ESA's Ice Sheets CCI: validation and inter-comparison of surface elevation changes derived from laser and radar altimetry over Jakobshavn Isbræ, Greenland – Round Robin results

Abstract. In order to increase the understanding of the changing climate, the European Space Agency has launched the Climate Change Initiative (ESA CCI), a program which joins scientists and space agencies into 13 projects either affecting or affected by the concurrent changes. This work is part of the Ice Sheets CCI and four parameters are to be determined for the Greenland Ice Sheet (GrIS), each resulting in a dataset made available to the public: Surface Elevation Changes (SEC), surface velocities, grounding line locations, and calving front locations. All CCI projects have completed a so-called Round Robin exercise in which the scientific community was asked to provide their best estimate of the sought parameters as well as a feedback sheet describing their work. By inter-comparing and validating the results, obtained from research institutions world-wide, it is possible to develop the most optimal method for determining each parameter. This work describes the SEC Round Robin and the subsequent conclusions leading to the creation of a method for determining GrIS SEC values. The participants used either Envisat radar or ICESat laser altimetry over Jakobshavn Isbræ drainage basin, and the submissions led to inter-comparisons of radar vs. altimetry as well as cross-over vs. repeat-track analyses. Due to the high accuracy of the former and the high spatial resolution of the latter, a method, which combines the two techniques will provide the most accurate SEC estimates. The data supporting the final GrIS analysis stem from the radar altimeters on-board Envisat, ERS-1 and ERS-2. The accuracy of laser data exceeds that of radar altimetry; the Round Robin analysis has, however, proven the latter equally capable of dealing with surface topography thereby making such data applicable in SEC analyses extending all the way from the interior ice sheet to margin regions. This shows good potential for a~future inclusion of ESA CryoSat-2 and Sentinel-3 radar data in the analysis, and thus for obtaining reliable SEC estimates throughout the entire GrIS.

Download Full-text

ESA ice sheet CCI: derivation of the optimal method for surface elevation change detection of the Greenland ice sheet – round robin results

International Journal of Remote Sensing ◽

10.1080/01431161.2014.999385 ◽

2015 ◽

Vol 36 (2) ◽

pp. 551-573 ◽

Cited By ~ 8

Author(s):

J.F. Levinsen ◽

K. Khvorostovsky ◽

F. Ticconi ◽

A. Shepherd ◽

R. Forsberg ◽

...

Keyword(s):

Change Detection ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Round Robin ◽

Surface Elevation ◽

Elevation Change ◽

Optimal Method ◽

Surface Elevation Change

Download Full-text

25 years of elevation changes of the Greenland Ice Sheet from ERS, Envisat, and CryoSat-2 radar altimetry

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2018.05.015 ◽

2018 ◽

Vol 495 ◽

pp. 234-241 ◽

Cited By ~ 15

Author(s):

Louise Sandberg Sørensen ◽

Sebastian B. Simonsen ◽

René Forsberg ◽

Kirill Khvorostovsky ◽

Rakia Meister ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Radar Altimetry ◽

Elevation Changes

Download Full-text

The importance of slope correction for studying Greenland ice change using radar altimetry (CryoSat-2)

10.5194/egusphere-egu2020-15264 ◽

2020 ◽

Author(s):

Katarzyna Sejan ◽

Bert Wouters ◽

Michiel van den Broeke

Keyword(s):

Slope Angle ◽

Greenland Ice Sheet ◽

Correction Method ◽

Surface Elevation ◽

Laser Altimetry ◽

Radar Altimetry ◽

Slope Correction ◽

Long Time ◽

Elevation Changes ◽

Satellite Radar

Satellite radar altimetry is one of the most important tools for monitoring changes in the mass balance of the world's ice sheets. Acquiring long time series of elevation changes is crucial, and the long lifetime of the CryoSat-2 mission has contributed wonderfully to this effort. However, once the CryoSat-2 mission ends, it will be important to bridge the gap between CryoSat-2 and future radar altimetry missions. IceSat2 data can help aid this effort, assuming that the appropriate processing techniques are used to allow the comparison of radar and laser altimetry. Furthermore, different altimetry techniques come with their own pitfalls, in radar altimetry signal penetration into the snowpack introduces ambiguity in the origin of reflected echo, a major issue not present in laser altimetry. It is therefore important to minimize this ambiguity by developing processing algorithms for the radar altimetry form CryoSat-2 mission, with a special attention on relating it to the IceSat2 mission. &#160;Focusing on Greenland Ice Sheet (GIS), we have developed a processing chain for the estimation of surface elevations and elevation changes from the ESA level-1 product (L1b) Baseline D. As a first step, we investigated the importance of Digital Elevation Model (DEM) in the slope correction algorithm and how it affects the estimated surface elevation.&#160;The waveform retracker algorithm was developed following the method by Nilsson (2015) with a range of thresholds in the threshold retracker applied to the waveform. Knowing the estimated range and the altitude of the satellite at the time of the measurement, we calculated the corresponding surface elevation at the point of the wavelet reflection.We apply a slope correction method by Hurkmans (2012), where displacement from the nadir location in x- and y- directions is calculated using the slope angle and aspect retrieved from a DEM, giving a new set of coordinates that represents the location of the estimated elevation. We use two sets of slope angle and aspect calculated from two DEMs, ArcticDEM Release 7 (Porter et al., 2018) and Greenland Ice Mapping Project (GIMP) DEM (Howat et al., 2017). Both DEMs are similar in terms of optical imagery data source, processing and resolution, however, they have been referenced to different laser altimetry data. We investigate this effect in the slope correction of radar altimetry from CryoSat2 mission.We checked the two sets of slope correction data using IceSat-2 data (Smith et al., 2019) corresponding to the same time period, and selected by nearest point calculation. We analyze and discuss the differences between IceSat-2 data and CryoSat-2 data with slope correction using GIMP DEM or ArcticDEM.

Download Full-text

Modeling of firn compaction for estimating ice-sheet mass change from observed ice-sheet elevation change

Annals of Glaciology ◽

10.3189/172756411799096321 ◽

2011 ◽

Vol 52 (59) ◽

pp. 1-7 ◽

Cited By ~ 40

Author(s):

Jun Li ◽

H. Jay Zwally

Keyword(s):

Accumulation Rate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Temporal Variations ◽

Mass Change ◽

Surface Elevation ◽

Effective Density ◽

Elevation Change ◽

Surface Elevation Change ◽

Elevation Changes

AbstractChanges in ice-sheet surface elevation are caused by a combination of ice-dynamic imbalance, ablation, temporal variations in accumulation rate, firn compaction and underlying bedrock motion. Thus, deriving the rate of ice-sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion, which do not involve changes in mass, and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability. We use a 25 year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model. We apply this formulation to ICESat measurements of surface elevation change at three locations on the Greenland ice sheet in order to separate the accumulation-driven changes from the ice-dynamic/ablation-driven changes, and thus to derive the corresponding mass change. Our calculated densities for the accumulation-driven changes range from 410 to 610 kgm–3, which along with 900 kgm–3 for the dynamic/ablation-driven changes gives average densities ranging from 680 to 790 kgm–3. We show that using an average (or ‘effective’) density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign.

Download Full-text

Ice-Sheet Surface Elevation and Changes Observable by Satellite Radar Altimetry

Journal of Glaciology ◽

10.1017/s0022143000015045 ◽

1979 ◽

Vol 24 (90) ◽

pp. 491-493 ◽

Cited By ~ 4

Author(s):

H. Jay Zwally ◽

R. L. Brooks ◽

H. Ray Stanley ◽

W. J. Campbell

Keyword(s):

Drainage Basin ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

The State ◽

Surface Elevation ◽

Radar Altimeter ◽

Major Question ◽

The Status ◽

Satellite Radar ◽

Ice Sheet Dynamics

Abstract A major question in ice-sheet dynamics is the state of balance between the net mass input and ice flow. Since an imbalance produces a change in surface elevation, the state of balance can be studied by monitoring the elevation, and this has been accomplished by surface-leveling techniques in a few locations. Due to the requirement for accurate and repetitive measurements over large areas, it is not practical to determine the status of balance of an entire ice sheet or even a major drainage basin by conventional techniques. Now, recent results from satellite-borne radar altimeter measurements over the Greenland ice sheet demonstrate the feasibility of accurately measuring and monitoring the topography of large ice masses. The application of this new technique offers the possibility of making a meaningful mass-balance determination and for detecting actual or potential ice-sheet surges.

Download Full-text

Drainage basins and glacier catchments for the Greenland Ice Sheet

10.5194/egusphere-egu21-10040 ◽

2021 ◽

Author(s):

Lukas Krieger ◽

Dana Floricioiu

Keyword(s):

Drainage Basin ◽

Ice Sheets ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Drainage Basins ◽

Catchment Areas ◽

Essential Input ◽

The Individual ◽

Seed Points ◽

The Antarctic

The drainage divides of ice sheets separate the overall glaciated area into multiple sectors and outlet glaciers. These catchments represent essential input data for partitioning glaciological measurements or modelling results to the individual glacier level. They specify the area over which basin specific measurements need to be integrated.The delineation of drainage basins on ice sheets is challenging due to their gentle slopes accompanied by local terrain disturbances and complex patterns of ice movement. Therefore, in Greenland the basins have been mostly delineated along the major ice divides, which results in large drainage sectors containing multiple outlet glaciers. In [1] we developed a methodology for delineating individual glaciers that was applied to the Northeast Greenland sector and proposed slightly changed separations between 79N and Zachariae basins driven by the ice flow lines. In the present study the method is extended to the entire Greenland Ice Sheet.We present a fully traceable approach that combines ice sheet wide velocity measurements by Sentinel-1 SAR and the 90 m TanDEM-X global DEM to derive individual glacier drainage basins for the entire Greenland Ice Sheet with a modified watershed algorithm. We delineate a total of 335 individual glacier catchments, a result triggered by the number and location of the selected seed points.The resulting dataset will be made publicly available online and is extensible by even more granular delineations of individual tributaries upon request. The proposed approach has the potential to produce catchment areas also for the entirety of the Antarctic Ice Sheet.&#160;[1] Krieger, L., D. Floricioiu, and N. Neckel (Feb. 1, 2020). &#8220;Drainage Basin Delineation for Outlet Glaciers of Northeast Greenland Based on Sentinel-1 Ice Velocities and TanDEM-X Elevations&#8221;.&#160; In:Remote&#160; Sensing&#160; of&#160; Environment 237,&#160; p.&#160; 111483.<img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.0a3a84f0b50066175890161/sdaolpUECMynit/12UGE&app=m&a=0&c=3dfbdc4652076318ef26948580f87415&ct=x&pn=gnp.elif&d=1" alt="">

Download Full-text

MASS BALANCE CHANGES AND ICE DYNAMICS OF GREENLAND AND ANTARCTIC ICE SHEETS FROM LASER ALTIMETRY

ISPRS - International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences ◽

10.5194/isprs-archives-xli-b8-481-2016 ◽

2016 ◽

Vol XLI-B8 ◽

pp. 481-487 ◽

Cited By ~ 2

Author(s):

G. S. Babonis ◽

B. Csatho ◽

T. Schenk

Keyword(s):

Sea Level ◽

Ice Sheets ◽

Ice Sheet ◽

Surface Shape ◽

Surface Elevation ◽

Laser Altimetry ◽

Ice Dynamics ◽

Small Surface ◽

Elevation Changes ◽

Antarctic Ice Sheets

During the past few decades the Greenland and Antarctic ice sheets have lost ice at accelerating rates, caused by increasing surface temperature. The melting of the two big ice sheets has a big impact on global sea level rise. If the ice sheets would melt down entirely, the sea level would rise more than 60 m. Even a much smaller rise would cause dramatic damage along coastal regions. In this paper we report about a major upgrade of surface elevation changes derived from laser altimetry data, acquired by NASA’s Ice, Cloud and land Elevation Satellite mission (ICESat) and airborne laser campaigns, such as Airborne Topographic Mapper (ATM) and Land, Vegetation and Ice Sensor (LVIS). For detecting changes in ice sheet elevations we have developed the Surface Elevation Reconstruction And Change detection (SERAC) method. It computes elevation changes of small surface patches by keeping the surface shape constant and considering the absolute values as surface elevations. We report about important upgrades of earlier results, for example the inclusion of local ice caps and the temporal extension from 1993 to 2014 for the Greenland Ice Sheet and for a comprehensive reconstruction of ice thickness and mass changes for the Antarctic Ice Sheets.

Download Full-text

Interannual variability of elevation on the Greenland ice sheet: effects of firn densification, and establishment of a multi-century benchmark

Journal of Glaciology ◽

10.3189/172756501781832151 ◽

2001 ◽

Vol 47 (158) ◽

pp. 369-377 ◽

Cited By ~ 7

Author(s):

K. M. Cuffey

Keyword(s):

Accumulation Rate ◽

Ice Core ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Elevation ◽

Elevation Change ◽

The Past ◽

Current Climate ◽

Elevation Changes ◽

Past Millennium

AbstractIn order to interpret measurements of ice-sheet surface elevation changes in terms of climatic or dynamic trends, it is necessary to establish the range of stochastic variability of elevation changes resulting from interannual fluctuations of accumulation rate and firn density. The analyses presented here are intended to facilitate such interpretations by defining benchmarks that characterize elevation-change variability in central Greenland, in the current climate and over the past millennium. We use a time- dependent firn-densification model coupled to an ice- and heat-flow model, forced by annual accumulation rate and temperature reconstructions from the Greenland Ice Sheet Project II (GISP2) ice core, to examine the elevation changes resulting from this climatic forcing. From these results, effective firn densities are calculated. These are factors that convert water-equivalent accumulation-rate variability to surface elevation variability. A current-climate benchmark is defined by applying this conversion to Van der Veen and Bolzan’s water-equivalent statistics, and to a 50 year accumulation variability estimate from the GISP2 core. Elevation-change statistics are compiled for the past millennium to define longer-term benchmarks, which show that multi-century variability has been substantially larger than current variability estimated by Van der Veen and Bolzan. It is estimated here that the standard deviation of net elevation change over 5 and 10 year intervals has been 0.27 and 0.38 m, respectively. An approximate method for applying these quantitative results to other dry-snow sites in Greenland is suggested.

Download Full-text