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  • 1.
    Charalampidis, Charalampos
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Geological Survey of Denmark and Greenland (GEUS).
    van As, Dirk
    Geological Survey of Denmark and Greenland (GEUS).
    Box, Jason E.
    Geological Survey of Denmark and Greenland (GEUS).
    van den Broeke, Michiel R.
    Institute for Marine and Atmospheric Research in Utrecht (IMAU).
    Colgan, William T.
    Geological Survey of Denmark and Greenland (GEUS).
    Doyle, Samuel H.
    Aberystwyth University.
    Hubbard, Alun L.
    University of Tromsø.
    MacFerrin, Michael
    Cooperative Institute for Research in Environmental Sciences (CIRES).
    Machguth, Horst
    Geological Survey of Denmark and Greenland (GEUS).
    Smeets, C. J. P. Paul
    Institute for Marine and Atmospheric Research in Utrecht (IMAU).
    Changing surface-atmosphere energy exchange and refreezing capacity of the lower accumulation area, West Greenland2015In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 9, no 6, p. 2163-2181Article in journal (Refereed)
    Abstract [en]

    We present 5 years (2009-2013) of automatic weather station measurements from the lower accumulation area (1840 m a.s.l. - above sea level) of the Greenland ice sheet in the Kangerlussuaq region. Here, the summers of 2010 and 2012 were both exceptionally warm, but only 2012 resulted in a strongly negative surface mass budget (SMB) and surface meltwater run-off. The observed run-off was due to a large ice fraction in the upper 10 m of firn that prevented meltwater from percolating to available pore volume below. Analysis reveals an anomalously low 2012 summer-averaged albedo of 0.71 (typically similar to 0.78), as meltwater was present at the ice sheet surface. Consequently, during the 2012 melt season, the ice sheet surface absorbed 28% (213 MJ m-2) more solar radiation than the average of all other years. A surface energy balance model is used to evaluate the seasonal and interannual variability of all surface energy fluxes. The model reproduces the observed melt rates as well as the SMB for each season. A sensitivity analysis reveals that 71% of the additional solar radiation in 2012 was used for melt, corresponding to 36% (0.64 m) of the 2012 surface lowering. The remaining 64% (1.14 m) of surface lowering resulted from high atmospheric temperatures, up to a + 2.6 degrees C daily average, indicating that 2012 would have been a negative SMB year at this site even without the melt-albedo feedback. Longer time series of SMB, regional temperature, and remotely sensed albedo (MODIS) show that 2012 was the first strongly negative SMB year, with the lowest albedo, at this elevation on record. The warm conditions of recent years have resulted in enhanced melt and reduction of the refreezing capacity in the lower accumulation area. If high temperatures continue, the current lower accumulation area will turn into a region with superimposed ice in coming years.

  • 2.
    Charalampidis, Charalampos
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Geol Survey Denmark & Greenland GEUS, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark.
    van As, Dirk
    Geol Survey Denmark & Greenland GEUS, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark.
    Colgan, William T.
    Geol Survey Denmark & Greenland GEUS, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark.; York Univ, Dept Earth & Space Sci & Engn, 4700 Keele St, Toronto, ON M3J 1P3, Canada.
    Fausto, Robert S.
    Geol Survey Denmark & Greenland GEUS, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark.
    MacFerrin, Michael
    Univ Colorado, CIRES, 216 UCB, Boulder, CO 80309 USA.
    Machguth, Horst
    Geol Survey Denmark & Greenland GEUS, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark.; Tech Univ Denmark, Arctic Technol Ctr ARTEK, Byg 118, DK-2800 Lyngby, Denmark.
    Thermal tracing of retained meltwater in the lower accumulation area of the Southwestern Greenland ice sheet2016In: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644, Vol. 57, no 72, p. 1-10, article id 6000021Article in journal (Refereed)
    Abstract [en]

    We present in situ firn temperatures from the extreme 2012 melt season in the southwestern lower accumulation area of the Greenland ice sheet. The upper 2.5 m of snow and firn was temperate during the melt season, when vertical meltwater percolation was inefficient due to a c. 5.5 m thick ice layer underlying the temperate firn. Meltwater percolation and refreezing beneath 2.5 m depth only occurred after the melt season. Deviations from temperatures predicted by pure conductivity suggest that meltwater refroze in discrete bands at depths of 2.0–2.5, 5.0–6.0 and 8.0–9.0 m. While we find no indication of meltwater percolation below 9 m depth or complete filling of pore volume above, firn at 10 and 15 m depth was respectively 4.2–4.5 degrees C and 1.7 degrees C higher than in a conductivity-only simulation. Even though meltwater percolation in 2012 was inefficient, firn between 2 and 15 m depth the following winter was on average 4.7 degrees C warmer due to meltwater refreezing. Our observations also suggest that the 2012 firn conditions were preconditioned by two warm summers and ice layer formation in 2010 and 2011. Overall, firn temperatures during the years 2009–13 increased by 0.6 degrees C.

  • 3.
    Machguth, Horst
    et al.
    Department of Geography, University of Zurich.
    MacFerrin, Michael
    Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado at Boulder.
    van As, Dirk
    Geological Survey of Denmark and Greenland (GEUS).
    Box, Jason E.
    Geological Survey of Denmark and Greenland (GEUS), København, Denmark.
    Charalampidis, Charalampos
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Geological Survey of Denmark and Greenland (GEUS).
    Colgan, William
    Department of Earth and Space Science and Engineering, York University.
    Fausto, Robert S.
    Geological Survey of Denmark and Greenland (GEUS).
    Meijer, Harro A. J.
    Univ Groningen, Energy & Sustainabil Res Inst Groningen, Ctr Isotope Res CIO, NL-9747 AG Groningen, Netherlands.
    Mosley-Thompson, Ellen
    Ohio State Univ, Byrd Polar & Climate Res Ctr, Columbus, OH 43210 USA.; Ohio State Univ, Dept Geog, Columbus, OH 43210 USA.
    van de Wal, Roderik S. W.
    Univ Utrecht, Inst Marine & Atmospher Res Utrecht IMAU, NL-3584 CC Utrecht, Netherlands.
    Greenland meltwater storage in firn limited by near-surface ice formation2016In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 6, no 4, p. 390-393Article in journal (Refereed)
    Abstract [en]

    Approximately half of Greenland's current annual mass loss is attributed to runoff from surface melt. At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn. Two recent studies suggest that all or most of Greenland's firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come. Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012, revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than similar to 1,900 m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient surface discharge system and intensifying ice sheet mass loss earlier than previously suggested.

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