Diurnal asymmetry in the GERB(-like) fluxes: an update

Cédric Bertrand

Royal Meteorological Institute of Belgium,Brussels, Belgium. Cedric.Bertrand@oma.be

ADM Normalization Factor(to account for the nonlinear variation of the radiance within an angular bin)

Radiance-to-flux conversion

Instantaneous SW flux estimation: (all scenes)

RMIB: (1)

(2)

CERES:

),,(),,(

)( .3.3

mvmsmadm

mvmsmSEVIRIsm

SEVIRI RLF

),,(),,(),,(

)(

mvmsmmvmsm

adm

mvmsmmsm NFF

RL

),,(*)()(.3.3/

mvmsmH

smSEVIRIsmSEVIRIGERB cFF

GERB SW correction factor (resolution enhancement factor)

Interpolated CERES-TRMMADM ACF

RMIB/CERES COMPARISON

Instantaneous flux estimation: (clear ocean)

CERES:

),,,(

),(),(),,,(

),,()(

mvmsmj

admth

mth

mvmsmadm

mvmsmmsm wsNF

LwsRLwsRwsR

LF

Aerosol correction term

where Radm(ws,sm,vm,m) = determined from the wind speed-dependent ADMs

Rth(ws, Lm) and Rth(ws, Ladm) = anisotropic factors inferred from the

measured and ADM interpolated radiances, respectively. determined by comparing Lm(sm,vm,m) and Ladm(sm,vm,m) with LUT of theoretical SW radiances stratified by aerosol optical depth

MS-7 DATA ALL AVAILABLE 2003/07/02 DAY TIME SLOTS 4 SELECTED ZONES outside sun glint occurrence

acquisition geometry

SBDART RTM

clear sky SW radiancetime series

RMIB SBDART CERES

COMPARISON

R2F CONV.

Monthly mean clear sky SW flux difference at l.n. ± 2 hours (April 04)

GERB-likeSEVIRI

GERB

CERES-TRMMADMs

mW 2.

mW 2.

mW 2.

mW 2.

mW 2.

mW 2.

mW 2.

l.n. – (l.n.-2) l.n. – (l.n.+2)

Max: < 1.33 W.m-2

Avg.: 0.11 W.m-2

West: Max FSW TOA after local noon

East: Max FSW TOA before l.n.

Local noon = Min(SZA)

Selected ocean footprints (3x3 SEVIRI pixel resolution)

[38.74oN, 41.08oW]

[37.00oN, 0.00o E]

[37.70oN, 25.41oE]

[11.97oS, 28.09oW]

[11.69oS, 0.00oE]

[12.43oS, 46.78oE]

[20.80oS, 26.62oW]

[20.36oS, 0.00oE]

[21.37oS, 41,29oE]

SEVIRI spectral conversion (NB-to-BB)

LufSW(s,v,)=co(s) + c1(s) L0.6 m (s,v,) + c2(s) L0.8 m (s,v,) + c3(s) L1.6 m (s,v,)

Data base of theoretical SBDART spectral radiances curves at TOA (150/600 clear ocean scene types)

Fictitious AOD diurnal cycle

Retrieved AOD from LufSEVIRI

distinct diurnal cycle pattern = artifact

AOD sinusoidal shape function of sin(RAA): AODmin = 180o ; AODmax = 90o

Azimuth dependence of the diffuse radiation scattering phase function of the atmos.Aerosol scattering phase function aerosol type. Wrong model scattering angle dependent error in the retrieved AOD

Fictitious AOD diurnal cycle

LUTs of theoretical SW radiances DISORT RTM (assuming the Hess et al. (1998) maritim tropical aerosols model)

SEVIRI spectral modeling SBDART RTM (assuming all the default SBDART aerosols models)

Estimated SEVIRI BB SW radiances theoretical SBDART SW radiances

IF SBDART ocean surface = very different from the real ocean

ADM ACF adjustment sensitive to more than just the presence of aerosols

Ex.: ocean too bright: Rth (LSBDART (No Aero))/Rth (LADM) > 1 for the wrong reason

Sensitivity to the DISORT ocean surface is a second-order effect since

Rth (LSBDART (No Aero))/Rth (LADM) = ratio of DISORT ACFs in a particular angle

Numerical noise Cloud contamination

Old response enhanced sensitivity in the blue

Adoption of the new response will GERB SW radiance ADM ACF partial adjustment

),,,(

),(

),,(),,(.3,),,,(

),,(),,(

*

*.3

vsws

admws

vsvsSEVIRIwsvsws

vsvsNF

LR

LRR

L

thH

th

adm

HSEVIRI

cc

),,(),,,(

),(

).3,(),,,(

),,( *.3

vsvsws

admws

SEVIRIwsvsws

vs cH

th

th

adm

SEVIRI NF

LRLRR

L

),,,(),,(

),(

).3,(),,,(

),,( *.3

vswsvs

admws

SEVIRIwsvsws

vs NF

LRLRR

L cH

th

th

adm

SEVIRI

F3.SEVIRI(s)

FGERB/3.SEVIRI(s) estimation:

Numerical noise Cloud contamination

Conclusions:1. asymmetry in the diurnal evolution of GERB(-like) SW

flux is an ocean surface related problem

2. there is a discrepancy between the theoretical NB-to-BB conversion from SEVIRI and the expected (CERES) BB radiances (used as input in the ADM aerosol adjustment procedure)

3. since the aerosol adjustment is a ratio of theoretical anisotropic factors that depends upon the departure of the observed BB radiance to the mean radiance used to develop the ADMs, introducing new “observed” BB radiances that are inconsistent with CERES radiances can lead to problems.

Conclusions:

4. correcting the SEVIRI based spectral modeling by the GERB measurement allows to reduce part of the disagreement between the estimated and the ADM radiances

5. The GERB TOA reflected SW flux over clear ocean surfaces should be estimated from the corrected SEVIRI BB radiance and the RGP should consider the new spectral response functions

IF SBDART ocean surface = very different from the real ocean

ADM ACF adjustment sensitive to more than just the presence of aerosols

Ex.: ocean too bright: Rth (LSBDART (No Aero))/Rth (LADM) > 1 for the wrong reason

Sensitivity to the DISORT ocean surface is a second-order effect since

Rth (LSBDART (No Aero))/Rth (LADM) = ratio of DISORT ACFs in a particular angle