Alexei V. Kouraev, Benoit Legrésy, Frédérique Remy

Studies of Austfonna ice cap (Svalbard)using radar altimetry withother satellite techniques

Laboratoire d'Etudes en Geophysique et Oceanographie Spatiales (LEGOS), Toulouse FranceState Oceanography Institute, St. Petersburg branch, St. Petersburg, Russia

15 Years of progress in Radar Altimetry SymposiumOcean surface topography science team (OSTST)

International Doris Service (IDS) Workshop, Argo Workshop

13-18 March 2006, Venice, Italy

AUSTFONNA

Etonbreen

Digital Elevation Model (DEM),courtesy of the Norwegian Polar Institute

(NPI), Tromsoe, Norway

Austfonna - largest ice cap in theEurasian Arctic

Austfonna - 8120 km2,Vestfonna - 2500 km2

0 m 767 m

555555555

1 km1 km1 km1 km1 km1 km1 km1 km1 km

EtonbreenEtonbreenEtonbreenEtonbreenEtonbreenEtonbreenEtonbreenEtonbreenEtonbreen111111111 222222222

444444444333333333

Basin 03Basin 03Basin 03Basin 03Basin 03Basin 03Basin 03Basin 03Basin 03

1 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-211 - 1987 April 15-21

2 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 282 - 1988 July 28

3 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-293 - 1991 July 18-29

4 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 194 - 1993 July 19

5 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 285 - 1998 March 28

ETONBREEN: RECENT CHANGESFROM SPOT

Period

Change,

km2

Rate,

km2/yr

15/04/1987-28/07/1988 -0.75 -0.58

28/07/1988-29/07/1991 -3.04 -1.01

29/07/1991-19/07/1993 -1.42 -0.72

19/07/1993-28/03/1998 -2.25 -0.48

Etonbreen + Basin 03:continuous rapid retreat

DATA: SPOT 1-4 imagery for 1987,1988, 1991, 1993, 1998 from ISIS

project 0506-778.

ENVISAT AUSTFONNACOVERAGE

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 2 3 4 5 6 7 8 9 10 11 12

Number of observations: monthly changes for cycles 10 to 36

Number of observations

25-3120-2415-1910-14

DATA: ENVISAT RA2 data for cycles 10 to 40(Oct 2002 - Aug 2005). ICE2 retracker.Data quality control:Non-blank values of backscatter and range in Kuband, heights more than -1 m above the geoid.

Eastern coast - few data(altimeter locked), centralpart - OK

Number of observationdecreases in summer

(melting)

CHOICE OF REFERENCE POINTSFOR TEMPORAL ANALYSIS

1500 m

1500 m

1500 m

1500 m

1500 m

1500 m

1500 m

1500 m

1500 m

400 m

Use of latitude orlongitude for study oftemporal changes is

misleading

Orbit changes across trackare much larger than distance

between 18 Hz measures(1500 m vs 400 m)

Solution:Choosing one cycle as

reference and referring datafor each cycle to the nearest

reference point Location of 18 Hz ENVISAT points for cycles 10 to 36.Track 20.

SPATIAL DISTRIBUTION:MEDIAN VALUES

Haight (Ku) Backscatter (Ku)

Backscatter:low values for ice,

low temporal variability

Height:good relation to DEM,

temporal variability low on top,high on slopes

JAN

79.2

79.4

79.7

79.9

80.2

80.4

Latitude

FEB MAR

APR

79.2

79.4

79.7

79.9

80.2

80.4

Latitude

MAY JUN

-50

-30

-10

0

10

20

30

40

50

JUL

79.2

79.4

79.7

79.9

80.2

80.4

Latitude

AUG SEP

18 20 22 24 26

Longitude

OCT

79.2

79.4

79.7

79.9

80.2

80.4

Latitude

18 20 22 24 26

Longitude

NOV

18 20 22 24 26

Longitude

DEC

SNOW DEPTHGR ratio: linear relation to snow depth

ENVISAT GR*1000 Monthly valuesGR= (TB365 - TB238) / (TB238 + TB365)

ENVISAT: maximal values NE, GPR -NE and SE

NPI Ground Penetrating Radar (GPR) snowdepth (2004) and interpolated map.

Courtesy of A. Taurisano, NPI

TEMPORAL VARIABILITYALONG THE TRACK

-480 -470 -460 -450 -440 -430 -420 -410 -400 -390 -380 -370 -360

2003.0

2003.2

2003.4

2003.6

2003.8

2004.0

2004.2

2004.4

2004.6

2004.8

2005.0

2005.2

-1000

200

400

600

800

1000

1250

1500

1750

2000

30000

-480 -470 -460 -450 -440 -430 -420 -410 -400 -390 -380 -370 -360

2003.0

2003.2

2003.4

2003.6

2003.8

2004.0

2004.2

2004.4

2004.6

2004.8

2005.0

2005.2

-16

-12

-8

-6

-4

-2

0

2

4

8

12

dTB (TB23.8-TB36.5)Backscatter (S)

Backscatter: high values in summer nearthe ice edge (melting), low values in winteron topdTB - strong melting signal in summer

SUMMER

SUMMER

A

CD

B

AC BD AC BD

Anomalies in mm (from median values for cycles 11 to 31)of height in Ku band for track 853.

Mean height (mm relative to referenceellipsoid) in Ku band for track 853.

Opposite changes (swingingmode) before and after top ofAID. No real seasonal signal,but something intriguing is

evident...

HEIGHT PROFILE - MEAN ANDANOMALIES

TOPA

C

B

D

A CB D

A CB D

Anomalies of Height in Ku band, colorscale is the same as previous slide

Artefact related tochanges in orbitover topography!

INFLUENCE OF ORBIT CHANGE ONALTIMETRIC MEASURES)

A

C

B

D

SLOPE AND TOPOGRAPHYINFLUENCE

Sloping and hilly terrain:complicated radar echo

Simulation of radar return echo at various time stepsfor different topography

RA

Nadir line(shortestdistance

in theory)

Shortestdistancein reality

Case 2: first echocomes not from the

nadir point

SLOPE AND CURVATUREINFLUENCE

Altimeter locked

Overestimation on slopes

Good agreement,topographyinfluence is low

Theoretical line (Measured = DEM)

Comparison of measured height in Ku bandvs NPI DEM

SLOPE AND CURVATURECORRECTIONS

Slope,cycle 18

Curvature,cycle 18

Altimeter X and Y position DEM

DEM subset in +-20 kmdirection from X and Y

Approach: for a given DEM subsetapproximate (least square solution)

Hdem=a*x2+ b*y2+ c*x*y + d*x + e*y + f

Coefficients (a,b,c,d,e,f)

Slope andcurvature values

for a given DEM subset

DEM SOURCE 2: SARINTERFEROMETRY (1995-1996)

Amplitude

Coherence DEM

Three ERS-1 and ERS-2tandem pairs from SPRI

November 1995, December 1995, January 1996

DEM SOURCE 3: SPOT STEREO

Austfonna (Etonbreen region):stereo pairs for 1987 (SPOT1) and 1991(SPOT2) from the ISIS project 0506-778

ITERATIVE APPROACH:SIMULATED AND

MEASURED RA DATA

DEM SIMULATOR ENVISAT

SIMULATEDWAVEFORM

MEASUREDWAVEFORM

COMPARISON

DEMCORRECTIONS

QC, DATA SELECTIONAND INTERPRETATION

Correcting errorsdue to DEM

inclination plane