Diurnal asymmetry in the GERB(-like) fluxes: an update Cédric Bertrand Royal Meteorological Institute of Belgium, Brussels, Belgium.

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: GERB SW correction factor (resolution enhancement factor) Interpolated CERES-TRMM ADM ACF

RMIB/CERES COMPARISON Instantaneous flux estimation: (clear ocean) CERES: Aerosol correction term where R adm (ws,  sm,  vm,  m ) = determined from the wind speed-dependent ADMs R th (ws, L m ) and R th (ws, L adm ) = anisotropic factors inferred from the measured and ADM interpolated radiances, respectively. determined by comparing L m (  sm,  vm,  m) and L adm (  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 radiance time series RMIB SBDART CERES COMPARISON R2F CONV.

Monthly mean clear sky SW flux difference at l.n. ± 2 hours (April 04) GERB-like SEVIRI GERB CERES-TRMM ADMs l.n. – (l.n.-2)l.n. – (l.n.+2) Max: < 1.33 W.m -2 Avg.:  0.11 W.m -2 West: Max F SW  TOA after local noon East: Max F SW  TOA before l.n. Local noon = Min(SZA)

Selected ocean footprints (3x3 SEVIRI pixel resolution) [38.74 o N, o W] [37.00 o N, 0.00 o E] [37.70 o N, o E] [11.97 o S, o W] [11.69 o S, 0.00 o E] [12.43 o S, o E] [20.80 o S, o W] [20.36 o S, 0.00 o E] [21.37 o S, 41,29 o E]

SEVIRI spectral conversion (NB-to-BB) L uf SW (  s,  v,  ) =c o (  s ) + c 1 (  s ) L 0.6  m (  s,  v,  ) + c 2 (  s ) L 0.8  m (  s,  v,  ) + c 3 (  s ) L 1.6  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 L uf SEVIRI  distinct diurnal cycle pattern = artifact AOD sinusoidal shape  function of sin(RAA): AOD min   = 180 o ; AOD max   = 90 o 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: R th (L SBDART (No Aero) ) /R th (L ADM ) > 1 for the wrong reason Sensitivity to the DISORT ocean surface is a second-order effect since R th (L SBDART (No Aero) ) /R th (L ADM ) = 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

F 3.SEVIRI (  s )    F GERB/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: R th (L SBDART (No Aero) ) /R th (L ADM ) > 1 for the wrong reason Sensitivity to the DISORT ocean surface is a second-order effect since R th (L SBDART (No Aero) ) /R th (L ADM ) = ratio of DISORT ACFs in a particular angle