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ASAR Wave mode first geophysical results

ASAR Wave mode first geophysical results. Introduction Instrument geophysical Calibration Summary. Achievements with ERS Wave Data: Theory and performance of SAR-wave measurements well known (Hasselmann et al, 1991 JGR), (Krogstad, 1992JGR)

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ASAR Wave mode first geophysical results

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  1. ASAR Wave mode first geophysical results • Introduction • Instrument geophysical Calibration • Summary

  2. Achievements with ERS Wave Data: Theory and performance of SAR-wave measurements well known (Hasselmann et al, 1991 JGR), (Krogstad, 1992JGR) Different inversion schemes exist (Hasselmann et al, 1996 JGR, Engen et al, 1994 TGARS, Krogstad et al, 1994 Atm.Ocean, Mastenbroek 1999 JGR) Assimilation experiments performed (Breivik et al, 1998 JGR),(Hasselmann et al, 1997 JGR) Spectral wave climate applications (Mastenbroek, CEOS SAR, 1998) Cross spectra inversion demonstrated (Engen et al, 1995 TGARS), (Dowd et al, 2001 TGARS) ENVISAT ASAR expected achievements Better data coverage Better processed and calibrated data Expected Assimilation to work well in near real time framework Complete Independent information in the swell part of SAR wave spectra

  3. Algorithm Philosophy / Product Inversion Model: Level 1 Product Processing Set-Up File Look-Up Table Cross Spectra Estimation where Clutter & SNR Estimation Wind Speed Retrieval Ocean Wave Spectra Retrieval Level 2 Product Generation Level 2 Product

  4. Cross spectral analysis Cross Spectra Estimation:

  5. Spatio-temporal analysis Look filters Bright targets only on one look Azimuth Variation of backscatter intensity with short duration within the integration time  Expected correlation to only apply on long duration patterns (longer waves) integration time

  6. ASAR Wave Mode Level 1b Imaginary Part Real Part Removal of propagation direction ambiguity SAR Ocean Images

  7. Level 2 processing to require absolute backscatter calibration (CMOD like) ASAR imagettes from May 2002(IS2 swath) June/July 2002 (IS2 swath) (Cross section derived from CMOD IFREMER and ECMWF colocated wind vectors)

  8. Geophysical calibrationassuming mean incidence angle of 22.9° for IS2,Calibration offset is 71.8dB for May 200250.2dB for June/July

  9. Azimuthal Cutoff estimationObserved systematic k-4decay

  10. Observed azimuthal resolution vs imagette cross sectionto allowindependantmonitoringof imagettecross section(kinematic propertiesinstead of roughnessproperties -> less sensitive to contamination(rain, slicks, …)

  11. Residual analysis versus image contrast (modulation)Ratio between azimuthalresolution and Imagettecross section related toimaged modulation strengthNb : ASAR observednormalised variance (contrast) is generallyhigher than ERS one.

  12. Doppler centroid estimation versus predicted (YSL) Follow the prediction-> enable residual analysis Prediction from ESA (B. R.)

  13. Doppler-centroid anomaly : geophysical interpretation Df = 2/l . U.sin (qinc)U is the horizontal velocity of scatterers Df = 50Hz -> U = 3.5m/s Scatterers advected 10 times faster than wind drift !

  14. Overall high quality of data (wave direction ambiguity removed in > 80% cases, frequent observation of wind streaks, …) Geophysical calibration implemented (ECMWF,QUIKSCAT wind, WAM, buoys) Kinematic properties from cutoff estimation to enable monitoring of radar cross-section Doppler-centroid anomaly strongly dependent on radial wind : to be further explored to retrieve wind vector (combined with cross-section and/or cutoff estimation) and to question actual sea surface scattering models ! Summary

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