We present a new icesheet model validation framework – the Cryospheric Model Comparison Tool (CmCt) – that aims to fill this gap. Broadly, the CmCt software is de- signed to post-process model output over a specified time frame, process and filter available spatially and temporally coincident observations from remote-sensing-based datasets, compare the two, and assess the model vs. observation mis- match using a number of proposed qualitative and quanti- tative metrics. Here, we demonstrate the CmCt using ob- servations over the Greenlandicesheet obtained between 2003 and 2013 from the Ice, Cloud, and land Elevation Satellite (ICESat) and from the Gravity Recovery and Cli- mate Experiment (GRACE) satellites. The design is, how- ever, intentionally extensible to allow for the incorporation of other similar observational datasets (e.g., for the Antarctic icesheet) covering similar or longer time periods (e.g., icesheet surface altimetry from other space and airborne mis- sions and follow-on missions to ICESat and GRACE). Im- plicit in our development of the CmCt is the understanding that the observational datasets of interest may be very large, may entail complex processing for which icesheet model- ers have little or no expertise, and may be updated, appended to, or altered in numerous ways at any point in the future. Therefore, data are accessed remotely via an online interface (https://ggsghpcc.sgt-inc.com/cmct/index.html) that insures a CmCt user is able to take advantage of data processing improvements and new datasets as they become available. While we acknowledge that validation should be “model ag- nostic” – that is, the framework makes no assumptions about the type of mesh used by the model (structured vs. unstruc- tured) – the prototype discussed below assumes regular grid-
In this study, we included additional dynamic processes in a thermomechanically coupled, three-dimensional ice flow model, with the aim of better assessing the impact of ice dy- namics on the future evolution of the Greenlandicesheet. We suggested parameterisations that link ice discharge increase to ocean warming and allow for runoff-induced lubrication. To assess the likely range of the future contribution from the Greenlandicesheet to sea-level change, climate anoma- lies were taken from a suite of 10 atmosphere–ocean general circulation models (Table B1). They were selected from the WCRP’s CMIP5 multi-model data set prepared for the IPCC AR5 (Taylor et al., 2012) and forced by four RCP climate scenarios. When considering climate forcing from ECMWF reanalysis data and ocean temperatures from an AOGCM that shows an expressed warming over the period 2005–2010, we find an ice loss rate of 0.62 mm yr −1 over the same period that is explained by ∼ 40 % from increased ice discharge, in agreement with the observational range. Changes in ice dis- charge are attributed to oceanic warming in the surrounding ocean basins. The mean ice volume loss for the CMIP5 en- semble is however biased low with 0.32 mm yr −1 . This bias arises from the spread in climate models that are not expected to correctly simulate the observed trend over such a short pe- riod of time. The ensemble maximum of the ice loss during this recent period is 0.71 mm yr −1 and equally covers val- ues inferred from observations. For the climate model en- semble, increased ice discharge also explains ∼ 40 % of the total mass loss during the last decade.
We will later demonstrate that, unlike absolute values of [ R ] , local variability in bed-echo reflectivity is highly insen- sitive to modelled values of < N > (Sect. 2.6). However, de- spite acting as a very weak constraint, an initial estimate for ice-sheet-scale variation in < N > is still necessary to cal- culate reflectivity variability. The estimate for < N > relies on previous work by Jordan et al. (2016) and uses the M07 Arrhenius equation MacGregor et al. (2007), the GreenlandIceSheet Model (GISM) temperature field from Huybrechts (1996) as updated in Goelzer et al. (2013), depth-averaged ionic concentrations from the GRIP ice core (MacGregor et al., 2015b), and the Greenlandice thickness data set in Bamber et al. (2013a). The temperature field derives from a full 3-D thermomechanical simulation over several glacial- interglacial cycles and resolves the flow on a model resolu- tion of 5 km, which causes some smoothing of the temper- ature field in narrow outlet glaciers near to the coast. In the calculation of < N > the temperature field was firstly inter- polated to a 1 km resolution, then vertically scaled using the 1 km representation of the Bamber et al. (2013a) ice thick- ness. The geothermal heat flux in GISM was initially taken from Shapiro and Ritzwoller (2004) but further adjusted with Gaussian functions around the deep ice core sites to match observed basal temperatures. Vertical temperature profiles are within 1–2 ◦ C when compared to available in situ mea- surements.
Assessing subglacial-roughness information with respect to ice motion, however, is not limited to basal traction, par- ticularly when defined at varying length scales. When con- sidering roughness signatures, Bingham and Siegert (2009) present a clear conceptual framework for examining the causes and controls of smooth and rough beds in both hard- and soft-bed scenarios. For example, the majority of rough- ness studies of the West Antarctic IceSheet bed have asso- ciated low roughness (i.e. smooth beds) with the presence of deformable sediment (e.g. Rippin et al., 2006, 2011, 2014; Bingham and Siegert, 2007, 2009; Schroeder et al., 2014); however, it is evident that streamlined bedrock (hard beds) can also promote smooth-bed signals (e.g. Siegert et al., 2005; Rippin et al., 2014; Jeofry et al., 2018). The link between the presence of saturated (wet), deformable sedi- ments, and ice motion was first identified on the Siple Coast, West Antarctica, by Blankenship et al. (1986) and Alley et al. (1986), where it is seen to control both the onset and magnitude of fast flow (Peters et al., 2006; Siegert et al., 2016). Whilst flow configuration of the GreenlandIceSheet is markedly different (with regard to streaming ice), recent regional studies have documented the presence of soft basal sediments underlying fast-flowing outlet glaciers (Christian- son et al., 2014; Kulessa et al., 2017; Hofstede et al., 2018), where it is, potentially, seen as an important control on ice flow in Greenland (Bougamont et al., 2014). Furthermore, recent characterisation of the majority of Greenland’s outlet glaciers implies that the role of effective basal water pressure (as well as the availability of deformable sediment) is more important and influential than basal friction itself (Stearns and van der Veen, 2018); however, it should be noted that this conclusion, and the role of friction in basal slip, is con- tested (Minchew et al., 2019). Altogether, this suggests not only that a consideration of orientation (or anisotropy) in the interpretation of subglacial roughness is necessary but also that basal motion relies on the influence of other factors (e.g. basal thermal state, geographic or geological setting, and/or the presence of sediment; Bingham and Siegert, 2009).
Abstract. We present a 1986 through 2017 estimate of GreenlandIceSheetice discharge. Our data include all discharging ice that flows faster than 100 m yr −1 and are generated through an automatic and adaptable method, as opposed to conventional hand-picked gates. We position gates near the present-year termini and estimate problematic bed topography (ice thickness) values where necessary. In addition to using annual time- varying ice thickness, our time series uses velocity maps that begin with sparse spatial and temporal coverage and end with near-complete spatial coverage and 6 d updates to velocity. The 2010 through 2017 average ice discharge through the flux gates is ∼ 488 ± 49 Gt yr −1 . The 10 % uncertainty stems primarily from uncertain ice bed location (ice thickness). We attribute the ∼ 50 Gt yr −1 differences among our results and previous studies to our use of updated bed topography from BedMachine v3. Discharge is approximately steady from 1986 to 2000, increases sharply from 2000 to 2005, and then is approximately steady again. However, regional and glacier variability is more pronounced, with recent decreases at most major glaciers and in all but one region offset by increases in the NW (northwestern) region. As part of the journal’s living archive option, all input data, code, and results from this study will be updated when new input data are accessible and made freely available at https://doi.org/10.22008/promice/data/ice_discharge.
Since the launch in 2002 of the Gravity Recovery and Climate Experiment (GRACE) satellites, several estimates of the mass balance of the Greenlandicesheet (GrIS) have been produced. To obtain ice mass changes, the GRACE data need to be corrected for the effect of deformation changes of the Earth’s crust. Recently, a new method has been proposed where ice mass changes and bedrock changes are simultane- ously solved. Results show bedrock subsidence over almost the entirety of Greenland in combination with ice mass loss which is only half of the currently standing estimates. This subsidence can be an elastic response, but it may however also be a delayed response to past changes. In this study we test whether these subsidence patterns are consistent with ice dynamical modeling results. We use a 3-D icesheet–bedrock model with a surface mass balance forcing based on a mass balance gradient approach to study the pattern and magnitude of bedrock changes in Greenland. Different mass balance forcings are used. Simulations since the Last Glacial Max- imum yield a bedrock delay with respect to the mass balance forcing of nearly 3000 yr and an average uplift at present of 0.3 mm yr −1 . The spatial pattern of bedrock changes shows a small central subsidence as well as more intense uplift in the south. These results are not compatible with the grav- ity based reconstructions showing a subsidence with a max- imum in central Greenland, thereby questioning whether the claim of halving of the ice mass change is justified.
sulting models all overestimate surface melt and violate the MBP criterion. This leads to a strong drop of the NEEM el- evation far below the one reconstructed from palaeo data, and the Greenlandicesheet becomes too sensitive to tem- perature anomalies. This is why Robinson et al. (2011) and Robinson et al. (2012) used the MBP criterion together with a palaeo constraint for calibration of their coarse resolution model, ensuring the correct long-term stability properties re- ported in this work. In our improved model with sub-grid scale discharge parameterization, we found a stability be- haviour similar to that found by Robinson et al. (2012) when using the MBP and NEEMup constraints, but now – still in the coarse resolution ice-sheet model – we can addition- ally fulfil a strong present-day shape constraint and achieve err(H ) < 20 %. We expect that all of our constraints would play a similar role in a model of the Greenland glacial sys- tem, which explicitly describes small-scale fast processes. Development of such a model is in our future plans.
Abstract. Over the last two decades, the Greenlandicesheet (GrIS) has been losing mass at an increasing rate, enhanc- ing its contribution to sea-level rise (SLR). The recent in- creases in ice loss appear to be due to changes in both the surface mass balance of the icesheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the dis- charge, but also alters ice-sheet geometry and so affects cli- mate and surface mass balance. Present-day ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new- generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essen- tial developments: the complete solution of the full system of equations governing ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate pos- sible bounds for the next century ice-sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the ice-
A trihedral reflector like response is noted for a distinct region at the near edge of the image for all the scenes and for all frequencies. This type of response, measured over a range of incidence angles adjoining the near edge of the image (up to -200 lines in) indicates that direct scattering occurs. It is not thought to be just an edge effect (from poor calibration of instrument) as it occurs over a sizeable part of each image (equivalent to an area -2km wide on the ground). This reflector like response at the near edge of the majority of the images suggests that direct scattering is the dominant mechanism for most of the images for a region at the near edge of the image. This occurs for images of the dry zone, the percolation zone and the soaked and ablation zones of the Greenlandicesheet, for all frequencies. The only exception to this is the C band, dry zone (i9) image, which shows a double dip probably due to low return power in this case (the measured value is - 22 dB total power at the near edge of i9).
Abstract. Recent studies indicate that the surface mass bal- ance will dominate the GreenlandIce Sheet’s (GrIS) con- tribution to 21st century sea level rise. Consequently, it is crucial to understand the liquid water balance (LWB) of the icesheet and its response to increasing surface melt. We therefore analyse a firn simulation conducted with the SNOWPACK model for the GrIS and over the period 1960– 2014 with a special focus on the LWB and refreezing. Eval- uations of the simulated refreezing climate with GRACE and firn temperature observations indicate a good model– observation agreement. Results of the LWB analysis reveal a spatially uniform increase in surface melt (0.16 m w.e. a −1 ) during 1990–2014. As a response, refreezing and run-off also indicate positive changes during this period (0.05 and 0.11 m w.e. a −1 , respectively), where refreezing increases at only half the rate of run-off, implying that the majority of the additional liquid input runs off the icesheet. This pattern of refreeze and run-off is spatially variable. For instance, in the south-eastern part of the GrIS, most of the additional liquid input is buffered in the firn layer due to relatively high snow- fall rates. Modelled increase in refreezing leads to a decrease in firn air content and to a substantial increase in near-surface firn temperature. On the western side of the icesheet, mod- elled firn temperature increases are highest in the lower ac- cumulation zone and are primarily caused by the exceptional melt season of 2012. On the eastern side, simulated firn tem- perature increases are more gradual and are associated with the migration of firn aquifers to higher elevations.
Cold-season release may in fact be a consequence of melt- water storage within the Greenlandicesheet according to the following hypothesis: meltwater retention builds up sub- glacial pressures such that CHS, in the cold-season near the ice margin, remains largely intact and can rapidly reactivate in response to short-lived melt events. This hypothesis is not unique to this study. Six years of energy and mass bal- ance studies from a Svalbard glacier reveal that years with lower than expected icesheet meltwater export (due to in- ternal storage) preceded years when less than usual energy was required for activating the subglacial system, perhaps due to increased subglacial pressure from larger than usual internal storage (Hodson, 2005). It is unclear how meltwater retention influences CHS evolution. Modeling and observa- tional studies suggest that CHS seasonally evolves from an un-channelized state dominated by distributed cavities to an efficient channelized system with large conduits controlled by meltwater fluxes to the subglacial hydrologic drainage system (Schoof, 2010; Sundal et al., 2011). This influences icesheet velocities, so that in years with strong surface melt- ing, and thus more efficient subglacial drainage, icesheet velocities decelerate early in the melt season (Sundal et al., 2011). To understand the importance of Greenlandicesheet meltwater retention on these processes, more observational and modeling studies must establish how common this is in other parts of Greenland, and how it is related to subglacial pressure, cold-season releases, and CHS development.
Abstract. We assess the recent contribution of the Greenlandicesheet (GrIS) to sea level change. We use the mass bud- get method, which quantifies icesheet mass balance (MB) as the difference between surface mass balance (SMB) and solid ice discharge across the grounding line (D). A com- parison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002–2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958–1995 average value of SMB are similar at 411 and 418 Gt yr −1 , respectively, suggesting that ice flow in the mid- 1990s was well adjusted to the average annual mass input, reminiscent of an icesheet in approximate balance. Starting in the early to mid-1990s, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwa- ter runoff, which is slightly compensated by a small (< 3 %) increase in snowfall. The excess runoff originates from low- lying (< 2000 m a.s.l.) parts of the icesheet; higher up, in- creased refreezing prevents runoff of meltwater from occur- ring, at the expense of increased firn temperatures and de- pleted pore space. With a 1991–2015 average annual mass loss of ∼ 0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has re- cently become a major source of global mean sea level rise.
This paper outlines results from some recent measure- ments of the icesheet carried out as part of the ECOGIS project (Elevation Changes of the GreenlandIceSheet), a cooperation between KMS, University of Copenhagen and the Danish Centre for Remote Sensing, Technical Univer- sity of Denmark, supported by the Danish research council’s TUPOLAR programme. In the ECOGIS project ice dynam- ics have been studied and methods compared in two areas at the centre of the icesheet (around the GRIP and NGRIP drilling camps), as well as at a local, high-elevation coastal ice cap in East Greenland (Geikie Plateau) with a very large annual snow accumulation. At the latter site both airborne and satellite SAR interferometry have been tested, to investi- gate if SAR interferometry could be useful in studying height changes. The location of all ECOGIS field sites are shown in Fig. 1. In this paper we give results from GRIP, NGRIP and Geikie, only.
Acknowledgements. Over the years the Watson River discharge monitoring has been (co)financed through various funding sources: the Commission for Scientific Research in Greenland grants 07-015998, 09-064628, and 2138-08-0003; the Danish Natural Science Research Council grant 272-07-0645; the Center for Per- mafrost (www.CENPERM.ku.dk); the Department of Geosciences and Natural Resource Management (www.IGN.ku.dk); the Green- land Analogue Project (GAP); and the Danish Energy Agency (www.ENS.dk). The weather stations monitoring the icesheet and SMB modeling are financed by the GAP with contributions from the Programme for Monitoring of the GreenlandIceSheet (www.PROMICE.dk). Kangerlussuaq meteorological data are collected by the Danish Meteorological Institute (www.DMI.dk). We acknowledge local support by the Centre for Ice and Climate (www.iceandclimate.nbi.ku.dk), CH2M HILL Polar Services (www.CPSpolar.com), Kangerlussuaq International Science Sup- port, and the Greenland Survey (www.Asiaq.gl). Support various in nature was kindly provided by Andreas Ahlstrøm, Sam Doyle, Emily Henkemans, Alun Hubbard (GAP subproject A lead), Anne Kontula (GAP co-lead), Horst Machguth, Sebastian Mernild, Paul Smeets, Larry Smith, Kisser Thorsøe, and many others. Edited by: L. Koenig
We have also used the model of supraglacial lake evolu- tion to estimate a maximum value for the area and volume of SGLs situated at elevations between 1100 and 1700 m a.s.l. within our study area. We agree with the findings of L¨uthje et al. (2006) that the local topographic setting limits poten- tial SGL area, and we find this limitation to be 6.4 % of the entire region in our model. We estimate that the maximum volume of water that can possibly be stored in SGLs in this sector of the Greenlandicesheet is 1.49 km 3 or 12 % of all runoff produced in 2003. This suggests that the vast majority of runoff passes through or over the icesheet without being stored temporarily in lakes. We neither simulate nor observe lake area coverage of this magnitude, even with an assumed doubling of runoff. We note that although simulated lake area coverage stabilises, not all lakes are brimful by the end of the melt season with respect to the sink in which they are located, especially at the higher elevations where melting begins later and ends sooner. If the ablation season were to lengthen, par- ticularly at these higher elevations, it is possible that this to- pographic limit could be reached.
Abstract. This paper presents autonomous drifting snow ob- servations performed on the GreenlandIceSheet in the fall of 2012. High-frequency snow particle counter (SPC) obser- vations at ∼ 1 m above the surface provided drifting snow number fluxes and size distributions; these were combined with meteorological observations at six levels. We identify two types of drifting snow events: katabatic events are rela- tively cold and dry, with prevalent winds from the southeast, whereas synoptic events are short lived, warm and wet. Pre- cipitating snow during synoptic events disturbs the drifting snow measurements. Output of the regional atmospheric cli- mate model RACMO2, which includes the drifting snow rou- tine PIEKTUK-B, agrees well with the observed near-surface climate at the site, as well as with the frequency and timing of drifting snow events. Direct comparisons with the SPC ob- servations at 1 m reveal that the model overestimates the hor- izontal snow transport at this level, which can be related to an overestimation of saltation and the typical size of drifting snow particles.
The most challenging aspect of quantifying refreezing is that both infiltration and refreezing of meltwater in cold firn is highly heterogeneous in space and time. Infiltration is char- acterized by a complex network of rapidly developing ice lenses and vertical pipe structures (Pfeffer and Humphrey, 1998). Vertical pipe flow can transport water deeper into the firn than uniform infiltration. Recent field observations in the percolation region of the Greenlandicesheet accumulation zone suggest that meltwater is able to penetrate cold firn to depths greater than 10 m and remain mobile throughout the winter (Humphrey et al., 2012; Forster et al., 2014). Piping of meltwater into cold firn occurs without warming the en- tire profile. The water moves down the pipes with only mini- mal heating of the surrounding firn, making a locally com- plex temperature field of both cold and near-melting tem- peratures. This deep penetration and heterogeneous heating make rudimentary parameterizations based on Pfeffer et al. (1991) questionable.
We studied the mean summer broadband albedo over the Greenlandicesheet between 1981 and 2012 as estimated from space-borne measurements and found that summer albedo decreased at a rate of 0.02 decade −1 between 1996 and 2012. The analysis of the outputs of the MAR regional climate model indicates that the observed darkening is as- sociated with increasing temperatures and enhanced melting occurring during the same period, which in turn promote in- creased surface snow grain size as well as the expansion and persistency of areas with exposed bare ice. The MAR model simulates the interannual variability in the retrieved GLASS albedo well, but the albedo trend is larger in the GLASS albedo product than in MAR, indicating that processes not represented in the MAR physics account for some of the de- clining albedo. Specifically, we suggest that the absence of the effects of light-absorbing impurities in MAR could ac- count for the difference. We also suggest that this hypothesis is supported by the trends observed along the ablation zone, where the differences between observed and modelled trends are more pronounced and the effect of the Terra sensor degra- dation plays a relatively small role. On the other hand, over the dry-snow zone, our hypothesis requires further testing, in view of the potentially higher impact of the sensor degra- dation on the observed albedo trend. The analysis of mod- elled fields and in situ data indicated an absence of trends in aerosol optical depth over Greenland, as well as no sig- nificant trend in particulate light-absorbing emissions (e.g. BC) from fires in likely source regions. This is consistent with the absence of trends in surface aerosol concentrations measured around the Arctic. Consequently, we suggest that the increased surface concentrations of LAI associated with the darkening are not related to increased deposition of LAI, but rather to post-depositional processes, including increased loss of snow water to sublimation and melt and the outcrop- ping of “dirty” underlying ice associated with snow/firn re- moval due to ablation.
The Arctic is the region of the Earth experiencing the largest increase in temperature since the pre-industrial era (Serreze and Barry, 2011), with consequences already perceptible on the mass evolution of the Arctic ice caps and the Greenlandicesheet (Rignot et al., 2011). The evolution of the Green- land icesheet (GrIS) is governed by variations of ice dynam- ics and surface mass balance (SMB), the latter being defined as the difference between snow accumulation that is further transformed into ice, and the ablation processes (i.e. surface melting and sublimation). While surface melting strongly de- pends on the surface energy balance, snowfall is primarily controlled by atmospheric conditions (wind, humidity con- tent, cloudiness, etc.). However, various feedbacks between the atmosphere and the GrIS impact the surface characteris- tics such as ice extent and thickness. This has potential con- sequences on ice dynamics (e.g. due to changes in surface slopes) and may lead to SMB variations that can therefore af- fect the total ice mass. These changes may in turn alter both local and global climate. As an example, changes in near- surface temperature and surface energy balance may occur in response to changes in orography (temperature–elevation feedback) or changes in ice-covered area (planetary albedo feedback; see Lunt et al., 2004 and Vizcaíno et al., 2008, 2015). On the other hand, topography changes may alter the atmospheric circulation patterns (Doyle and Shapiro, 1999; Petersen et al., 2003; Moore and Renfrew, 2005) causing changes in heat and humidity transports.
The same climate reanalysis data have been used to force four different models in order to estimate Greenlandicesheet surface mass balance and its sub-components. First, we con- sidered the effect of the icesheet mask. Total surface mass balance estimates from the four models for the icesheet show reasonable agreement once mapped onto the common mask. Applying the common mask to model output decreases the variation between models. However, part of this reduction may only be a result of reducing the amount of ablation area, where disagreement is larger, but leaving the accumulation area the same size. The largest inter-model variations remain for refreeze estimates. ECMWFd’s refreeze estimate deviates most from the other three models and therefore contributes most to the observed inter-model variation. ECMWFd’s low refreeze is partially attributable to its lower melt magnitude but the main difference with respect to the other models is the use of a degree-day approach instead of an energy bal- ance model. Modelled surface mass balance uncertainty may be smaller than previously thought due to variations in the native mask, and so we recommend the use of an accurate common mask for future modelling work.