studies of austfonna ice cap (svalbard) using radar ... · 203. 2003.4 2003.6 2003.8 2004.0 2004.2...
TRANSCRIPT
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