on the application of ceres sw adms
DESCRIPTION
On the application of CERES SW ADMs. C édric Bertrand. C oncern. Space-borne radiometers do not measure the instantaneous SW flux directly. ADMs required to relate radiance to flux: Instantaneous TOA albedo:. where s = solar zenith angle v = viewing zenith angle - PowerPoint PPT PresentationTRANSCRIPT
On the application of CERES SW ADMs
Cédric Bertrand
Concern
ADMs required to relate radiance to flux:
Instantaneous TOA albedo:
),,(),,()(
vs
vssRLF
where s = solar zenith angle v = viewing zenith angle = relative azimuth angle R = ADM anisotropic correction factor
)cos(.).(
)(0
2
s
ssS
dF
where d2 = Earth-Sun distance So = solar constant
Space-borne radiometers do not measure the instantaneous SW flux directly
CERES-TRMM ADMs vs. CERES-TERRA (AQUA) ADMs
Several years of CERES-TERRA data = enough sampling to define ADMs at a higher angular resolution than CERES-TRMM ADMs = extends the CERES-TRMM data (40ºN to 40º S) by adding mid-latitude and polar observations BUT:BUT: polar Sun-Synchronous orbit designed to cross the equator at the same local time each orbit do not provide observations of the angular radiation fields over the full range of s
empty ADMs angular bins
TRMM: 350-km circular, precessing orbit with a 35º inclination angle samples each grid box at different local time every day full range of s acquired every 46 days.
Anisotropy of Earth scenes varies with viewing geometry and cloud/clear sky properties in a continuous manner
CERES-TRMM ADMs are defined for discrete angular bins and scene types s and v : 9 angular bins from 0º to 90º in 10º step : 10 angular bins from 0º to 180º with a 10º or 20º step
ADMs angular resolution
An adjustement to the CERES anisotropic factors is needed to avoid introducing large instantaneous flux error or flux discontinuities between angular bins or scene types.
CERES: linear interpolation of bin-averaged ADM radiance Ladm(s,v,) and fluxes Fadm(s) to each observation angle (sa,va,a) and evaluate anisotropic factors from the interpolated quantities R(sa,va,a) = .Ladm(sa,va,a)/Fadm(sa)
RMIB: compute bin-averaged ADM anisotropic factors and use a tri-linear interpolation to estimate R(sa,va,a).
Reduce instantaneous flux errors but no guarantee that ensemble averages of the instantaneous fluxes will remain unbiased
R evaluation for specific angles (s,v,)
REDUCE ANGULAR BIN DISCRETIZATION ERRORS:
Bias in the mean flux will occur if: Linear interpolation used when actual radiance varies nonlinearly within an angular bin Theoretical models are used to supplement empirical ADMs.
The bias for a specific scene type j in angular bin (si,vk,l) is given by:
)(),,(
),,(),,( ,
,
sijadm
vsjadm
vs
ikl
lvksij FR
LF
Average of all instantaneous flux estimates falling in angular bin [si±(s)/2, vk±(v)/2,±()/2]
ADM mean flux
Bias estimation
Estimated mean flux
RMIB:
CERES:
),,(
),,()(
mvmsmadm
mvmsmmsm
RLF
),,(),,(
),,()(
mvmsm
mvmsmadm
mvmsmmsm CFFRL
),,(),,(
),,(
),,(
),,(),,(
mvmsmadm
mvmsmm
ikl
lvksiadm
mvmsmadm
mvmsmmmvmsm
L
L
F
L
LCF
where
Correction term to removethe bias
When the ensemble average of instantaneous TOA flux from the CERES
Eq. is determined, the mean flux is unbiased because CF(si,vk,l) = Fadm (si,vk,l)
Instantaneous SW flux computation
Clear ocean SW flux
Instantaneous flux estimation (clear ocean)
CERES:
),,,(
),(
),(),,,(
),,()(
mvmsmj
admjth
mjth
mvmsmjadm
mvmsmmsm wsCF
LwsR
LwsRwsR
LF
Aerosol correction term
where Radm(wsj,sm,vm,m) = determined from the wind speed-dependent ADMs
Rth(wsj, Lm) and Rth(wsj, 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
METHODOLOGY
(J155, I500)
(J416, I500)
(J750, I500)
MS-7 DATA ALL AVAILABLE 2003/07/02 SLOTS 3 SELECTED ZONES outside sun glint occurrence
acquisition geometry
SBDART RTM
clear sky SW radiancetime series
RMIB SBDART CERES
COMPARISON
R2F CONV.
! 18:30’ no MS-7 data !
underestimation
AERO_C + CF
AERO_C + CF
AERO_C + CF
Conclusions
Do not account for normalization and aerosol correction when estimating the reflected SW flux at TOA could explain the reported asymmetries in the diurnal variation of the GERB SW fluxes
Climatological wind speed values will be considered inorder to apply the aerosol correction (LUTs are wind speed dependent)
Both correction terms will be implemented in the RGP system as soon as possible …
CERES-TRMM (INT01)= R interpolated according to the CERES scheme
CERES-TRMM (INT02)= full CERES scheme
Green and Red curves = identical
Joanna Futyan: fit the CERES-TRMM SW ADMs TOA albedo to herderived clear sky fluxes to fill missing clear sky SW data
Systematic asymmetries in the diurnal variation of the GERB SW fluxes that are not represented by the CERES models
Residual between the fit the observations at local noon ± 2 hours for April 2004(the fit is constrained to match the data at local noon over the ocean regions)
adm(si) versus adm/bin(si,vi,i)
CERES-TRMMM SW ADM = 5
Wind speeds and ADMs
The wind speed dependence of the ADMs is only an issue near to sun glint:
(i) at low wind speeds, the range of angles affected by sunglint is small but intense.
(ii) at high wind speeds, the range of angles affected by sunglint is much larger, but not as intense.
MS-7 I500-J416 20030702
