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

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

<p>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.  </p><p>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.</p><p> </p><p>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.</p><p>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.</p><p>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.</p>

First steps to bridging the gap between CryoSat-2 and ICESat2: retrackers and slope induced error.

2021 ◽  
Author(s):  
Katarzyna Sejan ◽  
Bert Wouters ◽  
Michiel van den Broeke
Keyword(s):  
Nearest Neighbor ◽  
Greenland Ice Sheet ◽  
Processing Parameters ◽  
Fine Tuning ◽  
Surface Elevation ◽  
Data Sets ◽  
Altimetry Data ◽  
Laser Altimetry ◽  
Radar Altimetry ◽  
Slope Correction

<p>Satellite radar altimetry is one of the most important tools for monitoring changes in the mass balance of the world's ice sheets. Different altimetry techniques however, 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. Fine tuning the developed processing algorithms for the CryoSat-2 radar altimetry data, using the IceSat2 laser altimetry data as a benchmark, may allow for a more precise surface elevation and snowpack depth estimations. Furthermore, bridging the gap between radar and laser altimetry will result in larger spatial and temporal data coverage when the two data sets are combined.  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. We investigated the importance of a retracker type, retracker threshold, Digital Elevation Model (DEM) in the slope correction, and how these affect the estimated surface elevation as compared to the ICESat2 data.</p><p>Firstly, ESA L1b Baseline-D data was processed at several different thresholds and with various waveform retracker algorithms, including threshold first maxima retracker algorithm (TFMRA) (Helm, 2012; Nilsson, 2015) and the offset center of gravity (OCOG) retracker algorithm (Bamber, 1994; Ricker et al. 2014). We then apply slope correction to adjust for the slope induced error in the radar altimetry data (Hurkmans, 2012), the correction was applied using three different DEMs, ArcticDEM Release 7 (Porter et al., 2018), Greenland Ice Mapping Project (GIMP) DEM (Howat et al., 2017) and ‘Helm’ DEM (Helm, 2014). We checked all of the produced data sets against IceSat-2 data (Smith et al., 2019) corresponding to the same time period, and selected by nearest neighbor calculation for specified maximum distance. We analyze and discuss the differences between IceSat-2 data and CryoSat-2 data and their dependence on several radar altimetry processing parameters and methodologies.</p><p> </p>

scholarly journals Time-evolving mass loss of the Greenland ice sheet from satellite altimetry

2014 ◽  
Vol 8 (1) ◽  
pp. 1057-1093
Author(s):  
R. T. W. L. Hurkmans ◽  
J. L. Bamber ◽  
C. H. Davis ◽  
I. R. Joughin ◽  
K. S. Khvorostovsky ◽  
...  
Keyword(s):  
Mass Loss ◽  
Satellite Altimetry ◽  
Climate Model ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laser Altimetry ◽  
Radar Altimetry ◽  
Appropriate Treatment ◽  
Satellite Radar

Abstract. Mass changes of the Greenland ice sheet may be estimated by the Input Output Method (IOM), satellite gravimetry, or via surface elevation change rates (dH / dt). Whereas the first two have been shown to agree well in reconstructing mass changes over the last decade, there are few decadal estimates from satellite altimetry and none that provide a time evolving trend that can be readily compared with the other methods. Here, we interpolate radar and laser altimetry data between 1995 and 2009 in both space and time to reconstruct the evolving volume changes. A firn densification model forced by the output of a regional climate model is used to convert volume to mass. We consider and investigate the potential sources of error in our reconstruction of mass trends, including geophysical biases in the altimetry, and the resulting mass change rates are compared to other published estimates. We find that mass changes are dominated by SMB until about 2001, when mass loss rapidly accelerates. The onset of this acceleration is somewhat later, and less gradual, compared to the IOM. Our time averaged mass changes agree well with recently published estimates based on gravimetry, IOM, laser altimetry, and with radar altimetry when merged with airborne data over outlet glaciers. We demonstrate, that with appropriate treatment, satellite radar altimetry can provide reliable estimates of mass trends for the Greenland ice sheet. With the inclusion of data from CryoSat II, this provides the possibility of producing a continuous time series of regional mass trends from 1992 onward.

scholarly journals 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

2013 ◽  
Vol 7 (6) ◽  
pp. 5433-5460
Author(s):  
J. F. Levinsen ◽  
K. Khvorostovsky ◽  
F. Ticconi ◽  
A. Shepherd ◽  
R. Forsberg ◽  
...  
Keyword(s):  
Drainage Basin ◽  
European Space Agency ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Round Robin ◽  
Surface Elevation ◽  
Optimal Method ◽  
Radar Altimetry ◽  
Elevation Changes

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.

scholarly journals Time-evolving mass loss of the Greenland Ice Sheet from satellite altimetry

2014 ◽  
Vol 8 (5) ◽  
pp. 1725-1740 ◽  
Author(s):  
R. T. W. L. Hurkmans ◽  
J. L. Bamber ◽  
C. H. Davis ◽  
I. R. Joughin ◽  
K. S. Khvorostovsky ◽  
...  
Keyword(s):  
Mass Loss ◽  
Satellite Altimetry ◽  
Climate Model ◽  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laser Altimetry ◽  
Radar Altimetry ◽  
Surface Mass ◽  
Satellite Radar

Abstract. Mass changes of the Greenland Ice Sheet may be estimated by the input–output method (IOM), satellite gravimetry, or via surface elevation change rates (dH/dt). Whereas the first two have been shown to agree well in reconstructing ice-sheet wide mass changes over the last decade, there are few decadal estimates from satellite altimetry and none that provide a time-evolving trend that can be readily compared with the other methods. Here, we interpolate radar and laser altimetry data between 1995 and 2009 in both space and time to reconstruct the evolving volume changes. A firn densification model forced by the output of a regional climate model is used to convert volume to mass. We consider and investigate the potential sources of error in our reconstruction of mass trends, including geophysical biases in the altimetry, and the resulting mass change rates are compared to other published estimates. We find that mass changes are dominated by surface mass balance (SMB) until about 2001, when mass loss rapidly accelerates. The onset of this acceleration is somewhat later, and less gradual, compared to the IOM. Our time-averaged mass changes agree well with recently published estimates based on gravimetry, IOM, laser altimetry, and with radar altimetry when merged with airborne data over outlet glaciers. We demonstrate that, with appropriate treatment, satellite radar altimetry can provide reliable estimates of mass trends for the Greenland Ice Sheet. With the inclusion of data from CryoSat-2, this provides the possibility of producing a continuous time series of regional mass trends from 1992 onward.

scholarly journals Seasonal variation of snow-surface elevation in North Greenland as modeled and detected by satellite radar altimetry

2003 ◽  
Vol 37 ◽  
pp. 233-238 ◽  
Author(s):  
Jun Li ◽  
H. Jay Zwally ◽  
Helen Cornejo ◽  
Donghui Yi
Keyword(s):  
Seasonal Variation ◽  
Seasonal Variations ◽  
Accumulation Rate ◽  
Greenland Ice Sheet ◽  
Surface Elevation ◽  
Frequency Function ◽  
Radar Altimetry ◽  
Satellite Radar ◽  
Satellite Radar Altimetry ◽  
North Greenland

AbstractComparison of the distribution of seasonal variations in surface elevation derived from a firn-densification–elevation model with observed variations derived from ERS-1/-2 satellite radar altimetry shows close similarity in the patterns of the amplitude of the variations over the North Greenland ice sheet. The amplitudes of the seasonal variations decrease from west to east and from south to north, determined by the accumulation rate and the surface-temperature distribution pattern. Several methods of estimating the amplitude of the seasonal variation in the observations are compared, including the use of a three-frequency sinusoidal function derived from the modeled seasonal variation that is asymmetric. The resulting correlation coefficient between the observed amplitude, estimated with the three-frequency function, and the modeled amplitude is 0.66 and the slope is 0.7. Residual differences may be caused by interannual variability in accumulation and temperature and other approximations in the model.

scholarly journals Ice-Sheet Topography From Geosat Radar Altimetry (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 213 ◽  
Author(s):  
H. Jay Zwally ◽  
R. A. Bindschadler
Keyword(s):  
Standard Deviation ◽  
Ice Sheet ◽  
Satellite Orbit ◽  
Mean Value ◽  
Surface Elevation ◽  
Data Set ◽  
Radar Altimetry ◽  
Fitting Procedure ◽  
Optimal Spacing ◽  
Satellite Radar

Ice-sheet surface topography is the principal ice parameter obtainable from satellite radar altimetry. Surface-elevation maps of the East Antarctic ice sheet north of 72°S from Seasat data, collected between July and October 1978, and preliminary maps from Geosat data, collected between March 1985 and September 1986, are described. The Geosat data, obtained from the U.S. Navy as an unclassified data set, have greatly increased the density of elevation measurements. A principal correction to the altimeter measurements is obtained by applying a computer curve-fitting procedure to each radar waveform to correct for errors in the automatic range-tracking circuitry of the altimeter. The errors are caused by slow response to range variations due to undulations of the ice surface between successive measurements made at intervals of 662 m along track. The retracking correction for Geosat data has a standard deviation of 2.4 m and a mean value of 1.1m, values which are about 20% smaller than the corresponding values for Seasat. The positive mean correction indicates a common tendency of the altimeters' automatic tracking to give an excessive range to the surface. The precision of the measurements, given by the standard deviation of the range differences at cross-over points, is about 1.6 m before adjustment for errors in the radial position of the satellite orbit. The preliminary surface-elevation maps from Geosat data are improved over those produced from Seasat, mainly due to optimal spacing of successive ground tracks. The locations of ice divides and drainage basins along the East Antarctic coast are delineated by several methods, including vector plots of surface slope.

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

2018 ◽  
Vol 495 ◽  
pp. 234-241 ◽  
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

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

2011 ◽  
Vol 52 (59) ◽  
pp. 1-7 ◽  
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.

scholarly journals Airborne and spaceborne DEM- and laser altimetry-derived surface elevation and volume changes of the Bering Glacier system, Alaska, USA, and Yukon, Canada, 1972–2006

2009 ◽  
Vol 55 (190) ◽  
pp. 316-326 ◽  
Author(s):  
Reginald R. Muskett ◽  
Craig S. Lingle ◽  
Jeanne M. Sauber ◽  
Austin S. Post ◽  
Wendell V. Tangborn ◽  
...  
Keyword(s):  
Surface Elevation ◽  
Laser Altimetry ◽  
Volume Changes ◽  
The North ◽  
Glacier System ◽  
Outburst Floods ◽  
Digital Elevation ◽  
Bering Glacier ◽  
Elevation Changes ◽  
Terra Modis

AbstractUsing airborne and spaceborne high-resolution digital elevation models and laser altimetry, we present estimates of interannual and multi-decadal surface elevation changes on the Bering Glacier system, Alaska, USA, and Yukon, Canada, from 1972 to 2006. We find: (1) the rate of lowering during 1972–95 was 0.9 ± 0.1 m a−1; (2) this rate accelerated to 3.0 ± 0.7 m a−1 during 1995–2000; and (3) during 2000–03 the lowering rate was 1.5 ± 0.4 m a−1. From 1972 to 2003, 70% of the area of the system experienced a volume loss of 191 ± 17 km3, which was an area-average surface elevation lowering of 1.7 ± 0.2 m a−1. From November 2004 to November 2006, surface elevations across Bering Glacier, from McIntosh Peak on the south to Waxell Ridge on the north, rose as much as 53 m. Up-glacier on Bagley Ice Valley about 10 km east of Juniper Island nunatak, surface elevations lowered as much as 28 m from October 2003 to October 2006. NASA Terra/MODIS observations from May to September 2006 indicated muddy outburst floods from the Bering terminus into Vitus Lake. This suggests basal–englacial hydrologic storage changes were a contributing factor in the surface elevation changes in the fall of 2006.

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