CERES shortwave (SW), longwave (LW), and window (WN) channel radiative
fluxes are derived from empirical Angular Distribution Models (ADMs) that
convert a measured radiance in a given Sun-Earth-satellite viewing
configuration to a top-of-atmosphere (TOA) radiative flux. Recently, a new set
of ADMs were developed from 9 months of CERES-TRMM SSF measurements over the
Tropics (Loeb et al., 2003a,b). These ADMs were used to generate TOA fluxes on
the CERES-TRMM Edition2B SSF product. The TRMM ADMs were also used to produce
TOA fluxes for the CERES-Terra Edition1A SSF product while a new set of global
ADMs based on CERES/Terra data were being developed.
The new global CERES/Terra ADMs are now complete and are being used to
generate TOA fluxes on the CERES-Terra Edition2A SSF product. The new ADMs are
based on 32 months (Mar 2000 - Oct 2002) of CERES-Terra Edition1A SSF data. The
main features of the new Terra ADMs are:
Empirical ADMs over snow and sea-ice.
Increased angular resolution in nonpolar regions (2 degrees).
Use of "continuous" SW and LW ADM scene types over ocean, land,
and desert.
Monthly 1 degree regional clear land+desert ADMs.
Neural network scheme to improve TOA flux estimates for footprints with
excessive "no retrievals". No retrievals can occur when imager
data is missing or when the cloud algorithm cannot provide a physical retrieval.
If the scene characteristics over 35% or more of a CERES FOV are unknown, TOA
fluxes are estimated using a neural network scheme. Approximately 3% of the
CERES data fall in this category.
Caution: During the processing of SSF Edition2A, an error was
detected in the SW TOA flux code for CERES FOVs that are cloudy over land and
desert. The error is most pronounced for optically thin cloud over bright
background surfaces, with instantaneous TOA flux errors reaching 25%. [View a
summary of the error (PDF).] Regional
average errors of instantaneous TOA fluxes (at the Terra overpass time) are
determined by comparing 1° latitude-longitude averages of fluxes from the
Edition2A code ("original") and a corrected version of the
code ("corrected"). The regional errors are most pronounced over
central Africa, reaching 7 W m-2 in some 1° regions. The regional
average error over all land and desert is approximately 1 W m-2.
In terms of 24-hour average regional errors, these correspond to approximately
2 W m-2 errors over central Africa and 0.3 W m-2
for all land and desert surfaces. The global 24-hour average SW flux error
due to the code bug is approximately 0.1 - 0.2 W m-2.
Plans are underway to produce an updated SSF with a correct version of the
production code in Spring or Summer 2004. This new version will be called
Edition2B and will supersede SSF Edition2A.
The tables below provide additional details on Terra ADM scene
classification and validation results. Further details can be found in the
following CERES Science Team Meeting presentations:
A Portable Document Format (PDF) reader (such as
Adobe Acrobat Reader) is
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CERES/Terra Shortwave Channel ADMs for Different Scene Types
Scene Type
Description
Clear Ocean
Function of wind speed; Correction for aerosol optical depth included.
Cloud Ocean
Function of cloud phase; Continuous function of cloud fraction and cloud
optical depth (5-parameter sigmoid).
Land & Desert Clear
1°- regional monthly ADMs using Analytical Function of TOA BRDF
(Ahmad and Deering, 1992).
Land & Desert Cloud
Function of cloud phase; continuous function of cloud cover and cloud
optical depth; used 1°-regional clear-sky BRDFs to account for background
albedo.
Permanent Snow
Function of Cloud Fraction, Surface Brightness, cloud optical depth
Fresh Snow
Function of Cloud Fraction, Surface Brightness, Snow Fraction, cloud optical depth
Sea-Ice
Function of Cloud Fraction, Surface Brightness, Ice Fraction, cloud optical depth
CERES/Terra Longwave and Window Channel ADMs for Different Scene Types
Scene Type
Description
Clear Ocean, Land, Desert
Function of Ocean, Forest, Cropland/Grass, Savanna, Bright Desert, Dark Desert,
precip. water, lapse rate, skin temperature.
Clouds Over Ocean, Land Desert
Function of precip. water, skin temp, sfc-cloud temp. diff; continuous
function of parameterization involving cloud fraction, cloud and sfc emissivity,
sfc and cloud temp.
Permanent Snow, Fresh Snow, Sea-Ice
Each a function of cloud fraction, sfc temp, sfc-cld temp diff
CERES/Instantaneous SW TOA flux uncertainties for Terra and TRMM ADMs
Region
So W m-2
Terra ADMs W m-2 (%)
TRMM ADMs W m-2 (%)
Clear-Sky
All-Sky
Clear-Sky
All-Sky
Tropics
1150
5.2 (2.2)
14.3 (5.1)
7.7 (3.5)
14.3 (5.8)
Midlat
870
4.2 (3.0)
13.5 (3.9)
7.3 (5.6)
13.7 (4.1)
Polar
540
12.8 (4.3)
17.3 (5.9)
37.0 (11.7)
29.2 (9.8)
CERES/Instantaneous LW TOA flux uncertainties for Terra ADMs
Region
Terra ADMs W m-2 (%)
TRMM ADMs W m-2 (%)
Clear-Sky
All-Sky
Clear-Sky
All-Sky
Tropics
3.3 (1.1)
5.1 (1.8)
3.4 (1.1)
5.3 (1.9)
Midlat
2.9 (1.0)
5.4 (2.3)
2.7 (1.0)
5.6 (2.2)
Polar
3.4 (1.6)
4.0 (2.0)
3.3 (1.4)
5.0 (2.3)
Uncertainties in Regional Mean SW TOA fluxes (24-hour averages)
December 2001
June 2002
Mean Bias
Regional RMS
Mean Bias
Regional RMS
TRMM ADMs
0.21
1.77
-0.85
1.89
Terra ADMs
-0.42
0.89
-.056
0.96
Uncertainties in Regional Mean Daytime LW TOA fluxes
December 2001
June 2002
Mean Bias
Regional RMS
Mean Bias
Regional RMS
TRMM ADMs
-0.13
1.45
0.35
1.32
Terra ADMs
-0.04
0.93
0.26
0.82
Uncertainties in Regional Mean Nighttime LW TOA fluxes
