PALSAR studies of ice sheet motion in Antarctica

ALOS PI Symposium, Nov. 3-7 2008

PALSAR studies of ice sheet motion in Antarctica

Eric Rignot

University of California Irvine, Department of Earth System ScienceIrvine, California, USA

[email protected]

and Caltech’s Jet Propulsion Laboratory,Pasadena, California, USA.


Plan of presentation

How we use InSAR for ice sheet studies.Results from Greenland and AntarcticaSpeckle tracking with ALOS PALSARImplications for PIG and AntISStrengths and problems of ALOS PALSARConclusions and Recommendations.


Measuring 3-D ice motion with InSAR

• InSAR is a fundamental tool for measuring ice velocity, grounding line position and their temporal variability.

• First results obtained with ERS-1 (1992), ERS tandem (1996), continued with RSAT-1 (1997-2008), and now Envisat ASAR and ALOS PALSAR.


SAR Interferometry

Ice shelf motion around Hemmen Ice Rise, Antarctica

Images can be combined to form an interferogram that is sensitive to both topography and motion


Vector velocity of Lambert GlacierDescending track Ascending track

Range looking direction -->

Along-track direction -->

Along-track direction --

>


Asc./descending track combination and flow vectors.

Vector velocity of Lambert Glacier


ERS SAR 11/1995Track 92, 482, 81

•Longer time periods: speckle tracking (Michel and Rignot, 1999).

•Detect speckle offsets with autocorrelation technique.

•Precision = 1/128th pixel.

•5-10 m/yr with 24-day time separation (RSAT-1).

•Both x- and y- velocity with a single track.

•Tidal modulation 24 to 46 times less than with ERS-tandem data.

Vector velocity of Lambert Glacier


Glacier mass budget

Flux Gate

Mass balance = Accumulation - Discharge

Surface input : regional climate model calibrated or validated with ice cores and AWS (5-10%)

Outflow: Ice thickness from radio echo sounding (10 m) or hydrostatic equilibrium (80 m). Ice velocity from InSAR (5-20 m/yr).

Mass balance within 5-20%.


Hinge line↑

↓InSAR GL (~ 1km from HL)

Grounding lines

Complete mapping of GL north of 81oS.


Antarctic mass balance from InSAR and RACMO2/ANT 1996-2006

EAIS 1996 -4±61 Gt/yrEAIS 2000 -4±61 Gt/yrEAIS 2006 -4±61 Gt/yr

WAIS 1996 -83±59 Gt/yrWAIS 2000 -106±60 Gt/yrWAIS 2006 -132±50 Gt/yr

AP 1996 -25±45 Gt/yrAP 2000 -25±45 Gt/yrAP 2006 -60±46Gt/yr

Antarctic mass loss increased from-112 to -196 Gt/yr in 10 years.

Rignot et al., Nature Geosc. 2008


Greenland mass balance1957-2007.

• Mass loss: 150±65 Gt/yr in 1960s, 40±55 Gt/yr in 1970s-1980s, and tripled from 100±55Gt/yr to 290±55Gt/yr in 1996-2007.• 60% of the loss is due to an increase in ice discharge (D), 40% from an increase in runoff, precipitation is unchanged.

Rignot E., J. E. Box, E. Burgess, E. Hanna (2008), Mass balance of the Greenland ice sheet from 1958 to 2007, Geophys. Res. Lett., 35, L20502, doi:10.1029/2008GL035417.

-350

-300

-250

-200

-150

-100

-50

0

50

1957 1967 1977 1987 1997 2007

Gt/

yr

Surface mass balance anomaly

Ice discharge anomaly reconstructed

Ice discharge anomaly observed

Total mass balance reconstructed

Total mass balance observed


PALSAR approach

• Speckle tracking technique over 46 days FBS data.

• Pros: L-band procures better coherence than C-band (SIR-C 1994).

• Cons: L-band more sensitive to ionosphere and low SNR on wet snow; long repeat cycle.


PALSAR Speckle Tracking:Getz Ice Shelf, Antarctica

(Radarsat-1 and Envisat ASAR C-band show poor coherence)

Radar amplitude Filtered offsets Velocity magnitude

(slant-range geometry)

Crosson Ice Shelf

Dotson Ice Shelf

Low coherence

High coherence

ALOS PI Symposium, Nov. 3-7 2008Geocoded velocity Track Mosaic

Crosson Ice Shelf

Dotson Ice Shelf

Getz Ice Shelf, Antarctica


Crosson and Dotson ice shelves, West Antarctica


NASA/NSF Larsen C(UCI/JPL, Univ. Colorado, CECS, NASA/Wallops,

Univ. Utrecht)


Antarctic Peninsula


George VI north


Wilkins Ice Shelf


Pine Island Glacier, WAIS

Rignot, 1998.Thomas, Rignot et al., 2004


Pine Island 1996-2007

200619962007


ALOS PALSAR PIB ERS-1/2 tandem

RSAT-1PALSAR


Pine Island Glacier velocity

changes

Rignot, GRL 2008

Speed up predicted by Thomas et al (2004).

MODIS 2002-2007 suggests the glacier retreated over the entire ice plain.


Antarctic mass balance

GRACE from 2002-2005:-148±21 Gt/yr (Velicogna and Wahr, 2006)

Within error bars ofInSAR/RACMO2


Pine Island Glacier:

MODIS GL


Grounding line mapping with ALOS PALSAR


NASA Measure Project• Create Earth Science Data Record (ESDR): ice velocity of Antarctica and distribute them at NSIDC, Boulder, CO.• Data from ERS-1/2 tandem (1996), Radarsat-1 (1997, 2000), Envisat (2007, 2008) and ALOS PALSAR (2006/2007, 2008, 2009).• Three mosaics: 1996/1997 (ERS,RSAT), 2006/2007 (ALOS,Envisat), and 2008/2009 (ALOS,Envisat).


Lambert Glacier, East Antarctica ENVISAT ASAR

Very low accumulation, ASAR provides excellent mapping


Strengths and Problems of PALSAR

• Strengths: high coherence in critical (wet) sectors of Antarctica.• Strengths: mapping of grounding lines possible.• Strengths: highest precision in ice velocity mapping.• Strengths: systematic mappings of Antarctica (2006, 2007, 2008).

• Problems: effects of ionosphere, minor so far.• Problems: range ambiguities introduce artifacts, be careful in low backscatter

areas.• Problems: data acquisition is limited, no more than 3 consecutive cycles.• Problems: data distributed in individual frames instead of continuous swaths.

• Overall evaluation: ALOS PALSAR is one of the best mapping tool for ice motion.


Conclusions and recommendations

Conclusions:

• ALOS PALSAR extremely useful for ice sheet motion mapping. • ERS-1/2, Radarsat-1, Envisat ASAR, ALOS PALSAR will be our major

data source for the NASA Measure project.• Results obtained in Pine Island Bay, West Antarctica confirm

acceleration of mass loss.

Recommendations:

• Continue yearly Antarctic mapping, include 4 consecutive cycles instead of 3 consecutive cycles.

• Introduce a yearly mapping of Greenland ice velocities.• Establish a set of critical tracks in Greenland to monitor major glacier

evolution (same for Envisat ASAR).• Ice should be a major science objective of ALOS PALSAR and its follow

on missions.

PALSAR studies of ice sheet motion in Antarctica

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