1. Inter-calibrating polar imagers using SEVIRI as a transfer instrument
Jan Fokke Meirink, Piet Stammes (KNMI)
Rob Roebeling (EUMETSAT)

2. Procedure
Calibrate MSG-SEVIRI with Aqua-MODIS by regressing collocated reflectances
Apply same procedure for SEVIRI with other polar imager
Calibrate other polar imager with Aqua-MODIS by taking ratio of 1 and 2.

3. SEVIRI-MODIS inter-calibration
Collocated reflectances near MSG sub-sat. point
Regrid to 0.15-degree equal angle grid

4. Ray matching Criteria on viewing + scattering angle difference
Better correlation
More consistent fits

5. Correction for spectral response
MODTRAN-based LUT
CTH
Solar+viewing angle
Water vapour column
Ozone column

6. SRF correction validation (cf. Heidinger et al. 2002)
Main absorption in ch2 due to water vapour
MODIS ch18/ch17 ratio proportional to water vapour column along light path
MODTRAN-simulated SRF correction well correlated with MODIS ch18/ch17 ratio

7. Resulting re-calibration slopes
SEV<MOD by ≈ 8%
ch1
Consistent with Doelling et al. (2004), Ham & Sohn (2010)
SEV<MOD by ≈ 6%
ch2
SEV>MOD by ≈ 4%
ch3

8. Sensitivity study Re-calibration slopes ch3 slope corr.coef.
Default
No ray matching
0.3-deg vs 0.15-deg grid
0.1-deg vs 0.15-deg grid
Ocean-only data
8-daily vs daily data

9. Other polar imagers
Same can be done for SEVIRI w.r.t other polar imagers, e.g. AVHRR on NOAA17
AVHRR nominal calibration from Heidinger et al.

10. Polar-polar inter-calibration
Re-calibration slopes of other polar imagers with respect to Aqua-MODIS
Ch1
Ch2
Ch3
Terra-MODIS
1.017
1.003
0.999
NOAA17-AVHRR
1.013
1.031
0.970
NOAA18-AVHRR
1.009
1.036
n.a.

11. Conclusions
MSG-SEVIRI has very stable calibration with respect to Aqua-MODIS, but systematic offsets:
Ch1: -8%
Ch2: -6%
Ch3: +4%
SEVIRI can be used to transfer calibration from one polar imager to the other
Provides check of direct SNOs
Advantage could be: more collocations and lower solar zenith angles