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Remote sensing of supra-glacial lakes on the west Greenland Ice Sheet
Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. (Glaciologi)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Greenland Ice Sheet is the largest ice sheet in the northern hemisphere. Ongoing melting of the ice sheet, resulting in increased mass loss relative to the longer term trend, has raised concerns about the stability of the ice sheet. Melt water generated at the surface is temporarily stored in supra-glacial lakes on the ice sheet. Connections between melt water generation, storage and ice sheet dynamics highlight the importance of the surface hydrological system.

In this thesis different methods are used that improve our ability to observe the supra-glacial lake system on the west Greenland Ice Sheet. This region of the Greenland Ice Sheet has the most extensive supra-glacial hydrological system with a dense network of streams connecting lakes that can exceed several square kilometres in area. Synthetic Aperture Radar (SAR) and visible-near infrared (VNIR) images are used to explore the potential of different sensor systems for regular observations of the supra-glacial lakes. SAR imagery is found to be a useful complement to VNIR data. VNIR data from moderate resolution sensors are preferred as these provide high temporal resolution data, ameliorating problems with cloud cover.

The dynamic nature of the lakes makes automated classification difficult and manual mapping has been widely used. Here a new method is proposed that improves on existing methods by automating the identification and classification of lakes, and by introducing a flexible system that can capture the full range of lake forms. Applying our new method we are better able to analyse the evolution of lakes over a number of melt seasons. We find that lakes initiate after approximately 40 positive degree days. Most lakes exist for less than 20 days before draining, or later in the season, and less often, freezing over. Using the automated method developed in this thesis lakes have been mapped in imagery from 2001–2010 at approximately five day intervals.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography and Quaternary Geology (INK), Stockholm University , 2012. , 28 p.
Series
Dissertations from the Department of Physical Geography and Quaternary Geology, ISSN 1653-7211 ; 31
Keyword [en]
Supra-glacial lakes, Greenland Ice Sheet, Ice melting, Remote sensing, SAR, MODIS, Time-series analysis
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
URN: urn:nbn:se:su:diva-74509ISBN: 978-91-7447-465-7 (print)OAI: oai:DiVA.org:su-74509DiVA: diva2:510272
Public defence
2012-04-20, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.

Available from: 2012-03-29 Created: 2012-03-15 Last updated: 2013-04-02Bibliographically approved
List of papers
1. Multi-temporal, multi-sensor investigations of supra-glacial lakes on the Greenland Ice Sheet
Open this publication in new window or tab >>Multi-temporal, multi-sensor investigations of supra-glacial lakes on the Greenland Ice Sheet
2011 (English)In: Proceedings of ESA Living Planet Symposium: 28 June - 2 July 2010, Bergen, Norway(DVD) / [ed] H. Lacoste-Francis, Noordwijk: ESA (European Space Agency) , 2011Conference paper, Published paper (Other academic)
Abstract [en]

The stability of the ice sheets is affected by the ongoing climate change through changes in the meltwater budget and effects on ice sheet dynamics. Surface lakes on the Greenland Ice Sheet have attracted much attention, but assessing their number and size as well as the variability over time of these parameters is not straight-forward. We present a satellite image-based survey of the total lake area and number of supra-glacial lakes in a specific region on the western margin of the Greenland Ice Sheet. Optical images from two different sensors (Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat) and images from three different synthetic aperture radar (SAR) satellites (Radarsat, ERS and Envisat) from 2001, 2006 and 2007, were used in order to manually digitize the extent of supra-glacial lakes. The image spatial resolution ranges from high (Radarsat, ERS and Landsat) to moderate (Envisat and MODIS) where high resolution corresponds to 6.25 to 25 m, and a moderate resolution corresponds to 50 to 250 m. SAR imagery was tested as a supplement to the more commonly used optical data to fill gaps in the time series caused by frequent cloud cover. In total 709 individual lakes were studied. High resolution images were shown to be preferable in the beginning of the melt season, due to the smaller size of the lakes. In the middle of the melt season the resolution was of limited importance. Our work suggests that the use of a combination of active radar and optical images enables successful lake monitoring with high temporal and spatial resolution in both cloudy and clear weather conditions.

Place, publisher, year, edition, pages
Noordwijk: ESA (European Space Agency), 2011
Keyword
Greenland, Remote Sensing, SAR, supra-glacial lakes
National Category
Physical Geography
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-74504 (URN)978-92-9221-250-6 (ISBN)
Conference
ESA Living Planet Symposium: 28 June - 2 July 2010, Bergen, Norway
Available from: 2012-03-15 Created: 2012-03-15 Last updated: 2012-03-16Bibliographically approved
2. Observations of supra-glacial lakes in west Greenland using winter wide swath Synthetic Aperture Radar
Open this publication in new window or tab >>Observations of supra-glacial lakes in west Greenland using winter wide swath Synthetic Aperture Radar
2012 (English)In: Remote Sensing Letters, ISSN 2150-7058, Vol. 3, no 6, 531-539 p.Article in journal (Refereed) Published
Abstract [en]

Melt water gathered in supra-glacial lakes on the Greenland ice sheet contributes to reduced basal friction by the input of water from the surface to the bed during the melt season. Supra-glacial lakes change on both annual and interannual timescales. The high temporal resolution and moderate spatial resolution Moderate Resolution Imaging Spectroradiometer (MODIS) can be used to capture annual changes on the highly transient/fast-draining lakes. However, MODIS imagery lacks the ability to penetrate clouds. Using ENVISAT Synthetic Aperture Radar (SAR) winter images, we show that SAR images can be used to identify seasonal/inter-annual changes in the lake area. Furthermore, winter SAR images identify lakes obscured to optical data by ice and snow cover. However, the SAR imagery systematically failed to discriminate narrow lakes and lakes in corrugated topography. Thus, SAR images can be used as a complement to existing visible–near-infrared data but do not offer a viable replacement.

Place, publisher, year, edition, pages
London: Taylor & Francis, 2012
Keyword
Remote sensing, supra-glacial lakes, Greenland, MODIS, SAR
National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-74503 (URN)10.1080/01431161.2011.637527 (DOI)000302244100008 ()
Available from: 2012-03-15 Created: 2012-03-15 Last updated: 2012-06-08Bibliographically approved
3. Spatial and temporal variations of the Greenland Ice Sheet surface lake system
Open this publication in new window or tab >>Spatial and temporal variations of the Greenland Ice Sheet surface lake system
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Surface lakes on the Greenland Ice Sheet provide temporary storage for meltwater that influences both the surface and basal water fluxes. Thus, to understand the effects of variations in surface melt on ice sheet dynamics it is necessary to understand the surface hydrology. We have used satellite imagery, acquired at 5-day intervals, to map lake initiation and cessation on two sub-sections on the south west Greenland Ice Sheet over three melt seasons (2007-2009). We observe that lake initiation is closely tied to a threshold energy input of approximately 40 ± 18.5 positive-degree-days. This applies for all studied melt season, regardless of evolution and melt index anomalies. Lake longivity averages 24 days with little variation between different melt seasons. Our observed median lake area is larger than previously reported. Approximately 50% of all lakes have a life span of <10 days. Cessation of identified lakes is caused by two processes: drainage during the melt season (88% 2007, 78% 2008 and 88% 2009) and freeze-up at the end of the season (12% 2007, 22% 2008 and 12% 2009). Inclusion of energy needed for lake initiation and amount of freeze-up lakes into supra-glacial lake models will add further insight into the hydrological system dynamics.

National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-74505 (URN)
Available from: 2012-03-15 Created: 2012-03-15 Last updated: 2012-03-16Bibliographically approved
4. Adaptive Classification of Supra-Glacial Lakes on the West Greenland Ice Sheet
Open this publication in new window or tab >>Adaptive Classification of Supra-Glacial Lakes on the West Greenland Ice Sheet
2013 (English)In: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, ISSN 1939-1404, E-ISSN 2151-1535, Vol. 6, no 4, 1998-2007 p.Article in journal (Refereed) Published
Abstract [en]

Meltwater is stored in supra-glacial lakes on the Greenland Ice Sheet. Connections between the melt water, ice sheet dynamics and the extent of the surface hydrological system have been observed. This highlights the importance of being able to study the surface hydrology over large spatial scales with a high temporal resolution. In this study, we develop an adaptive classification method to identify and map these supra-glacial lakes using high temporal resolution satellite images from the Moderate-resolution Imaging Spectroradiometer (MODIS) sensor. Surface reflectance images from 2007 are used to extract information about the lakes, their morphologies and their surroundings. Using a multiclass approach we can recognize lake types that have previously been difficult to classify, such as deep lakes, lakes within cryoconite areas and lakes with floating ice. Given significant increases in melting in recent years relative to the long term record the inclusion of deep lakes might prove particularly important.

Keyword
Landsat, MODIS, multispectral satellite imagery, object tracking, remote sensing, remote sensing applications
National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-84503 (URN)10.1109/JSTARS.2012.2233722 (DOI)000322453800019 ()
Available from: 2012-12-23 Created: 2012-12-23 Last updated: 2017-12-06Bibliographically approved
5. Characteristics of supra-glacial lakes on the south-west Greenland Ice Sheet
Open this publication in new window or tab >>Characteristics of supra-glacial lakes on the south-west Greenland Ice Sheet
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Supra-glacial lake form and store melt water during the ablation season every year on the Greenland Ice Sheet (GrIS). This melt water influences both the surface and basal water fluxes. Using an adaptive object-oriented classification approach supra-glacial lakes on the west GrIS are mapped in Terra Moderate Resolution Imaging Spectroradiometer (MODIS) imagery 2001–2010. Lake size shape and distribution parameters are extracted for approximately 5-day intervals from the available cloud free imagery. Using temperature data measured within this region we correlate the lake parameters to melt proxies such ascumulative positive degree days (PDD). The analysis shows that simple parameters such as number of lakes and mean lake area, measures that have been used to describe the development of the surface hydrological system, are largely uncorrelated with melt proxies. Years with higher relative melt, expressed as negative surface mass balances and high PDDs, may exhibit long, predominantly mild melt seasons. Alternatively, a short but intense melt season may have occurred. Given the short duration of the time series, it is difficult to establish statistical relationships between parameters. However, parameters such as median lake size, reflectance and distance between lakes may provide valuable ancillary information to measures such as total lake area, median lake area and lake size distribution data.

Keyword
Greenland Ice Sheet, Supra-glacial lakes, Ice melting, Remote sensing, MODIS
National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-74507 (URN)
Available from: 2012-03-15 Created: 2012-03-15 Last updated: 2012-03-16

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