Intercomparison of SEVIRI data from MSG1 and MSG2 and implications for

the GERB data processing

Nicolas Clerbaux & RMIB GERB Team.

GIST 26, RAL

3 and 4 May 2007

MSG1 -> MSG2 transition

New GERB instrument but also new SEVIRI imager

• Are the SEVIRI scene identifications consistent?

– GERB unfiltering

– GERB SW ADM selection– GERB LW angular modelling

– GERB scene identification– GERB dust flag and AOD retrieval

• Are the GERB-likes (SEVIRI NB-to-BB) consistent?

– RGP GERB geolocation

– GERB unfiltering

– GERB resolution enhancement

– use of GERB-like data for monthly means

Structure of the talk

• Intercomparison of level 1.5 SEVIRI data– reflectance for visible channels– BT and radiance for thermal channels

• Intercomparison of GERB scene identification– cloud fraction, – cloud optical depth – cloud phase– clear sky reflectance– LW anisotropic factor– dust flag and AOD retrieval

• Intercomparison of the GERB-like BB quantities– solar and thermal radiances– solar and thermal fluxes

• Conclusions

The MSG1 and MSG2 data look very similar...

“natural color” “air mass”

Visible bands spectral response and effective central wavelength for MSG-1 and

MSG-2

MSGs=0.639µm

MSG2s=0.640µm

MSGs=0.809µm

MSG2s=0.807µm

MSGs=1.635µm

MSG2s=1.635µm

0.6 µm reflectance intercomparison

ratio MSG2/MSG1

0.6 µm reflectance intercomparison – saturations for MSG2

Max 0.6 µm reflectance at nadir and d=1 A.U. is

MSG1 : 1.082

MSG2 : 0.942

Software fix: accept saturation if SZA<20°

0.8 µm reflectance intercomparison

ratio MSG2/MSG1

1.6 µm reflectance intercomparison

ratio MSG2/MSG1

6.2 µm brightness temperature intercomparison

ratio MSG2/MSG1

7.3 µm brightness temperature intercomparison

ratio MSG2/MSG1

8.7 µm brightness temperature intercomparison

ratio MSG2/MSG1

9.7 µm brightness temperature intercomparison

ratio MSG2/MSG1

10.8 µm brightness temperature intercomparison

ratio MSG2/MSG1

12 µm brightness temperature intercomparison

ratio MSG2/MSG1

13.4 µm brightness temperature intercomparison

ratio MSG2/MSG1

Spectral radiance comparison

ratio MSG2/MSG1

• WV 6.2 : 1.002• WV 7.3 : 0.992• IR 8.7 : 0.999• IR 9.7 : 0.994• IR 10.8 : 1.000 • IR 12.0 : 0.996• IR 13.4 : 0.974

-> in general MSG-2 is close or a bit lower than MSG-1, CO2 is significantly lower

Summary for level 1.5 MSG-2/MSG-1 intercomparison

• visible 0.6µm and 0.8µm channels : 1 to 2% higher reflectance

• near IR channel (1.6µm): very close• thermal channels: very close or a bit lower except the

CO2 channel (13.4µm) which is significantly colder (0.5% in BT , 2.5% in radiance)

• 0.6µm is often saturated over thick cloud• residual stripes in the WV 6.2µm channel

Intercomparison of scene identification

• surface type is exactly the same (same rectification grid)• cloud fraction• cloud optical depth• cloud phase• clear sky reflectance images• LW angular modeling• Helen Brindley 's dust flag• Aerosol retrieval with Ignatov look-up-table for the

0.6µm, 0.8µm and 1.6µm channels

Cloud fraction

Cloud optical depth (log)

Cloud phase (0=pure water 1=pure ice)

Clear sky reflectance intercomparison (ratio to model)

VIS 0.6 VIS 0.8

LW ADM due to the change in NB radiances

F= L / R(,L6.2

, L10.8

, L12

,L13.4

)

FMSG1'

/FMSG1

Ratio of the flux for a same BB radiance L but for simulated differences between MSG1 and MSG2 NB radiances in the 6.2µm, 10.8µm, 12µm, 13.4µm channels

Small increase of the anisotropy (0.2%).

MSG2 MSG1Dust Flag

AOD in 0.6 µm channel

AOD in 0.8 µm channel

AOD in 1.6 µm channel

Summary for scene identification

• cloud fraction very similar• cloud optical depth slightly higher (2%) due to the 0.6µm

and 0.8µm reflectances• cloud phase very similar• LW angular modelling close, small difference in the “good

direction” (increase of the limb-darkening). • dust flag may differ in some semi-transparent situations • AOD 2% higher at 0.6µm , 0.6% higher at 0.8 µm and

1.6% lower at 1.6 µm. Ratio of AOD does not correspond to ratio in reflectance.

Intercomparison of broadband quantities

• GERB-like BB radiances (NB->BB)– reflected solar– emitted thermal

• GERB-like BB fluxes (NB->BB + ADMs) – reflected solar– emitted thermal

GERB-like Reflected Solar Radiance

GERB-like Emitted Thermal Radiance

GERB-like Reflected Solar Flux

ratio MSG2/MSG1

GERB-like Emitted Thermal Flux

ratio MSG2/MSG1

Summary for the broadband products

• solar radiance MSG2/MSG1 = 1.012• thermal radiance MSG2/MSG1 = 0.9964• solar flux MSG2/MSG1 = 1.012• thermal flux MSG2/MSG1 = 0.9954

Conclusions and future work

• start of GERB-1 processing with SEVIRI on MSG2• good consistency with MSG1, most of the

differences are explained by the level 1.5 SEVIRI data intercomparison,

• GERB-like radiances and fluxes are a bit closer to the actual GERB products

• To be done: analysis of the influence of the SEVIRI radiance definition (change from spectral radiance to effective radiance in April 2008).