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Changing surface-atmosphere energy exchange and refreezing capacity of the lower accumulation area, West Greenland
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Geological Survey of Denmark and Greenland (GEUS). (Ice, Climate and Environment)ORCID iD: 0000-0003-0853-697X
Geological Survey of Denmark and Greenland (GEUS).ORCID iD: 0000-0002-6553-8982
Geological Survey of Denmark and Greenland (GEUS).ORCID iD: 0000-0003-0052-8705
Institute for Marine and Atmospheric Research in Utrecht (IMAU).ORCID iD: 0000-0003-4662-7565
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2015 (English)In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 9, no 6, 2163-2181 p.Article in journal (Refereed) Published
Resource type
Text
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.

Place, publisher, year, edition, pages
2015. Vol. 9, no 6, 2163-2181 p.
Keyword [en]
energy balance, Greenland, ice sheet, melt, albedo, feedback, percolation, refreezing
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:uu:diva-275890DOI: 10.5194/tc-9-2163-2015ISI: 000367523400010OAI: oai:DiVA.org:uu-275890DiVA: diva2:901571
Projects
Stability and Variations of Arctic Land Ice (SVALI)Programme for Monitoring of the Greenland Ice Sheet (PROMICE)Greenland Analogue Project (GAP)
Available from: 2016-02-08 Created: 2016-02-08 Last updated: 2017-11-30
In thesis
1. Climatology and firn processes in the lower accumulation area of the Greenland ice sheet
Open this publication in new window or tab >>Climatology and firn processes in the lower accumulation area of the Greenland ice sheet
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Greenland ice sheet is the largest Northern Hemisphere store of fresh water, and it is responding rapidly to the warming climate. In situ observations document the changing ice sheet properties in the lower accumulation area, Southwest Greenland. Firn densities from 1840 meters above sea level retrieved in May 2012 revealed the existence of a 5.5-meter-thick, near-surface ice layer in response to the recent increased melt and refreezing in firn. As a consequence, vertical meltwater percolation in the extreme summer 2012 was inefficient, resulting in surface runoff. Meltwater percolated and refroze at six meters depth only after the end of the melt season. This prolonged autumn refreezing under the newly accumulated snowpack resulted in unprecedented firn warming with temperature at ten meters depth increased by more than four degrees Celsius. Simulations confirm that meltwater reached nine meters depth at most. The refrozen meltwater was estimated at 0.23 meters water equivalent, amounting to 25 % of the total 2012 ablation.

A surface energy balance model was used to evaluate the seasonal and interannual variability of all surface energy fluxes at that elevation in the years 2009 to 2013. Due to the meltwater presence at the surface in 2012, the summer-averaged albedo was significantly reduced (0.71 in 2012; typically 0.78). A sensitivity analysis revealed that 71 % of the subsequent additional solar radiation in 2012 was used for melt, corresponding to 36 % of the total 2012 surface lowering. This interplay between melt and firn properties highlights that the lower accumulation area of the Greenland ice sheet will be responding rapidly in a warming climate.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 81 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1372
Keyword
climate change, Greenland ice sheet, accumulation area, automatic weather stations, surface energy balance, melt–albedo feedback, surface mass budget, snow, firn, meltwater, percolation, refreezing, runoff
National Category
Climate Research Environmental Sciences Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-284365 (URN)978-91-554-9571-8 (ISBN)
Public defence
2016-06-10, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:15 (English)
Opponent
Supervisors
Projects
Stability and Variations of Arctic Land Ice (SVALI)Programme for Monitoring of the Greenland Ice Sheet (PROMICE)Greenland Analogue Project (GAP)
Available from: 2016-05-20 Created: 2016-04-17 Last updated: 2016-06-15

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Charalampidis, Charalamposvan As, DirkBox, Jason E.van den Broeke, Michiel R.Colgan, William T.Hubbard, Alun L.MacFerrin, MichaelMachguth, Horst

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