Thomas Nagler

Rapid collapse of Northern Larsen Ice Shelf, Antarctica

In January 1995, 4200 square kilometers of the northern Larsen Ice Shelf, Antarctic Peninsula, broke away. Radar images from the ERS-1 satellite, complemented by field observations, showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days. This breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming. The observations imply that after an ice shelf retreats beyond a critical limit, it may collapse rapidly as a result of perturbated mass balance.

Retrieval of wet snow by means of multitemporal SAR data

An algorithm has been developed for mapping wet snow in mountainous terrain using repeat pass synthetic aperture radar (SAR) images. As a basis for algorithm development, backscattering properties of snow-free and snow-covered alpine surfaces were investigated using ERS SAR data and field measurements at test sites in the Austrian Alps. The incidence angle dependence of backscattering derived from SAR data is compared with simulations for snow-free surfaces and for surfaces covered by dry snow and wet snow. Significant seasonal changes of backscattering are observed, which are mainly caused by variations of the snow liquid water content and of the surface roughness. The importance of surface roughness for backscattering of wet snow is demonstrated by a surface roughness experiment. The algorithm for mapping wet snow applies change detection using ratios of wet snow versus snow-free or dry snow surfaces. The main steps include coregistration, speckle reduction, thresholding of ratio images, geocoding, and, optionally, combination of crossing passes to reduce the loss of information due to layover. A threshold of -3 dB was found to be appropriate for both Radarsat and ERS SAR to separate wet snow from other surfaces. Postprocessing steps, based on historic snow maps or topographic information, are used to correct for dry snow areas at high elevations. Effects of imaging geometry are investigated by comparing ERS SAR images with a look angle of 19/spl deg/ and Radarsat SAR Beam Mode S7 images with a look angle of 40/spl deg/. The comparison of snow maps from SAR and Landsat-5 Thematic Mapper images shows good agreement in areas of closed snow cover, whereas near the snow line/SAR tends to slightly underestimate the snow extent.

Cold Regions Hydrology High-Resolution Observatory for Snow and Cold Land Processes

Snow is a critical component of the global water cycle and climate system, and a major source of water supply in many parts of the world. There is a lack of spatially distributed information on the accumulation of snow on land surfaces, glaciers, lake ice, and sea ice. Satellite missions for systematic and global snow observations will be essential to improve the representation of the cryosphere in climate models and to advance the knowledge and prediction of the water cycle variability and changes that depend on snow and ice resources. This paper describes the scientific drivers and technical approach of the proposed Cold Regions Hydrology High-Resolution Observatory (CoReH2O) satellite mission for snow and cold land processes. The sensor is a synthetic aperture radar operating at 17.2 and 9.6 GHz, VV and VH polarizations. The dual-frequency and dual-polarization design enables the decomposition of the scattering signal for retrieving snow mass and other physical properties of snow and ice.

The Sentinel-1 Mission: New Opportunities for Ice Sheet Observations

The Sentinel satellite constellation series, developed by the European Space Agency, represents the dedicated space component of the European Copernicus program, committed to long-term operational services in a wide range of application domains. Here, we address the potential of the Sentinel-1 mission for mapping and monitoring the surface velocity of glaciers and ice sheets. We present an ice velocity map of Greenland, derived from synthetic aperture radar (SAR) data acquired in winter 2015 by Sentinel-1A, the first satellite of the Copernicus program in orbit. The map is assembled from about 900 SAR scenes acquired in Interferometric Wide swath (IW) mode, applying the offset tracking technique. We discuss special features of IW mode data, describe the procedures for producing ice velocity maps, and assess the uncertainty of the ice motion product. We compare the Sentinel-1 ice motion product with velocity maps derived from high resolution SAR data of the TerraSAR-X mission and from PALSAR data. Beyond supporting operational services, the Sentinel-1 mission offers enhanced capabilities for comprehensive and long-term observation of key climate variables, such as the motion of ice masses.

Using a Ground-Based SAR Interferometer and a Terrestrial Laser Scanner to Monitor a Snow-Covered Slope: Results From an Experimental Data Collection in Tyrol (Austria)

In this paper, we report on an experimental activity aimed at investigating the potential of two terrestrial remote-sensing techniques, namely, ground-based SAR (GB SAR) interferometry and terrestrial laser scanning, in order to retrieve snow-depth (SD) measurements in mountainous regions. Terrestrial laser scanning is a more consolidated technique based on the measurement of the optical (near infrared) reflectivity, and it is affected by the surface of the snow layer: a temporal data sequence allows us to estimate the absolute SD variation. Recent use of SAR interferometry to evaluate snow-mass characteristics is based on relating the measured interferometric phase shift to a change in the snow mass. Interferometric GB SAR measurements and terrestrial laser scanner scans were collected together with pointwise conventional measurements of physical snow parameters during the winters of 2005/2006 and 2006/2007. The experiment was carried out in the Wattener Lizum, a high Alpine area at about 2000-m elevation north of the main ridge of the Austrian Alps in Tyrol. Notwithstanding the difficulty of providing both lengthy data record in dry snow conditions and detailed knowledge of the observed snow characteristics, the obtained results confirmed the presence of a clearly measurable interferometric phase variation in relation to the growing height of the snow layer. A comparison of the SD maps obtained through the two techniques shows differences partly due to the different nature of the two observations.

Mass changes of outlet glaciers along the Nordensjköld Coast, northern Antarctic Peninsula, based on TanDEM-X satellite measurements

We analyzed volume change and mass balance of outlet glaciers of the northern Antarctic Peninsula over the period 2011 to 2013, using topographic data of high vertical accuracy and great spatial detail, acquired by bistatic radar interferometry of the TanDEM-X/TerraSAR-X satellite formation. The study area includes glaciers draining into the Larsen-A, Larsen Inlet, and Prince-Gustav-Channel embayments. After collapse of buttressing ice shelves in 1995 the glaciers became tidewater calving glaciers and accelerated, resulting in increased ice export. Downwasting of most glaciers is going on, but at reduced rates compared to previous years in accordance with deceleration of ice flow. The rate of mass depletion is 4.2 ± 0.4 Gt a−1, with the largest contribution by Drygalski Glacier amounting to 2.2 ± 0.2 Gt a−1. On the technological side, the investigations demonstrate the capability of satellite-borne single-pass radar interferometry as a new tool for accurate and detailed monitoring of glacier volume change.

Glacier Remote Sensing Using Sentinel-2. Part I: Radiometric and Geometric Performance, and Application to Ice Velocity

With its temporal resolution of 10 days (five days with two satellites, and significantly more at high latitudes), its swath width of 290 km, and its 10 m and 20 m spatial resolution bands from the visible to the shortwave infrared, the European Sentinel-2 satellites have significant potential for glacier remote sensing, in particular mapping of glacier outlines and facies, and velocity measurements. Testing Level 1C commissioning and ramp-up phase data for initial sensor quality experiences, we find a high radiometric performance, but with slight striping effects under certain conditions. Through co-registration of repeat Sentinal-2 data we also find lateral offset patterns and noise on the order of a few metres. Neither of these issues will complicate most typical glaciological applications. Absolute geo-location of the data investigated was on the order of one pixel at the time of writing. The most severe geometric problem stems from vertical errors of the DEM used for ortho-rectifying Sentinel-2 data. These errors propagate into locally varying lateral offsets in the images, up to several pixels with respect to other georeferenced data, or between Sentinel-2 data from different orbits. Finally, we characterize the potential and limitations of tracking glacier flow from repeat Sentinel-2 data using a set of typical glaciers in different environments: Aletsch Glacier, Swiss Alps; Fox Glacier, New Zealand; Jakobshavn Isbree, Greenland; Antarctic Peninsula at the Larsen C ice shelf.

Glacier Remote Sensing Using Sentinel-2. Part II: Mapping Glacier Extents and Surface Facies, and Comparison to Landsat 8

Mapping of glacier extents from automated classification of optical satellite images has become a major application of the freely available images from Landsat. A widely applied method is based on segmented ratio images from a red and shortwave infrared band. With the now available data from Sentinel-2 (S2) and Landsat 8 (L8) there is high potential to further extend the existing time series (starting with Landsat 4/5 in 1982) and to considerably improve over previous capabilities, thanks to increased spatial resolution and dynamic range, a wider swath width and more frequent coverage. Here, we test and compare a variety of previously used methods to map glacier extents from S2 and L8, and investigate the mapping of snow facies with S2 using top of atmosphere reflectance. Our results confirm that the band ratio method works well with S2 and L8. The 15 m panchromatic band of L8 can be used instead of the red band, resulting in glacier extents similar to S2 (0.7% larger for 155 glaciers). On the other hand, extents derived from the 30 m bands are 4%–5% larger, indicating a more generous interpretation of mixed pixels. Mapping of snow cover with S2 provided accurate results, but the required topographic correction would benefit from a better orthorectification with a more precise DEM than currently used.

Velocities of Major Outlet Glaciers of the Patagonia Icefield Observed by TerraSAR-X

The capabilities of TerraSAR-X data for feature tracking by amplitude correlation over glacier surfaces are investigated. Methodical aspects of the amplitude correlation approach are described. The TerraSAR-X based velocity fields are compared with former InSAR derived velocities and field measurements on three outlet glaciers on the South Patagonia ice field.

Analysis of landslides in Alpine areas by means of SAR interferometry

Methods and applications of differential SAR interferometry (DINSAR) for detecting and monitoring slow movements of mountain slopes on the order of centimeters per year were investigated in the Austrian and Swiss Alps, using SAR images from the European ERS-1 and ERS-2 satellites. The DINSAR analysis methods and criteria for selecting SAR image pairs suitable for landslide monitoring are briefly described. Most of the detected landslides are above the tree line, because on surfaces with sparse vegetation and bare soil or rock the coherence is preserved over long periods. The investigations confirm the operational potential of DINSAR for detecting and monitoring mass movements in high Alpine areas.