CERES Angular Dependence Models for Radiance to Flux Conversion

40 Pages·2003·2.95 MB·English

by  Norman G. Loeb

TOA Radiative Flux Estimation From
CERES Angular Distribution Models
Norman G. Loeb
Hampton University/NASA Langley Research Center
Hampton, VA
Acknowledgements: K. Loukachine, S. Kato, N. M. Smith
January 29, 2003
Outline
1. Introduction
2. TOA flux retrieval strategy – ADM definition
3. CERES/TRMM Validation Results
4. Plans for CERES/Terra ADMs
5. Summary
Top-of-Atmosphere Radiation Budget
(Incoming Solar = Outgoing Longwave):
S
− o = σ 4 ≈ +
(1 A) T 208 2T
e s
4
=
A Planetary Albedo
=
S Solar Irradiance
o
=
T Earth Radiative Temperature
e
=
T Equilibrium Surface Temperature
s
F I F I
∆
1 S A
ο
∆ = − H o K ∆ ≈ − H K
T A 0.5 C 100
s
2 4 A
1% relative error in A
≈ ≈ °
⇒ 1 W m-2 flux error ⇒ 0.5 C error in T
s
2xCO => +4 W m-2
2
Instantaneous Fluxes at TOA
and Angular Distribution Models
CERES Radiance Measurement TOA Flux Estimate SW
LW
WN
⇒
Satellite
Sun
•
θ
θ
o
φ
Instantaneous Fluxes at TOA
and Angular Distribution Models
TOA flux estimate from CERES radiance:
where,
θ θ φ
R ( , , ) is the Angular Distribution Model (ADM) for the “jth” scene type.
j o

ADM Scene Identification
• The main reason for defining ADMs by scene type is to
reduce the error in the albedo estimate.
=> Earth scenes have distinct anisotropic characteristics
which depend on their physical and optical properties.
(e.g. thin vs thick clouds; cloud-free, broken, overcast
etc.).
=> Scene identification must be self-consistent. Biases in
cloud property retrievals (e.g. due to 3D cloud effects)
should not introduce biases in flux/albedo estimates.
CERES/TRMM Overcast Ice Cloud ADMs vs ERBE
θ
( =53.1-60)
o
Overcast LW ADMs
(Precipitable Water 4.63 – 10.00 cm)
Anisotropic Model Scene Type Stratification
Spacecraft/Mission Cloud Surface Type Total
TIROS 2, 3, 4 N/A N/A isotropy
TIROS 7
Global Global 1
(Arking and Levine, 1967)
Nimbus 2, 3 Cloud/Land Ocean 3
(Rashke et al. 1973) Snow
Ocean
All Cloud
Nimbus-6, 7 4
Land
(Taylor and Stowe, 1984;
Snow/Ice
Jacobowitz et al., 1984)
Ocean
ERBE Clear
Land 12
(Smith et al., 1986; Partly cloudy
Desert
Suttles et al., 1988) Mostly cloudy
Snow
Overcast
Land-Ocean Mix