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Stefan R M Ligtenberg - One of the best experts on this subject based on the ideXlab platform.
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brief communication improved simulation of the present day greenland Firn layer 1960 2016
The Cryosphere, 2018Co-Authors: Stefan R M Ligtenberg, Peter Kuipers Munneke, Brice Noel, Michiel R Van Den BroekeAbstract:Abstract. By providing pore space for storage or refreezing of meltwater, the Greenland ice sheet Firn layer strongly modulates runoff. Correctly representing the Firn layer is therefore crucial for Greenland (surface) mass balance studies. Here, we present a simulation of the Greenland Firn layer with the Firn model IMAU-FDM forced by the latest output of the regional climate model RACMO2, version 2.3p2. In the percolation zone, much improved agreement is found with Firn density and temperature observations. A full simulation of Greenland Firn at high temporal (10 days) and spatial (11 km) resolution is available for the period 1960–2016.
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Firn model intercomparison experiment Firnmice
Journal of Glaciology, 2017Co-Authors: Jessica Lundin, Anais Orsi, Max C Stevens, Stefan R M Ligtenberg, Robert J Arthern, Christo Buizert, Sebastian B Simonsen, Evan Cummings, Richard Essery, Will LeahyAbstract:Evolution of cold dry snow and Firn plays important roles in glaciology; however, the physical formulation of a densification law is still an active research topic. We forced eight Firn-densification models and one seasonal-snow model in six different experiments by imposing step changes in temperature and accumulation-rate boundary conditions; all of the boundary conditions were chosen to simulate Firn densification in cold, dry environments. While the intended application of the participating models varies, they are describing the same physical system and should in principle yield the same solutions. The Firn models all produce plausible depth-density profiles, but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research. These differences demonstrate that Firn-densification models are incorrectly or incompletely representing physical processes. We quantitatively characterize the differences among the results from the various models. For example, we find depth-integrated porosity is unlikely to be inferred with confidence from a Firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature. Firn Model Intercomparison Experiment can provide a benchmark of results for future models, provide a basis to quantify model uncertainties and guide future directions of Firn-densification modeling.
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hydraulic conductivity of a Firn aquifer in southeast greenland
Frontiers in Earth Science, 2017Co-Authors: Ol Miller, Stefan R M Ligtenberg, Richard R Forster, Lora S Koenig, Clément Miège, Kip D Solomon, Lynn Montgomery, Nicholas Schmerr, Anatoly Legchenko, Ludovic BruckerAbstract:Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the Firn pore space throughout the year. These Firn aquifers, found between ~10-30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the Firn. Here we describe the hydraulic conductivity of the Firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5x10-5 and 1.1x10-3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7x10-4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the Firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.
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Firn meltwater retention on the greenland ice sheet a model comparison
Frontiers in Earth Science, 2017Co-Authors: Christian Steger, Peter Kuipers Munneke, Michiel R Van Den Broeke, C H Reijmer, Nander Wever, Richard R Forster, Lora S Koenig, Michael Lehning, Stef Lhermitte, Stefan R M LigtenbergAbstract:Runoff has recently become the main source of mass loss from the Greenland Ice Sheet and is an important contributor to global sea level rise. Linking runoff to surface meltwater production is complex, as meltwater can be retained within the Firn by refreezing or perennial liquid water storage. To constrain these uncertainties, the outputs of two offline snow/Firn models of different complexity (IMAU-FDM and SNOWPACK) are compared to assess the sensitivity of meltwater retention to the model formulation (e.g., densification, irreducible water content, vertical resolution). Results indicate that model differences are largest in areas where Firn aquifers form, i.e., particularly along the south-eastern margin of the ice sheet. The IMAU-FDM simulates higher densification rates for such climatic conditions and prescribes a lower irreducible water content than SNOWPACK. As a result, the model predicts substantially lower amounts of refreezing and liquid water storage. SNOWPACK performs better for this area, confirmed both by density profiles from Firn cores and radar-inferred observations. Refreezing integrated over the entire ice sheet and averaged for the period 1960 - 2014 amounts to 216 Gt a-1 (IMAU-FDM) and 242 Gt a-1 (SNOWPACK), which is 41% and 46% of the total liquid water input (snowmelt and rainfall). The mean areal extents of perennial Firn aquifers for 2010 - 2014 simulated by the models are 55,700 km2 (IMAU-FDM) and 90,200 km2 (SNOWPACK). Discrepancies between modeled Firn profiles and observations emphasize the importance of processes currently not accounted for in most snow/Firn models, such as vertical heterogeneous percolation, ponding of water on impermeable layers, lateral (sub-)surface water flow, and the issue of ill-constrained refreezing conditions at the base of Firn aquifers.
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elevation change of the greenland ice sheet due to surface mass balance and Firn processes 1960 2014
The Cryosphere, 2015Co-Authors: Stefan R M Ligtenberg, Brice Noel, Kuipers P Munneke, Ian M Howat, Jason E Box, Ellen Mosleythompson, Joseph R Mcconnell, Konrad Steffen, Joel T HarperAbstract:Abstract. Observed changes in the surface elevation of the Greenland Ice Sheet are caused by ice dynamics, basal elevation change, basal melt, surface mass balance (SMB) variability, and by compaction of the overlying Firn. The last two contributions are quantified here using a Firn model that includes compaction, meltwater percolation, and refreezing. The model is forced with surface mass fluxes and temperature from a regional climate model for the period 1960–2014. The model results agree with observations of surface density, density profiles from 62 Firn cores, and altimetric observations from regions where ice-dynamical surface height changes are likely small. In areas with strong surface melt, the Firn model overestimates density. We find that the Firn layer in the high interior is generally thickening slowly (1–5 cm yr−1). In the percolation and ablation areas, Firn and SMB processes account for a surface elevation lowering of up to 20–50 cm yr−1. Most of this Firn-induced marginal thinning is caused by an increase in melt since the mid-1990s and partly compensated by an increase in the accumulation of fresh snow around most of the ice sheet. The total Firn and ice volume change between 1980 and 2014 is estimated at −3295 ± 1030 km3 due to Firn and SMB changes, corresponding to an ice-sheet average thinning of 1.96 ± 0.61 m. Most of this volume decrease occurred after 1995. The computed changes in surface elevation can be used to partition altimetrically observed volume change into surface mass balance and ice-dynamically related mass changes.
Baptiste Vandecrux - One of the best experts on this subject based on the ideXlab platform.
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the Firn meltwater retention model intercomparison project retmip evaluation of nine Firn models at four weather station sites on the greenland ice sheet
The Cryosphere, 2020Co-Authors: Robert S Fausto, Max C Stevens, Vincent Verjans, Baptiste Vandecrux, Ruth Mottram, Peter L Langen, Martin Olesen, Amber LeesonAbstract:Abstract. Perennial snow, or Firn, covers 80 % of the Greenland ice sheet and has the capacity to retain part of the surface meltwater, buffering the ice sheet’s contribution to sea level. Multi-layer Firn models are traditionally used to simulate the Firn processes and estimate meltwater retention. We present the output from nine Firn models, forced by weather-station-derived mass and energy fluxes at four sites representative of the dry snow, percolation, ice slab and Firn aquifer areas. We compare the model outputs and evaluate them against in situ observations. Models that explicitly account for deep meltwater percolation overestimate percolation depth and consequently Firn temperature at the percolation and ice slab sites although they accurately simulate the recharge of the Firn aquifer. Models using Darcy's law and a bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate Firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models, that transfer Firn through fixed layers, smooth sharp gradients in Firn temperature and density over time. From the model spread, we find that simulated densities (respectively temperature) have an uncertainty envelope of ±60 kg m−3 (resp. ±14 °C) in the dry snow area and up to ±280 kg m−3 (resp. ±15–18 °C) at warmer sites.
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Firn cold content evolution at nine sites on the greenland ice sheet between 1998 and 2017
Journal of Glaciology, 2020Co-Authors: Baptiste Vandecrux, Robert S Fausto, William Colgan, Dirk Van As, Achim Heilig, Peter L Langen, K Haubner, Thomas Ingemannielsen, C Stevens, Michael MacferrinAbstract:Current sea-level rise partly stems from increased surface melting and meltwater runoff from the Greenland ice sheet. Multi-year snow, also known as Firn, covers about 80% of the ice sheet and retains part of the surface meltwater. Since the Firn cold content integrates its physical and thermal characteristics, it is a valuable tool for determining the meltwater-retention potential of Firn. We use gap-filled climatological data from nine automatic weather stations in the ice-sheet accumulation area to drive a surface-energy-budget and Firn model, validated against Firn density and temperature observations, over the 1998–2017 period. Our results show a stable top 20 m Firn cold content (CC20) at most sites. Only at the lower-elevation Dye-2 site did CC20 decrease, by 24% in 2012, before recovering to its original value by 2017. Heat conduction towards the surface is the main process feeding CC20 at all nine sites, while CC20 reduction occurs through low-cold-content fresh-snow addition at the surface during snowfall and latent-heat release when meltwater refreezes. Our simulations suggest that Firn densification, while reducing pore space for meltwater retention, increases the Firn cold content, enhances near-surface meltwater refreezing and potentially sets favourable conditions for ice-slab formation.
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Firn data compilation reveals widespread decrease of Firn air content in western Greenland
Copernicus Publications, 2019Co-Authors: A. Heilig, M. Macferrin, Baptiste Vandecrux, Horst Machguth, Dirk Van As, W. T. Colgan, C. M. Stevens, Charalampos CharalampidisAbstract:A porous layer of multi-year snow known as Firn covers the Greenland-ice-sheet interior. The Firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet Firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire Firn column before interpolating FAC over the entire ice-sheet Firn area as an empirical function of long-term mean air temperature (Ta‾) and net snow accumulation (c˙‾). We estimate a total ice-sheet-wide FAC of 26 800±1840 km3, of which 6500±450 km3 resides within the uppermost 10 m of Firn, for the 2010–2017 period. In the dry snow area (Ta‾≤-19 ∘C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (Ta‾>-19 ∘C and c˙‾≤600 mm w.e. yr−1), FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of Firn retention capacity of between 150±100 Gt and 540±440 Gt, respectively, from the top 10 m and entire Firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and Firn retention capacity.
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brief communication Firn data compilation reveals the evolution of the Firn air content on the greenland ice sheet
The Cryosphere Discussions, 2018Co-Authors: Max C Stevens, Baptiste Vandecrux, Michael Macferrin, Horst Machguth, William Colgan, Dirk Van As, Achim Heilig, Charalampos CharalampidisAbstract:Abstract. The Firn covering the Greenland ice sheet interior can retain part of the surface melt, buffering the ice sheet’s contribution to sea level, but its characteristics are still little known. Using remote-sensing observations from 2000–2017, we estimate that Firn covers 1,405,500 ± 17,250 km 2 of the ice sheet. We present 344 Firn-core-derived observations of the top 10 m Firn air content (FAC 10 ), indicative of the Firn’s meltwater retention capacity. FAC 10 remained stable in the coldest 74 % of the Firn area during 1953–2017, while FAC 10 decreased in the warmest and driest 12 % of the Firn area between 1997–2008 and 2011–2017, resulting in a loss of 180 ± 78 km 3 (−26 ± 11 %) of air from the near-surface Firn.
Max C Stevens - One of the best experts on this subject based on the ideXlab platform.
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numerical experiments on Firn isotope diffusion with the community Firn model
Journal of Glaciology, 2021Co-Authors: Vasileios Gkinis, Max C Stevens, Emma C Kahle, Christian Holme, Eric J Steig, Bo M VintherAbstract:Advances in analytical methods have made it possible to obtain high-resolution water isotopic data from ice cores. Their spectral signature contains information on the diffusion process that attenuated the isotopic signal during the Firn densification process. Here, we provide a tool for estimating Firn-diffusion rates that builds on the Community Firn Model. Our model requires two main inputs, temperature and accumulation, and it calculates the diffusion lengths for δ17O, δ18O and δD. Prior information on the isotopic signal of the precipitation is not a requirement. In combination with deconvolution techniques, diffusion lengths can be used in order reconstruct the pre-diffusion isotopic signal. Furthermore, the temperature dependence of the isotope diffusion and Firn densification makes the diffusion length an interesting candidate as a temperature proxy. We test the model under steady state and transient scenarios and compare four densification models. Comparisons with ice core data provide an evaluation of the four models and indicate that there are differences in their performance. Combining data-based diffusion length estimates with information on past accumulation rates and ice flow thinning, we reconstruct absolute temperatures from three Antarctic ice core sites.
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effect of horizontal divergence on estimates of Firn air content
Journal of Glaciology, 2021Co-Authors: Annika N Horlings, Knut Christianson, Max C Stevens, Nicholas Holschuh, Edwin D WaddingtonAbstract:Ice-sheet mass-balance estimates derived from repeat satellite-altimetry observations require accurate calculation of spatiotemporal variability in Firn-air content (FAC). However, Firn-compaction models remain a large source of uncertainty within mass-balance estimates. In this study, we investigate one process that is neglected in FAC estimates derived from Firn-compaction models: enhanced layer thinning due to horizontal divergence. We incorporate a layer-thinning scheme into the Community Firn Model. At every time step, Firn layers first densify according to a Firn-compaction model and then thin further due to an imposed horizontal divergence rate without additional density changes. We find that horizontal divergence on Thwaites (THW) and Pine Island Glaciers can reduce local FAC by up to 41% and 18%, respectively. We also assess the impact of temporal variability of horizontal divergence on FAC. We find a 15% decrease in FAC between 2007 and 2016 due to horizontal divergence at a location that is characteristic of lower THW. This decrease accounts for 16% of the observed surface lowering, whereas climate variability alone causes negligible changes in FAC at this location. Omitting transient horizontal divergence in estimates of FAC leads to an overestimation of ice loss via satellite-altimetry methods in regions of dynamic ice flow.
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the Firn meltwater retention model intercomparison project retmip evaluation of nine Firn models at four weather station sites on the greenland ice sheet
The Cryosphere, 2020Co-Authors: Robert S Fausto, Max C Stevens, Vincent Verjans, Baptiste Vandecrux, Ruth Mottram, Peter L Langen, Martin Olesen, Amber LeesonAbstract:Abstract. Perennial snow, or Firn, covers 80 % of the Greenland ice sheet and has the capacity to retain part of the surface meltwater, buffering the ice sheet’s contribution to sea level. Multi-layer Firn models are traditionally used to simulate the Firn processes and estimate meltwater retention. We present the output from nine Firn models, forced by weather-station-derived mass and energy fluxes at four sites representative of the dry snow, percolation, ice slab and Firn aquifer areas. We compare the model outputs and evaluate them against in situ observations. Models that explicitly account for deep meltwater percolation overestimate percolation depth and consequently Firn temperature at the percolation and ice slab sites although they accurately simulate the recharge of the Firn aquifer. Models using Darcy's law and a bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate Firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models, that transfer Firn through fixed layers, smooth sharp gradients in Firn temperature and density over time. From the model spread, we find that simulated densities (respectively temperature) have an uncertainty envelope of ±60 kg m−3 (resp. ±14 °C) in the dry snow area and up to ±280 kg m−3 (resp. ±15–18 °C) at warmer sites.
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the community Firn model cfm v1 0
Geoscientific Model Development, 2020Co-Authors: Max C Stevens, Vincent Verjans, Jessica M D Lundin, Emma C Kahle, Annika N Horlings, Brita I Horlings, Edwin D WaddingtonAbstract:Abstract. Models that simulate evolution of polar Firn are important for several applications in glaciology, including converting ice-sheet elevation-change measurements to mass change and interpreting climate records in ice cores. We have developed the Community Firn Model (CFM), an open-source, modular model framework designed to simulate numerous physical processes in Firn. The modules include Firn densification, heat transport, meltwater percolation and refreezing, water-isotope diffusion, and Firn-air diffusion. The CFM is designed so that new modules can be added with ease. In this paper, we first describe the CFM and its modules. We then demonstrate the CFM's usefulness in two model applications that utilize two of its novel aspects. The CFM currently has the ability to run any of 13 previously published Firn-densification models, and in the first application we compare those models' results when they are forced with regional climate model outputs for Summit, Greenland. The results show that the models do not agree well (spread greater than 10 %) when predicting depth-integrated porosity, Firn age, or trend in surface-elevation change trend. In the second application, we show that the CFM's coupled Firn-air and Firn-densification models can simulate noble-gas records from an ice core better than a Firn-air model alone.
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bayesian calibration of Firn densification models
The Cryosphere, 2020Co-Authors: Vincent Verjans, Max C Stevens, Peter Kuipers Munneke, Brice Noel, Amber Leeson, Christopher Nemeth, Jan Melchior Van WessemAbstract:Abstract. Firn densification modelling is key to understanding ice sheet mass balance, ice sheet surface elevation change, and the age difference between ice and the air in enclosed air bubbles. This has resulted in the development of many Firn models, all relying to a certain degree on parameter calibration against observed data. We present a novel Bayesian calibration method for these parameters and apply it to three existing Firn models. Using an extensive dataset of Firn cores from Greenland and Antarctica, we reach optimal parameter estimates applicable to both ice sheets. We then use these to simulate Firn density and evaluate against independent observations. Our simulations show a significant decrease (24 % and 56 %) in observation–model discrepancy for two models and a smaller increase (15 %) for the third. As opposed to current methods, the Bayesian framework allows for robust uncertainty analysis related to parameter values. Based on our results, we review some inherent model assumptions and demonstrate how Firn model choice and uncertainties in parameter values cause spread in key model outputs.
Richard R Forster - One of the best experts on this subject based on the ideXlab platform.
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hydraulic conductivity of a Firn aquifer in southeast greenland
Frontiers in Earth Science, 2017Co-Authors: Ol Miller, Stefan R M Ligtenberg, Richard R Forster, Lora S Koenig, Clément Miège, Kip D Solomon, Lynn Montgomery, Nicholas Schmerr, Anatoly Legchenko, Ludovic BruckerAbstract:Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the Firn pore space throughout the year. These Firn aquifers, found between ~10-30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the Firn. Here we describe the hydraulic conductivity of the Firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5x10-5 and 1.1x10-3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7x10-4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the Firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.
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Firn meltwater retention on the greenland ice sheet a model comparison
Frontiers in Earth Science, 2017Co-Authors: Christian Steger, Peter Kuipers Munneke, Michiel R Van Den Broeke, C H Reijmer, Nander Wever, Richard R Forster, Lora S Koenig, Michael Lehning, Stef Lhermitte, Stefan R M LigtenbergAbstract:Runoff has recently become the main source of mass loss from the Greenland Ice Sheet and is an important contributor to global sea level rise. Linking runoff to surface meltwater production is complex, as meltwater can be retained within the Firn by refreezing or perennial liquid water storage. To constrain these uncertainties, the outputs of two offline snow/Firn models of different complexity (IMAU-FDM and SNOWPACK) are compared to assess the sensitivity of meltwater retention to the model formulation (e.g., densification, irreducible water content, vertical resolution). Results indicate that model differences are largest in areas where Firn aquifers form, i.e., particularly along the south-eastern margin of the ice sheet. The IMAU-FDM simulates higher densification rates for such climatic conditions and prescribes a lower irreducible water content than SNOWPACK. As a result, the model predicts substantially lower amounts of refreezing and liquid water storage. SNOWPACK performs better for this area, confirmed both by density profiles from Firn cores and radar-inferred observations. Refreezing integrated over the entire ice sheet and averaged for the period 1960 - 2014 amounts to 216 Gt a-1 (IMAU-FDM) and 242 Gt a-1 (SNOWPACK), which is 41% and 46% of the total liquid water input (snowmelt and rainfall). The mean areal extents of perennial Firn aquifers for 2010 - 2014 simulated by the models are 55,700 km2 (IMAU-FDM) and 90,200 km2 (SNOWPACK). Discrepancies between modeled Firn profiles and observations emphasize the importance of processes currently not accounted for in most snow/Firn models, such as vertical heterogeneous percolation, ponding of water on impermeable layers, lateral (sub-)surface water flow, and the issue of ill-constrained refreezing conditions at the base of Firn aquifers.
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spatial extent and temporal variability of greenland Firn aquifers detected by ground and airborne radars
Journal of Geophysical Research, 2016Co-Authors: Clément Miège, Jason E Box, Richard R Forster, Lora S Koenig, Evan W. Burgess, Ludovic Brucker, Kip D Solomon, John Paden, Julie MillerAbstract:We document the existence of widespread Firn aquifers in an elevation range of ~1200–2000 m, in the high snow-accumulation regions of the Greenland ice sheet. We use NASA Operation IceBridge accumulation radar data from five campaigns (2010–2014) to estimate a Firn-aquifer total extent of 21,900 km2. We investigate two locations in Southeast Greenland, where repeated radar profiles allow mapping of aquifer-extent and water table variations. In the upper part of Helheim Glacier the water table rises in spring following above-average summer melt, showing the direct Firn-aquifer response to surface meltwater production changes. After spring 2012, a drainage of the Firn-aquifer lower margin (5 km) is inferred from both 750 MHz accumulation radar and 195 MHz multicoherent radar depth sounder data. For 2011–2014, we use a ground-penetrating radar profile located at our Ridgeline field site and find a spatially stable aquifer with a water table fluctuating less than 2.5 m vertically. When combining radar data with surface topography, we find that the upper elevation edge of Firn aquifers is located directly downstream of locally high surface slopes. Using a steady state 2-D groundwater flow model, water is simulated to flow laterally in an unconfined aquifer, topographically driven by ice sheet surface undulations until the water encounters crevasses. Simulations suggest that local flow cells form within the Helheim aquifer, allowing water to discharge in the Firn at the steep-to-flat transitions of surface topography. Supported by visible imagery, we infer that water drains into crevasses, but its volume and rate remain unconstrained.
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the effects of dating uncertainties on net accumulation estimates from Firn cores
Journal of Glaciology, 2015Co-Authors: Summer Rupper, Lora S Koenig, Clément Miège, William F Christensen, Barry R Bickmore, Landon Burgener, Michelle Koutnik, Richard R ForsterAbstract:The mean, trend and variability of net snow accumulation in Firn cores are often used to validate model output, develop remote-sensing algorithms and quantify ice-sheet surface mass balance. Thus, accurately defining uncertainties associated with these in situ measurements is critical. In this study, we apply statistical simulation methods to quantify the uncertainty in Firn-core accumulation data due to the uncertainty in depth–age scales. The methods are applied to a suite of Firn cores from central West Antarctica. The results show that uncertainty in depth–age scales can give rise to spurious trends in accumulation that are the same order of magnitude as accumulation trends reported in West Antarctica. The depth–age scale uncertainties also significantly increase the apparent interannual accumulation variability, so these uncertainties must first be accounted for before using Firn-core data to assess such processes as small-spatial-scale variability. Better quantification of error in accumulation will improve our ability to meaningfully compare Firn-core data across different regions of the ice sheet, and provide appropriate targets for calibration and/or validation of model output and remote-sensing data.
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extensive liquid meltwater storage in Firn within the greenland ice sheet
Nature Geoscience, 2014Co-Authors: Richard R Forster, Michiel R Van Den Broeke, Jason E Box, Lora S Koenig, Clément Miège, Evan W. Burgess, J H Van Angelen, Jan T M Lenaerts, John PadenAbstract:Surface melt water from the Greenland ice sheet can become trapped in Firn, delaying its journey to the sea. Radar and ice-core observations provide direct evidence of a perennial aquifer in the Firn layer in southern Greenland that represents a potentially significant contribution to the Greenland mass budget.
Clément Miège - One of the best experts on this subject based on the ideXlab platform.
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sentinel 1 detects Firn aquifers in the greenland ice sheet
Geophysical Research Letters, 2020Co-Authors: Isis Brangers, Clément Miège, Ludovic Brucker, Hans Lievens, Matthias Demuzere, G De LannoyAbstract:Firn aquifers in Greenland store liquid water within the upper ice sheet and impact the hydrological system. Their location and area have been estimated with airborne radar sounder surveys (Operation IceBridge, OIB). However, the OIB coverage is limited to narrow flight lines, offering an incomplete view. Here, we show the ability of satellite radar measurements from Sentinel-1 to map Firn aquifers across all of Greenland at 1 km(2) resolution. The detection of aquifers relies on a delay in the freezing of meltwater within the Firn above the water table, causing a distinctive pattern in the radar backscatter. The Sentinel-1 aquifer locations are in very good agreement with those detected along the OIB flight lines (Cohen's kappa = 0.84). The total aquifer area is estimated at 54,800 km(2). With continuity of Sentinel-1 ensured until 2030, our study lays a foundation for monitoring the future response of Firn aquifers to climate change.
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hydraulic conductivity of a Firn aquifer in southeast greenland
Frontiers in Earth Science, 2017Co-Authors: Ol Miller, Stefan R M Ligtenberg, Richard R Forster, Lora S Koenig, Clément Miège, Kip D Solomon, Lynn Montgomery, Nicholas Schmerr, Anatoly Legchenko, Ludovic BruckerAbstract:Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the Firn pore space throughout the year. These Firn aquifers, found between ~10-30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the Firn. Here we describe the hydraulic conductivity of the Firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5x10-5 and 1.1x10-3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7x10-4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the Firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.
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spatial extent and temporal variability of greenland Firn aquifers detected by ground and airborne radars
Journal of Geophysical Research, 2016Co-Authors: Clément Miège, Jason E Box, Richard R Forster, Lora S Koenig, Evan W. Burgess, Ludovic Brucker, Kip D Solomon, John Paden, Julie MillerAbstract:We document the existence of widespread Firn aquifers in an elevation range of ~1200–2000 m, in the high snow-accumulation regions of the Greenland ice sheet. We use NASA Operation IceBridge accumulation radar data from five campaigns (2010–2014) to estimate a Firn-aquifer total extent of 21,900 km2. We investigate two locations in Southeast Greenland, where repeated radar profiles allow mapping of aquifer-extent and water table variations. In the upper part of Helheim Glacier the water table rises in spring following above-average summer melt, showing the direct Firn-aquifer response to surface meltwater production changes. After spring 2012, a drainage of the Firn-aquifer lower margin (5 km) is inferred from both 750 MHz accumulation radar and 195 MHz multicoherent radar depth sounder data. For 2011–2014, we use a ground-penetrating radar profile located at our Ridgeline field site and find a spatially stable aquifer with a water table fluctuating less than 2.5 m vertically. When combining radar data with surface topography, we find that the upper elevation edge of Firn aquifers is located directly downstream of locally high surface slopes. Using a steady state 2-D groundwater flow model, water is simulated to flow laterally in an unconfined aquifer, topographically driven by ice sheet surface undulations until the water encounters crevasses. Simulations suggest that local flow cells form within the Helheim aquifer, allowing water to discharge in the Firn at the steep-to-flat transitions of surface topography. Supported by visible imagery, we infer that water drains into crevasses, but its volume and rate remain unconstrained.
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the effects of dating uncertainties on net accumulation estimates from Firn cores
Journal of Glaciology, 2015Co-Authors: Summer Rupper, Lora S Koenig, Clément Miège, William F Christensen, Barry R Bickmore, Landon Burgener, Michelle Koutnik, Richard R ForsterAbstract:The mean, trend and variability of net snow accumulation in Firn cores are often used to validate model output, develop remote-sensing algorithms and quantify ice-sheet surface mass balance. Thus, accurately defining uncertainties associated with these in situ measurements is critical. In this study, we apply statistical simulation methods to quantify the uncertainty in Firn-core accumulation data due to the uncertainty in depth–age scales. The methods are applied to a suite of Firn cores from central West Antarctica. The results show that uncertainty in depth–age scales can give rise to spurious trends in accumulation that are the same order of magnitude as accumulation trends reported in West Antarctica. The depth–age scale uncertainties also significantly increase the apparent interannual accumulation variability, so these uncertainties must first be accounted for before using Firn-core data to assess such processes as small-spatial-scale variability. Better quantification of error in accumulation will improve our ability to meaningfully compare Firn-core data across different regions of the ice sheet, and provide appropriate targets for calibration and/or validation of model output and remote-sensing data.
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extensive liquid meltwater storage in Firn within the greenland ice sheet
Nature Geoscience, 2014Co-Authors: Richard R Forster, Michiel R Van Den Broeke, Jason E Box, Lora S Koenig, Clément Miège, Evan W. Burgess, J H Van Angelen, Jan T M Lenaerts, John PadenAbstract:Surface melt water from the Greenland ice sheet can become trapped in Firn, delaying its journey to the sea. Radar and ice-core observations provide direct evidence of a perennial aquifer in the Firn layer in southern Greenland that represents a potentially significant contribution to the Greenland mass budget.