• MABEL photon-counting laser altimetry data in Alaska for ICESat-2 simulations and development

      Brunt, Kelly M.; Neumann, Thomas A.; Amundson, Jason M.; Kavanaugh, Jeffrey L.; Moussavi, Mahsa S.; Walsh, Kaitlin M.; Cook, William B.; Markus, Thorsten (Copernicus Publications on behalf of the European Geosciences Union, 2016-08-10)
      Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is scheduled to launch in late 2017 and will carry the Advanced Topographic Laser Altimeter System (ATLAS) which is a photon-counting laser altimeter and represents a new approach to satellite determination of surface elevation. Given the new technology of ATLAS, an airborne instrument, the Multiple Altimeter Beam Experimental Lidar (MABEL), was developed to provide data needed for satellite-algorithm development and ICESat-2 error analysis. MABEL was deployed out of Fairbanks, Alaska, in July 2014 to provide a test dataset for algorithm development in summer conditions with water-saturated snow and ice surfaces. Here we compare MABEL lidar data to in situ observations in Southeast Alaska to assess instrument performance in summer conditions and in the presence of glacier surface melt ponds and a wet snowpack. Results indicate the following: (1) based on MABEL and in situ data comparisons, the ATLAS 90m beam-spacing strategy will provide a valid assessment of across-track slope that is consistent with shallow slopes (< 1) of an ice-sheet interior over 50 to 150m length scales; (2) the dense along-track sampling strategy of photon counting systems can provide crevasse detail; and (3) MABEL 532 nm wavelength light may sample both the surface and subsurface of shallow (approximately 2m deep) supraglacial melt ponds. The data associated with crevasses and melt ponds indicate the potential ICESat-2 will have for the study of mountain and other small glaciers.
    • A mass-flux perspective of the tidewater glacier cycle

      Amundson, Jason M. (International Glaciological Society, 2016-04-06)
      I explore the tidewater glacier cycle with a 1-D, depth- and width-integrated flow model that includes a mass-flux calving parameterization. The parameterization is developed from mass continuity arguments and relates the calving rate to the terminus velocity and the terminus balance velocity. The model demonstrates variable sensitivity to climate. From an advanced, stable configuration, a small warming of the climate triggers a rapid retreat that causes large-scale drawdown and is enhanced by positive glacier-dynamic feedbacks. Eventually, the terminus retreats out of deep water and the terminus velocity decreases, resulting in reduced drawdown and the potential for restabilization. Terminus readvance can be initiated by cooling the climate. Terminus advance into deep water is difficult to sustain, however, due to negative feedbacks between glacier dynamics and surface mass balance. Despite uncertainty in the precise form of the parameterization, the model provides a simple explanation of the tidewater glacier cycle and can be used to evaluate the response of tidewater glaciers to climate variability. It also highlights the importance of improving parameterizations of calving rates and of incorporating sediment dynamics into tidewater glacier models.
    • Meltwater Intrusions Reveal Mechanisms for Rapid Submarine Melt at a Tidewater Glacier

      Kienholtz, C.; Sutherland, D. A.; Jackson, R. H.; Nash, J. D.; Amundson, Jason M.; Motyka, R. J.; Winters, D.; Skyllingstad, E.; Pettit, E. C. (American Geophysical Union, 2019-11-25)
      Submarine melting has been implicated as a driver of glacier retreat and sea level rise, but to date melting has been difficult to observe and quantify. As a result, melt rates have been estimated from parameterizations that are largely unconstrained by observations, particularly at the near-vertical termini of tidewater glaciers. With standard coefficients, these melt parameterizations predict that ambient melting (the melt away from subglacial discharge outlets) is negligible compared to discharge-driven melting for typical tidewater glaciers. Here, we present new data from LeConte Glacier, Alaska, that challenges this paradigm. Using autonomous kayaks, we observe ambient meltwater intrusions that are ubiquitous within 400 m of the terminus, and we provide the first characterization of their properties, structure, and distribution. Our results suggest that ambient melt rates are substantially higher (×100) than standard theory predicts and that ambient melting is a significant part of the total submarine melt flux. We explore modifications to the prevalent melt parameterization to provide a path forward for improved modeling of ocean-glacier interactions.
    • The morphology of supraglacial lake ogives

      Darnell, K.N.; Amundson, Jason M.; Cathles, L.M.; MacAyeal, D.R. (International Glaciological Society, 2013-02-12)
      Supraglacial lakes on grounded regions of the Greenland and Antarctic ice sheets sometimes produce ‘lake ogives’ or banded structures that sweep downstream from the lakes. Using a variety of remote-sensing data, we demonstrate that lake ogives originate from supraglacial lakes that form each year in the same bedrock-fixed location near the equilibrium-line altitude. As the ice flows underneath one of these lakes, an ‘image’ of the lake is imprinted on the ice surface both by summer- season ablation and by superimposed ice (lake ice) formation. Ogives associated with a lake are sequenced in time, with the downstream ogives being the oldest, and with spatial separation equal to the local annual ice displacement. In addition, lake ogives can have decimeter- to meter-scale topographic relief, much like wave ogives that form below icefalls on alpine glaciers. Our observations highlight the fact that lake ogives, and other related surface features, are a consequence of hydrological processes in a bedrock-fixed reference frame. These features should arise naturally from physically based thermodynamic models of supraglacial water transport, and thus they may serve as fiducial features that help to test the performance of such models.
    • Non-linear glacier response to calving events, Jakobshavn Isbræ, Greenland

      Cassoto, Ryan; Fahnestock, Mark; Amundson, Jason M.; Truffer, Martin; Boettcher, Margaret S.; De La Pena, Santiago; Howat, Ian (International Glaciological Society, 2018-11-29)
      Jakobshavn Isbræ, a tidewater glacier that produces some of Greenland’s largest icebergs and highest speeds, reached record-high flow rates in 2012 (Joughin and others, 2014). We use terrestrial radar interferometric observations from August 2012 to characterize the events that led to record-high flow. We find that the highest speeds occurred in response to a small calving retreat, while several larger calving events produced negligible changes in glacier speed. This non-linear response to calving events suggests the terminus was close to flotation and therefore highly sensitive to terminus position. Our observations indicate that a glacier’s response to calving is a consequence of two competing feedbacks: (1) an increase in strain rates that leads to dynamic thinning and faster flow, thereby promoting desta- bilization, and (2) an increase in flow rates that advects thick ice toward the terminus and promotes restabilization. The competition between these feedbacks depends on temporal and spatial variations in the glacier’s proximity to flotation. This study highlights the importance of dynamic thinning and advective processes on tidewater glacier stability, and further suggests the latter may be limiting the current retreat due to the thick ice that occupies Jakobshavn Isbræ’s retrograde bed.
    • Observing calving-generated ocean waves with coastal broadband seismometers, Jakobshavn Isbræ, Greenland

      Amundson, Jason M.; Clinton, John F.; Fahnestock, Mark; Truffer, Martin; Luthi, Martin P.; Motyka, Roman J. (International Glaciological Society, 2012)
      We use time-lapse photography, MODIS satellite imagery, ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from Jakobshavn Isbræ, Greenland, can generate ocean waves that are detectable over 150 km from their source. The waves, which are recorded seismically, have distinct spectral peaks, are not dispersive and persist for several hours. On the basis of these observations, we suggest that calving events at Jakobshavn Isbræ can stimulate seiches, or basin eigenmodes, in both Ilulissat Icefjord and Disko Bay. Our observations furthermore indicate that coastal, land-based seismometers located near calving termini (e.g. as part of the new Greenland Ice Sheet Monitoring Network (GLISN)) can aid investigations into the largely unexplored, oceanographic consequences of iceberg calving.
    • Outlet glacier response to forcing over hourly to interannual timescales, Jakobshavn Isbræ, Greenland

      Podrasky, David; Truffer, Martin; Fahnestock, Mark; Amundson, Jason M.; Cassoto, Ryan; Joughin, Ian (International Glaciological Society, 2012-09-07)
      The loss of the floating ice tongue on Jakobshavn Isbræ, Greenland, in the early 2000s has been concurrent with a pattern of thinning, retreat and acceleration leading to enhanced contribution to global sea level. These changes on decadal timescales have been well documented. Here we identify how the glacier responds to forcings on shorter timescales, such as from variations in surface melt, the drainage of supraglacial lakes and seasonal fluctuations in terminus position. Ice motion and surface melt were monitored intermittently from 2006 to 2008. Dual-frequency GPS were deployed 20–50 km upstream of the terminus along the glacier center line. Gaps in surface melt measurements were filled using a temperature-index model of ablation driven by surface air temperatures recorded during the same time period. Our results corroborate the premise that the primary factors controlling speeds on Jakobshavn Isbræ are terminus position and geometry. We also observe that surface speeds demonstrate a complex relationship with meltwater input: on diurnal timescales, velocities closely match changes in water input; however, on seasonal timescales a longer, more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year.
    • Quantifying flow and stress in ice mélange, the world’s largest granular material.

      Burton, J. C.; Amundson, Jason M.; Cassotto, R.; Kuo, C. C.; Dennin, M. (PNAS Proceedings of the National Academy of Sciences, 2018-03-29)
      Tidewater glacier fjords are often filled with a collection of calved icebergs, brash ice, and sea ice. For glaciers with high calving rates, this “m ́elange” of ice can be jam-packed, so that the flow of ice fragments is mostly determined by granular interactions. In the jammed state, ice m ́elange has been hypothesized to influence iceberg calving and capsize, dispersion and attenuation of ocean waves, injection of freshwater into fjords, and fjord circulation. However, detailed measurements of ice m ́elange are lacking due to difficulties in instrumenting remote, ice-choked fjords. Here we characterize the flow and associated stress in icem ́elange, using a combination of terrestrial radar data, laboratory experiments, and numerical simulations. We find that, during periods of terminus quiescence, ice m ́elange experiences laminar flow over timescales of hours to days. The uniform flow fields are bounded by shear margins along fjord walls where force chains between granular icebergs terminate. In addition, the average force per unit width that is transmitted to the glacier terminus, which can exceed 107N/m, increases exponentially with them ́elange length-to-width ratio. These “buttressing” forces are sufficiently high to inhibit the initiation of large-scale calving events, supporting the notion that ice m ́elange can be viewed as a weak granular ice shelf that transmits stresses from fjord walls back to glacier termini.
    • Quasi-static granular flow of ice mélange

      Amundson, Jason M.; Burton, J. C. (American Geophysical Union, 2018-09-11)
      We use Landsat 8 imagery to generate ice mélange velocity fields at Greenland’s three most productive outlet glaciers: Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow at Jakobshavn Isbræ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in the context of quasi-static flow using continuum rheologies for granular materials and find that the force per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange dynamics with continuum models.
    • Rapid submarine melting driven by subglacial discharge, LeConte Glacier, Alaska

      Motyka, R. J.; Dryer, William P.; Amundson, Jason M.; Truffer, Martin; Fahnestock, Mark (American Geophysical Union, 2013-09-27)
      We show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers. This buoyant discharge draws in warm seawater, entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice. To capture the effects of subglacial discharge on submarine melting, we conducted 4 days of hydrographic transects during late summer 2012 at LeConte Glacier, Alaska. A major rainstorm allowed us to document the influence of large changes in subglacial discharge. We found strong submarine melt fluxes that increased from 9.1 ± 1.0 to 16.8 ± 1.3 m d1 (ice face equivalent frontal ablation) as a result of the rainstorm. With projected continued global warming and increased glacial runoff, our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems. These effects must be considered when modeling glacier response to future warming and increased runoff.
    • Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbræ, Greenland

      Cassotto, Ryan; Fahnestock, Mark; Amundson, Jason M.; Truffer, Martin; Joughin, Ian (International Glaciological Society, 2014-09-29)
      We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland. Freezing of the melange-covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with (1) a decrease in ice melange mobility and (2) a drastic reduction in iceberg production. Vigorous calving resumes in spring, typically abruptly, following the steady up-fjord retreat of the sea-ice/ice-melange margin. An analysis of pixel displacement from time-lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events. We find that secular changes in ice melange extent, character and persistence can influence iceberg calving, and therefore glacier dynamics over daily-to-monthly timescales, which, if sustained, will influence the mass balance of an ice sheet.
    • Seismic Tremor Reveals Spatial Organization and Temporal Changes of Subglacial Water System

      Vore, Margot E.; Bartholomaus, Timothy, C.; Winberry, J. Paul; Walter, Jacob I.; Amundson, Jason M. (American Geophysical Union, 2019-02-09)
      Subglacial water flow impacts glacier dynamics and shapes the subglacial environment. However, due to the challenges of observing glacier beds, the spatial organization of subglacial water systems and the time scales of conduit evolution and migration are largely unknown. To address these questions, we analyze 1.5‐ to 10‐Hz seismic tremor that we associate with subglacial water flow, that is, glaciohydraulic tremor, at Taku Glacier, Alaska, throughout the 2016 melt season. We use frequency‐dependent polarization analysis to estimate glaciohydraulic tremor propagation direction (related to the subglacial conduit location) and a degree day melt model to monitor variations in melt‐water input. We suggest that conduit formation requires sustained water input and that multiconduit flow paths can be distinguished from single‐conduit flow paths. Theoretical analysis supports our seismic interpretations that subglacial discharge likely flows through a single‐conduit in regions of steep hydraulic potential gradients but may be distributed among multiple conduits in regions with shallower potential gradients. Seismic tremor in regions with multiple conduits evolves through abrupt jumps between stable configurations that last 3–7 days, while tremor produced by single‐conduit flow remains more stationary. We also find that polarized glaciohydraulic tremor wave types are potentially linked to the distance from source to station and that multiple peak frequencies propagate from a similar direction. Tremor appears undetectable at distances beyond 2–6 km from the source. This new understanding of the spatial organization and temporal development of subglacial conduits informs our understanding of dynamism within the subglacial hydrologic system.
    • Subglacial discharge at tidewater glaciers revealed by seismic tremor

      Bartholomaus, Timothy C.; Amundson, Jason M.; Walter, Jacob I.; O'Neel, Shad; West, Michael E.; Larsen, Christopher F. (American Geophysical Union, 2015-08-10)
      Subglacial discharge influences glacier basal motion and erodes and redeposits sediment. At tidewater glacier termini, discharge drives submarine terminus melting, affects fjord circulation, and is a central component of proglacial marine ecosystems. However, our present inability to track subglacial discharge and its variability significantly hinders our understanding of these processes. Here we report observations of hourly to seasonal variations in 1.5–10 Hz seismic tremor that strongly correlate with subglacial discharge but not with basal motion, weather, or discrete icequakes. Our data demonstrate that vigorous discharge occurs from tidewater glaciers during summer, in spite of fast basal motion that could limit the formation of subglacial conduits, and then abates during winter. Furthermore, tremor observations and a melt model demonstrate that drainage efficiency of tidewater glaciers evolves seasonally. Glaciohydraulic tremor provides a means by which to quantify subglacial discharge variations and offers a promising window into otherwise obscured glacierized environments.
    • Subseasonal changes observed in subglacial channel pressure, size, and sediment transport

      Gimbert, Florent; Tsai, Victor C.; Amundson, Jason M.; Bartholomaus, Timothy, C.; Walter, Jacob I. (American Geophysical Union, 2016-04-07)
      Water that pressurizes the base of glaciers and ice sheets enhances glacier velocities and modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial channel evolution, which remains observationally limited. Here we demonstrate that detailed analysis of seismic ground motion caused by subglacial water flow at Mendenhall Glacier (Alaska) allows for continuous measurement of daily to subseasonal changes in basal water pressure gradient, channel size, and sediment transport. We observe intermittent subglacial water pressure gradient changes during the melt season, at odds with common assumptions of slowly varying, low-pressure channels. These observations indicate that changes in channel size do not keep pace with changes in discharge. This behavior strongly affects glacier dynamics and subglacial channel erosion at Mendenhall Glacier, where episodic periods of high water pressure gradients enhance glacier surface velocity and channel sediment transport by up to 30% and 50%, respectively. We expect the application of this framework to future seismic observations acquired at glaciers worldwide to improve our understanding of subglacial processes.
    • Testing a glacial erosion rule using hang heights of hanging valleys, Jasper National Park, Alberta, Canada

      Amundson, Jason M.; Iverson, N. R. (American Geophysical Union, 2006)
      In most models of glacial erosion, glacier sliding velocity is hypothesized to control rates of bedrock erosion. If this hypothesis is correct, then the elevation difference between hanging and trunk valley floors, the hang height, should be dictated by the relative sliding velocities of the glaciers that occupied these valleys. By considering sliding velocity to be proportional to balance velocity and using mass continuity, hang height is expressed in terms of glacier catchment areas, slopes, and widths, which can be estimated for past glaciers from the morphology of glacial valleys. These parameters were estimated for 46 hanging valleys and their trunk valleys in three adjacent regions of Jasper National Park. The variability in valley morphology can account for 55–85% of the hang height variability if erosion rate scales with balance velocity raised to a power of 1/3. This correspondence is in spite of spatial variations in glaciation duration, snow accumulation rates, and other variables that likely affected hang heights but cannot be readily estimated and so are not included in our formulation. Thus it appears that balance velocity, and by extension, sliding velocity if the two are proportional, may be a reasonable control variable for assessing erosion rate.
    • Time-dependent basal stress conditions beneath Black Rapids Glacier, Alaska, USA, inferred from measurements of ice deformation and surface motion

      Amundson, Jason M.; Truffer, Martin; Luthi, Martin P. (International Glaciological Society, 2006-06-23)
      Observations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales. Basal shear stresses in a well-defined zone north of the center line (orographic left) were approximately 7% and 16% lower in spring and summer, respectively, than in winter. Correspondingly higher stresses were found near the margins. These changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1.2 and 1.5 times larger in spring and summer than in winter. These results were inferred with a simple inverse finite-element flow model that can successfully reproduce bulk surface velocities and tiltmeter data. Stress redistribution between the well-defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions (ice–bed decoupling or enhanced till deformation), thereby causing large variations in surface velocity and strongly influencing the glacier’s net motion during summer.
    • Tracking icebergs with time-lapse photography and sparse optical flow, LeConte Bay, Alaska, 2016–2017

      Kienholtz, Christian; Amundson, Jason M.; Motyka, Roman J.; Jackson, Rebecca H.; Mickett, John B.; Sutherland, David A.; Nash, Jonathan D.; Winters, Dylan S.; Dryer, William P.; Truffer, Martin (International Glaciological Society, 2019-03-07)
      We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1 . Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (∼ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1 ) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay).
    • A unifying framework for iceberg-calving models

      Amundson, Jason M.; Truffer, Martin (International Glaciological Society, 2010-07-09)
      We propose a general framework for iceberg-calving models that can be applied to any calving margin. The framework is based on mass continuity, the assumption that calving rate and terminus velocity are not independent and the simple idea that terminus thickness following a calving event is larger than terminus thickness at the event onset. The theoretical, near steady-state analysis used to support and analyze the framework indicates that calving rate is governed, to first order, by ice thickness, thickness gradient, strain rate, mass-balance rate and backwards melting of the terminus; the analysis furthermore provides a physical explanation for a previously derived empirical relationship for ice-shelf calving (Alley and others, 2008). In the calving framework the pre- and post-calving terminus thicknesses are given by two unknown but related functions. The functions can vary independently of changes in glacier flow and geometry, and can therefore account for variations in calving behavior due to external forcings and/or self-sustaining calving processes (positive feedbacks). Although the calving framework does not constitute a complete calving model, any thickness-based calving criterion can easily be incorporated into the framework. The framework should be viewed as a guide for future attempts to parameterize calving.