The purpose of this document is to inform users of the accuracy of this data
product which has been determined by the CERES Team. This document briefly
summarizes key validation results, provides cautions where users might easily
misinterpret the data, provides helpful links to further information about
the data product, algorithms, and accuracy, gives information about planned
data improvements, and, finally, automates registration in order to keep
users informed of new validation results, cautions, or improved data sets
as they become available.
This document is a high-level summary and represents the minimum information
for scientific users of this data product. It is strongly suggested that
authors, researchers, and reviewers of research papers re-check this document
for the latest status before publication of any scientific papers using
this data product.
The quality of the CERES TRMM ES4 data is comparable to the quality of
the ERBE ERBS single-satellite S4 data in terms of monthly regional, zonal,
and global mean fluxes and scene identification. The major differences between
CERES/TRMM and ERBE/ERBS are the field of view resolution, the spectral
response of the instruments, the inclusion of rotating scanner plane data in
the CERES product, and the tropical-only coverage of CERES/TRMM.
This document discusses the ERBE-Like Science Product
[ES4] data set version Edition1. Additional information is in
the Description/Abstract
Guide. The CERES ES4 data product contains the "ERBE-like"
temporally and spatially averaged shortwave (SW) and longwave (LW)
top-of-the-atmosphere (TOA) fluxes derived from one month of CERES data from
the Tropical Rainfall Measuring Mission (TRMM) spacecraft. Instantaneous TOA
fluxes from the ES8 product have been spatially averaged on the same 2.5°
equal-angle grid used by the Earth Radiation Budget Experiment (ERBE). Temporal
interpolation algorithms identical to those used by ERBE have been applied to
produce daily, monthly-hourly, and monthly mean fluxes from the instantaneous
gridded data. The ES4 contains the temporally averaged values of TOA total-sky
LW, total-sky SW, clear-sky LW, and clear-sky SW flux, total-sky albedo and
clear-sky albedo for each 2.5° region observed during the month. In
addition, the 2.5° regional means have been combined to produce 5°
regional, 10° regional, 2.5° zonal, 5° zonal, 10° zonal, and
global mean fluxes.
When referring to a CERES data set, please include the satellite name and/or
the CERES instrument name, the data set version, and the data product. Multiple
files which are identical in all aspects of the filename except for the 6 digit
configuration code (see Collection Guide) differ little, if any,
scientifically. Users may, therefore, analyze data from the same
satellite/instrument, data set version, and data product without regard to
configuration code. This data set may be referred to as "CERES TRMM
Edition1 ES4."
The resolution of CERES TRMM is 10 km at nadir and the resolution of ERBE
ERBS is 40 km at nadir so that the surface area observed by ERBS is 16 times
larger than the area observed by TRMM.
The nominal scan mode for ERBE was crosstrack to provide good area
coverage. TRMM has two scan modes. The Fixed Azimuth Plane scan mode is similar
to ERBE. The Rotating Azimuth Plane (RAP) scan mode was added to TRMM to
provide angular coverage for construction of Angular Distribution Models (ADMs).
TRMM is in a low inclination (35°) orbit that precesses through all
local times in 46 days. The ERBS had an inclination of 57° and a
precessionary period of 72 days.
The longwave channel on ERBE was replaced by an 8 to 12 µm window
channel on TRMM.
The data rate on ERBS was 30 measurements per second. The data rate on
CERES is 100 measurements per second.
The ERBE ERBS S4 data product is a binary file of about 15 MB. The CERES
TRMM ES4 product is an HDF file of about 27 MB.
There are several cautions the CERES Team notes regarding the use of the ES4
TRMM Edition1 data:
CERES TRMM is observing more clear sky than ERBE due in part to the
difference in footprint size. The resolution of TRMM is 10 km at nadir and the
resolution of ERBS is 40 km at nadir so that the surface area observed by ERBS
is 16 times larger than the area observed by TRMM. For the time period of
January through July, ~17% of ERBS footprints and ~28% of TRMM footprints are
classified as clear-sky. ERBS also observed about 17% overcast and TRMM
observed about 16% overcast. It is not fully understood why the overcast for
TRMM decreased instead of increasing as for clear sky. Overall, the cloud
fraction was 46% for ERBS and 40% for TRMM.
The ERBE scene identification algorithm (Maximum Likelihood Estimator,
MLE) in conjunction with the ERBE angular distribution models (ADMs) are known
to erroneously produce albedo growth from nadir to the limb. The ERBE ADMs are
probably insufficiently limb-darkened in longwave and insufficiently
limb-brightened in shortwave. The TRMM fluxes also have these biases with
viewing angle.
The spectral response of the CERES shortwave and total channels differs
from that on ERBE at wavelengths below 1 µm. CERES uses silver mirrors,
which offer a more uniform spectral response from 0.4 µm to 100 µm
than the ERBE aluminum mirrors, but are less responsive below 0.4 µm. The
spectral correction has therefore been modified from that on ERBE to account
for these differences. As a result, the CERES radiances are less sensitive to
spectral correction for land, desert, and cloudy scenes. The ERBE radiances are
less sensitive than CERES for clear-sky ocean. Further studies are underway to
evaluate the impact of spectral correction on the use of the CERES clear ocean
radiances and shortwave fluxes to study aerosol radiative effects over ocean
backgrounds. The current spectral correction algorithm over ocean slightly
overestimates unfiltered SW radiance for large optical depths and slightly
underestimates the SW radiance for low optical depths. The net effect is likely
to cause an overestimate of aerosol radiative forcing of roughly 10%. In other
words, if the time-averaged SW radiative forcing for ocean aerosols is
2 Wm-2, the current ERBE-Like spectral correction algorithm
will cause the value to incorrectly increase to 2.2 Wm-2.
Improved spectral correction methods are under development, and for clear ocean
conditions are expected to reduce this uncertainty by a factor of 5 to 10;
however, the current ERBE-Like products do not include this improvement.
The TRMM spacecraft is in a 46-day precessing 35° orbit that is
designed to provide good coverage of the tropics. For regions poleward of
20°N and 20°S, the temporal sampling patterns are very different from
ERBS. In general, extratropical regions are viewed in daytime only during part
of the month and nighttime during the remainder. The typical ERBE sampling
pattern of alternating day and night observations only occurs in the tropics
with TRMM. Users should be aware that this temporal sampling can cause:
Large regional bias errors due to not sampling all local times during a
month. These errors can be reduced by a factor of 2 by using seasonal means
instead of monthly means.
Large errors in the modeling of diurnal variations of flux, particularly
for extratropical land and desert regions.
Insufficient coverage for calculating global means since there are no
data poleward of ±45°.
The CERES TRMM instrument has operated in a standard mode of 2 days of
crosstrack scanning followed by 1 day of rotating azimuth plane (RAP) scanning.
Both the crosstrack and RAP data have been used in the computation of CERES
monthly mean fluxes. ERBE data were exclusively crosstrack.
The Earth may have real variations in longwave and shortwave radiation
properties between the ERBE time period and the CERES TRMM time period. The
major factors noted are:
The substantial and widespread increase in ocean temperature due to the
strong 1998 El Niño event lies outside the range of conditions
encountered in the ERBE time period
Systematic changes in tropospheric water vapor between the 1998 El
Niño period and the ERBE period may have an influence on LW fluxes
Errors in scene identification due to the use of climatological values
for LW cloud thresholds that are inadequate for strong El Niño events.
Increased temperatures in the tropics will be interpreted as less cloud which
will introduce errors in the inversion from radiance to flux.
The possible darkening of some deserts owing to increased rainfall early
in 1998, again owing to El Niño
The potential changes in radiation over the tropics due to smoke from
fires in exceptionally dry forests, where the smoke may be confused with
clouds
The CERES Team has performed the following validation and quality assurance
processes on this data set:
Pre-Launch
The CERES ERBE-like operational code has been tested for consistency with
the historical ERBE algorithm. The CERES code was run using ERBE data as input.
Monthly mean SW and LW fluxes have been calculated that reproduce ERBE values
to better than 0.1%.
An error analysis of spatial averaging and temporal interpolation errors
has been performed using one month of 1-hourly, 4-km GOES data. In summary:
Spatial errors have been computed using simulated CERES footprints
constructed by convolving the GOES pixels with the CERES point spread
function. These footprints can be averaged on a grid and compared with
regional averages of the GOES pixels. Currently, results are only available
for the CERES 1.0° grid. For crosstrack data, the rms SW and LW flux
spatial gridding errors are 10.1 Wm-2 (5%) and 2.3 Wm-2
(1%), respectively, with no bias error for either. Errors for RAP data
are twice as large with SW errors of 23.1 Wm-2 and LW errors of
5.6 Wm-2. Currently, the best estimate for instantaneous gridding
error for the 2.5° ERBE-like grid is given by Stowe et al., (J. of Atmos.
& Ocean. Tech, 1994). For CERES-like footprints, Stowe et al. calculated
crosstrack errors of ~8.5 Wm-2 and ~1.3 Wm-2 for SW and
LW, respectively.
Temporal errors were calculated by temporally sampling GOES data
and comparing monthly means computed from these data with means from the
complete time series. SW and LW rms monthly mean errors are <11
Wm-2 (<12%) and <5 Wm-2 (<2%),
respectively. Bias errors for LW are < 0.5Wm-2. For SW, mean
biases can be ±3 Wm-2depending on the particular TRMM
sampling pattern for the month. The effects of the spatial gridding errors
on monthly mean errors are negligible in the LW and only increase monthly
SW rms errors by ~0.5 Wm-2.
Post-Launch
The CERES ERBE-like data have been compared with ERBS non-scanner data for
verification of calibration. Tropical monthly mean ocean total-sky LW fluxes
have been averaged for all available months of ERBS scanner (1/85 - 12/89),
ERBS non-scanner (1/85 - 2/98), SCARAB scanner (3/94 - 2/95), and CERES scanner
(1/98 - 2/98) data. Scanner and non-scanner differences for each of the 3
scanners agree to < 1%. In addition, instantaneous CERES ERBE-like fluxes
have been compared with ERBS non-scanner data. Preliminary comparisons using
data from January and February 1998 have demonstrated agreement to better than
1% for both LW flux at night and SW flux. However, additional data are
necessary to establish agreement within CERES error limits due to limited
sampling. (ERBS non-scanner data from other 1998 months are expected to become
available late in 1998).
Directional models of the variation of albedo with solar zenith angle (SZA)
have been constructed using CERES TRMM and ERBE ERBS data for each of the 12
ERBE scene types. Comparisons of these models reveal no statistically
significant differences.
Six months of instantaneous rotating azimuth plane (RAP) and crosstrack
fluxes have been averaged as a function of SZA and scene type. These fluxes
agree to <1% in all cases with no statistically significant biases.
Seasonally averaged regional fluxes computed from crosstrack data alone and
from combined RAP and crosstrack data also show no systematic biases.
The first seven months of CERES ERBE-like data have been compared with the
historical ERBE ERBS scanner data from 1985-1989. The emphasis of this study
has been on comparisons of tropical mean fluxes (defined as the average of all
regions between 20°N and 20°S) in order to minimize temporal sampling
differences.
The main results include:
Total-sky LW flux - CERES LW fluxes are 5-10 Wm-2 (2-4%)
higher than ERBE. The difference maximizes in February, which is also the
maximum of the 1998 El Niño event. The difference is minimized in July
when El Niño had essentially disappeared. As explained above, a similar
increase in total-sky LW flux from ERBE (1985-1989) to 1998 is also seen in the
ERBS non-scanner data.
Clear-sky LW flux - The CERES clear-sky LW fluxes are
1-3.5 Wm-2 (0.3-1.2%) higher than ERBE. This difference also
maximizes in February and minimizes in July. The differences have been shown to
be consistent with variations in sea surface temperature and atmospheric
humidity associated with El Niño.
Total-sky SW flux - The difference between CERES and the 5-year mean
ERBE data varies between +0.3 and -5 Wm-2 (+0.3 and -5%).
However, the 2σ bound for the month-to-month temporal sampling
variability of the total-sky SW tropical mean for this time period is 5%.
Therefore, the observed difference is within the temporal sampling error
limits.
Clear-sky SW flux - The difference between CERES and ERBE in
clear-sky SW flux varies with geographical scene type. CERES fluxes are on the
average 0.8 Wm-2 (1.8%), 4.1 Wm-2 (6.1%), and
7.3 Wm-2 (8.7%) lower than ERBE for ocean, land and desert
regions, respectively. In January, the clear ocean difference can be reduced
from -1.5 Wm-2 to +0.2 Wm-2 when the CERES
spatial resolution is reduced to simulate the ERBS field of view. The land and
desert differences are reduced only slightly by changing the spatial
resolution. The archived ES4 products were produced using the full resolution
CERES data.
Scene identification - In general, CERES classifies more footprints
as clear than ERBE. This difference is also greatest in February with CERES
classifying 33% of the observations as clear, while ERBE classifies only 20% as
clear. The difference in August is decreased to 25% vs 17%. Only 2-3% of the
remaining difference can be attributed to the smaller CERES footprint size.
The CERES team expects to reprocess the S4 data product for ERBS, NOAA-9,
NOAA-10, and the ES4 data product for TRMM. The purpose of the reprocessing is
to generate a consistent, long-term climate record where advances in the data
calibration and processing will be incorporated to remove former errors. The
major contributions to reprocessing will be an improved set of Angular
Distribution Models based on CERES data and the MLE as the scene identifier.
Other improvements will be more accurate scanner offsets for NOAA-9 and
NOAA-10, correction of the low daytime longwave flux for NOAA-9, drift
corrections, and a possible resolution correction for CERES so that CERES and
ERBS footprints will be similar in size.
The CERES Team has gone to considerable trouble to remove major errors and
to verify the quality and accuracy of this data. Please provide a reference
to the following paper when you publish scientific results with the data:
Wielicki, B. A., B. R. Barkstrom, E. F. Harrison, R. B.
Lee III, G. L. Smith, and J. E. Cooper, 1996: Clouds and the Earth's Radiant
Energy System (CERES): An Earth Observing System Experiment,
Bull. Amer. Meteor. Soc., 77, 853-868.
When Langley DAAC data are used in a publication, we request the following
acknowledgment be included:
"These data were obtained from the NASA Langley Research
Center EOSDIS Distributed Active Archive Center."
The Langley Data Center requests a reprint of any published papers or
reports or a brief description of other uses (e.g., posters, oral
presentations, etc.) of data that we have distributed. This will help us
determine the use of data that we distribute, which is helpful in optimizing
product development. It also helps us to keep our product-related references
current.
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Document Creation Date: October 21, 1998
Modification History: 07/22/1999; 02/04/2000; 04/28/2000; 06/22/2000;
11/21/2000; 08/27/2001 (non-science related update)
Most Recent Modification: December 12, 2001
