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Spatial variability in water content at the cold-temperate transition surface of the polythermal Storglaciären, Sweden
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
2004 (English)In: Journal of Geophysical Research, ISSN 0148-0227, Vol. 109, no F02009Article in journal (Refereed) Published
Abstract [en]

The volume fraction of liquid water in temperate glacier ice is important not only for the flow of glaciers and the analysis and processing of ground penetrating radar data from glaciers but also for the stability of the thermal layering in polythermal glaciers. However, little is known about the spatial variations of water content in glaciers. We use relative backscatter strength of ground-penetrating radar signals to estimate the spatial distribution of water content close to the cold-temperate transition on Storglaciären, northern Sweden, in an area close to the equilibrium line. The values of relative backscatter strength are calibrated using determinations of absolute water content from temperature measurements across the cold-temperate transition and the thermodynamic boundary condition at the freezing front. The results show a water content of 0.80%, 0.75%, and 0.58% at three calibration points and a mean water content of 0.8% with a standard deviation of ±0.26% for the extrapolated water content. The extrapolated water content shows a distinct pattern, with lower water content on one side of the glacier center line and higher water content on the other side, with higher water content on the northern side. We hypothesize that the different water contents result from the fact that the ice on either side of the center line originates from different cirques, thus implying spatial variations in the entrapment of water in the firn-ice transition process in the different cirques.

Place, publisher, year, edition, pages
Washington D.C.: American Geophysical Union. , 2004. Vol. 109, no F02009
Keyword [en]
spatial variability, water content, polythermal glacier, cold surface layer, Storglaciären, Sweden.
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-23133DOI: 10.1029/2003JF000110OAI: oai:DiVA.org:su-23133DiVA: diva2:190318
Note
Part of urn:nbn:se:su:diva-161Available from: 2004-05-13 Created: 2004-05-13 Last updated: 2009-12-21Bibliographically approved
In thesis
1. Dynamics of the cold surface layer of polythermal Storglaciären, Sweden
Open this publication in new window or tab >>Dynamics of the cold surface layer of polythermal Storglaciären, Sweden
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polythermal glaciers, i.e. glaciers with a combination of ice at and below the freezing point, are widespread in arctic and subarctic environments. The polythermal structure has major implications for glacier hydrology, ice flow and glacial erosion. However, the interplay of factors governing its spatial and temporal variations such as net mass balance, ice advection and water content in the ice is poorly investigated and as yet not fully understood. This study deals with a thorough investigation of the polythermal regime on Storglaciären, northern Sweden, a small valley glacier with a cold surface layer in the ablation area. Extensive field work was performed including mapping of the cold surface layer using ground-penetrating radar, ice temperature measurements, mass balance and ice velocity measurements. Analyses of these data combined with numerical modelling were used specifically to investigate the spatial and temporal variability of the cold surface layer, the spatial distribution of the water content just below the cold surface layer transition, the effect of radar frequency on the detection of the surface layer, and the sensitivity of the cold surface layer to changes in forcing.

A comparison between direct temperature measurements in boreholes and ground-penetrating surveys shows that the radar-inferred cold-temperate transition depth is within ±1 m from the melting point of ice at frequencies above ~300 MHz. At frequencies below ~155 MHz, the accuracy degrades because of reduced scattering efficiency that occurs when the scatterers become much smaller compared to the wavelength. The mapped spatial pattern of the englacial cold-temperate transition boundary is complex. This pattern reflects the observed spatial variation in net loss of ice at the surface by ablation and vertical advection of ice, which is suggested to provide the predominant forcing of the cold surface layer thickness pattern. This is further supported by thermomechanical modeling of the cold surface layer, which indicates high sensitivity of the cold surface layer thickness to changes in vertical advection rates.

The water content is the least investigated quantity that is relevant for the thermal regime of glaciers, but also the most difficult to assess. Spatial variability of absolute water content in the temperate ice immediately below the cold surface layer on Storglaciären was determined by combining relative estimates of water content from ground-penetrating radar data with absolute determination from temperature measurements and the thermal boundary condition at the freezing front. These measurements indicate large-scale spatial variability in the water content, which seems to arise from variations in entrapment of water at the firn-ice transition. However, this variability cannot alone explain the spatial pattern in the thermal regime on Storglaciären.

Repeated surveys of the cold surface layer show a 22% average thinning of the cold surface layer on Storglaciären between 1989 and 2001. Transient thermomechanical modeling results suggest that the cold surface layer adapts to new equilibrium conditions in only a few decades after a perturbation in the forcing is introduced. An increased winter air temperature since mid-1980s seems to be the cause of the observed thinning of the cold surface layer. Over the last decades, mass balance measurements indicate that the glacier has been close to a steady state. The quasi-steady state situation is also reflected in the vertical advection, which shows no significant changes during the last decades. Increased winter temperatures at the ice surface would result in a slow-down of the formation of cold ice at the base of the cold surface layer and lead to a larger imbalance between net loss of ice at the surface and freezing of temperate ice at the cold-temperate transition.

Place, publisher, year, edition, pages
Stockholm: Institutionen för naturgeografi och kvartärgeologi, 2004. 96 p.
Series
Avhandling i geografi med naturgeografisk inriktning, ISSN 1650-4992 ; 31
Keyword
polythermal glacier, cold surface layer, ground-penetrating radar, spatial variability, water content, Storglaciären
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-161 (URN)91-7265-907-6 (ISBN)
Public defence
2004-06-03, sal G, Arrheniuslaboratorierna, Svante Arrhenius väg 14-18, Stockholm, 10:00
Opponent
Supervisors
Available from: 2004-05-13 Created: 2004-05-13Bibliographically approved

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