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Validation of TOA radiative fluxes from the GERB instrument

Validation of TOA radiative fluxes from the GERB instrument. S. Dewitte Royal Meteorological Institute of Belgium. Overview. Methodology Used data Validation results Theoretical interpretation Conclusions. Methodology. Radiative flux at the top of the atmosphere: F (W/m 2 )

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Validation of TOA radiative fluxes from the GERB instrument

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  1. Validation of TOA radiative fluxes from the GERB instrument S. Dewitte Royal Meteorological Institute of Belgium

  2. Overview • Methodology • Used data • Validation results • Theoretical interpretation • Conclusions

  3. Methodology • Radiative flux at the top of the atmosphere: F (W/m2) • Satellite observations: radiances L (W/m2sr) • Satellite viewing zenith angle qvz F = p L (qvz) / R(qvz) • GERB: fixed qvz • Validation GERB fluxes: comparison with CERES fluxes with variable qvz

  4. Used data • GERB: ARG fluxes, SEVIRI as imager, Version 2 • CERES FM2 and FM3: RAPS or GERB mode or special scan, use of inflight calibration • 19/12/2003-31/3/2004 • use of night data for thermal fluxes • CERES data is colocated to nearest GERB ARG pixel

  5. 95% confidence intervals • GERB/(CERES ES8 FM2) = 0.989 +/- 0.002 • GERB/(CERES ES8 FM3) = 0.982 +/- 0.003 • (CERES SSF)/(CERES ES8) = 0.992 • GERB/(CERES SSF FM2) = 0.997 +/- 0.002 • GERB/(CERES SSF FM3) = 0.990 +/- 0.003

  6. Regional distribution (FM3)

  7. Viewing zenith angle dependence • ‘Cold’ GERB pixel: mean OLR < = 220 W/m2 • ‘Warm’ GERB pixel: mean OLR > 220 W/m2 • Separately for warm and cold pixels: calculate (mean GERB OLR)/ (mean CERES OLR) per 5 degree viewing zenith angle interval

  8. Coldest/intermediate scenes Flux < 150 W/m2 150 W/m2 < Flux < 250 W/m2

  9. Expected theoretical error

  10. Conclusions • On the average and for viewing zenith angles near 50 degrees, the GERB and CERES FM2 and FM3 fluxes agree within the required 1%. • A limb darkening remains present in the GERB fluxes • Within +/-1% for the warm scenes. • Up to +20% at nadir for the coldest scenes ! • For a possible improvement, a better detection of thin cirrus seems crucial.

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