Data from three continuously operating GPS sites located in the interior of the Greenland ice sheet are analyzed. Traditionally these kinds of GPS installations (where the GPS antenna is placed on a pole deployed into the firn) are used to estimate the local horizontal speed and direction of the ice sheet. However, these data are also sensitive to the vertical displacement of the pole as it moves through the firn layer. A new method developed to measure snow depth variations with reflected GPS signals is applied to these GPS data from Greenland. This method provides a constraint on the vertical distance between the GPS antenna and the surface snow layer. The vertical positions and snow surface heights are then used to assess output from surface accumulation and firn densification models, showing agreement better than 10% at the sites with the longest records. Comparisons between the GPS reflection method and in situ snow sensors at the Dye-2 site show good agreement, capturing the dramatic changes observed in Greenland during the 2012 summer melt season. The geocentric elevation of the snow surface can be inferred by subtracting the snow surface height estimates from the vertical position measurements. It should be possible to use those surface elevation estimates to help validate elevation results obtained from satellite altimetry.

In an effort to improve the reliability of Arctic sea-ice predictions, an ensemble-based Arctic Ice Ocean Prediction System (ArcIOPS) has been developed to meet operational demands. The system is based on a regional Arctic configuration of the Massachusetts Institute of Technology general circulation model. A localized error subspace transform ensemble Kalman filter is used to assimilate the weekly merged CryoSat-2 and Soil Moisture and Ocean Salinity sea-ice thickness data together with the daily Advanced Microwave Scanning Radiometer 2 (AMSR2) sea-ice concentration data. The weather forecasts from the Global Forecast System of the National Centers for Environmental Prediction drive the sea ice–ocean coupled model. The ensemble mean sea-ice forecasts were used to facilitate the Chinese National Arctic Research Expedition in summer 2017. The forecasted sea-ice concentration is evaluated against AMSR2 and Special Sensor Microwave Imager/Sounder sea-ice concentration data. The forecasted sea-ice thickness is compared to the in-situ observations and the Pan-Arctic Ice-Ocean Modeling and Assimilation System. These comparisons show the promising potential of ArcIOPS for operational Arctic sea-ice forecasts. Nevertheless, the forecast bias in the Beaufort Sea calls for a delicate parameter calibration and a better design of the assimilation system.

We present an overview and the first systematic results on the backscatter properties of snow samples with different grain properties. During a 2 year study we investigated the effect of the apparent characteristics of the samples on the intensity enhancement in the direction of exact backscatter (retroreflection), using a specific instrument designed for backscatter measurements. We observed that a sharp peak in intensity (the hot spot) occurs for most types of snow, which is not observable with traditional goniometers. The key factors in the peak properties are the temperature (related to the changes in grain structure) and grain shape and size. These results form the basis of a larger backscatter data archive which is being applied in the systematic study and exploitation of ‘hot spots’ in remote sensing of natural targets, as well as in the development of airborne laser intensity measurement.

We investigate how temperature gradient and initial density influence depth-hoar growth in snow and seek to better define the range of conditions under which cohesive, or hard, depth hoar forms. Samples of 400 kg m−3 sieved snow were exposed to temperature gradients of 20–80°C m−1, and samples of four different densities were exposed to a 40°C m temperature gradient. Following exposure to temperature gradients, penetrometer tests were made on samples to determine the presence of solid and/or hard depth hoar. Grain bond orientation was analyzed in section planes by two-dimensional stereological techniques where hard depth hoar developed. Results indicate that hard cohesive depth hoar forms from rounded-grain snow having a density of 400 kg m−3 or greater, following exposure to a temperature gradient of 20°C m−1 or greater. Hard depth hoar appears to consist of solid-type depth-hoar grains connected by necks, with vertically preferred directions of grain elongation and organization of grain-to-grain chains. This work corroborates Atikaya’s (1974) results, but extends his observation of formation of hard depth hoar to weaker temperature gradients for high-density snow. Our results also indicate that hard depth hoar is composed of faceted solid-type (anhedral) grains.

Negribreen, a tidewater glacier located in central eastern Svalbard, began actively surging after it experienced an initial collapse in summer 2016. The surge resulted in horizontal surface velocities of more than 25 m d−1, making it one of the fastest-flowing glaciers in the archipelago. The last surge of Negribreen likely occurred in the 1930s, but due to a long quiescent phase, investigations of this glacier have been limited. As Negribreen is part of the Negribreen Glacier System, one of the largest glacier systems in Svalbard, investigating its current surge event provides important information on surge behaviour among tidewater glaciers within the region. Here, we demonstrate the surge development and discuss triggering mechanisms using time series of digital elevation models (1969–2018), surface velocities (1995–2018), crevasse patterns and glacier extents from various data sources. We find that the active surge results from a four-stage process. Stage 1 (quiescent phase) involves a long-term, gradual geometry change due to high subglacial friction towards the terminus. These changes allow the onset of Stage 2, an accelerating frontal destabilization, which ultimately results in the collapse (Stage 3) and active surge (Stage 4).

Two non-invasive optical Schlieren methods have been adapted to visualize brine channel development and convective processes in experimentally grown sea ice obtained when a NaCl aqueous solution is cooled from above in a quasi-two-dimensional Hele–Shaw cell. The two different visualization methods, i.e. traditional and synthetic Schlieren optical imaging, produce high spatial resolution images of transport processes during ice growth, without any external perturbation. These images allow observations of the flow dynamics simultaneously within the ice layer, around the ice/water interface, and in the liquid water layer, revealing connections between the processes occurring within the two phases. Results from these methods show that desalination of the growing ice layer occurs by two concurrent, yet independent, mechanisms: (1) boundary layer convection persisting throughout the ice growth period, with short fingers present just below the ice/water interface, and (2) gravity-driven drainage from the brine channels producing deep penetrating convective streamers, which appear after a given time from the beginning of ice growth. The improved visualization and qualitative characterization of these processes show that Schlieren optical methods have exciting potential applications for future study of convective processes during sea-ice growth.

Ice at depth in ice sheets can be softer in bed-parallel shear than Glen’s flow law predicts. For example, at Dye 3, Greenland, enhancement factors of 3 4 are needed in order to explain the rate of borehole tilting Previous authors have identified crystal fabric as the dominant contributor, but the role of impurities and crystal size is still incompletely resolved. Here we use two formulations of anisotropic flow laws for ice (Azuma’s and Sachs’ models) to account for the effects of anisotropy, and show that the measured anisotropy of the ice at Dye 3 cannot explain all the detailed variations in the measured strain rates, the jump in enhancement across the Holocene–Wisconsin boundary is larger than expected from the measured fabrics alone. Dust and soluble-ion concentration divided by crystal size correlates well with the residual enhancement, indicating that most of the “excess deformation” may be due to impurities or crystal size. While the major features of the deformation at Dye 3 are explained by anisotropy and temperature, results also suggest that further research into the role of impurities and crystal size is warranted.

An ice core extracted from Holtedahlfonna ice cap, western Spitsbergen, record spanning the period 1700–2005, was analyzed for major ions. The leading empirical orthogonal function (EOF) component is correlated with an index of summer melt (log([Na+]/[Mg2+]) from 1850 and shows that almost 50% of the variance can be attributed to seasonal melting since the beginning of the industrial revolution. The Holtedahlfonna δ18O value is less negative than in the more easterly Lomonosovfonna ice core, suggesting that moist air masses originate from a closer source, most likely the Greenland Sea. During the Little Ice Age the lower methanesulfonic acid (MSA) concentration and MSA non-sea-salt sulfate fraction are consistent with the Greenland Sea as the main source for biogenic ions in the ice cores. Both the melt index and the MSA fraction suggest that the early decades of the 18th century may have exhibited the coldest summers of the last 300 years in Svalbard. Ammonium concentrations rise from 1880, which may result from the warming of the Greenland Sea or from zonal differences in atmospheric pollution transport over Svalbard. During winter, neutralized aerosols are trapped within the tropospheric inversion layer, which is usually weaker over open seas than over sea ice, placing Holtedahlfonna within the inversion more frequently than Lomonosovfonna.

A new model, the variable width/depth ratio (VWDR) model, is used to analyze longitudinal variations in cross-section morphology along glacial valleys. In the VWDR, the cross-sectional shape of a valley is expressed as a function of the width/depth ratio at various heights above the valley floor. Two parameters, m, a measure of the breadth of the valley floor, and n, a measure of the steepness of the valley sides, appear in the model. We have used the VWDR model to study morphological variations of cross-sections along glacial valleys in the middle Tien Shan mountains, China, and find that: (1) in valleys without tributaries, m increases (the valley floor becomes wider) and n becomes more negative (valley sides become steeper) from the head of the valley to the equilibrium line, and then m decreases and n becomes less negative to the end of the valley; (2) in valleys with tributaries, a similar pattern is observed, with an oscillating maximum in m and minimum in n in those sections where a tributary enters the main valley. These characteristics are believed to reflect a maximization of glacial erosion potential in the vicinity of the equilibrium line and in locations of confluence.

Many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood. Here we develop the theory of grain growth in ice that is not deforming rapidly, and in the succeeding paper we use this theory to explain observations from glacial ice. In the absence of significant strain energy, the driving force for grain growth arises from grain-boundary curvature. Grain growth is slowed by the interaction of grain boundaries with extrinsic materials (microparticles, bubbles, and dissolved impurities). If the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material, then the grain-boundary velocity is determined by the velocity characteristic of the extrinsic material (low-velocity regime). If the driving force is large enough to cause separation, then boundaries migrate more rapidly than the extrinsic material (high-velocity regime) but the net driving force is reduced through transient pinning by the extrinsic material. Polar ice is typically in the low-velocity regime relative to dissolved impurities and the high-velocity regime relative to microparticles and bubbles. Cross-sectional area of grains is predicted to increase linearly with time under most but not all circumstances.

A combination of remotely sensed data, field mapping, bathymetric survey and geophysical soundings is used to describe the recent changes in the terminal area of Petrov glacier, the largest glacier in the Akshiirak massif, central Tien Shan. According to our results, three periods of accelerated glacier retreat (1977-80, 1990-95 and 2008-09) can be distinguished since 1977. The largest recession occurred in 1990-95 when the glacier retreated by 54 ± 8ma-1. Accelerated glacier retreat affected the enlargement rate of Petrov lake, which increased by 0.04-0.1 km2 a-1 and by 1.3-2.2 × 106m3a-1 in the last three decades. Since 1995, the mean annual retreat rate of the lake-calving northern glacier section has been up to three times higher than the retreat rate of the land-terminating southern section. The calving flux ranged from 2.5 to 4.6 × 106m3a-1 in 2003-09, resulting in a total glacier mass loss of (17.7 ± 0.4) × 106m3. The calving terminus of Petrov glacier was >65 m thick in 2009 according to ground-penetrating radar measurements.

Ortles mountain (3905 m a.s.l.), South Tyrol, Italy, is the highest mountain of the Eastern European Alps, and its upper glacier, Alto dell’Ortles, presents a unique opportunity to obtain the first paleoenvironmental record from an ice core in this area. To study the suitability of this glacier as a drilling site, in 2009 we performed the first preliminary study of its glaciological characteristics at ˜3830 m a.s.l. The maximum thickness is ˜75 m, and lamination of the exposed ice layers is excellent down to bedrock. Firn and ice lenses were observed in a 10 m shallow core, and the firn/ice transition was below ˜24m. The seasonal chemical signal is clearly preserved only within the uppermost 2008 and 2009 snow/firn. A simple mass-balance model, the incipient ‘smoothing’ of the chemical record, and the observed ice lenses provide evidence that melting, infiltration and refreezing cycles occurred within the firn layers formed before 2008. Nevertheless, the mass balance of the upper part of Alto dell’Ortles was positive (˜800mma_1) during the last few years. We suggest that an environmental history is likely to be well preserved only within the ice layers formed before ˜1980, when summer air temperature was ˜2°C colder than today in this area. Clearly the continued warming trend predicted to occur over the next few decades, and the consequent increase in frequency and/or intensity of infiltration processes, will endanger the preservation of the glacial archive conserved in the deep ice layers of Ortles mountain.

The status of the theory of glacier sliding is reviewed in this paper. The theory of Nye and Kamb essentially solves the problem of glacier sliding when the basal ice is in intimate contact with a non-deformable bed. But experimental work is needed on cyclic creep and the regelation process in order to put better numbers into the theory. The outstanding problem that remains to be solved, called in this paper the general glacier sliding problem, is the case of sliding when basal ice is not in intimate contact with a non-deformable bed because of the presence of cavities and a water film of finite thickness. This problem involves solving an intricately coupled basal water-flow problem. A new field of research is the study of the motion of glaciers over deformable beds.

Clasts modified by glacial erosion are described from lodgement tills in front of the glacier Mýrdalsjökull, south Iceland. Many clasts show modification of their lower surfaces in the same way as their upper ones. However, the lower surfaces have a smoothed down-glacier face and a truncated up-glacier face, which is the opposite orientation to that of the upper surfaces. This so-called double stoss-lee form is interpreted as a response to basal transport over abrading materials, following deposition of the clast and succeeded by glacial erosion. It is suggested that clasts with a double stoss-lee form are a diagnostic criterion for subglacial deposition by lodgement. Furthermore, the distribution and orientation of clasts with a stoss-lee form was investigated on a ground-moraine surface. 17.3% of 2199 clasts with an a-axis diameter > 30 cm had a stoss-lee form. The proportion of clasts with their smoothed ends facing up-glacier within ± 22.5° of the ice-flow direction was 72.7%. Thus, the preferred stoss-side orientation is closely related to the ice movement and indicates the direction from which the ice came. Only 11.7% of boulders with a divergent stoss-side orientation are located in connection with annual moraines. It is suggested that such boulders have mainly been re-deposited beneath the ice and not at the ice front by minor advances of the glacier.

Ice rises and rumples, locally grounded features adjacent to ice shelves, are relatively small yet play significant roles in Antarctic ice dynamics. Their roles generally depend upon their location within the ice shelf and the stage of the ice-sheet retreat or advance. Large, long-stable ice rises can be excellent sites for deep ice coring and paleoclimate study of the Antarctic coast and the Southern Ocean, while small ice rises tend to respond more promptly and can be used to reveal recent changes in regional mass balance. The coasts of Dronning Maud Land (DML) and Enderby Land in East Antarctica are abundant with these features. Here we review existing knowledge, presenting an up-to-date status of research in these regions with focus on ice rises and rumples. We use regional datasets (satellite imagery, surface mass balance and ice thickness) to analyze the extent and surface morphology of ice shelves and characteristic timescales of ice rises. We find that large parts of DML have been changing over the past several millennia. Based on our findings, we highlight ice rises suitable for drilling ice cores for paleoclimate studies as well as ice rises suitable for deciphering ice dynamics and evolution in the region.

The mass flux from marine terminating glacier outlets from the Greenland ice sheet is a significant mass budget term. Yet, glacier calving sensitivity to climate parameters is not well understood. In this study, the front area changes of 42 marine terminating Greenland outlet glaciers were measured annually from 1999 to 2013 using optical satellite imagery. Thirty-five glaciers retreated, while seven remained stable. The average front position change was −240 m a−1 and the total rate of front area change was −114 km2 a−1. The yearly average area changes of all measured glaciers are compared with interannual variations in sea surface temperature (SST), sea-ice concentration (SIC), surface melt, Northern Hemisphere (NH) surface air temperatures and the North Atlantic Oscillation (NAO) index. We find: (1) northwestern glaciers area change have a significant correlation with surface melt and NH land temperature variations; (2) southeastern glaciers have a significant correlation with local SST, SIC and NH temperatures; and (3) southwestern glaciers area change correlate significantly with local SST and NAO index. We conclude that a climate sensitivity signal emerges considering a population of glaciers. Further, we find a significant correlation between the date of minimum extent and glacier latitude. Area change data are available from http://GAC.geus.dk.

The paper tries to reconcile the facts that there are often useful correlations between ablation or run-off and air temperature while net radiation is usually the major source of ablation energy. Equations are derived from the energy balance to describe statistics for the ablation-temperature relation in terms of statistics for the relations between individual energy fluxes and air temperature. As examples, statistics are evaluated for four published series from Arctic Canada. Although the net radiation is the largest energy source in all four cases, the ablation rate is moderately well correlated with temperature and poorly correlated with net radiation. This is because the sensible heat flux is more variable than the radiation in three cases and is itself better correlated with temperature in all four cases. The major contributions to the increase of ablation rate with temperature (on average 6.3 kg m−2 d−1 deg−1) are due to sensible heat, followed by latent heat with a small contribution from net radiation. The resulting ablation-temperature model explains about half the variance of ablation rate. The main application of such a simple model is for the estimation of ablation totals in areas where glaciological and hydrological data are sparse.

Impact pressures of snow avalanches have been measured at the Swiss Vallée de la Sionne experimental test site using two kinds of sensor placed at different locations in the avalanche flow. Pressures measured in a fast dry-snow avalanche and a slow wet-snow avalanche are compared and discussed. The pressures recorded using the two types of sensor in the dense flow of a dry-snow avalanche agree well, showing negligible dependence on the measurement device. On the other hand, significantly different pressures are measured in the slow dense flow of a wet-snow avalanche. This is attributed to the slow drag and bulk flow of this type of avalanche, leading to the formation and collapse of force-chain structures against the different surfaces of the sensors. At a macroscopic scale, limit state analysis can be used to explain such a mechanism by a shear failure occurring between freely flowing snow and a confined snow volume against the sensor, according to a Mohr–Coulomb failure criterion. The proposed model explains (1) how impact pressure can be up to eight times higher than hydrostatic snow pressure in wet cohesive slow avalanches and (2) its dependence on sensor geometry.

The effect of subglacial lakes upon ice-sheet topography and the velocity patterns of subglacial water-sheet floods is investigated. A subglacial lake in the combined Michigan–Green Bay basin, Great Lakes, North America, leads to: (1) an ice-sheet lobe in the lee of Lake Michigan; (2) a change in orientations of flood velocities across the site of a supraglacial trough aligned closely with Green Bay, in agreement with drumlin orientations; (3) low water velocities in the lee of Lake Michigan where drumlins are absent; and (4) drumlinization occurring in regions of predicted high water velocities. The extraordinary divergence of drumlin orientations near Lake Ontario is explained by the presence of subglacial lakes in the Ontario and Erie basins, along with ice-sheet displacements of up to 30 km in eastern Lake Ontario. The megagrooves on the islands in western Lake Erie are likely to be the product of the late stage of a water-sheet flood when outflow from eastern Lake Ontario was dammed by displaced ice and instead flowed westward along the Erie basin. The Finger Lakes of northern New York state, northeastern U.S.A., occur in a region of likely ice-sheet grounding where water sheets became channelized. Green Bay and Grand Traverse Bay are probably the products of erosion along paths of strongly convergent water-sheet flow.