the wind has been changed more meridionally over the exit region of the Fram Strait, which causes sea ice drift and discharge through that region. In addition, the response of surface albedo and the net surface heat flux becomes larger and much clearer, suggesting a positive sea-ice–albedo feedback in the sea ice variability associated with the AD. The analysis also reveals that the zonal shift of the centers of SLP anomalies and associated circulation change affects a significant reduction in sea ice concentration over the Pacific sector of the Arctic Ocean. This study further suggests that the Pacific Decadal Oscillation (PDO) phase change could influence the spatial pattern change in the AD.

Beschreibung: Owing to increase in snowfall, the Antarctic Ice Sheet surface mass balance is expected to increase by the end of the current century. Assuming no associated response of ice dynamics, this will be a negative contribution to sea-level rise. However, the assessment of these changes using dynamical downscaling of coupled climate model projections still bears considerable uncertainties due to poorly represented high-southern-latitude atmospheric circulation and sea surface conditions (SSCs), that is sea surface temperature and sea ice concentration. This study evaluates the Antarctic surface climate simulated using a global high-resolution atmospheric model and assesses the effects on the simulated Antarctic surface climate of two different SSC data sets obtained from two coupled climate model projections. The two coupled models from which SSCs are taken, MIROC-ESM and NorESM1-M, simulate future Antarctic sea ice trends at the opposite ends of the CMIP5 RCP8.5 projection range. The atmospheric model ARPEGE is used with a stretched grid configuration in order to achieve an average horizontal resolution of 35 km over Antarctica. Over the 1981–2010 period, ARPEGE is driven by the SSCs from MIROC-ESM, NorESM1-M and CMIP5 historical runs and by observed SSCs. These three simulations are evaluated against the ERA-Interim reanalyses for atmospheric general circulation as well as the MAR regional climate model and in situ observations for surface climate. For the late 21st century, SSCs from the same coupled climate models forced by the RCP8.5 emission scenario are used both directly and bias-corrected with an anomaly method which consists in adding the future climate anomaly from coupled model projections to the observed SSCs with taking into account the quantile distribution of these anomalies. We evaluate the effects of driving the atmospheric model by the bias-corrected instead of the original SSCs. For the simulation using SSCs from NorESM1-M, no significantly different climate change signals over Antarctica as a whole are found when bias-corrected SSCs are used. For the simulation driven by MIROC-ESM SSCs, a significant additional increase in precipitation and in winter temperatures for the Antarctic Ice Sheet is obtained when using bias-corrected SSCs. For the range of Antarctic warming found (+3 to +4 K), we confirm that snowfall increase will largely outweigh increases in melt and rainfall. Using the end members of sea ice trends from the CMIP5 RCP8.5 projections, the difference in warming obtained (∼ 1 K) is much smaller than the spread of the CMIP5 Antarctic warming projections. This confirms that the errors in representing the Southern Hemisphere atmospheric circulation in climate models are also determinant for the diversity of their projected late 21st century Antarctic climate change.

Beschreibung: Following the general warming trend in Greenland, an increase in calving rates, retreat and ice flow has been observed at ocean-terminating outlet glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice flow is needed, in particular on the rapidly changing tongues of ocean-terminating outlet glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset-tracking method based on synthetic aperture radar (SAR) intensity data with a collection of three independent reference measurements of glacier flow. These reference datasets are comprised of measurements from differential GPS, a terrestrial radar interferometer (TRI) and repeated unmanned aerial vehicle (UAV) surveys. Our approach to SAR velocity processing aims at achieving a relatively fine grid spacing and a high temporal resolution in order to best resolve the steep velocity gradients in the terminus area and aims to exploit the 12 d repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean-terminating outlet glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 11.5 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity fields with independently generated velocity maps from TRI and UAV surveys showed good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 m d−1. In comparison with existing operational velocity products, our SAR-derived velocities show an improved spatial velocity pattern near the margins and calving front. There 8 % to 30 % higher surface ice velocities are produced, which has implications on ice fluxes and on mass budget estimates of similarly sized outlet glaciers. Further, we show that offset tracking from SAR intensity data at relatively low spatio-temporal sampling intervals is a valid method to derive glacier flow fields for fast-flowing glacier termini of outlet glaciers and, given the repeat period of 12 d of the Sentinel-1A sensor (6 d with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.

Beschreibung: We present an updated, spatially resolved estimate of 2003–2008 glacier surface elevation changes for the entire region of High Mountain Asia (HMA) from ICESat laser altimetry data. The results reveal a diverse pattern that is caused by spatially greatly varying glacier sensitivity, in particular to precipitation availability and changes. We introduce a spatially resolved zonation where ICESat samples are grouped into units of similar glacier behaviour, glacier type and topographic settings. In several regions, our new zonation reveals local differences and anomalies that have not been described previously. Glaciers in the Eastern Pamirs, Kunlun Shan and central TP were thickening by 0.1–0.7 m a−1, and the thickening anomaly has a crisp boundary in the Eastern Pamirs that continues just north of the central Karakoram. Glaciers in the south and east of the TP were thinning, with increasing rates towards southeast. We attribute the glacier thickening signal to a stepwise increase in precipitation around ∼1997–2000 on the Tibetan Plateau (TP). The precipitation change is reflected by growth of endorheic lakes in particular in the northern and eastern TP. We estimate lake volume changes through a combination of repeat lake extents from Landsat data and shoreline elevations from ICESat and the Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) for over 1300 lakes. The rise in water volume contained in the lakes corresponds to 4–25 mm a−1, when distributed over entire catchments, for the areas where we see glacier thickening. The precipitation increase is also visible in sparse in situ measurements and MERRA-2 climate reanalysis data but less visible in ERA-Interim reanalysis data. Taking into account evaporation loss, the difference between average annual precipitation during the 1990s and 2000s suggested by these datasets is 34–100 mm a−1, depending on region, which can fully explain both lake growth and glacier thickening (Kunlun Shan) or glacier geometry changes such as thinning tongues while upper glacier areas were thickening or stable (eastern TP). The precipitation increase reflected in these glacier changes possibly extended to the northern slopes of the Tarim Basin, where glaciers were nearly in balance in 2003–2008. Along the entire Himalaya, glaciers on the first orographic ridge, which are exposed to abundant precipitation, were thinning less than glaciers in the dryer climate of the inner ranges. Thinning rates in the Tien Shan vary spatially but are rather stronger than in other parts of HMA.

Beschreibung: We report cosmogenic-nuclide measurements from two isolated groups of nunataks in West Antarctica: the Pirrit Hills, located midway between the grounding line and the divide in the Weddell Sea sector, and the Whitmore Mountains, located along the Ross–Weddell divide. At the Pirrit Hills, evidence of glacial-stage ice cover extends ∼320 m above the present ice surface. Subsequent thinning mostly occurred after ∼14 kyr BP, and modern ice levels were established some time after ∼4 kyr BP. We infer that, like at other flank sites, these changes were primarily controlled by the position of the grounding line downstream. At the Whitmore Mountains, cosmogenic 14C concentrations in bedrock surfaces demonstrate that ice there was no more than ∼190 m thicker than present during the past ∼30 kyr. Combined with other constraints from West Antarctica, the 14C data imply that the divide was thicker than present for a period of less than ∼8 kyr within the past ∼15 kyr. These results are consistent with the hypothesis that the divide initially thickened due to the deglacial rise in snowfall and subsequently thinned in response to retreat of the ice-sheet margin. We use these data to evaluate several recently published ice-sheet models at the Pirrit Hills and Whitmore Mountains. Most of the models we consider do not match the observed timing and/or magnitude of thickness change at these sites. However, one model performs relatively well at both sites, which may, in part, be due to the fact that it was calibrated with geological observations of ice-thickness change from other sites in Antarctica.

Beschreibung: The early 21st century retreat of Jakobshavn Isbræ into its overdeepened bedrock trough was accompanied by acceleration to unprecedented ice stream speeds. Such dramatic changes suggested the possibility of substantial mass loss over the rest of this century. Here we use a three-dimensional ice sheet model with parameterizations to represent the effects of ice mélange buttressing, crevasse-depth-based calving and submarine melting to adequately reproduce its recent evolution. We are the first study on Jakobshavn Isbræ that solves for three-dimensional ice flow coupled with representations of hydro-fracturing-induced calving and mélange buttressing. Additionally, the model can accurately replicate interannual variations in grounding line and terminus position, including seasonal fluctuations that emerged after arriving at the overdeepened basin and the disappearance of its floating ice shelf. Our simulated ice viscosity variability due to shear margin evolution is particularly important in reproducing the large observed interannual changes in terminus velocity. We use this model to project Jakobshavn's evolution over this century, forced by ocean temperatures from seven Earth system models and surface runoff derived from RACMO, all under the IPCC RCP4.5 climate scenario. In our simulations, Jakobshavn's grounding line continues to retreat ∼18.5 km by the end of this century, leading to a total mass loss of ∼2068 Gt (5.7 mm sea level rise equivalent). Despite the relative success of the model in simulating the recent behavior of the glacier, the model does not simulate winter calving events that have become relatively more important.