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Alexei V. Kouraev, Benoit Legrésy, Frédérique Remy

15 Years of progress in Radar Altimetry Symposium Ocean surface topography science team (OSTST) International Doris Service (IDS) Workshop, Argo Workshop 13-18 March 2006, Venice, Italy. Alexei V. Kouraev, Benoit Legrésy, Frédérique Remy.

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Alexei V. Kouraev, Benoit Legrésy, Frédérique Remy

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  1. 15 Years of progress in Radar Altimetry Symposium Ocean surface topography science team (OSTST)International Doris Service (IDS) Workshop, Argo Workshop 13-18 March 2006, Venice, Italy Alexei V. Kouraev, Benoit Legrésy, Frédérique Remy Studies of Austfonna ice cap (Svalbard) using radar altimetry with other satellite techniques Laboratoire d'Etudes en Geophysique et Oceanographie Spatiales (LEGOS), Toulouse France State Oceanography Institute, St. Petersburg branch, St. Petersburg, Russia

  2. 0 m 767 m Austfonna - largest ice cap in the Eurasian Arctic AUSTFONNA Austfonna - 8120 km2, Vestfonna - 2500 km2 Etonbreen Digital Elevation Model (DEM), courtesy of the Norwegian Polar Institute (NPI), Tromsoe, Norway

  3. ETONBREEN: RECENT CHANGES FROM SPOT Etonbreen + Basin 03: continuous rapid retreat DATA: SPOT 1-4 imagery for 1987, 1988, 1991, 1993, 1998 from ISIS project 0506-778.

  4. Number of observations 25-31 20-24 15-19 10-14 ENVISAT AUSTFONNA COVERAGE 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 Ku band, heights more than -1 m above the geoid. Eastern coast - few data (altimeter locked), central part - OK Number of observation decreases in summer (melting)

  5. CHOICE OF REFERENCE POINTS FOR TEMPORAL ANALYSIS Orbit changes across track are much larger than distance between 18 Hz measures (1500 m vs 400 m) Use of latitude or longitude for study of temporal changes is misleading Solution:Choosing one cycle as reference and referring data for each cycle to the nearest reference point Location of 18 Hz ENVISAT points for cycles 10 to 36. Track 20.

  6. SPATIAL DISTRIBUTION: MEDIAN VALUES Height: good relation to DEM, temporal variability low on top, high on slopes Backscatter: low values for ice, low temporal variability Haight (Ku) Backscatter (Ku)

  7. ENVISAT GR*1000 Monthly values GR= (TB365 - TB238) / (TB238 + TB365) SNOW DEPTH GR ratio: linear relation to snow depth ENVISAT: maximal values NE, GPR - NE and SE NPI Ground Penetrating Radar (GPR) snow depth (2004) and interpolated map. Courtesy of A. Taurisano, NPI

  8. TEMPORAL VARIABILITY ALONG THE TRACK D C Backscatter: high values in summer near the ice edge (melting), low values in winter on top B dTB - strong melting signal in summer A dTB (TB23.8-TB36.5) Backscatter (S) D C B A D C B A SUMMER SUMMER

  9. HEIGHT PROFILE - MEAN AND ANOMALIES D C B TOP A A B C D A B C D Mean height (mm relative to reference ellipsoid) in Ku band for track 853. Opposite changes (swinging mode) before and after top of AID. No real seasonal signal, but something intriguing is evident... Anomalies in mm (from median values for cycles 11 to 31) of height in Ku band for track 853.

  10. INFLUENCE OF ORBIT CHANGE ON ALTIMETRIC MEASURES) Artefact related to changes in orbit over topography! Anomalies of Height in Ku band, color scale is the same as previous slide D C B A

  11. RA Shortest distance in reality Nadir line (shortest distance in theory) SLOPE AND TOPOGRAPHY INFLUENCE Sloping and hilly terrain: complicated radar echo Case 2: first echo comes not from the nadir point Simulation of radar return echo at various time steps for different topography

  12. SLOPE AND CURVATURE INFLUENCE Overestimation on slopes Theoretical line (Measured = DEM) Good agreement, topography influence is low Altimeter locked Comparison of measured height in Ku band vs NPI DEM

  13. Altimeter X and Y position DEM DEM subset in +-20 km direction from X and Y Approach: for a given DEM subset approximate (least square solution)Hdem=a*x2+ b*y2+ c*x*y + d*x + e*y + f Coefficients (a,b,c,d,e,f) Slope and curvature values for a given DEM subset SLOPE AND CURVATURE CORRECTIONS Slope, cycle 18 Curvature, cycle 18

  14. DEM SOURCE 2: SAR INTERFEROMETRY (1995-1996) Amplitude Coherence DEM Three ERS-1 and ERS-2 tandem pairs from SPRI November 1995, December 1995, January 1996

  15. DEM SOURCE 3: SPOT STEREO Austfonna (Etonbreen region): stereo pairs for 1987 (SPOT1) and 1991 (SPOT2) from the ISIS project 0506-778

  16. DEM SIMULATOR ENVISAT SIMULATED WAVEFORM MEASUREDWAVEFORM COMPARISON DEM CORRECTIONS QC, DATA SELECTION AND INTERPRETATION ITERATIVE APPROACH: SIMULATED AND MEASURED RA DATA Correcting errors due to DEM inclination plane

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