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

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.

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

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

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

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

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

Sensitivity study Re-calibration slopes ch3 slope corr.coef. Default 0.964 0.987 No ray matching 1.009 0.979 0.3-deg vs 0.15-deg grid 0.966 0.990 0.1-deg vs 0.15-deg grid 0.964 0.983 Ocean-only data 0.955 0.984 8-daily vs daily data 0.962 0.988

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.

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.

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