EUMETSAT conference, Praag, 4/6/2004

Validation of TOA radiative fluxes from the GERB instrument

S. Dewitte

Royal Meteorological Institute of Belgium

EUMETSAT conference, Praag, 4/6/2004

Overview

Methodology Used data Validation results Theoretical interpretation Conclusions

EUMETSAT conference, Praag, 4/6/2004

Methodology Radiative flux at the top of the atmosphere: F (W/m2) Satellite observations: radiances L (W/m2sr) Satellite viewing zenith angle vz

F = L (vz) / R(vz)

GERB: fixed vz

Validation GERB fluxes: comparison with CERES fluxes with variable vz

EUMETSAT conference, Praag, 4/6/2004

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

EUMETSAT conference, Praag, 4/6/2004

EUMETSAT conference, Praag, 4/6/2004

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

EUMETSAT conference, Praag, 4/6/2004

Regional distribution (FM3)

EUMETSAT conference, Praag, 4/6/2004

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

EUMETSAT conference, Praag, 4/6/2004

EUMETSAT conference, Praag, 4/6/2004

Coldest/intermediate scenes

Flux < 150 W/m2 150 W/m2 < Flux < 250 W/m2

EUMETSAT conference, Praag, 4/6/2004

Expected theoretical error

EUMETSAT conference, Praag, 4/6/2004

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.