12 research outputs found
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
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
Time-evolving mass loss of the Greenland Ice Sheet from satellite altimetry
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
Изменчивость морского льда в Арктике по данным Арктического портала
The Arctic Portal of the Laboratory of Satellite Oceanography of the Russian State Hydrometeorological University is an open geo-informational system designed for operational monitoring of the sea ice–ocean–atmosphere system in the Arctic. Possibilities to use the Arctic Portal for the Arctic sea ice monitoring on the basis of satellite data, as well as the types of satellite measurements suitable for studying the properties of sea ice: active and passive microwave data; data of spectral radiometers in the infrared (IR) and visible ranges are considered. Every type of satellite data has certain limitations. For the study of sea ice the most suitable are the all-season remote sensing data – measurements of microwave instruments, independent of clouds and time of a day. Existing in the world resources of the sea ice maps and satellite data on sea ice are either closed for users or limited in their informational content. Several types of satellite data are currently available on the Arctic portal: Sentinel-1 synthetic aperture radar (SAR) images, 8-day averaged MODIS reflectivity data, optical and IR MODIS data of original time and spatial resolution, Norwegian Meteorological University sea ice maps, and data on consolidation of sea ice, based on passive microwave radiometer measurements. Some data is additionally available in the test mode. The effectiveness of combined use of various satellite data on the sea ice is proved by the examples of sea ice analyses.Представлены возможности Арктического портала (геоинформационного сервиса) для мониторинга ледяного покрова Арктики на основе спутниковых данных. Дан обзор основных типов спутниковых инструментов, позволяющих изучать морской лёд. Обоснована эффективность совместного применения результатов обработки разных спутниковых данных, имеющихся в среде геосервиса
ESA ice sheet CCI: derivation of the optimal method for surface elevation change detection of the Greenland ice sheet - round robin results
For more than two decades, radar altimetry missions have provided continuous elevation estimates of the Greenland ice sheet (GrIS). Here, we propose a method for using such data to estimate ice-sheet-wide surface elevation changes (SECs). The final data set will be based on observations acquired from the European Space Agency’s Environmental Satellite (ENVISAT), European Remote Sensing (ERS)-1 and -2, CryoSat-2, and, in the longer term, Sentinel-3 satellites. In order to find the best-performing method, an intercomparison exercise has been carried out in which the scientific community was asked to provide their best SEC estimates as well as feedback sheets describing the applied method. Due to the hitherto few radar-based SEC analyses as well as the higher accuracy of laser data, the participants were asked to use either ENVISAT radar or ICESat (Ice, Cloud, and land Elevation Satellite) laser altimetry over the Jakobshavn Isbræ drainage basin. The submissions were validated against airborne laser-scanner data, and intercomparisons were carried out to analyse the potential of the applied methods and to find whether the two altimeters were capable of resolving the same signal. The analyses found great potential of the applied repeat-track and cross-over techniques, and, for the first time over Greenland, that repeat-track analyses from radar altimetry agreed well with laser data. Since topography-related errors can be neglected in cross-over analyses, it is expected that the most accurate, ice-sheet-wide SEC estimates are obtained by combining the cross-over and repeat-track techniques. It is thus possible to exploit the high accuracy of the former and the large spatial data coverage of the latter. Based on CryoSat’s different operation modes, and the increased spatial and temporal data coverage, this shows good potential for a future inclusion of CryoSat-2 and Sentinel-3 data to continuously obtain accurate SEC estimates both in the interior and margin ice sheet
On retrieving sea ice freeboard from ICESat laser altimeter
Sea ice freeboard derived from satellite altimetry is the basis for the
estimation of sea ice thickness using the assumption of hydrostatic
equilibrium. High accuracy of altimeter measurements and freeboard retrieval
procedure are, therefore, required. As of today, two approaches for
estimating the freeboard using laser altimeter measurements from Ice, Cloud,
and land Elevation Satellite (ICESat), referred to as tie points (TP) and
lowest-level elevation (LLE) methods, have been developed and applied in
different studies. We reproduced these methods for the ICESat observation
periods (2003–2008) in order to assess and analyse the sources of
differences found in the retrieved freeboard and corresponding thickness
estimates of the Arctic sea ice as produced by the Jet Propulsion
Laboratory (JPL) and Goddard Space Flight Center (GSFC). Three main factors
are found to affect the freeboard differences when applying these methods:
(a) the approach used for calculation of the local sea surface references in
leads (TP or LLE methods), (b) the along-track averaging scales used for this
calculation, and (c) the corrections for lead width relative to the ICESat
footprint and for snow depth accumulated in refrozen leads. The LLE method
with 100 km averaging scale, as used to produce the GSFC data set, and the
LLE method with a shorter averaging scale of 25 km both give larger
freeboard estimates comparing to those derived by applying the TP method with
25 km averaging scale as used for the JPL product. Two factors, (a) and (b),
contribute to the freeboard differences in approximately equal proportions,
and their combined effect is, on average, about 6–7 cm. The effect of using
different methods varies spatially: the LLE method tends to give lower
freeboards (by up to 15 cm) over the thick multiyear ice and higher
freeboards (by up to 10 cm) over first-year ice and the thin part of
multiyear ice; the higher freeboards dominate. We show that the freeboard
underestimation over most of these thinner parts of sea ice can be reduced to
less than 2 cm when using the improved TP method proposed in this paper. The
corrections for snow depth in leads and lead width, (c), are applied only for
the JPL product and increase the freeboard estimates by about 7 cm on
average. Thus, different approaches to calculating sea surface references and
different along-track averaging scales from one side and the freeboard
corrections as applied when producing the JPL data set from the other side
roughly compensate each other with respect to freeboard estimation.
Therefore, one may conclude that the difference in the mean sea ice thickness
between the JPL and GSFC data sets reported in previous studies should be
attributed mostly to different parameters used in the freeboard-to-thickness
conversion
Waveform classification of airborne synthetic aperture radar altimeter over Arctic sea ice
Sea ice thickness is one of the most sensitive
variables in the Arctic climate system. In order to quan-
tify changes in sea ice thickness, CryoSat-2 was launched in
2010 carrying a Ku-band radar altimeter (SIRAL) designed
to measure sea ice freeboard with a few centimeters accu-
racy. The instrument uses the synthetic aperture radar tech-
nique providing signals with a resolution of about 300 m
along track. In this study, airborne Ku-band radar altime-
ter data over different sea ice types have been analyzed. A
set of parameters has been defined to characterize the dif-
ferences in strength and width of the returned power wave-
forms. With a Bayesian-based method, it is possible to clas-
sify about 80 % of the waveforms from three parameters:
maximum of the returned power waveform, the trailing edge
width and pulse peakiness. Furthermore, the maximum of
the power waveform can be used to reduce the number of
false detections of leads, compared to the widely used pulse
peakiness parameter. For the pulse peakiness the false clas-
sification rate is 12.6 % while for the power maximum it is
reduced to 6.5 %. The ability to distinguish between differ-
ent ice types and leads allows us to improve the freeboard
retrieval and the conversion from freeboard into sea ice thick-
ness, where surface type dependent values for the sea ice den-
sity and snow load can be used
輸送包装のための製品衝撃強さ評価に関する研究
博士(工学)神戸大