The meltwater feedbacks on ice dynamics, elevation versus lubrication

2020 ◽  
Author(s):  
Basile de Fleurian ◽  
Petra Langebroek ◽  
Paul Halas
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
Ice Dynamics ◽  
Ice Surface ◽  
Basal Sliding ◽  
Surface Melt ◽  
The One ◽  
Different Response

<p>In recent years, temperatures over the Greenland ice sheet have been rising leading to an increase in surface melt.  Projections show that this augmentation of surface melt will continue in the future and spread to higher elevations. As it increases, melt leads to two different feedbacks on the dynamic of the Greenland ice sheet. This augmentation of melt lowers the ice surface and changes its overall geometry hence impacting the ice dynamics through ice deformation. The other feedback comes into play at the base of glaciers. Here, the increase of water availability will impact the distribution of water pressure at the base of glaciers and hence their sliding velocity. The first feedback is relatively well known and relies on our knowledge of the rheology and deformation of ice. The lubrication feedback acting at the bed of glaciers is however highly uncertain on time scales longer than a season. Here we apply the  Ice  Sheet  System  Model  (ISSM)  to  a  synthetic  glacier  which  geometry  is  similar to the one of a Greenland ice sheet land terminating glacier. The dynamic contributions from ice deformation and sliding are separated to study their relative evolution. This is permitted by the use of a dynamical subglacial hydrology model that allows to link the basal sliding to the meltwater production through an appropriate friction law. The  model  is  forced  through  a  simple  temperature  distribution  and  a  Positive  Degree  Day  model which allows to apply a large range of different forcing scenarios. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system and their different response to the distribution of melt during the year which directly impact the sliding regime at the base of the glacier.</p>

Greenland land-terminating glaciers velocity trends during the last two decades

2021 ◽  
Author(s):  
Paul Halas ◽  
Jeremie Mouginot ◽  
Basile de Fleurian ◽  
Petra Langebroek
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Melting ◽  
Limited Area ◽  
Ice Dynamics ◽  
Basal Sliding ◽  
Surface Melt ◽  
Ice Sheet Dynamics ◽  
The Impact

<div> <p>Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise. Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding. The sliding component of glaciers has been observed to be strongly related to surface melting, as water can eventually reach the bed and impact the subglacial water pressure, affecting the basal sliding.  </p> </div><div> <p>The link between ice velocities and surface melt on multi-annual time scale is still not totally understood even though it is of major importance with expected increasing surface melting. Several studies showed some correlation between an increase in surface melt and a slowdown in velocities, but there is no consensus on those trends. Moreover those investigations only presented results in a limited area over Southwest Greenland.  </p> </div><div> <p>Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 & 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.  </p> </div><div> <p>Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration. This trend however does not seem to be observed on the whole ice sheet and is probably specific to this region’s climate forcing. </p> </div><div> <p>Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.</p> </div>

scholarly journals Ice sheet mass loss caused by dust and black carbon accumulation

2015 ◽  
Vol 9 (2) ◽  
pp. 2563-2596
Author(s):  
T. Goelles ◽  
C. E. Bøggild ◽  
R. Greve
Keyword(s):  
Mass Loss ◽  
Black Carbon ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Carbon Accumulation ◽  
Model Framework ◽  
Ice Dynamics ◽  
Ablation Area ◽  
Ice Surface ◽  
Surface Melt

Abstract. Albedo is the dominating factor governing surface melt variability in the ablation area of ice sheets and glaciers. Aerosols such as mineral dust and black carbon (soot) accumulate on the ice surface and cause a darker surface and therefore a lower albedo. The dominant source of these aerosols in the ablation area is melt-out of englacial material which has been transported via ice flow. The darkening effect on the ice surface is currently not included in sea level projections, and the effect is unknown. We present a model framework which includes ice dynamics, aerosol transport, aerosol accumulation and the darkening effect on ice albedo and its consequences for surface melt. The model is applied to a simplified geometry resembling the conditions of the Greenland ice sheet, and it is forced by several temperature scenarios to quantify the darkening effect of aerosols on future mass loss. The effect of aerosols depends non-linearly on the temperature rise due to the feedback between aerosol accumulation and surface melt. The effect of aerosols in the year 3000 is up to 12% of additional ice sheet volume loss in the warmest scenario.

The meltwater feedbacks on ice dynamics, influence of melt amplitude, duration and extent.

2021 ◽  
Author(s):  
Basile de Fleurian ◽  
Petra M. Langebroeke ◽  
Richard Davy
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
System Model ◽  
Ice Dynamics ◽  
Mountain Glaciers ◽  
Different Characteristics ◽  
Crucial Parameter ◽  
Subglacial Drainage

<p>In recent years, temperatures over the Greenland ice sheet have been rising, leading to an increase in surface melt. This increase however can not be reduced to a simple number. Throughout the recent years we have seen some extreme melt seasons with melt extending over the whole surface of the ice sheet (2012) or melt seasons of lower amplitudes but with a longer duration (2010). The effect of those variations on the subglacial system and hence on ice dynamic are poorly understood and are still mainly deduced from studies based on mountain glaciers.</p><p>Here we apply the Ice-sheet and Sea-level System Model (ISSM) to a synthetic glacier with a geometry similar to a Greenland ice sheet land terminating glacier. The forcing is designed such that it allows to investigate different characteristics of the melt season: its length, intensity or the spatial extension of the melt. Subglacial hydrology and ice dynamics are coupled within ISSM is coupled to a subglacial hydrology model, allowing to study the response of the system in terms of subglacial water pressure and the final impact on ice dynamics. Of particular interest is the evolution of the distribution of the efficient and inefficient component of the subglacial drainage system which directly impacts the water pressure evolution at the base of the glacier.</p><p>We note that the initiation of the melt season and the intensity of the melt at this period is a crucial parameter when studying the dynamic response of the glacier to different melt season characteristics. From those results, we can infer a more precise evolution of the dynamics of land terminating glaciers that are heavily driven by their subglacial drainage system. We also highlight which changes in the melt season pattern would be the most damageable for glacier stability in the future.</p>

Greenland ice sheet hydrology

2013 ◽  
Vol 38 (1) ◽  
pp. 19-54 ◽  
Author(s):  
Vena W. Chu
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Field Studies ◽  
Ice Dynamics ◽  
Surface Water Hydrology ◽  
Basal Sliding ◽  
Hydrologic System ◽  
Global Sea Level

Understanding Greenland ice sheet (GrIS) hydrology is essential for evaluating response of ice dynamics to a warming climate and future contributions to global sea level rise. Recently observed increases in temperature and melt extent over the GrIS have prompted numerous remote sensing, modeling, and field studies gauging the response of the ice sheet and outlet glaciers to increasing meltwater input, providing a quickly growing body of literature describing seasonal and annual development of the GrIS hydrologic system. This system is characterized by supraglacial streams and lakes that drain through moulins, providing an influx of meltwater into englacial and subglacial environments that increases basal sliding speeds of outlet glaciers in the short term. However, englacial and subglacial drainage systems may adjust to efficiently drain increased meltwater without significant changes to ice dynamics over seasonal and annual scales. Both proglacial rivers originating from land-terminating glaciers and subglacial conduits under marine-terminating glaciers represent direct meltwater outputs in the form of fjord sediment plumes, visible in remotely sensed imagery. This review provides the current state of knowledge on GrIS surface water hydrology, following ice sheet surface meltwater production and transport via supra-, en-, sub-, and proglacial processes to final meltwater export to the ocean. With continued efforts targeting both process-level and systems analysis of the hydrologic system, the larger picture of how future changes in Greenland hydrology will affect ice sheet glacier dynamics and ultimately global sea level rise can be advanced.

scholarly journals Direct measurements of meltwater runoff on the Greenland ice sheet surface

2017 ◽  
Vol 114 (50) ◽  
pp. E10622-E10631 ◽  
Author(s):  
Laurence C. Smith ◽  
Kang Yang ◽  
Lincoln H Pitcher ◽  
Brandon T. Overstreet ◽  
Vena W. Chu ◽  
...  
Keyword(s):  
Sea Level ◽  
Surface Runoff ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Dynamics ◽  
Sheet Surface ◽  
Ice Surface ◽  
Meltwater Runoff ◽  
Direct Measurements ◽  
Peak Discharges

Meltwater runoff from the Greenland ice sheet surface influences surface mass balance (SMB), ice dynamics, and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-h trial for a 63.1-km2moulin-terminating internally drained catchment (IDC) on Greenland’s midelevation (1,207–1,381 m above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6, RACMO2.3, MERRA-2, and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins but are improved using synthetic unit hydrograph (SUH) theory. Retroactive SUH applications to two older field studies reproduce their findings, signifying that remotely sensed IDC area, shape, and supraglacial river length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6, and RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment, models overestimated runoff by +21 to +58%, linked to overestimated surface ablation and possible meltwater retention in bare, porous, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of SMB with ice dynamics and subglacial systems.

Modelling the basal hydrology under the Greenland Ice Sheet

2020 ◽  
Author(s):  
Alexander Vanhulle ◽  
Sébastien Le Clec’h ◽  
Philippe Huybrechts
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Grid Cell ◽  
Sensitivity Analyses ◽  
Water Model ◽  
Ice Dynamics ◽  
Continental Scale ◽  
Basal Sliding ◽  
Smoothing Procedure ◽  
Individual Grid

<p>Subglacial hydrology plays an important role in the evolution of ice dynamics. Primarily, it affects basal processes such as basal sliding. Further, subglacial water exiting a calving front incites submarine melt, increasing calving, resulting in a thinning of the interior ice sheet. Knowledge of it is therefore crucial towards the development and improvement of ice sheet models. We implement a model representing the routing of subglacial water below the Greenland ice sheet in either a one, four or eight directional manner. Due to its computational efficiency, the model is suited for coupling with continental scale ice sheet models on very high resolutions (e.g. 150 m).</p><p>Routing depends on the hydraulic potential of individual grid cells which is therefore heavily dependent on accurate estimates of the ice thickness as well as the grid utilized. Sensitivity analyses brought to life that the routing exhibits artefacts resulting in significant flow diversions on high resolutions if the gradients are only considered over the distance of a single grid cell, this is overcome by incorporating a smoothing procedure.</p><p>With the basal water model in place and input of the basal melt rate from the VUB Greenland Ice Sheet Model (GISM) as well as runoff input from the Modèle Athmospherique Régional (MAR), we calculate the inflow of freshwater to several reference fjords for the last thirty years and investigate its temporal and spatial patterns. Jakobshavn Isbrae experiences by far the most freshwater inflow compared to the other reference fjords. Despite limited runoff in the northeast of Greenland, high basal melt rates and a significant catchment area provide the outlets of the Northeast Greenland Ice Stream (NEGIS) with substantial inflow too.</p>

scholarly journals The impact of jökulhlaups on basal sliding observed by SAR interferometry on Vatnajökull, Iceland

2007 ◽  
Vol 53 (181) ◽  
pp. 232-240 ◽  
Author(s):  
Eyjólfur Magnússon ◽  
Helmut Rott ◽  
Helgi Björnsson ◽  
Finnur Pálsson
Keyword(s):  
Initial Phase ◽  
Water Pressure ◽  
Drainage System ◽  
Sar Interferometry ◽  
Ice Dynamics ◽  
Velocity Increase ◽  
Basal Sliding ◽  
Tunnel System ◽  
The Impact ◽  
Local Uplift

AbstractWe have analyzed InSAR data from the ERS-1/ERS-2 tandem mission, to study the ice dynamics of Vatnajökull, Iceland, during jökulhlaups from the Skaftá cauldrons and the Grímsvötn geothermal area, which drained under the Tungnaárjökull and Skeiðarárjökull outlets, respectively. During the initial phase of a Grímsvötn jökulhlaup in March 1996, the velocity of Skeiðarárjökull increased up to three-fold (relative to observed velocities in December 1995) over an area up to 8 km wide around the subglacial flood path. Accumulation of water was observed at one location in the flood path. During a small jökulhlaup from the Skaftá cauldrons in October 1995 the velocity on Tungnaárjökull increased up to four-fold over a 9 km wide area. The velocity increase was observed 1.5 days before the floodwater was detected in the river Skaftá. A reduced glacier speed as the flood peaked in Skaftá indicates evolution of the subglacial drainage system from sheet to tunnel flow. The glacier acceleration and local uplift, observed in the early phase of both jökulhlaups, supports the concept that increased water inflow in a narrow tunnel system causes water pressure to rise and forces water into areas outside the channels, thus reducing the coupling of ice with the glacier bed.

scholarly journals Ice sheet mass loss caused by dust and black carbon accumulation

2015 ◽  
Vol 9 (5) ◽  
pp. 1845-1856 ◽  
Author(s):  
T. Goelles ◽  
C. E. Bøggild ◽  
R. Greve
Keyword(s):  
Mass Loss ◽  
Black Carbon ◽  
Ice Sheet ◽  
Carbon Accumulation ◽  
Dominant Factor ◽  
Model Framework ◽  
Ice Dynamics ◽  
Ice Surface ◽  
Surface Melt ◽  
The Impact

Abstract. Albedo is the dominant factor governing surface melt variability in the ablation area of ice sheets and glaciers. Aerosols such as mineral dust and black carbon (soot) accumulate on the ice surface and cause a darker surface and therefore a lower albedo. The darkening effect on the ice surface is currently not included in sea level projections, and the effect is unknown. We present a model framework which includes ice dynamics, aerosol transport, aerosol accumulation and the darkening effect on ice albedo and its consequences for surface melt. The model is applied to a simplified geometry resembling the conditions of the Greenland ice sheet, and it is forced by several temperature scenarios to quantify the darkening effect of aerosols on future mass loss. The effect of aerosols depends non-linearly on the temperature rise due to the feedback between aerosol accumulation and surface melt. According to our conceptual model, accounting for black carbon and dust in future projections of ice sheet changes until the year 3000 could induce an additional volume loss of 7 %. Since we have ignored some feedback processes, the impact might be even larger.

A coupled model of rapidly-evolving subglacial hydrology with ice dynamics

2021 ◽  
Author(s):  
Michael McPhail ◽  
Ian Hewitt
Keyword(s):  
Large Scale ◽  
Water Pressure ◽  
Ice Sheet ◽  
Drainage System ◽  
Momentum Equation ◽  
Effective Pressure ◽  
Short Term ◽  
Ice Dynamics ◽  
The Impact

<p>The presence of subglacial water can have a significant effect on the motion of an ice sheet. The rate at which the ice slides over the bedrock is mediated by subglacial water pressure. Meltwater on the surface of the sheet can drain through cracks and moulins; drastically increasing the amount of water under the sheet. This source of water fluctuates seasonally and diurnally, much faster than the timescale associated with large-scale glacier evolution. We are interested in the effect that this short-term variation in the subglacial hydrology, and therefore water pressure, has on the long-term behaviour of the ice sheet.<span>  </span>In particular, we are interested in how important it is to resolve the short-timescale variations in ice sliding speed.</p><p> </p><p>We use a mathematical model to study the response of the subglacial drainage system to time-varying surface melt input. By coupling this subglacial hydrology through an effective-pressure-dependent sliding law to the momentum equation for the overlying ice sheet, we study the impact of short-term meltwater fluctuations on the ice velocity.<span>  </span>We study these interactions using a one-dimensional (1D) flowline model representing a confined glacier, allowing us to explore a range of couplings between the ice flow and hydrology.<span>  </span>This enables us to assess the influence of the fluctuating meltwater input on the long-term behaviour of the ice sheet. We find that using a time-averaged effective pressure with an asynchronous coupling to the momentum equation gives a reasonable estimate for the time-averaged ice-sheet velocity, despite the nonlinearity of the governing equations. We use the results to suggest how hydrological coupling might be achieved in larger-scale models where resolving the short-term fluctuations is likely to be infeasible. <span> </span></p>

scholarly journals Assessing the summer water budget of a moulin basin in the Sermeq Avannarleq ablation region, Greenland ice sheet

2011 ◽  
Vol 57 (205) ◽  
pp. 954-964 ◽  
Author(s):  
Daniel McGrath ◽  
William Colgan ◽  
Konrad Steffen ◽  
Phillip Lauffenburger ◽  
James Balog
Keyword(s):  
Water Budget ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Time Lapse ◽  
Balance Model ◽  
Diurnal Variability ◽  
Surface Uplift ◽  
Ice Surface ◽  
Basal Sliding

AbstractWe provide an assessment of the supraglacial water budget of a moulin basin on the western margin of the Greenland ice sheet for 15 days in August 2009. Meltwater production, the dominant input term to the 1.14 ± 0.06 km2 basin, was determined from in situ ablation measurements. The dominant water-output terms from the basin, accounting for 52% and 48% of output, respectively, were moulin discharge and drainage into crevasses. Moulin discharge exhibits large diurnal variability (0.017–0.54 m3 s−1) with a distinct late-afternoon peak at 16:45 local time. This lags peak meltwater production by ∼2.8 ± 4.2 hours. An Extreme Ice Survey time-lapse photography sequence complements the observations of moulin discharge. We infer, from in situ observations of moulin geometry, previously published borehole water heights and estimates of the temporal lag between meltwater production and observed local ice surface uplift (‘jacking’), that the transfer of surface meltwater to the englacial water table via moulins is nearly instantaneous (<30 min). We employ a simple crevasse mass-balance model to demonstrate that crevasse drainage could significantly dampen the surface meltwater fluctuations reaching the englacial system in comparison to moulin discharge. Thus, unlike crevasses, moulins propagate meltwater pulses to the englacial system that are capable of overwhelming subglacial transmission capacity, resulting in enhanced basal sliding.

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