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Author: Daniel J. Sturgis
Publisher:
ISBN: 9781109532845
Category : Ice sheets
Languages : en
Pages : 80
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Book Description
Surface meltwater generated in the accumulation zone of the Greenland Ice Sheet (GrIS) will either be retained by refreezing or connect to the glacial drainage system and contribute to annual runoff. The fate of this meltwater is controlled by the infiltration process, which occurs in the presence of subfreezing snow/firnpack temperatures and ice layers. Ice layers are typically treated as impermeable horizons. However, dye-trace observations suggest ice layers do not impede flow but rather accelerate flow by destabilizing the wetting-front, forming preferential flow paths termed pipes. Until the 2008 field season, the permeability of ice layers formed in the snow/firnpack on GrIS was unmeasured. Air permeameter measurements show ice layer permeability to range from 10 -15 m 2 to 10 -12 m 2 and firn to be approximately 10 -11 m 2 . Temperature profile measurements of the snow/firnpack were recorded every 30 min during the 2007 melt season. Temperature profile data confirms piping as a mechanism for meltwater delivery to 10+ m depths at T1 without increasing the full snow/firnpack temperature to 0°C. Meltwater that is piped to the glacier-ice surface can connect to the glacial drainage system and runoff. Current models used to estimate annual runoff from GrIS do not consider the infiltration process; possibly underestimating the actual runoff.
![Meltwater Infilltration [sic] in the Accumulation Zone, West Greenland Ice Sheet PDF](https://books.google.com/books/content/images/frontcover/5hSqnQAACAAJ?fife=w80-h100)
Author: Daniel J. Sturgis
Publisher:
ISBN: 9781109532845
Category : Ice sheets
Languages : en
Pages : 80
Get Book Here
Book Description
Surface meltwater generated in the accumulation zone of the Greenland Ice Sheet (GrIS) will either be retained by refreezing or connect to the glacial drainage system and contribute to annual runoff. The fate of this meltwater is controlled by the infiltration process, which occurs in the presence of subfreezing snow/firnpack temperatures and ice layers. Ice layers are typically treated as impermeable horizons. However, dye-trace observations suggest ice layers do not impede flow but rather accelerate flow by destabilizing the wetting-front, forming preferential flow paths termed pipes. Until the 2008 field season, the permeability of ice layers formed in the snow/firnpack on GrIS was unmeasured. Air permeameter measurements show ice layer permeability to range from 10 -15 m 2 to 10 -12 m 2 and firn to be approximately 10 -11 m 2 . Temperature profile measurements of the snow/firnpack were recorded every 30 min during the 2007 melt season. Temperature profile data confirms piping as a mechanism for meltwater delivery to 10+ m depths at T1 without increasing the full snow/firnpack temperature to 0°C. Meltwater that is piped to the glacier-ice surface can connect to the glacial drainage system and runoff. Current models used to estimate annual runoff from GrIS do not consider the infiltration process; possibly underestimating the actual runoff.
Author: W. Tad Pfeffer
Publisher: Frontiers Media SA
ISBN: 2889456196
Category :
Languages : en
Pages : 160
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Book Description
Melt takes place where the surface of glaciers or ice sheets interacts with the atmosphere. While the processes governing surface melt are fairly well understood, the pathways of the meltwater, from its origin to the moment it leaves a glacier system, remain enigmatic. It is not even guaranteed that meltwater leaves a glacier or ice sheet. On Greenland, for example, only slightly more than 50% of the meltwater runs off. The remainder mostly refreezes within the so-called firn cover of the ice sheet. This eBook contains 11 studies which tackle the challenge of understanding meltwater retention in snow and firn from various angles. The studies focus both on mountain glaciers and on the Greenland ice sheet and address challenges such as measuring firn properties, quantifying their influence on meltwater retention, modelling firn processes and meltwater refreezing as well as unravelling the mechanisms within the recently discovered Greenland firn aquifers.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 9
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Atmospheric warming over the Greenland Ice Sheet during the last 2 decades has increased the amount of surface meltwater production, resulting in the migration of melt and percolation regimes to higher altitudes and an increase in the amount of ice content from refrozen meltwater found in the firn above the superimposed ice zone. Here we present field and airborne radar observations of buried ice layers within the near-surface (0-20 m) firn in western Greenland, obtained from campaigns between 1998 and 2014. We find a sharp increase in firn-ice content in the form of thick widespread layers in the percolation zone, which decreases the capacity of the firn to store meltwater. The estimated total annual ice content retained in the near-surface firn in areas with positive surface mass balance west of the ice divide in Greenland reached a maximum of 74 " 25 Gt in 2012, compared to the 1958-1999 average of 13 " 2 Gt, while the percolation zone area more than doubled between 2003 and 2012. Increased melt and column densification resulted in surface lowering averaging -0.80 " 0.39 m yr−1 between 1800 and 2800 m in the accumulation zone of western Greenland. Since 2007, modeled annual melt and refreezing rates in the percolation zone at elevations below 2100 m surpass the annual snowfall from the previous year, implying that mass gain in the region is retained after melt in the form of refrozen meltwater. If current melt trends over high elevation regions continue, subsequent changes in firn structure will have implications for the hydrology of the ice sheet and related abrupt seasonal densification could become increasingly significant for altimetry-derived ice sheet mass balance estimates.
Author:
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ISBN:
Category :
Languages : en
Pages :
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The temporal variability in surface snow and glacier melt flux and runoff were investigated for the ablation area of lakobshavn Isbrae, West Greenland. High-resolution meteorological observations both on and outside the Greenland Ice Sheet (GrIS) were used as model input. Realistic descriptions of snow accumulation, snow and glacier-ice melt, and runoff are essential to understand trends in ice sheet surface properties and processes. SnowModel, a physically based, spatially distributed meteorological and snow-evolution modeling system was used to simulate the temporal variability of lakobshavn Isbrre accumulation and ablation processes for 2000/01-2006/07. Winter snow-depth observations and MODIS satellite-derived summer melt observations were used for model validation of accumulation and ablation. Simulations agreed well with observed values. Simulated annual surface melt varied from as low as 3.83 x 109 m3 (2001/02) to as high as 8.64 x 109 m3 (2004/05). Modeled surface melt occurred at elevations reaching 1,870 m a.s.l. for 2004/05, while the equilibrium line altitude (ELA) fluctuated from 990 to 1,210 m a.s.l. during the simulation period. The SnowModel meltwater retention and refreezing routines considerably reduce the amount of meltwater available as ice sheet runoff; without these routines the lakobshavn surface runoff would be overestimated by an average of 80%. From September/October through May/June no runoff events were simulated. The modeled interannual runoff variability varied from 1.81 x 109 m3 (2001/02) to 5.21 x 109 m3 (2004/05), yielding a cumulative runoff at the Jakobshavn glacier terminus of ≈2.25 m w.eq. to ≈4.5 m w.eq., respectively. The average modeled lakobshavn runoff of ≈3.4 km3 y−1 was merged with previous estimates of Jakobshavn ice discharge to quantify the freshwater flux to Illulissat Icefiord. For both runoff and ice discharge the average trends are similar, indicating increasing (insignificant) influx of freshwater to the Illulissat Icefiord for the period 2000/01-2006/07. This study suggests that surface runoff forms a minor part of the overall Jakobshavn freshwater flux to the fiord: around 7% (≈3.4 km3 y−1) of the average annual freshwater flux of ≈51.0 km3 y−1 originates from the surface runoff.
Author: Laura A. Stevens
Publisher:
ISBN:
Category : Glaciers
Languages : en
Pages : 220
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Seasonal fluxes of meltwater control ice-flow processes across the Greenland Ice Sheet ablation zone and subglacial discharge at marine-terminating outlet glaciers. With the increase in annual ice sheet meltwater production observed over recent decades and predicted into future decades, understanding mechanisms driving the hourly to decadal impact of meltwater on ice flow is critical for predicting Greenland Ice Sheet dynamic mass loss. This thesis investigates a wide range of meltwater-driven processes using empirical and theoretical methods for a region of the western margin of the Greenland Ice Sheet. I begin with an examination of the seasonal and annual ice flow record for the region using in situ observations of ice flow from a network of Global Positioning System (GPS) stations. Annual velocities decrease over the seven-year time-series at a rate consistent with the negative trend in annual velocities observed in neighboring regions. Using observations from the same GPS network, I next determine the trigger mechanism for rapid drainage of a supraglacial lake. In three consecutive years, I find precursory basal slip and uplift in the lake basin generates tensile stresses that promote hydrofracture beneath the lake. As these precursors are likely associated with the introduction of meltwater to the bed through neighboring moulin systems, our results imply that lakes may be less able to drain in the less crevassed, interior regions of the ice sheet. Expanding spatial scales to the full ablation zone, I then use a numerical model of subglacial hydrology to test whether model-derived effective pressures exhibit the theorized inverse relationship with melt-season ice sheet surface velocities. Finally, I pair near-ice fjord hydrographic observations with modeled and observed subglacial discharge for the Saqqardliup sermia–Sarqardleq Fjord system. I find evidence of two types of glacially modified waters whose distinct properties and locations in the fjord align with subglacial discharge from two prominent subcatchments beneath Saqqardliup sermia. Continued observational and theoretical work reaching across discipline boundaries is required to further narrow our gap in understanding the forcing mechanisms and magnitude of Greenland Ice Sheet dynamic mass loss.
Author: Jianhua Zou
Publisher:
ISBN:
Category : Ice sheets
Languages : en
Pages : 462
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Author: Christopher R. Cox
Publisher:
ISBN: 9781267675880
Category : Climatic changes
Languages : en
Pages : 93
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Firn temperature profiles from the accumulation zone of the Greenland ice sheet are used to estimate thermal conductivities and melt water refreezing quantities. Firn thermal conductivity values are calculated from temperature profiles using a new optimization method. The results indicate that polar snowpack thermal conductivities may be significantly higher as a function of density than previously published empirical regressions would suggest. The quantity of melt water refreezing at each site is determined using temperature profiles and a heat conduction model. The heat conduction model is used to partition the seasonal heating of firn into heat conducted from the surface and heat released during refreezing. The heat from refreezing is then converted to a quantity of water using the latent heat of fusion. The refreezing values can be used to verify previously published refreezing estimates, thereby helping reduce uncertainty in Greenland surface mass balance calculations.
Author: Samiah Moustafa
Publisher:
ISBN:
Category : Climatic changes
Languages : en
Pages : 207
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Author: Kristin Poinar
Publisher:
ISBN:
Category : Glaciers
Languages : en
Pages : 152
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As the climate has warmed over the past decades, the amount of melt on the Greenland Ice Sheet has increased, and areas higher on the ice sheet have begun to melt regularly. This increase in melt has been hypothesized to enhance ice flow in myriad ways, including through basal lubrication and englacial refreezing. By developing and interpreting thermal ice-sheet models and analyzing remote sensing data, I evaluate the effect of these processes on ice flow and sea-level rise from the Greenland Ice Sheet. I first develop a thermal ice sheet model that is applicable to western Greenland. Key components of this model are its treatment of multiple phases (solid ice and liquid water) and its viscosity-dependent velocity field. I apply the model to Jakobshavn Isbræ, a fast-flowing outlet glacier. This is an important benchmark for my model, which I next apply to the topics outlined above. I use the thermal model to calculate the effect of englacial latent-heat transfer (meltwater refreezing within englacial features such as firn and crevasses) on ice dynamics in western Greenland. I find that in slow-moving areas, this can significantly warm the ice, but that englacial latent heat transfer has only a minimal effect on ice motion (10%). By contrast, in fast-flowing regions, which contribute most (60%) of the ice flux into the ocean, evidence of deep englacial warming is virtually absent. Thus, the effects of englacial latent heat transfer on ice motion are likely limited to slow-moving regions, which limits its importance to ice-sheet mass balance. Next, I couple a model for ice fracture to a modified version of my thermal model to calculate the depth and shape evolution of water-filled crevasses that form in crevasse fields. At most elevations and for typical water input volumes, crevasses penetrate to the top ~200–300 meters depth, warm the ice there by ~10°C, and may persist englacially, in a liquid state, for multiple decades. The surface hydrological network limits the amount of water that can reach most crevasses. We find that the depth and longevity of such crevasses is relatively robust to realistic increases in melt volumes over the coming century, so that we should not expect large changes in the englacial hydrological system under near-future climate regimes. These inferences put important constraints on the timescales of the Greenland supraglacial-to-subglacial water cycle. Finally, I assess the likelihood that higher-elevation surface melt could deliver water to regions where the bed is currently frozen. This hypothetical process is important because it could potentially greatly accelerate the seaward motion of the ice sheet. By analyzing surface strain rates and comparing them to my modeled basal temperature field, I find that this scenario is unlikely to occur: the conditions necessary to form surface-to-bed conduits are rarely found at higher elevations (~1600 meters) that may overlie frozen beds.
Author: Clément Miège
Publisher:
ISBN:
Category : Ice sheets
Languages : en
Pages : 167
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