This paper presents an overview of the CERES project. It demonstrates how algorithm improvements have lead to improved top-of-atmosphere (TOA) radiative flux accuracy. CERES shortwave flux anomalies are compared with those from Earthshine and ISCCP-FD

Loeb, Norman G.

Wielicki, Bruce A.

NASA Technical Reports Server

Overview of the Clouds and the Earth’s Radiant Energy 
System (CERES) Experiment 
 
Norman G. Loeb and Bruce A. Wielicki 
NASA Langley Research Center, Mail Stop 420, Hampton, VA 
n.g.loeb@larc.nasa.gov 
 
Abstract: This paper presents an overview of the CERES project. It demonstrates how algorithm 
improvements have lead to improved top-of-atmosphere (TOA) radiative flux accuracy. CERES 
shortwave flux anomalies are compared with those from Earthshine and ISCCP-FD. 
OCIS codes: (010.1310) Atmospheric scattering 
1. Introduction 
Clouds and the Earth’s Radiant Energy System (CERES) investigates the critical role that clouds and aerosols play 
in modulating the radiative energy flow within the Earth-atmosphere system. CERES builds upon the foundation 
laid by previous missions, such as the Earth Radiation Budget Experiment (ERBE), to provide highly accurate top-
of-atmosphere (TOA), within-atmosphere and surface radiative fluxes together with coincident cloud and aerosol 
properties inferred from high-resolution imager measurements. The CERES instrument is a scanning broadband 
radiometer that measures filtered radiances in the SW (wavelengths between 0.3-5 μm), total (TOT) (wavelengths 
between 0.3-200 μm) and window (WN) (wavelengths between 8-12 μm) regions. The spatial resolution of CERES 
at nadir is 10 km on the Tropical Rainfall Measuring Mission (TRMM) and 20 km on the Terra and Aqua 
spacecrafts. One of the unique features of CERES is its ability to scan in several different modes. Typically, CERES 
is placed in one of three scan modes: crosstrack, alongtrack, and rotating azimuth plane (RAP). The crosstrack scan 
is perpendicular to the ground track and optimizes spatial sampling but has limited angular sampling; the alongtrack 
scan provides measurements near the satellite orbital plane from several near-simultaneous viewing zenith angles 
over the same region; the RAP scan provides multiangle measurements from a number of viewing zenith and 
relative azimuth angles by scanning in elevation as it rotates in azimuth. Figure 1 shows the data processing flow of 
CERES. In order to maintain continuity with the TOA radiation budget from ERBE, one subsystem of CERES 
produces ERBE-Like products obtained by applying algorithms developed during ERBE to CERES data. The new 
generation of data products from CERES takes full advantage of coincident imager measurements from the Visible 
Infrared Radiometer (VIRS) on TRMM and the Moderate Resolution Imaging Spectroradiometer (MODIS) on Terra 
and Aqua. The imager measurements are used to infer cloud properties [1], such as cloud optical depth, cloud 
fraction, cloud phase, cloud height, effective radius, etc., as well as aerosol properties [2,3]. The high-resolution 
cloud and aerosol properties are weighted by the CERES point-spread function to provide a spatially matched cloud-
aerosol-radiation dataset. The improved scene identification from the imager and the ability of CERES to optimize 
angular sampling of the 
radiation field through the RAP 
scan mode was used to produce 
improved Angular Distribution 
Models (ADMs) for converting 
measured radiances to radiative 
fluxes [4-7]. The imager-based 
cloud and aerosol properties 
and meteorological information 
from the Global Modeling and 
Assimilation Office (GMAO)'s 
Goddard Earth Observing 
System DAS (GEOS-DAS 
V4.0.3) product [8], are used as 
input to a radiative transfer 
model in order to compute 
radiative fluxes at the surface 
and at various levels within the 
atmosphere [9]. Since the 
satellites that carry CERES instruments sample a given location only a few times per day (e.g., twice per day at the 
 
Figure 1 CERES data processing flow. 
1
https://ntrs.nasa.gov/search.jsp?R=20060051633 2019-08-30T00:04:40+00:00Z
equator), 3-hourly geostationary data are used in conjunction with CERES in order to account for the influence of 
diurnal changes in the clouds and radiation in CERES monthly mean data products [10]. To produce its data 
products, up to 11 instruments on 7 spacecraft are all integrated to obtain climate accuracy in top to bottom fluxes. 
Clearly, it is not possible to summarize all of the exciting new results from CERES in a 3-page report. Therefore, 
we restrict the discussion to recent results that demonstrate how improvements in CERES ADMs have lead to 
improved TOA flux accuracy. Validation results include TOA flux self-consistency tests and comparisons with 
MISR albedos. We also compare global SW TOA flux anomalies with other datasets such as Earthshine and ISCCP. 
2. Results 
A useful consistency test of ADMs is to stratify regional or global mean TOA fluxes by satellite viewing zenith 
angle (Figs. 2a-d). Since radiative flux is by definition independent of viewing geometry, flux retrievals obtained by 
applying the ADMs at different angles should not depend on viewing geometry. As shown in Fig. 2, albedos based  
on the ERBE ADMs systematically increase by 4% and 8% with viewing zenith angle in January and July 2003, 
respectively. In contrast, albedos inferred from the CERES Terra ADMs show a smaller increase of 1%-2%. ERBE-
like LW TOA fluxes show a systematic decrease 
with viewing zenith angle of 2%-2.4%, whereas 
fluxes from the CERES Terra ADMs remain 
within 0.7%-0.8% at all angles.  
Figure 3 presents the first direct comparison 
of CERES and MISR albedos based on an 
analysis by [11]. In order to compare CERES 
and MISR albedos, an algorithm for converting 
MISR spectral albedos to broadband albedos is 
derived using a new merged CERES-MISR-
MODIS dataset [12]. For overcast 1°×1° ocean 
regions, the relative differences and the relative 
root-mean-square (RMS) differences between 
MISR and CERES albedos are ~0.81% and 
~4.29%, respectively. Since there is a ~2% error 
due to the narrow-to-broadband albedo 
conversion, the RMS differences between MISR 
and CERES albedos due to ADM differences 
alone is estimated to be smaller than ~3.8%.  
 Recently a number of publications have 
emerged showing contradictory changes in 
planetary albedo during the first part of the 21st century. Using ground-based measurements of Earthshine at the Big 
Bear Solar Observatory (BBSO) in California, [13], find that the Earth’s reflectance has increased by as much as 
0.018 between 1999 and 2003, corresponding to a change of ~6 Wm-2 in TOA radiative flux.  This approach 
estimates the Earth’s albedo using telescope measurements of the visible solar radiation that is first reflected by the 
Earth towards the Moon and then 
back from the Moon to an observer 
on the night-side of the Earth. The 
BBSO Earthshine measurements 
sample only ~1/3 of the Earth on 
any given day, and cover at most 
~1/3 of each lunar month provided 
the night-sky is cloud-free. A 6 
Wm-2 change is climatologically 
significant as it exceeds the 
radiative forcing caused by Mount 
Pinatubo in the early 1990s by a 
factor of 2.5, and is twice as large 
as the longwave radiative forcing 
by greenhouse gases since 1850. In  
Figure 2 SSF and ES8 global albedo against viewing zenith angle 
for (a) January and (b) July 2003. LW TOA flux against 
viewing zenith angle for (c) January and (d) July 2003. 
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
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0.25
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on
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Figure 3 (a) The monthly zonal mean of the MISR (black) and CERES (red) 
albedos for overcast ocean scenes in October 2001; (b) the monthly zonal 
mean of the relative RMS differences between the instantaneous 1o x 1o-
region-averaged MISR and CERES albedos. 
2
contrast, the global CERES observations 
show a very different result, as illustrated in 
Figure 4. In fact, none of the other methods 
considered (including ISCCP-FD, [14]) 
support the dramatic increase in reflectance 
obtained by BBSO Earthshine between 2000 
and 2003. The standard deviation in the 
BBSO Earthshine anomalies is a factor of 5 
greater than simulated Earthshine anomalies 
(using CERES data), and over an order-of-
magnitude greater than those from CERES 
global results. Interestingly, anomalies 
based on the simulated CERES Earthshine 
approach bear little resemblance to the 
CERES global results. In fact, the standard 
deviation in the simulated CERES Earthshine anomalies is a factor of 2 larger than the CERES global results. While 
ISCCP and CERES SW TOA flux anomalies also differ from one another from year to year, the standard deviation 
in the anomalies is within 40% of each other. 
3. Conclusions 
The CERES project provides cloud-aerosol-radiation data products for studying the influence of clouds and aerosols 
on the Earth’s radiation budget.  CERES goes well beyond previous radiation budget projects (e.g., ERBE) by 
merging broadband CERES radiative fluxes with coincident imager-based cloud and aerosol properties. The new 
algorithms created during CERES have lead to improved accuracy in TOA fluxes by a factor of 2-5 compared to 
ERBE. The additional imager data allow accurate estimates of radiative flux profiles within the atmosphere and at 
the surface. Merging CERES with geostationary data leads to improved characterization of diurnal variations in 
clouds and radiation which in turn leads to improved monthly mean radiative fluxes and cloud properties. Global 
shortwave radiative flux anomalies from CERES during the first 5 years of the 21st century are in stark contrast to 
those derived from ground-based measurements of Earthshine. The CERES data do not support the 6 Wm-2 increase 
in reflected solar radiation obtained from Earthshine. 
4. References 
 [1] P. Minnis, et al., "CERES Cloud Property Retrievals from Imagers on TRMM, Terra, and Aqua" Proc. SPIE 10th International Symposium 
on Remote Sensing: Conference on Remote Sensing of Clouds and the Atmosphere VII, Barcelona, Spain, September 8-12, 2003, 37-48. 
[2] L.A. Remer, et al., The MODIS Aerosol Algorithm, Products and Validation, J. Atmos. Sci., 62, 947-973 (2005). 
[3] A. Ignatov and L. L. Stowe, Aerosol retrievals from individual AVHRR channels: I. Retrieval algorithm and transition from Dave to 6S 
radiative transfer model. J. Atmos. Sci., 59, 313-334 (2002). 
[4] N.G. Loeb, et al., Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth's Radiant Energy 
System instrument on the Tropical Rainfall Measuring Mission Satellite. Part I: Methodology, J. Appl. Meteor., 42, 240-265 (2003). 
[5] N.G. Loeb, et al., Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth’s Radiant 
Energy System instrument on the Tropical Rainfall Measuring Mission Satellite. Part II: Validation, J. Appl. Meteor., 42, 1,748-1,769 
(2003). 
[6] N.G. Loeb, et al., Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth’s Radiant 
Energy System instrument on the Terra Satellite. Part I: Methodology. J. Atmos. Ocean. Tech., 22, 338-351 (2005). 
[6] N.G. Loeb, et al., Angular distribution models for top-of-atmosphere radiative flux estimation from the Clouds and the Earth’s Radiant 
Energy System instrument on the Terra Satellite. Part II: Validation. J. Atmos. Ocean. Tech., (in press) (2005). 
[8] M.J. Suarez, Technical report series on global modeling and data assimilation, Vol. 26: Documentation and validation of the Goddard Earth 
Observing System (GEOS) data assimilation system⎯Version 4, NASA/TM⎯2005⎯104606, Vol. 26, 181 pp., Washington, DC. (2005) 
[9] T.P. Charlock, et al., Compute Surface and Atmospheric Fluxes (System 5.0), CERES Algorithm Theoretical Basis Document. 84 pp., 
[Available from http://asd-www.larc.nasa.gov/ATBD/pdf_docs/r2_2/ceres-atbd2.2-s5.0.pdf] (1997). 
[10] D.F. Young, et al., Temporal interpolation methods for the Clouds and the Earth’s Radiant Energy System (CERES) experiment. J. Appl. 
Meteor., 37, 572-590 (1998). 
[11] W. Sun, et al., Comparison of MISR and CERES Top-of-Atmosphere Albedo, Geophys. Res. Lett., submitted (2006). 
[12] N.G. Loeb, et al., Fusion of CERES, MISR and MODIS measurements for top-of-atmosphere radiative flux validation, J. Geophy. Res., in 
press (2006) 
[13] E. Pallé, Changes in the Earth’s reflectance over the past two decades, Science, 304, 1299–1301, doi:10.1126/science.1094070 (2004). 
[14] Y. Zhang, et al., Calculation of radiative fluxes from the surface to top of atmosphere based on ISCCP and other global data sets: 
Refinements of the radiative transfer model and the input data, J. Geophys. Res., 109, D19105, doi:10.1029/2003JD004457 (2004). 
[15] N.G. Loeb, et al., Variability in Global Top-of-Atmosphere Shortwave Radiation between 2000 and 2005, Geophys. Res. Lett, submitted 
(2006). 
 
Figure 4 Global annual mean SW TOA flux anomalies from 
Earthshine BBSO, CERES Terra global fluxes, ISCCP-FD, and a 
simulation of Earthshine. 
Year
2000 2001 2002 2003 2004
SW
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Fl
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(W
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Earthshine BBSO
CERES Global
ISCCP-FD Global
CERES Earthshine Simulation
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Overview of the Clouds and the Earth's Radiant Energy System (CERES) Experiment

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