CERES-GERB meeting, Boulder, 3/2004

Validation of GERB unfiltered radiances

S. Dewitte

Royal Meteorological Institute of Belgium

CERES-GERB meeting, Boulder, 3/2004

Overview

GERB-CERES comparison– Reflected solar radiance– Emitted thermal radiance - Night

GERB-SEVIRI comparison– Day-night consistency

CERES-GERB meeting, Boulder, 3/2004

GERB-CERES: Used data 19/12/2003-29/2/2004

– Only night time used for thermal comparisons GERB: L2 ARG

– Unfiltering with SEVIRI– Version 2 : geolocation from matching

CERES FM2– GERB mode, RAPS, Principal plane scan– Edition 1: pre-flight gains

Coangular: Difference viewing direction < 20 deg.

CERES-GERB meeting, Boulder, 3/2004

Reflected solar radiance comparison

Clear sky/overcast separated by cloud amount C=(L-Lcs)/(Lov-Lcs)– CS: C<50%, OV: C>50%

Clear sky separated by ADM surface type– Ocean, dark/bright vegetation, dark/bright

desert

CERES-GERB meeting, Boulder, 3/2004

CERES-GERB meeting, Boulder, 3/2004

95% confidence intervals

GERB /CERES reflected solar radiance=Ocean: 0.931+/-0.009=[0.922-0.941]

Dark veg.: 0.951+/-0.006=[0.944-0.957]

Bright veg.: 0.969+/-0.011=[0.958-0.980]

Dark desert: 1.001+/-0.010=[0.991-1.011]

Bright desert: 0.980+/-0.006=[0.974-0.986]

Clouds: 0.959+/-0.009=[0.950-0.968]

CERES-GERB meeting, Boulder, 3/2004

Spectral Response

Reprocessed measured average detector and witness sample response

250

270

290

310

330

350

370

390

410

430

450

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7

wavelength (um)

No

rmal

ize

d r

es

po

ns

e

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CERES-GERB meeting, Boulder, 3/2004

CERES-GERB meeting, Boulder, 3/2004

CERES FM2 LW 0.9 % too high

CERES-GERB meeting, Boulder, 3/2004

95% confidence interval

GERB/(ed. 1 CERES) emitted thermal radiance=0.988+/-0.007=[0.982-0.995]

After in-flight correction CERES=0.997+/-0.007=[0.991-1.004]

CERES-GERB meeting, Boulder, 3/2004

GERB LW day night consistency

(GERB LW)/(SEVIRI BB LW) known for every ARG pixel

Calculate mean for every image Plot mean ratio as function of hour of day Time interval: 12/2003-2/2004

CERES-GERB meeting, Boulder, 3/2004

CERES-GERB meeting, Boulder, 3/2004

Day-night consistency

Discontinuity of 0.5 % between 5:45 and 6:00 UTC– Origin: geolocation from matching ?

Off-scale values around midnight– Origin: straylight ?

Mean daily variation of order 0.5 % -> OK

CERES-GERB meeting, Boulder, 3/2004

Conclusions GERB/CERES reflected solar radiance differences are within

target accuracy only for clear sky desert.– Cause: Spectral response ?

GERB/CERES night time emitted thermal radiance differences are within target accuracy.

GERB/SEVIRI emitted thermal radiance differences show day/night consistency within 0.5 %– Exception: stray light ?

Next steps: – Check reflected solar radiance differences with updated spectral response.– Validate BARG, SHI unfiltered radiances.