GERB SW fluxes: an update (Clear ocean SW radiances and fluxes in the reprocessed V999 June 04 data) Cédric Bertrand GIST th December, Imperial College, London, U.K.

RECAP: unfiltering New spectral response function New spectral conversion: where: = wavelength (  m) I = reflected solar radiance (Wm -2.Sr -1.  m -1 ) S =spectral response function (0  S  1) L SEVIRI = a + b 1 L b 2 L b 3 L c 11 L c 21 L 0.8 L c 22 L !!!! fct of SZA ONLY !!!! c 31 L 1.6 L c 32 L 1.6 L c 33 L !!!! fct of SZA ONLY !!!!  = unfilter factor V002

RECAP: unfiltering Database of co-angular CERES and SEVIRI measurements (March, April and July 04 FM2/FM3 ES8 Edition 2 CERES data) L SEVIRI UF (L CERES UF = a + b L c L d L e SZA sza  60°) with a, b, c, d and e surface types dependent L SEVIRI F CAN NOT BE DETERMINED FROM CERES DATA General case: L SEVIRI F = a’ + b’ L c’ L d’ L e’ SZA ( a", b" and c" = f(SZA,VZA,RAA) ) V999: Clear Ocean: L SEVIRI F = L SEVIRI UF /  SBDART

RECAP: angular conversion R adm = clear ocean wind speed dependent CERES-TRMM BB ADM anisotropic correction factor If  (glint angle) < 25°

 F ADM  F DUF  F UF  F ADM FFFF  F DUF  F UF FFFF F l.n – F l.n-2:00 (W.m -2 ) F l.n – F l.n+2:00 (W.m -2 ) ARG JUNE 04

F l.n – F l.n-2:00 (W.m -2 )  F ADM  F S3  F G3 F l.n – F l.n+2:00 (W.m -2 )  F S3 3x3 SEVIRI JUNE 04

RECAP: resolution enhancement (I) Up-sampling of the filtered GERB radiances from the nominal GERB footprint resolution (i, j) to the 3 x 3 SEVIRI pixels resolution (x, y). BUT: rather than L f GERB we use CF = L f GERB /L f SEVIRI FIRST: FIRST: Fluxes at the GERB footprint resolution (i, j) are derived from the 3 x 3 SEVIRI pixels (x, y) based flux estimates F GERB (i, j) = (  x  y PSF(i, j, x, y). F S3 (x, y) ). CF L (i, j)  CF L (i, j) = correction of SEVIRI by GERB SECOND: SECOND: improvement of the spatial resolution of GERB fluxes by use of SEVIRI high resolution information F GERB (i, j) =  x  y PSF(i, j, x, y). CF H (x, y). F S3 (x, y)

RECAP: resolution enhancement (II) FINALLY: FINALLY: GERB flux at the 3 x 3 SEVIRI pixels resolution is given by F G3 (x, y) = CF H (x, y). F S3 (x, y) use of L f ADM (x,y) rather than L f SEVIRI (x,y) in the CF L (i,j) computation. To reduce the impact of the SEVIRI spectral conversion in the estimation of the GERB SW fluxes at the 3x3 SEVIRI pixels resolution: Limitation: while  SBDART is a function of the 3 acquisition angles (sza, vza, and raa) it is still dependent of the assumed GERB spectral response function

F l.n – F l.n-2:00 (W.m -2 )  F S3  F G3  F G3_ADM_L F l.n – F l.n+2:00 (W.m -2 )  F G3 3x3 SEVIRI JUNE 04

L S3 /L ADM L G3 /L ADM SZARAAVZA UNFILTERED RADIANCE RATIO L G3_ADM_L /L ADM

F S3 /F ADM F G3 /F ADM SZARAAVZA FLUX RATIO F G3_ADM_L /F ADM

ARG: L UF /L ADM F UF /F ADM

GERB and SEVIRI images recorded at  times  temporal matching needed Scene types independent Resolution enhancement limitations: GERB pixel 3x3 SEVIRI pixels footprint 100% clear 100% cloudy Clear ocean S3 footprint for which the CF H (x,y) will be cloud contaminated

Conclusions Problems over clear land surfaces which were not found in the V002 version Asymmetry problem in the diurnal evolution of the SW fluxes over clear ocean surface still present.  mainly due to deficiencies in the SEVIRI spectral modeling  but also possible impact of the - GERB spectral response function through its influence on the estimated L f SEVIRI and thus on the CFs - resolution enhancement  reduced clear ocean pixels at the GERB nominal spatial resolution (  temporal matching between GERB and SEVIRI)

ARG Comparison Unscreened Cloudy pixels

ARG Comparison Unscreened Cloudy pixels GERB spectral response function ?

ARG Comparison Unscreened Cloudy pixels

Direct unfiltering Surface type dependent: ocean, vegetation and desert surfaces The normalized unfilter factor is estimated constaint: X = 0  Y = 1 X = 1  Y = 0 Where a, b, c and d are SZA dependent X is the normalized filtered radiance SBDART filtered SW radiance SBDART unfilter factor Mean of the 5% highest values L cld  cld Mean of the 5% lowest values L oc  oc

ARG F F, F DUF and F UF computation  RAL G2_L15A files  L F  SZA, VZA, RAA  RMIB G2_SEV1_L20S files  Surface type   Cloud cover, cloud amount, cloud phase   L UF, F UF   Assuming that the cloud optical depth was retrieved in the VIS 0.8 SEVIRI channel over ocean surface ( less sensitive to the Rayleight scattering) F F and F DUF

ARG F F, F DUF and F UF computation WARNING: The GERB PSF is applied in each of the RMIB G2_SEV1_L20S file fields. Not defined F F F DUF F UF