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Diurnal asymmetry in the GERB(-like) fluxes: an update. C édric Bertrand Royal Meteorological Institute of Belgium, Brussels, Belgium. [email protected] Radiance-to-flux conversion. Instantaneous SW flux estimation: (all scenes) RMIB: (1) (2) CERES:.

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

Cédric Bertrand

Royal Meteorological Institute of Belgium,

Brussels, Belgium.

[email protected]

slide2

Radiance-to-flux conversion

Instantaneous SW flux estimation: (all scenes)

RMIB: (1)

(2)

CERES:

Interpolated CERES-TRMM

ADM ACF

GERB SW correction factor (resolution enhancement factor)

ADM Normalization Factor

(to account for the nonlinear variation of the radiance

within an angular bin)

slide3

RMIB/CERES COMPARISON

Instantaneous flux estimation: (clear ocean)

CERES:

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

slide4

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 radiance

time series

R2F CONV.

RMIB SBDART CERES

COMPARISON

slide5

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

l.n. – (l.n.-2)

l.n. – (l.n.+2)

GERB-like

SEVIRI

East: Max FSW TOA before l.n.

West: Max FSW TOA

after local noon

GERB

CERES-TRMM

ADMs

Max: < 1.33 W.m-2

Avg.:  0.11 W.m-2

Local noon = Min(SZA)

slide6

Selected ocean footprints (3x3 SEVIRI pixel resolution)

[38.74oN, 41.08oW]

[37.70oN, 25.41oE]

[11.69oS, 0.00oE]

[37.00oN, 0.00o E]

[11.97oS, 28.09oW]

[12.43oS, 46.78oE]

[21.37oS, 41,29oE]

[20.80oS, 26.62oW]

[20.36oS, 0.00oE]

slide10

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)

slide11

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

slide12

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) > 1for 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

slide14

Numerical

noise

Cloud contamination

slide15

Old response  enhanced sensitivity in the blue

Adoption of the new response will  GERB SW radiance

 ADM ACF partial adjustment

slide16

FGERB/3.SEVIRI(s) estimation:

F3.SEVIRI(s)

slide17

Numerical

noise

Cloud contamination

slide18

Conclusions:

  • 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.
slide19

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

slide20

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) > 1for 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

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