scholarly journals Hourly surface meltwater routing for a Greenlandic supraglacial catchment across hillslopes and through a dense topological channel network

2020 ◽  
Author(s):  
Colin J. Gleason ◽  
Kang Yang ◽  
Dongmei Feng ◽  
Laurence C. Smith ◽  
Kai Liu ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Complex Surface ◽  
Calibration Model ◽  
Channel Measurements ◽  
Channel Network ◽  
Surface Mass ◽  
Ice Surface ◽  
Meltwater Runoff

Abstract. Recent work has identified complex perennial supraglacial stream/river networks in areas of the Greenland Ice Sheet (GrIS) ablation zone. Current surface mass balance (SMB) models appear to overestimate meltwater runoff in these networks compared to in-channel measurements of supraglacial discharge. Here, we constrain SMB models using the Hillslope River Routing Model (HRR), a spatially explicit flow routing model used in terrestrial hydrology, in a 63 km2 supraglacial river catchment in southwest Greenland. HRR conserves water mass and momentum and explicitly accounts for hillslope routing, and we produce hourly flows for nearly 10,000 channels given inputs of an ice surface DEM, a remotely sensed supraglacial channel network, SMB-modelled runoff, and an in situ discharge dataset used for calibration. Model calibration yields a Nash Sutcliffe Efficiency as high as 0.92 and physically realistic parameters. We confirm earlier assertions that SMB runoff exceeds the conserved mass of water routed to match measured flows in this catchment (by 12–59 %) and that large channels do not dewater overnight despite a diurnal shutdown of SMB runoff production. We further test hillslope routing and network density controls on channel discharge and conclude that explicitly including hillslope flow and routing runoff through a realistically fine channel network produces the most accurate results. Modelling complex surface water processes is thus both possible and necessary to accurately simulate the timing and magnitude of supraglacial channel flows, and we highlight a need for additional in situ discharge datasets to better calibrate and apply this method elsewhere on the ice sheet.

scholarly journals Hourly surface meltwater routing for a Greenlandic supraglacial catchment across hillslopes and through a dense topological channel network

2021 ◽  
Vol 15 (5) ◽  
pp. 2315-2331
Author(s):  
Colin J. Gleason ◽  
Kang Yang ◽  
Dongmei Feng ◽  
Laurence C. Smith ◽  
Kai Liu ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Complex Surface ◽  
Calibration Model ◽  
Channel Measurements ◽  
Channel Network ◽  
Surface Mass ◽  
Meltwater Runoff ◽  
Elevation Model

Abstract. Recent work has identified complex perennial supraglacial stream and river networks in areas of the Greenland Ice Sheet (GrIS) ablation zone. Current surface mass balance (SMB) models appear to overestimate meltwater runoff in these networks compared to in-channel measurements of supraglacial discharge. Here, we constrain SMB models using the hillslope river routing model (HRR), a spatially explicit flow routing model used in terrestrial hydrology, in a 63 km2 supraglacial river catchment in southwest Greenland. HRR conserves water mass and momentum and explicitly accounts for hillslope routing (i.e., flow over ice and/or firn on the GrIS), and we produce hourly flows for nearly 10 000 channels given inputs of an ice surface digital elevation model (DEM), a remotely sensed supraglacial channel network, SMB-modeled runoff, and an in situ discharge dataset used for calibration. Model calibration yields a Nash–Sutcliffe efficiency as high as 0.92 and physically realistic parameters. We confirm earlier assertions that SMB runoff exceeds the conserved mass of water measured in this catchment (by 12 %–59 %) and that large channels do not dewater overnight despite a diurnal shutdown of SMB runoff production. We further test hillslope routing and network density controls on channel discharge and conclude that explicitly including hillslope flow and routing runoff through a realistic fine-channel network (as opposed to excluding hillslope flow and using a coarse-channel network) produces the most accurate results. Modeling complex surface water processes is thus both possible and necessary to accurately simulate the timing and magnitude of supraglacial channel flows, and we highlight a need for additional in situ discharge datasets to better calibrate and apply this method elsewhere on the ice sheet.

scholarly journals GrSMBMIP: Intercomparison of the modelled 1980-2012 surface mass balance over the Greenland Ice sheet

2020 ◽  
Author(s):  
Xavier Fettweis ◽  
Keyword(s):  
Mass Loss ◽  
Mass Balance ◽  
Climate Models ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Surface Mass Balance ◽  
Surface Mass ◽  
Meltwater Runoff

<p>The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 +/- 10 Gt/yr<sup>2</sup> since the end of the 1990's, with around 60% of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980-2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 +/- 112 Gt/yr, but has decreased at an average rate of -7.3 Gt/yr<sup>2</sup> (with a significance of 96%), mainly driven by an increase of 8.0 Gt/yr<sup>2</sup> (with a significance of 98%) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. It is also interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models. Finally, results from MAR forced by ERA5 will be added in this intercomparison to evaluate the added value of using this new reanalysis as forcing vs the former ERA-Interim reanalysis (used in SMBMIP). </p>

scholarly journals GrSMBMIP: Intercomparison of the modelled 1980–2012 surface mass balance over the Greenland Ice sheet

2020 ◽  
Author(s):  
Xavier Fettweis ◽  
Stefan Hofer ◽  
Uta Krebs-Kanzow ◽  
Charles Amory ◽  
Teruo Aoki ◽  
...  
Keyword(s):  
Mass Loss ◽  
Mass Balance ◽  
Climate Models ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Surface Mass Balance ◽  
Surface Mass ◽  
Meltwater Runoff

Abstract. The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 ± 10 Gt/yr2 since the end of the 1990's, with around 60 % of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980–2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 ± Gt/yr, but has decreased at an average rate of −7.3 Gt/yr2 (with a significance of 96 %), mainly driven by an increase of 8.0 Gt/yr2 (with a significance of 98 %) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. Finally, it is interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models.

scholarly journals Seasonal mass variations show timing and magnitude of meltwater storage in the Greenland Ice Sheet

2018 ◽  
Vol 12 (9) ◽  
pp. 2981-2999 ◽  
Author(s):  
Jiangjun Ran ◽  
Miren Vizcaino ◽  
Pavel Ditmar ◽  
Michiel R. van den Broeke ◽  
Twila Moon ◽  
...  
Keyword(s):  
Mass Balance ◽  
Climate Model ◽  
Spatial Scales ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Mass Balance ◽  
High Accumulation ◽  
Outlet Glacier ◽  
Surface Mass ◽  
Meltwater Runoff

Abstract. The Greenland Ice Sheet (GrIS) is currently losing ice mass. In order to accurately predict future sea level rise, the mechanisms driving the observed mass loss must be better understood. Here, we combine data from the satellite gravimetry mission Gravity Recovery and Climate Experiment (GRACE), surface mass balance (SMB) output of the Regional Atmospheric Climate Model v. 2 (RACMO2), and ice discharge estimates to analyze the mass budget of Greenland at various temporal and spatial scales. We find that the mean rate of mass variations in Greenland observed by GRACE was between −277 and −269 Gt yr−1 in 2003–2012. This estimate is consistent with the sum (i.e., -304±126 Gt yr−1) of individual contributions – surface mass balance (SMB, 216±122 Gt yr−1) and ice discharge (520±31 Gt yr−1) – and with previous studies. We further identify a seasonal mass anomaly throughout the GRACE record that peaks in July at 80–120 Gt and which we interpret to be due to a combination of englacial and subglacial water storage generated by summer surface melting. The robustness of this estimate is demonstrated by using both different GRACE-based solutions and different meltwater runoff estimates (namely, RACMO2.3, SNOWPACK, and MAR3.9). Meltwater storage in the ice sheet occurs primarily due to storage in the high-accumulation regions of the southeast and northwest parts of Greenland. Analysis of seasonal variations in outlet glacier discharge shows that the contribution of ice discharge to the observed signal is minor (at the level of only a few gigatonnes) and does not explain the seasonal differences between the total mass and SMB signals. With the improved quantification of meltwater storage at the seasonal scale, we highlight its importance for understanding glacio-hydrological processes and their contributions to the ice sheet mass variability.

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.

scholarly journals Quantifying meltwater refreezing along a transect of sites on the Greenland ice sheet

2015 ◽  
Vol 9 (2) ◽  
pp. 691-701 ◽  
Author(s):  
C. Cox ◽  
N. Humphrey ◽  
J. Harper
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
High Temporal Resolution ◽  
Temperature Profiles ◽  
Lateral Migration ◽  
Conservation Of Energy ◽  
Surface Mass ◽  
The Relationship ◽  
Heat Released

Abstract. On the Greenland ice sheet, a significant quantity of surface meltwater refreezes within the firn, creating uncertainty in surface mass balance estimates. This refreezing has the potential to buffer seasonal runoff to future increases in melting, but direct measurement of the process remains difficult. We present a method for quantifying refreezing at point locations using in situ firn temperature observations. A time series of sub-hourly firn temperature profiles were collected over the course of two melt seasons from 2007 to 2009 along a transect of 11 sites in the accumulation zone of Greenland. Seasonal changes in temperature profiles combined with heat flux estimates based on high-temporal-resolution temperature gradients enable us to isolate the heat released by refreezing using conservation of energy. Our method is verified from winter data when no refreezing takes place, and uncertainty is estimated using a Monte Carlo technique. While we limit our method to a subsection of firn between depths of 1 and 10 m, our refreezing estimates appear to differ significantly from model-based estimates. Furthermore, results indicate that a significant amount of refreezing takes place at depths greater than 1 m and that lateral migration of meltwater significantly complicates the relationship between total surface melt and total refreezing.

Improved modelling of the present-day Greenland firn layer

2021 ◽  
Author(s):  
Max Brils ◽  
Peter Kuipers Munneke ◽  
Willem Jan van de Berg ◽  
Achim Heilig ◽  
Baptiste Vandercrux ◽  
...  
Keyword(s):  
Mass Loss ◽  
Sea Level Rise ◽  
Sea Level ◽  
Climate Model ◽  
Pore Space ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Mass ◽  
Meltwater Runoff

<p>Recent studies indicate that a declining surface mass balance will dominate the Greenland Ice Sheet’s (GrIS) contribution to 21<sup>st</sup> century sea level rise. It is therefore crucial to understand the liquid water balance of the ice sheet and its response to increasing temperatures and surface melt if we want to accurately predict future sea level rise. The ice sheet firn layer covers ~90% of the GrIS and provides pore space for storage and refreezing of meltwater. Because of this, the firn layer can retain up to ~45% of the surface meltwater and thus act as an efficient buffer to ice sheet mass loss. However, in a warming climate this buffer capacity of the firn layer is expected to decrease, amplifying meltwater runoff and sea-level rise. Dedicated firn models are used to understand how firn layers evolve and affect runoff. Additionally, firn models are used to estimate the changing thickness of the firn layer, which is necessary in altimetry to convert surface height change into ice sheet mass loss.</p><p>Here, we present the latest version of our firn model IMAU-FDM. With respect to the previous version, changes have been made to the handling of the freshly fallen snow, the densification rate of the firn and the conduction of heat. These changes lead to an improved representation of firn density and temperature. The results have been thoroughly validated using an extensive dataset of density and temperature measurements that we have compiled covering 126 different locations on the GrIS. Meltwater behaviour in the model is validated with upward-looking GPR measurements at Dye-2. Lastly, we present an in-depth look at the evolution firn characteristics at some typical locations in Greenland.</p><p>Dedicated, stand-alone firn models offer various benefits to using a regional climate model with an embedded firn model. Firstly, the vertical resolution for buried snow and ice layers can be larger, improving accuracy. Secondly, a stand-alone firn model allows for spinning up the model to a more accurate equilibrium state. And thirdly, a stand-alone model is more cost- and time-effective to use. Firn models are increasingly capable of simulating the firn layer, but areas with large amounts of melt still pose the greatest challenge.</p>

The role of an interactive Greenland Ice Sheet in abrupt 4xCO2 forcing experiments.

2021 ◽  
Author(s):  
Victoria Lee ◽  
Robin S. Smith ◽  
Antony J. Payne
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ocean Model ◽  
Surface Melting ◽  
Surface Mass ◽  
Meltwater Runoff ◽  
Iceberg Calving

<p><span xml:lang="EN-US" data-contrast="auto"><span>We compare the response of a</span></span><span xml:lang="EN-US" data-contrast="auto"><span> coupled atmosphere-ocean-Greenland Ice Sheet (</span><span>GrIS</span><span>) model forced with an abrupt quadrupling of CO</span></span><sub><span xml:lang="EN-US" data-contrast="auto"><span>2 </span></span></sub><span xml:lang="EN-US" data-contrast="auto"><span>from greenhouse gas concentrations in 1970 with the response of the</span></span> <span xml:lang="EN-US" data-contrast="auto"><span>atmosphere-ocean model with a static </span><span>GrIS</span><span> . The model, UKESM1.ice.N</span><span>96.ORCA</span><span>1, consists of </span><span>HadGEM</span><span> GC3.1 coupled to the BISICLES ice sheet model with mean annual surface mass balance</span></span> <span xml:lang="EN-US" data-contrast="auto"><span>(SMB) passed to BISICLES and orography and cumulated iceberg flux passed back to the atmosphere and ocean, respectively, at the end of each year. The differences in the surface temperature and atmospheric fields between the two experiments are confined to Greenland, with no discernible global effects from the evolving orography</span></span><span xml:lang="EN-US" data-contrast="auto"><span>. The volume of the </span><span>GrIS</span><span> decreases by 15 % in 330 years. The surface height decreases the most (over 800m in 330 years) in southwest </span><span>GrIS</span><span> due to surface melting enhanced by feedbacks between elevation, air temperature and albedo. </span></span><span xml:lang="EN-US" data-contrast="auto"><span>The input of freshwater to the ocean from Greenland is enhanced</span></span><span xml:lang="EN-US" data-contrast="auto"><span> due to increased meltwater runoff, but the flux from melting icebergs decays to zero as calving from glaciers declines. The resulting sea level rise is dominated by SMB</span></span><span xml:lang="EN-US" data-contrast="auto"><span>, where the equivalent sea level rise is 1179 mm (5.0 mm/</span><span>yr</span><span>) for the static </span><span>GrIS</span><span> and </span></span><span xml:lang="EN-US" data-contrast="auto"><span>1120 mm</span></span><span xml:lang="EN-US" data-contrast="auto"><span> (4.4 mm/</span><span>yr</span><span>) for the interactive ice sheet at 2300.  There is less sea level rise in the interactive GrIS experiment, even though more mass is lost through surface melting, because the amount lost through iceberg calving decreases as the grounding line of marine-terminating glaciers retreat inland whereas calving in the static experiment is constant.   </span></span><span> </span></p>

scholarly journals Algal photophysiology drives darkening and melt of the Greenland Ice Sheet

2020 ◽  
Vol 117 (11) ◽  
pp. 5694-5705 ◽  
Author(s):  
Christopher J. Williamson ◽  
Joseph Cook ◽  
Andrew Tedstone ◽  
Marian Yallop ◽  
Jenine McCutcheon ◽  
...  
Keyword(s):  
Energy Absorption ◽  
Regional Scale ◽  
Negative Relationship ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Short Wave ◽  
Wave Radiation ◽  
Ice Surface ◽  
Meltwater Runoff ◽  
Algal Abundance

Blooms of Zygnematophycean “glacier algae” lower the bare ice albedo of the Greenland Ice Sheet (GrIS), amplifying summer energy absorption at the ice surface and enhancing meltwater runoff from the largest cryospheric contributor to contemporary sea-level rise. Here, we provide a step change in current understanding of algal-driven ice sheet darkening through quantification of the photophysiological mechanisms that allow glacier algae to thrive on and darken the bare ice surface. Significant secondary phenolic pigmentation (11 times the cellular content of chlorophylla) enables glacier algae to tolerate extreme irradiance (up to ∼4,000 µmol photons⋅m−2⋅s−1) while simultaneously repurposing captured ultraviolet and short-wave radiation for melt generation. Total cellular energy absorption is increased 50-fold by phenolic pigmentation, while glacier algal chloroplasts positioned beneath shading pigments remain low-light–adapted (Ek∼46 µmol photons⋅m−2⋅s−1) and dependent upon typical nonphotochemical quenching mechanisms for photoregulation. On the GrIS, glacier algae direct only ∼1 to 2.4% of incident energy to photochemistry versus 48 to 65% to ice surface melting, contributing an additional ∼1.86 cm water equivalent surface melt per day in patches of high algal abundance (∼104cells⋅mL−1). At the regional scale, surface darkening is driven by the direct and indirect impacts of glacier algae on ice albedo, with a significant negative relationship between broadband albedo (Moderate Resolution Imaging Spectroradiometer [MODIS]) and glacier algal biomass (R2= 0.75,n= 149), indicating that up to 75% of the variability in albedo across the southwestern GrIS may be attributable to the presence of glacier algae.

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