Ice and snow

Zachariae Isstrom, in Northeast Greenland, is retreating and accelerating,
most probably because of enhanced melting at its ice shelf bottom followed
by its break-up (Mouginot et al. 2015). Nioghalvfjerdfjorden, its
neighbor, is also showing sign of thinning close to its grounding line as is
Peterman Gletscher located 800 km more to the west (Münchow et al.
2014).
Here, we investigate dynamical and geometrical changes of all the other
glaciers located along the Northern coast of Greenland, namely Humboldt
Gletscher, Steensby Gletscher, Ryder Gletscher, Ostenfeld Gletscher, Marie
Sophie Gletscher, Academy Gletscher and Hagen Brae. Using satellite and
airborne-based remote sensing sensors, we reconstruct the time series of
speed, grounding line position, ice thickness and surface elevation
changes since the 80s. We will provide an update of the glacier ice discharges and
will discuss any large scale pattern of enhanced melting of the northern
Greenlandic ice shelves . We will conclude on the possibility of actual or
future destabilization -or lack thereof- of the glaciers in this sector of
Greenland.

Observations of surface elevation lowering, and an associated change in ice mass, in Western Palmer Land, has raised the prospect that dynamic instability is now occurring in this marine-based sector of the Antarctic Ice Sheet. To assess this, we track changes in the speed of the regions glaciers over the past 25 years using optical and synthetic aperture radar satellite imagery. Ice velocities were computed using a combination of SAR and optical feature tracking and SAR interferometry. We tracked the motion of features in sequential SAR images acquired by the ERS-1 and -2 in 1992, 1994, and 1996, by the ALOS PALSAR satellite in 2006, 2007, 2008 and 2010, and the Sentinel-1 satellite in 2014, 2015, and 2016; and in sequential optical images acquired by the Landsat-8 satellite in 2014. We applied the interferometric technique to repeat SAR acquisitions acquired by the ERS-1 and ERS-2 satellites in 1995 and 1996. More than 30 unnamed glaciers drain the 800 km coastline of Western Palmer Land at speeds ranging from 0.5 to 2.5 m/day, interspersed with near stagnant ice. Since 1992, most of these glaciers have speeded up by 0.25 to 0.50 m/day, leading to an 11 % increase in ice flow across the sector as a whole over the past 25 years. With the aid of an optimised ice flow model, we estimate that ice discharge from the sector has increased by 13 km³/yr since 1992. Although this increase is significant, it is far too small to account for observed changes in ice volume and mass inland, which must instead be related to a contemporary shortfall in snowfall. We show that ice the observed speedup is greatest where glaciers are grounded more than 300 m below sea level, suggesting that ocean warming in the Bellingshausen Sea is the dominant forcing mechanism. 

Supraglacial lakes are playing an important role in ice dynamics, once they drain through moulins or hydrofracture and deliver water acting as lubricant to the base of the ice sheets. During the summer month, supraglacial lakes are developing in topographic depressions and either refreeze during winter, drain along the surface or drain to the ice sheet base. A particular lake might refreeze over several years but once a critical threshold in volume and stress is reached, a sudden drainage event with subsequent refilling can occur. Thus their formation, drainage and size is of particular interest to glaciologists. Here we present three different aspects: (1) detection of supraglacial lakes at 79°N Glacier (79NG) using Sentinel-1A/B data from 2015 to present, (2) supraglacial lake filling and drainage based on TanDEM-X DEMs over a time period from 2011 to 2015 and (3) the comparison of the detected location of supraglacial lakes with sinks in surface topography. The detection of supraglacial lakes is based on thresholds in backscatter values and we present selected time series over prominent lakes. The monitoring of lake filling and drainage events is based on a dense time series of TanDEM-X DEMs derived by single-pass SAR interferometry. For the third aspect we interprete local depressions in a digital elevation model as possible lake locations. The three different aspects are then interpreted to give an overall assessment of supraglacial lake dynamics at 79NG.

In the mountainous landscape of Norway, historical catastrophic collapses are documented by numerous deposits in fjords and valleys, some resulting in devastating tsunamis. Today, increasing ground temperatures and thawing of permafrost could lead to more frequent rockslides and destabilized permafrost landforms. Future consequences threatening settlement and infrastructure span from road closures to loss of human lives.

Spaceborne radar instruments provide all-day and all-weather regular coverage of large spatial areas, which make them very useful for mapping and monitoring of slope related hazards. However, interpretation of displacement processes based on single one-dimensional radar datasets can be challenging because of the instruments reduced sensitivity to displacement diverging from the line-of-sight direction. As a consequence, displacement patterns of rockslides/landforms are not fully understood or high-risk objects located remain undetected.

By combining extensive high-resolution stripmap TerraSAR-X satellite and terrestrial radar interferometric data from a Gamma Portable Radar Interferometer (GPRI), we present 3D surface displacement for a destabilized rock glacier (Adjet), located in Troms county, northern Norway. In order to estimate surface velocity of dm/day, we apply a cross-correlation-based method (offset-tracking) on the TerraSAR-X dataset (2009–2016). TerraSAR-X satellite offset-tracking data were combined with GRPI data (2014 and 2015) to produce 3D surface displacement, documenting rates up to dm/day for the fast moving Adjet rock glacier.

Our results show the need for using radar data both from spaceborne and terrestrial platforms in order to investigate the complex displacement patterns of permafrost landforms. Only the combination of these different sensor systems allow for the development of 3D displacement maps that further enhance our process understanding of a changing periglacial landscape. This will contribute to improve the quality of future risk assessments.

Fedchenko Glaciers in the Pamir Mountains is one of the largest mountain glaciers outside the polar regions. The elevation of this more than 70 km long glacier ranges from about 5400 m in the highest basins to 2900 m at the terminus. The glacier has several tributaries including the 25 km long Bivachny Glacier.

We use TanDEM-X and TerraSAR-X data acquired over the glacier system between 2011 and 2016 to investigate different aspects of mass balance and glacier dynamics. While differentiating digital elevation models (DEMs) to derive the geodetic mass balance of a glacier is a well-established method, InSAR derived DEM differences have to be interpreted with care due to the penetration of the radar signal into ice and snow. The penetration depth depends on the radar wavelength of the sensor and the dielectric properties of the surface varying mainly with the wetness of snow and ice. Thus, even using data from the same sensor, penetration depth may differ depending on the season and the location on the glacier. Comparison of elevation changes in different seasons amongst each other and with GPS data acquired in the summers 2015 and 2016 show penetration depths of the TanDEM-X data in the range from zero up to 6 m.

Bivachny Glacier, the largest tributary of Fedchenko Glacier, is one of the many surge-type glaciers in the Pamir Mountains and surged between 2011 and 2015. A time series of 9 DEMs during this period showed the development of a surge bulge and its progression down the glacier until it eventually reached the confluence with Fedchenko Glacier in late 2014. Elevation increased by up to 90 m in the receiving area and lowered by 70 m in the reservoir area in the upper ablation zone. Ice flow velocities during the surge were monitored by feature tracking on the basis of optical and radar data. These data reveal the evolution of the flow velocities with a stepwise acceleration during early summer in three consecutive years and a maximum in early summer 2014 before the surge slowly tapered off. Both, elevation and velocity information were used to estimate the volume flux rate and the total volume mobilized by the surge.

The example of the Fedchenko glacier system shows how the availability of high-resolution DEMs with a high temporal repetition facilitates the detailed study of glacier mass balance and especially the monitoring of the evolution of glacier surges such as the one observed at Bivachny Glacier.