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. 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) New: Empirical SEVIRI NB-2-BB 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. 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 Clear Ocean NB-2-BB Problem MethodSlope(a)RMS error (%) Radiance from ADM0.98218.2% V999 NB2BB0.97416.7% V999 NB2BB + Corr1.01613.0% Dubrovnik NB2BB1.00414.1% Dubrovnik NB2BB + Corr0.99112.9% See RMIB TN: “Clear ocean correction for SEVIRI NB-to-BB Conversion”

10. 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 To do before release - SW

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 -2 sr -1 ) See RMIB TN: “Cross validation of GERB LW radiance with CERES”

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

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

16. FM3 June/July 2004

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

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

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 !