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Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data. Paper in press at “Remote Sensing of Environment”, proofs available via http://gerb.oma.be (rubrique “News”) Collaboration RMIB/Imperial College. Outline. Objectives GERB and CERES data used Methodology

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Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

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  1. Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data Paper in press at “Remote Sensing of Environment”, proofs available via http://gerb.oma.be (rubrique “News”) Collaboration RMIB/Imperial College

  2. Outline • Objectives • GERB and CERES data used • Methodology • SW radiance comparison results • SW flux comparison results • Summary SW • LW radiance comparison results • LW flux comparison results • Summary LW • Extra: GERB aging? Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  3. Objectives • Validations of the GERB products before release • Absolute level of GERB SW and LW wrt CERES • Scene type dependencies • Validations of the processing • Spectral response and unfiltering • Spatial processing, correction of the PSF in the BARG and HR formats • Radiance-to-flux conversion • Is the BARG format ready for release? Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  4. GERB data used • GERB-2 ED01 (ARG) and V003 (BARG,HR) • June and December 2004 • Accuracies of GERB radiance at 1SD (J.E. Russell, pers. comm.): • 1.99% for SW • 0.9% for LW Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  5. CERES data used • SSF Edition-2 Rev. 1 • Flux scaled from 20km to the surface level • FM-1 & FM-4: cross-track scanning, • FM-2 : mostly RAPS + GERB PAPS mode, • FM-3 : mostly RAPS • Accuracies of GERB radiance at 1SD (Loeb et al.): • 1% for SW • 1% for LW (day), 0.5% for LW (night) Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  6. Methodology : collocation • ARG and BARG: averaging of the CERES observations that fall in each GERB “pixel” • ARG: • square of 44 x 44 km (i.e. grid spacing), no correction for PSF • time interval : 17 min • BARG: • square of 45 x 45 km • time interval : 15 min • HR : average of the GERB observations that fall in each CERES pixel • CERES: circle of 20km / cos(VZAceres) • time interval : +/- 5 min Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  7. GERB CERES a Methodology: co-angularity for radiance comparisons • Selection based on the angle a • Threshold at 2°, 5° and 8° • Sensitivity study done • Localization of co-angular observations • Numbers of matches Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  8. Methodology: scene type dependency • Surface type from the GERB products • Ocean, dark and bright vegetation, dark and bright desert (no snow) • Cloud type dependency: “AND” operator between the GERB and CERES cloud fractions: • Clear: 0% cloud fractions for both GERB and CERES • Overcast: 100% cloud fractions and cloud optical depth > 7.39 for both GERB and CERES • Additional independent check: • Comparison within radiance (LW) or reflectance (SW) bins Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  9. Methodology: statistical analysis • Idea: 24h are needed to cover the full FOV • Evaluation of the daily values • Best estimate over N days with Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  10. Methodology: regional analysis • Averaging GERB and CERES values in boxes of S x S BARG pixels. S values: • 10 for radiance comp. (450km) • 7 for SW flux comp. • 4 for LW flux comp. • To improve the statistics: • Radiance comp. with a < 8° • Clear sky : cloud fraction < 10% S=10 S=7 S=4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  11. SW Radiance Comparison • Ratio more appropriated than difference • Overall ratio 1.059 • FM1: 1.045 • FM2: 1.054 • FM3: 1.071 • FM4: 1.067 • Scene type dependency higher for ARG (5.9%) than for BARG (2%) and HR (2.1%) Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  12. SW Radiance Comparison – reflectance bin analysis FM1 FM2 FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  13. SW Radiance Comparison: angular dependencies Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  14. SW radiance: regional analysis all aky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  15. SW Flux Comparison • Overall ratio 1.075 • more than the radiance by 1.6% (FM) but • FM1: +2.1% • FM2: +1.2% • FM3: +0.9% • FM4: +1.9% • Reduced scene type dependency for the ARG wrt the radiance comp Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  16. SW flux: regional analysis all sky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  17. SW flux: regional analysis clear sky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  18. Summary SW comparison • Comparison show ratio rather than difference • Overall ratio • radiance 1.059 • flux ratio 1.075 • Not explained by the SW channel accuracies of 1% at 1SD for CERES and 1.99% at 1 SD for GERB • Better agreement with FM1/FM2 than FM3/FM4 • Apparent scene type dependency in the ARG due to the GERB PSF, bilinear rectification, geolocation (?) • BARG and HR agree better with CERES -> should be released ASAP Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  19. LW Radiance Comparison • Ratio and difference are appropriated • Overall ratio 0.987 • FM1: 0.989 • FM2: 0.993 • FM3: 0.983 • FM4: 0.981 • Better agreement with Terra than Aqua (as for the SW) • Scene type dependency higher for ARG (1.7%) than for BARG (0.5%) and HR (0.7%) • Day/Night difference: • FM1: 1.2% • FM2: 0.5% • FM3: 0.0% • FM4: 0.1% Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  20. LW Radiance Comparison – reflectance bin analysis FM1 FM2 FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  21. LW radiance: regional analysis all aky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  22. LW radiance comparison North-South effect • 0.5% North/South effect expected from the telescope throughput. Can be corrected in Edition2 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  23. LW Flux Comparison • Overall ratio 0.987 • Similar to the radiance ratio • Consistent with CERES/GERB accuracies of 1% at 1SD. • Scene type dependency: • ARG: 1.7% • BARG:0.3% • HR:0.1% Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  24. LW flux: regional analysis all sky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  25. LW flux: regional analysis clear sky FM2 FM1 FMX FM3 FM4 Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  26. Summary LW comparison • Comparison show ratio rather than difference • Overall ratio 0.987 • explained by the LW channel accuracies of 1% (day time) at 1SD for CERES and 0.9% at 1 SD for GERB • Better agreement with FM1/FM2 than FM3/FM4 (as SW) • As for the SW, apparent scene type dependency in the ARG • (extreme-) North-South difference of 0.5% due to the telescope • BARG and HR agree better with CERES -> should be released ASAP • ADM problem for cloudy scene (anisotropy underestimated) • Proposal to use a dedicated ADM for high semi-transparent clouds Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

  27. Extra: GERB aging at short wavelengths? • NANRG versus CERES-TRMM models • Similar “aging” observed in the • SEVIRI GERB-like/GERB values • Meteosat-7 GERB-like/GERB values • GERB-1/GERB-2 comparisons? Comparison of GERB Instantaneous Radiance and Flux Products with CERES Edition-2 Data

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