Satellite altimeter remote sensing of ice caps

This thesis investigates the use of satellite altimetry techniques for measuring surface elevation changes of ice caps. Two satellite altimeters, Radar Altimeter 2 (RA-2) and Geoscience Laser Altimeter System (GLAS) are used to assess the surface elevation changes of three Arctic ice caps. This is the first time the RA-2 has been used to assess the elevation changes of ice caps - targets much smaller than the ice sheets which are the instrument’s primary land ice targets. Algorithms for the retrieval of elevation change rates over ice caps using data acquired by RA-2 and GLAS are presented. These algorithms form a part of a European Space Agency (ESA) glacier monitoring system GlobGlacier. A comparison of GLAS elevation data to those acquired by the RA-2 shows agreement between the two instruments. Surface elevation change rate estimates based on RA-2 are given for three ice caps: Devon Ice Cap in Arctic Canada (−0.09 ± 0.29 m/a), Flade Isblink in Greenland (0.03 ± 0.03 m/a) and Austfonna on Svalbard (0.33 ± 0.08 m/a). Based on RA-2 and GLAS measurements it is shown that the areas of Flade Isblink below the late summer snow line have been thinning whereas the areas above the late summer snow line have been thickening. Also GLAS observed dynamic thickening rates of more than 3 m/a are presented. On Flade Isblink and Austfonna RA-2 measurements are compared to surface mass balance (SMB) estimates from a regional atmospheric climate model RACMO2. The comparison shows that SMB is the driver of interannual surface elevation changes at Austfonna. In contrast the comparison reveals areas on Flade Isblink where ice dynamics have an important effect on the surface elevation. Furthermore, RACMO2 estimates of surface mass budget at Austfonna before the satellite altimeter era are presented. This thesis shows that both traditional radar and laser satellite altimetry can be used to quantify the response of ice caps to the changing climate. Direct altimeter measurements of surface elevation and, in consequence volume change of ice caps, can be used to improve their mass budget estimates

Elevation change and mass balance of Svalbard glaciers from geodetic data

This thesis uses ground-based, airborne and spaceborne elevation measurements to estimate elevation change and mass balance of glaciers and ice caps on the Svalbard archipelago in the Norwegian Arctic. Remote sensing data are validated against field measurements from annual campaigns at the Austfonna ice cap. A new and more accurate DEM of the ice cap is constucted by combining SAR interferometry with ICESat laser altimetry. The precision of the DEM is sufficient to correct ICESat near repeat-tracks for the cross-track topography such that multitemporal elevation profiles can be compared along each reference track. The calculated elevation changes agree well with more accurate elevation change data from airborne laser scanning and GNSS surface profiling. The average mass balance of Austfonna between 2002 and 2008 is estimated to -1.3 ± 0.5 Gt y-1, corresponding to an area-averaged water equivalent elevation change of -0.16 ± 0.06 m w.e. y-1. The entire net loss is due to a retreat of the tidewater fronts. Earlier time periods are difficult to assess due to limitations in the amount and quality of previous elevation data sets. Other Svalbard regions have been precisely mapped by aerial photogrammetry, so the ICESat profiles from 2003-2008 can be compared with existing topographic maps and DEMs from 1965-1990. The mass balance for this period is estimated to -9.7 ± 0.6 Gt y-1 (or -0.36 ± 0.02 m w.e. y-1), excluding Austfonna. Repeat-track ICESat data are also analysed for the entire Svalbard yielding an average 2003-2008 mass balance of -4.3 ± 1.4 Gt y-1 (or -0.12 ± 0.04 m w.e. y-1) when tidewater front retreat is not accounted for. The most accurate elevation change estimates are obtained using all available ICESat data in a joint regression where surface slope and elevation change are estimated for rectangular planes that are fitted to the data along each track. The good performance of the plane method implies that it can also be used in other Arctic regions where accurate DEMs typically are not available

Quantification and interpretation of glacier elevation changes

Glaciers, ice caps and ice sheets constitute a large reservoir in the global hydrological cycle and provide a coupling between climate and sea-level. Observations of glacial change is important for constraining their contribution to sea-level fluctuations and to better understand the interactions between glaciers and climate. This thesis focuses on glacier observations through measurements of elevation change. The research in this thesis is oriented towards the methodological detection of elevation changes using remote sensing techniques. The quality of glacier elevation change measurements is dependent on controlling the potential errors and biases within the data. Therefore, one aspect is focused on a universal co-registration method for elevation products and further identification and correction of biases that remain, specifically in satellite stereo products. For glaciological studies, elevation changes require conversion into volume and mass changes. This is sometimes complicated when the data available is not spatially continuous and/or temporally consistent. Therefore, another aspect of this thesis explores methods for estimating regional glacier volume change. Specifically, Svalbard glacial contribution to sea-level has been estimated using regionalization techniques from scattered elevation measurements over roughly two time epochs. We observed that Svalbard glaciers over the past few decades have had a negative mass balance, contributing approximately 0.026 mm per year to the oceans. During the past few years, the sea-level contribution from Svalbard glaciers decreased slightly to 0.013 mm per year. Interpretations of elevation changes are convoluted by their dependence on climatic and dynamic forces operating on glacier systems. The last aspect of this thesis experiments with surface mass balance modelling for quantifying the climatic component of an elevation change. Combining this with observed elevation changes using theory of mass continuity can yield estimates of the calving flux of icebergs into the ocean. We observed on one particular fast flowing glacier in Svalbard that the average calving flux in the 1966-1990 epoch increased in the 1990-2007 epoch

Cryosphere Applications

Synthetic aperture radar (SAR) provides large coverage and high resolution, and it has been proven to be sensitive to both surface and near-surface features related to accumulation, ablation, and metamorphism of snow and firn. Exploiting this sensitivity, SAR polarimetry and polarimetric interferometry found application to land ice for instance for the estimation of wave extinction (which relates to sub surface ice volume structure) and for the estimation of snow water equivalent (which relates to snow density and depth). After presenting these applications, the Chapter proceeds by reviewing applications of SAR polarimetry to sea ice for the classification of different ice types, the estimation of thickness, and the characterisation of its surface. Finally, an application to the characterisation of permafrost regions is considered. For each application, the used (model-based) decomposition and polarimetric parameters are critically described, and real data results from relevant airborne campaigns and space borne acquisitions are reported

Ice loss in High Mountain Asia and the Gulf of Alaska observed by CryoSat-2 swath altimetry between 2010 and 2019

We report the first application of a novel approach to retrieve spatially-resolved elevation change from radar altimetry over entire mountain glaciers areas. We apply interferometric swath altimetry to CryoSat-2 data acquired between July 2010 and July 2019 over High Mountain Asia (HMA) and in the Gulf of Alaska (GoA). We bin swath elevation data into 100 x 100 km bins, remove the topography with a reference DEM and generate linear rates of elevation changes for each bin individually using a weighted regression model. We exclude solutions that that did not fulfil a set of quality criteria based on elevation change uncertainties, temporal completeness, interannual changes and stability of regression results. To extrapolate missing data, hypsometric averaging is applied. We find that during the study period, HMA and GoA have lost an average of –28.0 ± 3.0 Gt yr–1 (–0.29 ± 0.03 m w.e. yr–1) and –76.3 ± 5.7 Gt yr–1 (–0.89 ± 0.07 m w.e. yr–1) respectively. Glacier thinning is ubiquitous except for the Karakoram-Kunlun region experiencing stable or slightly positive mass balanc

Flow and Temperature Dynamics in the Hydrologic Response of Alpine Catchments: Travel Time Formulation and Geomorphologic Signatures

We present a travel time formulation of water and energy transport at sub-catchment scale. The derived equations are implemented in Alpine3D, a physically-based model of snow processes, which provides the necessary boundary conditions to perform hydro-thermal response simulations of Alpine catchments. The model set-up accounts for advective and non- advective energy fluxes to perform spatially distributed simulations of streamflow and temperature in river networks having an arbitrary degree of geomorphological complexity. The model gives reliable predictions of streamflow and tem- perature, as shown by comparing modeled and measured hydrographs and thermographs at the outlet of the Dischma catchment (45 km2) in the Swiss Alps. Our model setup is applied to investigate the role of hillslope aspects, representing the main control on radiation and snowmelt patterns, in the flow regime of the study catchment. The distributed simu- lation results show that snowmelt-induced discharge exhibits a visible geomorphologic signature of aspects at sub-catch- ment scale, but this progressively fades out going from headwater streams to the outlet. Accordingly, the geomorphologic signature is scale-dependent: it is significant at small scales where the high aspect correlation generates predominant orientations but is lost at larger scales where aspects are de-correlated and different orientations are averaged out. We further apply the model to investigate the geomorphologic signature of drainage density in the thermal regime of the study catchment. The results show that the contribution of the advective energy fluxes becomes progressively smaller when the drainage density increases, while the one of the non-advective energy fluxes becomes larger. Moreover, such variations balance out at the catchment outlet, where the temperature signal is not sensitive to the increasing drainage density. The relevance of the performed invetigations stems from the increasing scientific interest concerning the impacts of the warming climate on water resources management and temperature-influenced ecological processes