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  • 1
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    A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air (2013)
    Copernicus
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    Publication Date: 2013-08-06
    Description: We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140–150 nmol mol−1, which is higher than today's values. CO mole fractions rose by 10–15 nmol mol−1 from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a ≈ 30 nmol mol−1 decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.
    Print ISSN: 1680-7316
    Electronic ISSN: 1680-7324
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 2
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    Climate of the Greenland ice sheet using a high-resolution climate model – Part 1: Evaluation (2010)
    Copernicus
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    Publication Date: 2010-12-01
    Description: A simulation of 51 years (1957–2008) has been performed over Greenland using the regional atmospheric climate model (RACMO2/GR) at a horizontal grid spacing of 11 km and forced by ECMWF re-analysis products. To better represent processes affecting ice sheet surface mass balance, such as meltwater refreezing and penetration, an additional snow/ice surface module has been developed and implemented into the surface part of the climate model. The temporal evolution and climatology of the model is evaluated with in situ coastal and ice sheet atmospheric measurements of near-surface variables and surface energy balance components. The bias for the near-surface air temperature (−0.8 °C), specific humidity (0.1 g kg−1), wind speed (0.3 m s−1) as well as for radiative (2.5 W m−2 for net radiation) and turbulent heat fluxes shows that the model is in good accordance with available observations on and around the ice sheet. The modelled surface energy budget underestimates the downward longwave radiation and overestimates the sensible heat flux. Due to their compensating effect, the averaged 2 m temperature bias is small and the katabatic wind circulation well captured by the model.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 3
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    The role of radiation penetration in the energy budget of the snowpack at Summit, Greenland (2009)
    Copernicus
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    Publication Date: 2009-07-03
    Description: Measurements of the summer surface energy balance at Summit, Greenland, are presented (8 June–20 July 2007). These measurements serve as input to an energy balance model that searches for a surface temperature for which closure of all energy terms is achieved. A good agreement between observed and modelled surface temperatures was found, with an average difference of 0.45°C and an RMSE of 0.85°C. It turns out that penetration of shortwave radiation into the snowpack plays a small but important role in correctly simulating snow temperatures. After 42 days, snow temperatures in the first meter are 3.6–4.0°C higher compared to a model simulation without radiation penetration. Sensitivity experiments show that these results cannot be reproduced by tuning the heat conduction process alone, by varying snow density or snow diffusivity. We compared the two-stream radiation penetration calculations with a sophisticated radiative transfer model and discuss the differences. The average diurnal cycle shows that net shortwave radiation is the largest energy source (diurnal average of +61 W m−2), net longwave radiation the largest energy sink (−42 W m−2). On average, subsurface heat flux, sensible and latent heat fluxes are the remaining, small heat sinks (−5, −5 and −7 W m−2, respectively), although these are more important on a subdaily timescale.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 4
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    Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers (2012)
    Copernicus
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    Publication Date: 2012-02-13
    Description: 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.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 5
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    Climate of the Greenland ice sheet using a high-resolution climate model – Part 1: Evaluation (2010)
    Copernicus
    Details
    Publication Date: 2010-04-21
    Description: A simulation of 51 years (1957–2008) has been performed over Greenland using the regional atmospheric climate model RACMO2 at a horizontal grid spacing of 11 km forced by ECMWF analysis products. To better represent processes affecting ice sheet surface mass balance, such as melt water refreezing and penetration, an additional snow/ice surface module has been developed and implemented into the surface parameterisation of RACMO2v1. The temporal evolution and climatology of the model is evaluated with in situ coastal and ice sheet atmospheric measurements of near-surface variables and surface energy balance components. The bias for the near-surface air temperature (0.9 °C), specific humidity (0.1 g kg−1), wind speed (0.3 m s−1) as well as for radiative (2.5 W m−2 for net radiation) and turbulent heat fluxes shows that the model is in good accordance with available observations. The modeled surface energy budget underestimates the downward longwave radiation and overestimates the sensible heat flux. Due to their compensating effect, the averaged 2 m temperature bias is less than −0.9°C. The katabatic wind circulation is well captured by the model.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
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    Increasing runoff from the Greenland Ice Sheet at Kangerlussuaq (Søndre Strømfjord) in a 30-year perspective, 1979–2008 (2010)
    Copernicus
    Details
    Publication Date: 2010-03-24
    Description: This observation and modeling study provides insights into runoff exiting the Watson River drainage basin, Kangerlussuaq, West Greenland during a 30 year period (1978/79–2007/08) when the climate experienced increasing temperatures and precipitation. The 30-year simulations quantify the terrestrial freshwater output from part of the Greenland Ice Sheet (GrIS) and the land between the GrIS and the ocean, in the context of global warming and increasing GrIS surface melt. We used a snow-evolution modeling system (SnowModel) to simulate the winter accumulation and summer ablation processes, including runoff and surface mass balance (SMB), of the ice sheet. To a large extent, the SMB fluctuations could be explained by changes in net precipitation (precipitation minus evaporation and sublimation), with 8 out of 30 years having negative SMB, mainly because of relatively low annual net precipitation. The overall trend in net precipitation and runoff increased significantly, while SMB increased insignificantly throughout the simulation period, leading to enhanced precipitation of 0.59 km3 w.eq. (or 60%), runoff of 0.43 km3 w.eq. (or 54%), and SMB of 0.16 km3 w.eq. (or 86%). Runoff rose on average from 0.80 km3 w.eq. in 1978/79 to 1.23 km3 w.eq. in 2007/08. The percentage of catchment outlet runoff explained by runoff from the GrIS decreased on average ∼10%.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
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    The role of radiation penetration in the energy budget of the snowpack at Summit, Greenland (2009)
    Copernicus
    Details
    Publication Date: 2009-04-08
    Description: Measurements of the summer surface energy balance at Summit, Greenland, are presented (8 June–20 July 2007). These measurements serve as input to an energy balance model that searches for a surface temperature for which closure of all energy terms is achieved. A good agreement between observed and modeled surface temperatures was found, with an average difference of 0.45°C and an RMSE of 0.85°C. It turns out that penetration of shortwave radiation into the snowpack plays a small but important role in correctly simulating snow temperatures. After 42 days, snow temperatures in the first meter are 3.6–4.0°C higher compared to a model simulation without radiation penetration. Sensitivity experiments show that these results cannot be reproduced by tuning the heat conduction process alone, by varying snow density or snow diffusivity. We compared the two-stream radiation penetration calculations with a sophisticated radiative transfer model and discuss the differences. The average diurnal cycle shows that net shortwave radiation is the largest energy source (+61 W m−2 on average), net longwave radiation the largest energy sink (−42 W m−2). On average, subsurface heat flux, sensible and latent heat fluxes are the remaining, small heat sinks (−5, −5 and −7 W m−2, respectively), although these are more important on a subdaily timescale.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 8
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    Elevation change of the Greenland ice sheet due to surface mass balance and firn processes, 1960–2013 (2015)
    Copernicus
    Details
    Publication Date: 2015-06-30
    Description: Observed changes in the surface elevation of the Greenland ice sheet are caused by ice dynamics, basal elevation change, surface mass balance (SMB) variability, and by compaction of the overlying firn. The latter 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 1960–2013. 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. 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 2013 is estimated at −3900 ± 1030 km3 due to firn and SMB, corresponding to an ice-sheet average thinning of 2.32 ± 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.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 9
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    Freshwater flux to Sermilik Fjord, SE Greenland (2010)
    Copernicus
    Details
    Publication Date: 2010-07-30
    Description: Fluctuations in terrestrial surface freshwater flux to Sermilik Fjord, SE Greenland, were simulated and analyzed. SnowModel, a state-of-the-art snow-evolution, snow and ice melt, and runoff modeling system, was used to simulate the temporal and spatial terrestrial runoff distribution to the fjord based on observed meteorological data (1999–2008) from stations located on and around the Greenland Ice Sheet (GrIS). Simulated runoff was compared and verified against independent glacier catchment runoff observations (1999–2005). Modeled runoff to Sermilik Fjord was highly variable, ranging from 2.9×109 m3 y−1 in 1999 to 5.9×109 m3 y−1 in 2005. The uneven spatial runoff distribution produced an areally-averaged annual maximum runoff at the Helheim glacier terminus of more than 3.8 m w.eq. The sub-catchment runoff of the Helheim glacier region accounted for 25% of the total runoff to Sermilik Fjord. The runoff distribution from the different sub-catchments suggested a strong influence from the spatial variation in glacier coverage. To assess the Sermilik Fjord freshwater flux, simulated terrestrial runoff and net precipitation (precipitation minus evaporation and sublimation) for the fjord area were combined with satellite-derived ice discharge and subglacial geothermal and frictional melting due to basal ice motion. A terrestrial freshwater flux of ~40.4×109 m3 y−1 was found for Sermilik Fjord, with an 11% contribution originated from surface runoff. For the Helheim glacier sub-catchment only 4% of the flux originated from terrestrial surface runoff.
    Print ISSN: 1994-0432
    Electronic ISSN: 1994-0440
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 10
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    Elevation change of the Greenland Ice Sheet due to surface mass balance and firn processes, 1960–2014 (2015)
    Copernicus
    Details
    Publication Date: 2015-11-02
    Description: 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.
    Print ISSN: 1994-0416
    Electronic ISSN: 1994-0424
    Topics: Geography , Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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