Modeling surface-roughness/solar-ablation feedback: application to small-scale surface channels and crevasses of the Greenland ice sheet

AbstractSurface roughness enhances the net ablation rate associated with direct solar radiation relative to smooth surfaces, because roughness allows solar energy reflected from one part of the surface to be absorbed by another part. In this study we examine the feedback between solar-radiation-driven ablation and growth of surface roughness on the Greenland ice sheet, using a numerical model of radiative transfer. Our experiments extend previous work by examining: (1) the effects of diurnal and seasonal variation of solar zenith angle and azimuth relative to incipient roughness features, (2) the evolution of roughness geometry in response to radiatively driven ablation and (3) the relative solar energy collection efficiencies of various roughness geometries and geographic locations and orientations. A notable result of this examination is that the time evolution of the aspect ratio of surface features under solar-driven ablation collapses onto a roughly universal curve that depends only on latitude, not the detailed shape of the feature. The total enhancement of surface melt relative to a smooth surface over a full ablation season varies with this ratio, and this dependence suggests a way to parameterize roughness effects in large-scale models that cannot treat individual roughness features. Overall, our model results suggest that surface roughness at the latitudes spanned by the Greenland ice sheet tends to dissipate as the ablation season progresses.

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Climate of the Greenland ice sheet using a high-resolution climate model – Part 2: Near-surface climate and energy balance

The Cryosphere ◽

10.5194/tc-4-529-2010 ◽

2010 ◽

Vol 4 (4) ◽

pp. 529-544 ◽

Cited By ~ 62

Author(s):

J. Ettema ◽

M. R. van den Broeke ◽

E. van Meijgaard ◽

W. J. van de Berg

Keyword(s):

Energy Balance ◽

Large Scale ◽

Climate Model ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Small Scale ◽

Katabatic Wind ◽

Radiative Loss ◽

Radiative Heat Loss ◽

Near Surface

Abstract. The spatial variability of near-surface variables and surface energy balance components over the Greenland ice sheet are presented, using the output of a regional atmospheric climate model for the period 1958–2008. The model was evaluated in Part 1 of this paper. The near-surface temperature over the ice sheet is affected by surface elevation, latitude, longitude, large-scale and small-scale advection, occurrence of summer melt and mesoscale topographical features. The atmospheric boundary layer is characterised by a strong temperature inversion, due to continuous longwave cooling of the surface. In combination with a gently sloping surface the radiative loss maintains a persistent katabatic wind. This radiative heat loss is mainly balanced by turbulent sensible heat transport towards the surface. In summer, the surface is near radiative balance, resulting in lower wind speeds. Absorption of shortwave radiation and a positive subsurface heat flux due to refreezing melt water are heat sources for surface sublimation and melt. The strongest temperature deficits (>13 °C) are found on the northeastern slopes, where the strongest katabatic winds (>9 m s−1) and lowest relative humidity (<65%) occur. Due to strong large scale winds, clear sky (cloud cover <0.5) and a concave surface, a continuous supply of cold dry air is generated, which enhances the katabatic forcing and suppresses subsidence of potentially warmer free atmosphere air.

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A new bedrock and surface elevation dataset for modelling the Greenland ice sheet

Annals of Glaciology ◽

10.3189/172756403781815456 ◽

2003 ◽

Vol 37 ◽

pp. 351-356 ◽

Cited By ~ 15

Author(s):

Jonathan L. Bamber ◽

Duncan J. Baldwin ◽

S. Prasad Gogineni

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Detailed Comparison ◽

Radar Data ◽

The Arctic ◽

Small Scale ◽

Surface Model ◽

Rock Outcrops ◽

Bed Topography ◽

Elevation Model

AbstractA new digital elevation model of the surface of the Greenland ice sheet and surrounding rock outcrops has been produced from a comprehensive suite of satellite and airborne remote-sensing and cartographic datasets. The surface model has been regridded to a resolution of 5 km, and combined with a new ice-thickness grid derived from ice-penetrating radar data collected in the 1970s and 1990s. A further dataset, the International Bathymetric Chart of the Arctic Ocean, was used to extend the bed elevations to include the continental shelf. The new bed topography was compared with a previous version used for ice-sheet modelling. Near the margins of the ice sheet and, in particular, in the vicinity of small-scale features associated with outlet glaciers and rapid ice motion, significant differences were noted. This was highlighted by a detailed comparison of the bed topography around the northeast Greenland ice stream.

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High-frequency ground-motion variability for rough-fault ruptures

10.5194/egusphere-egu21-11998 ◽

2021 ◽

Author(s):

Jagdish Chandra Vyas ◽

Martin Galis ◽

Paul Martin Mai

Keyword(s):

Ground Motion ◽

Large Scale ◽

Earthquake Ground Motion ◽

Small Scale ◽

Dynamic Rupture ◽

Roughness Effects ◽

Ground Shaking ◽

Fault Surface ◽

Fault Roughness ◽

Ground Motion Variability

Geological observations show variations in fault-surface topography not only at large scale (segmentation) but also at small scale (roughness). These geometrical complexities strongly affect the stress distribution and frictional strength of the fault, and therefore control the earthquake rupture process and resulting ground-shaking. Previous studies examined fault-segmentation effects on ground-shaking, but our understanding of fault-roughness effects on seismic wavefield radiation and earthquake ground-motion is still limited.&#160;&#160;In this study we examine the effects of fault roughness on ground-shaking variability as a function of distance based on 3D dynamic rupture simulations. We consider linear slip-weakening friction, variations of fault-roughness parametrizations, and alternative nucleation positions (unilateral and bilateral ruptures). We use generalized finite difference method to compute synthetic waveforms (max. resolved frequency 5.75 Hz) at numerous surface sites&#160; to carry out statistical analysis.&#160;&#160;Our simulations reveal that ground-motion variability from unilateral ruptures is almost independent of&#160; distance from the fault, with comparable or higher values than estimates from ground-motion prediction equations (e.g., Boore and Atkinson, 2008; Campbell and Bozornia, 2008). However, ground-motion variability from bilateral ruptures decreases with increasing distance, in contrast to previous studies (e.g., Imtiaz et. al., 2015) who observe an increasing trend with distance. Ground-shaking variability from unilateral ruptures is higher than for bilateral ruptures, a feature due to intricate seismic radiation patterns related to fault roughness and hypocenter location. Moreover, ground-shaking variability for rougher faults is lower than for smoother faults. As fault roughness increases the difference in ground-shaking variabilities between unilateral and bilateral ruptures increases. In summary, our simulations help develop a fundamental understanding of ground-motion variability at high frequencies (~ 6 Hz) due small-scale geometrical fault-surface variations.

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The effect of the north-east ice stream on the Greenland ice sheet in changing climates

The Cryosphere Discussions ◽

10.5194/tcd-1-41-2007 ◽

2007 ◽

Vol 1 (1) ◽

pp. 41-76 ◽

Cited By ~ 10

Author(s):

R. Greve ◽

S. Otsu

Keyword(s):

Large Scale ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Simulation Code ◽

North East ◽

The North ◽

Ice Stream ◽

Basal Sliding ◽

Speed Up ◽

Fast Flow

Abstract. The north-east Greenland ice stream (NEGIS) was discovered as a large fast-flow feature of the Greenland ice sheet by synthetic aperture radar (SAR) imaginary of the ERS-1 satellite. In this study, the NEGIS is implemented in the dynamic/thermodynamic, large-scale ice-sheet model SICOPOLIS (Simulation Code for POLythermal Ice Sheets). In the first step, we simulate the evolution of the ice sheet on a 10-km grid for the period from 250 ka ago until today, driven by a climatology reconstructed from a combination of present-day observations and GCM results for the past. We assume that the NEGIS area is characterized by enhanced basal sliding compared to the "normal", slowly-flowing areas of the ice sheet, and find that the misfit between simulated and observed ice thicknesses and surface velocities is minimized for a sliding enhancement by the factor three. In the second step, the consequences of the NEGIS, and also of surface-meltwater-induced acceleration of basal sliding, for the possible decay of the Greenland ice sheet in future warming climates are investigated. It is demonstrated that the ice sheet is generally very susceptible to global warming on time-scales of centuries and that surface-meltwater-induced acceleration of basal sliding can speed up the decay significantly, whereas the NEGIS is not likely to dynamically destabilize the ice sheet as a whole.

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Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers

The Cryosphere Discussions ◽

10.5194/tcd-6-593-2012 ◽

2012 ◽

Vol 6 (1) ◽

pp. 593-634 ◽

Cited By ~ 12

Author(s):

J. E. Box ◽

X. Fettweis ◽

J. C. Stroeve ◽

M. Tedesco ◽

D. K. Hall ◽

...

Keyword(s):

Solar Energy ◽

Climate Model ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Dominant Factor ◽

Water Runoff ◽

Albedo Feedback ◽

Ablation Area ◽

Regional Climate Model Output ◽

Surface Melt

Abstract. Greenland ice sheet mass loss has accelerated in the past decade responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. Using satellite observations of albedo and melt extent with calibrated regional climate model output, we determine the spatial dependence and quantitative impact of the ice sheet albedo feedback in twelve summer periods beginning in 2000. We find that while the albedo feedback is negative over 70 % of the ice sheet, concentrated in the accumulation area above 1500 m, positive feedback prevailing over the ablation area accounts for more than half of the overall increase in melting. Over the ablation area, year 2010 and 2011 absorbed solar energy was more than twice as large as in years 2000–2004. Anomalous anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007 enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward solar irradiance, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net radiation for the high elevation accumulation area approached positive values during this period.

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Dependence of Eemian Greenland temperature reconstructions on the ice sheet topography

Climate of the Past Discussions ◽

10.5194/cpd-9-6683-2013 ◽

2013 ◽

Vol 9 (6) ◽

pp. 6683-6732

Author(s):

N. Merz ◽

A. Born ◽

C. C. Raible ◽

H. Fischer ◽

T. F. Stocker

Keyword(s):

Energy Balance ◽

Surface Energy ◽

Large Scale ◽

Surface Energy Balance ◽

Climate Model ◽

Surface Wind ◽

Sensible Heat Flux ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Lapse Rate

Abstract. The influence of a reduced Greenland ice sheet (GrIS) on Greenland's surface climate during the Eemian interglacial is studied using a comprehensive climate model. We find a distinct impact of changes in the GrIS topography on Greenland's surface air temperatures (SAT) even when correcting for changes in surface elevation which influences SAT through the lapse rate effect. The resulting lapse rate corrected SAT anomalies are thermodynamically driven by changes in the local surface energy balance rather than dynamically caused through anomalous advection of warm/cold air masses. The large-scale circulation is indeed very stable among all sensitivity experiments and the NH flow pattern does not depend on Greenland's topography in the Eemian. In contrast, Greenland's surface energy balance is clearly influenced by changes in the GrIS topography and this impact is seasonally diverse. In winter, the variable reacting strongest to changes in the topography is the sensible heat flux (SHFLX). The reason is its dependence on surface winds, which themselves are controlled to a large extent by the shape of the GrIS. Hence, regions where a receding GrIS causes higher surface wind velocities also experience anomalous warming through SHFLX. Vice-versa, regions that become flat and ice-free are characterized by low wind speeds, low SHFLX and anomalous cold winter temperatures. In summer, we find surface warming induced by a decrease in surface albedo in deglaciated areas and regions which experience surface melting. The Eemian temperature records derived from Greenland proxies, thus, likely include a temperature signal arising from changes in the GrIS topography. For the NEEM ice core site, our model suggests that up to 3.2 °C of the annual mean Eemian warming can be attributed to these topography-related processes and hence is not necessarily linked to large-scale climate variations.

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21st century ocean forcing of the Greenland Ice Sheet for modeling of sea level contribution

10.5194/tc-2019-222 ◽

2019 ◽

Cited By ~ 1

Author(s):

Donald A. Slater ◽

Denis Felikson ◽

Fiamma Straneo ◽

Heiko Goelzer ◽

Christopher M. Little ◽

...

Keyword(s):

Sea Level ◽

21St Century ◽

General Circulation ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

General Circulation Models ◽

Glacier Retreat ◽

Small Scale ◽

Intergovernmental Panel ◽

Continental Scale

Abstract. Changes in the ocean are expected to be an important determinant of the Greenland Ice Sheet's future sea level contribution. Yet representing these changes in continental-scale ice sheet models remains challenging due to the small scale of the key physics, and limitations in processing understanding. Here we present the ocean forcing strategy for Greenland Ice Sheet models taking part in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), the primary community effort to provide 21st century sea level projections for the Intergovernmental Panel on Climate Change 6th Assessment Report. Beginning from global atmosphere-ocean general circulation models, we describe two complementary approaches to provide ocean boundary conditions for Greenland Ice Sheet models, termed the retreat and submarine melt implementations. The retreat implementation parameterizes glacier retreat as a function of projected submarine melting, is designed to be implementable by all ice sheet models, and results in retreat of around 1 and 15 km by 2100 in RCP2.6 and 8.5 scenarios respectively. The submarine melt implementation provides estimated submarine melting only, leaving the ice sheet model to solve for the resulting calving and glacier retreat, and suggests submarine melt rates will change little under RCP2.6 but will approximately triple by 2100 under RCP8.5. Both implementations have necessarily made use of simplifying assumptions and poorly-constrained parameterisations and as such, further research on submarine melting, calving and fjord-shelf exchange should remain a priority. Nevertheless, the presented framework will allow an ensemble of Greenland Ice Sheet models to be systematically and consistently forced by the ocean for the first time, and should therefore result in a significant improvement in projections of the Greenland ice sheet's contribution to future sea level change.

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Freshwater input into a low-Arctic Fjord in West Greenland: Timing, drivers and model evaluation

10.5194/egusphere-egu21-14149 ◽

2021 ◽

Author(s):

Jakob Abermann ◽

Kirsty Langley ◽

Sille Myreng ◽

Dorthe Petersen ◽

Kerstin Rasmussen ◽

...

Keyword(s):

Large Scale ◽

Temporal Trends ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Base Flow ◽

Minor Role ◽

Freshwater Flux ◽

Freshwater Input ◽

West Greenland ◽

Specific Discharge

The majority of the freshwater input from Greenland stems from the Greenland Ice Sheet. Despite its importance in terms of freshwater totals, there is a much higher number of individual catchments disconnected from the ice sheet contributing on average about 26% of the total Greenland freshwater flux. Most of those catchments have local glacier cover, only very few of them are instrumented and little scientific literature exists. We present a dataset of 12 years of discharge of four catchments less than 15 km apart, that are different in size (between 7 and 32 km&#178;), local glacier coverage (4-11%) and lake cover (0-5%). They all drain into Kobbefjord, a well-studied fjord in West Greenland, near Greenland&#8217;s capital Nuuk. We find that annual specific discharge totals vary greatly (between 1.2 and 1.9 m/yr on a 12-year average within 15 km) due to a general climatic gradient and different strengths of orographic shading. The seasonal cycle differs among the sites mainly due to different exposure to solar radiation as a driver for major snowmelt; small ice coverage in the catchments plays only a minor role in discharge variability. Dry years generally increase the magnitude of spatial gradients in specific discharge and no significant temporal trends have been found in the studied catchments. On the sub-daily scale, the presence and elevation of lakes determines the catchment&#8217;s response during sunny days, leading to a difference in the timing of maximum discharge of between 7 and 12 hours depending on the site and the time of the year. The response of discharge to major precipitation events is discussed, where uniform reaction is found for the catchments with no lakes near the gauge and a delay of between 5 and 7 hours in the catchment with low-lying lakes. A comparison with a recently published modelled discharge time series on individual catchment scale shows the model&#8217;s capability of reproducing both snowmelt and large-scale storm events; however, the strong spatial heterogeneity of discharge magnitude and timing as well as the presence and variability of base-flow is not captured. We discuss methods to combine observational data with existing model output in order to improve the potential of their combined usage on the Greenland-scale.

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Economics of the disintegration of the Greenland ice sheet

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1814990116 ◽

2019 ◽

Vol 116 (25) ◽

pp. 12261-12269 ◽

Cited By ~ 5

Author(s):

William Nordhaus

Keyword(s):

Climate Change ◽

Large Scale ◽

Social Cost ◽

Climate Models ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Reduced Form ◽

Small Contribution ◽

Wide Range ◽

The Impact

Concerns about the impact on large-scale earth systems have taken center stage in the scientific and economic analysis of climate change. The present study analyzes the economic impact of a potential disintegration of the Greenland ice sheet (GIS). The study introduces an approach that combines long-run economic growth models, climate models, and reduced-form GIS models. The study demonstrates that social cost–benefit analysis and damage-limiting strategies can be usefully extended to illuminate issues with major long-term consequences, as well as concerns such as potential tipping points, irreversibility, and hysteresis. A key finding is that, under a wide range of assumptions, the risk of GIS disintegration makes a small contribution to the optimal stringency of current policy or to the overall social cost of climate change. It finds that the cost of GIS disintegration adds less than 5% to the social cost of carbon (SCC) under alternative discount rates and estimates of the GIS dynamics.

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Variations in snowpack melt on the Greenland ice sheet based on passive-microwave measurements

Journal of Glaciology ◽

10.1017/s0022143000017755 ◽

1995 ◽

Vol 41 (137) ◽

pp. 51-60 ◽

Cited By ~ 90

Author(s):

Thomas L. Mote ◽

Mark R. Anderson

Keyword(s):

Climate Change ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ablation Rate ◽

Passive Microwave ◽

Microwave Emission ◽

The Arctic ◽

Emission Model ◽

The Impact ◽

Microwave Data

AbstractA simple microwave-emission model is used to simulate 37 GHz brightness temperatures associated with snowpack-melt conditions for locations across the Greenland ice sheet. The simulated values are utilized as threshold values and compared to daily, gridded SMMR and SSM/I passive-microwave data, in order to reveal regions experiencing melt. The spatial extent of the area classified as melting is examined on a daily, monthly and seasonal (May-August) basis for 1979–91. The typical seasonal cycle of melt coverage shows melt beginning in late April, a rapid increase in the melting area from mid-May to mid-July, a rapid decrease in melt extent from late July through mid-August, and cessation of melt in late September. Seasonal averages of the daily melt extents demonstrate an apparent increase in melt coverage over the 13 year period of approximately 3.8% annually (significant at the 95% confidence interval). This increase is dominated by statistically significant positive trends in melt coverage during July and August in the west and southwest of the ice sheet. We find that a linear correlation between microwave-derived melt extent and a surface measure of ablation rate is significant in June and July but not August, so caution must be exercised in using the microwave-derived melt extents in August. Nevertheless, knowledge of the variability of snowpack melt on the Greenland ice sheet as derived from microwave data should prove useful in detecting climate change in the Arctic and examining the impact of climate change on the ice sheet.

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