Data Product: ERBE-like Monthly Regional Averages (ES9)
Data Set: TRMM
Data Set Version: Edition1
The purpose of this document is to inform users of the best current
understanding of the accuracy of this CERES data product, to briefly summarize
key validation results, to provide cautions where users might easily
misinterpret the data, to provide helpful links to further information about
the data product, algorithms, and accuracy, to give information about planned
data improvements, and finally to register users of this data product so that
we can automate the process of keeping users informed of new validation
results, cautions, or improved data sets that become available in the future.
This document is a high-level summary and represents the minimum information
that all scientific users of this data product should be familiar with. We
strongly suggest that users re-check this document for the latest status before
publication of any scientific papers using this data product: this would apply
to both authors and reviewers of such research papers.
The quality of the CERES TRMM ES9 data is comparable to the quality of the
ERBE ERBS single-satellite S9 data in terms of instantaneous gridded and
monthly 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
[ES9] data set version Edition1. Additional information is in
the Description/Abstract
Guide. The CERES ES9 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 ES9 files contain both the temporally averaged and the
instantaneous gridded mean 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.
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 ES9."
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 Angular Distribution Models construction.
The TRMM orbit 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 S9 data product is a binary file of about 75 MB. The CERES
TRMM ES9 product is an HDF file of about 72 MB.
There are several cautions the CERES Team notes regarding the use of the ES9
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 (MLE) in conjunction with the ERBE
angular distribution models (ADM) 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 micron. CERES uses silver mirrors,
which offer much 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 use of
the CERES clear ocean radiances and shortwave fluxes to study aerosol radiative
effects over ocean backgrounds. CERES unfiltered clear-sky ocean radiances can
vary by up to 4% for very thin or thick aerosol conditions. As a result TOA
clear ocean albedos may vary by up to 0.005. Future improved spectral
corrections for clear ocean conditions are expected to reduce this uncertainty
to 1% or less: i.e. albedo uncertainty of 0.001 or less, but 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 that we can note are:
The substantial and widespread increase in ocean temperature due to the
strong 1998 El Niño event that 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
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-2 depending 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 to 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 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
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 has essentially disappeared.
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 0.2 Wm-2 (1.8%), 1.0 Wm-2 (6.1%), and
1.8 Wm-2 (8.7%) lower than ERBE for ocean, land and desert
regions, respectively. The clear ocean difference is reduced to
0.0 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.
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 July is decreased to 22%
vs. 16%. Much of the remaining difference can be attributed to the smaller
CERES footprint size.
The CERES team expects to reprocess the S9 data product for ERBS, NOAA 9,
NOAA 10, and the ES9 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: 07/27/1999; 01/05/2000; 01/13/2000; 04/28/2000;
06/22/2000, 12/20/2000 (non-science related update), 08/27/2001
(non-science related update); Dec 12, 2001
Most Recent Modification: December 12, 2001