P. Riesen

Deducing the thermal structure in the tongue of Gornergletscher, Switzerland, from radar surveys and borehole measurements

Abstract We present the thermal distribution in the confluence area of Gorner- and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1–5 and 40 MHz. The higher-frequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold–temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

Short-term surface ice motion variations measured with a ground-based portable real aperture radar interferometer

We report measurements using a portable real aperture radar (Gamma Portable Radar Interferometer (GPRI)) for interferometric imaging of the surface ice motion on Gornergletscher, Switzerland, during the drainage of the adjacent ice-marginal lake Gornersee. The GPRI tracked the surface ice motion in line of sight over an area of � 3k m 2 down-glacier of Gornersee almost continuously during the drainage event. The displacement maps derived from the acquired interferograms capture the spatial distribution of the surface ice motion. Due to fast acquisition times of the microwave images, the GPRI was able to record sub-daily variations of the ice displacements, most likely caused by the impact of the Gornersee drainage on the ice motion of Gornergletscher. In situ point measurements of the ice displacement agree reasonably well with the results obtained by the GPRI and highlight the use of the GPRI for high-resolution measurements of glacier surface ice motion. motion of Gornergletscher. These variations in surface ice motion were triggered by the subglacial drainage of the adjacent marginal, ice-dammed lake Gornersee. We present ice displacement maps derived from the GPRI interfero- grams and analyze the performance of the radar device for remote sensing of surface ice motions of a glacier on spatial and temporal scales lower than in conventional D-InSAR applications. We compare the radar displacements against local in situ ice displacement measurements. To complete the study, a brief discussion of the observed displacements in relation to the drainage phenomena is given.

Correction to “Triggering and drainage mechanisms of the 2004 glacier‐dammed lake outburst in Gornergletscher, Switzerland”

[1] To investigate the triggering and the drainage mechanisms of a glacier-dammed lake outburst, we conducted high-frequency measurements of the ice surface motion in the vicinity of Gornersee, an ice marginal lake on Gornergletscher, Switzerland. During the outburst event in July 2004, the ice surface within a distance of 400 m from the lakeshore moved vertically upward by up to 0.1 m. This vertical surface motion cannot be explained by vertical straining of ice which was measured in one of the boreholes; therefore, we suggest the separation of the glacier sole from the bed was caused by subglacially drained lake water. Our observation indicates that the lake water drained as a sheet-like flow through the space created by the basal separation. The upward surface motion was greater in the region where the ice flotation level was exceeded by the lake level, implying that the ice barrier was breached when the lake water hydraulically connected to the bed and lifted up the glacier. In addition to the centimeter-scale vertical ice motion, three survey stakes located within 100 m from the lake showed extraordinarily large vertical displacement of 0.5–3.0 m associated with abrupt changes in horizontal flow direction. A plausible interpretation is that the marginal ice wedge bent upward because of the buoyancy force generated by the drained water. Such bending is possible if subglacial and englacial fractures formed at about 200 m from the glacier margin and acted as a hinge. The newly formed and preexisting englacial fractures probably took the role of inducing englacial water drainage which preceded the outburst.