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GERB-2 V999 Unfiltering

GERB-2 V999 Unfiltering. Nicolas Clerbaux & RMIB GERB Team. GIST 24, Imperial College, London 15 december 2005. Outline. Shortwave V999 spectral response V999 unfiltering New : Empirical SEVIRI NB-2-BB New : Clear ocean unfiltering Validations/main results

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GERB-2 V999 Unfiltering

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  1. GERB-2 V999 Unfiltering Nicolas Clerbaux & RMIB GERB Team. GIST 24, Imperial College, London 15 december 2005

  2. Outline • Shortwave • V999 spectral response • V999 unfiltering • New : Empirical SEVIRI NB-2-BB • New : Clear ocean unfiltering • Validations/main results • Clear desert unfiltering problem • Clear ocean NB-to-BB problem • Solution for release • Longwave • V999 spectral response • V999 Unfiltering • Unfiltered radiance comparison with CERES

  3. Comparison V002/V999 SW spectral response curves • V999 (green) seems flatter but... • ... unfiltering more difficult

  4. V999 SW Unfiltering • Same unfiltering method as before: • New empirical BB estimation with parameters dependent on surface type (CERES ST used: ocean, dark vegetation, …): -> Unfiltering parameters are dependent on surface type • Unfiltering parameterization based on a new data base of spectral radiance curves (SBDART 2.4) • Implementation of a dedicated clear ocean unfiltering (to try to solve the asymmetry problem).

  5. New: Empirical SEVIRI NB-2-BB • Database of 3 months (March, April and July 2004) of coangular SEVIRI NB and CERES BB radiances (FM2 and FM3), (more than 4 millions observations), • Derive regressions in terms of reflectance, for V999: • Further improvement presented during the EUMETSAT conference at Dubrovnik (more info in the proceedings) sun glint angle

  6. New: Dedicated clear ocean unfiltering • As different as possible from the previous version to see if the asymmetry still there • Direct estimation of the unfiltering factor as a function of0.6: =a+b/0.6+c/0.6 with a,b,c dependent on (). • Best fit on the ocean scenes (clear or cloudy) in the database.

  7. SW Unfiltering Validations Analysis of • Dave Doelling • Richard Allan • Cedric Bertrand • Steven Dewitte Main results: • Clear desert overestimation, • “Diurnal cycle” of error for the SEVIRI BB estimate (GERB-like), specially over the clear ocean, • This error is only partly corrected by GERB in the high resolution products (ARCH, SHI), • Dedicated clear ocean unfiltering not needed (direct unfiltering seems sufficient) • Diurnal cycle of unfiltering error for dark scene?

  8. Clear desert problem • V999 clear desert overestimation wrt to the other scene types (about 2%?) • Reason: the desert unfiltering was parameterized using the “Soils” ASTER library category and not the “Rocks” • Preliminary simulations using “Rocks” explain about 1.5% of the overestimation • Solution: add new simulations in the data base and re-parameterize the unfiltering for the desert surfaces

  9. Clear Ocean NB-2-BB Problem • See presentation of Cedric Bertrand. • Possible solutions: • Do not use NB-2-BB but well the CERES TRMM ADM radiance, • Add to the NB-2-BB a correction c(SZA,VZA,RAA) dependent on the full angular geometry (parameterization using RTM), • Improve the NB-2-BB (more proxi) • Validations • Comparison diurnal cycle with model • Data base of 132 695 coangular CERES and SEVIRI clear ocean observations: L_ceres = a L_nb2bb See RMIB TN: “Clear ocean correction for SEVIRI NB-to-BB Conversion”

  10. To do before release - SW • Select/implement on method to improve clear ocean BB estimation • SBDART simulations with ASTER rock category + check the clear desert improvement • Additional unfiltering validations needed for dark scenes (dark vegetation- tropical forest). If needed, use direct unfiltering instead of the ratio of the BB estimations

  11. Comparison V002/V999 LW spectral response curves • V999 more sensitive to cold scene • Bug in the V002 solar LW contamination • Spectral response extended up to 500µm

  12. LW Unfiltering • Same as before except that the unfiltering factor is now estimated over [2.5:500]µm instead of [2.5:100]µm for V002

  13. Validation with CERES FM3 & FM2 • V999 ARCH data for June, July & December 2004. • Analysis of the averaged coangular LW radiance: • at regional scale (10*10 ARCH pixel boxes) • in radiance bin (5Wm-2sr-1) See RMIB TN: “Cross validation of GERB LW radiance with CERES”

  14. FM3 daytime June/July 2004 <Lceres> [Wm-2sr-1] ratio : <Lgerb>/<Lceres>

  15. FM3 nighttime June/July 2004 <Lceres> [Wm-2sr-1] ratio : <Lgerb>/<Lceres>

  16. FM3 June/July 2004

  17. FM2 daytime June/July 2004 <Lceres> [Wm-2sr-1] ratio : <Lgerb>/<Lceres>

  18. FM2 night time June/July 2004 <Lceres> [Wm-2sr-1] ratio : <Lgerb>/<Lceres>

  19. FM2 June/July 2004

  20. Discussion- LW • about 2% scene type dependency • A bit higher for V999 than for V002 • Should be confirmed with ARG data • Over daytime hot desert ratio GERB/FM2 not consistent with GERB/FM3. Reason? Thank you !

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