The Earth Radiation Budget Experiment (ERBE) included non-scanning radiometers (Luther, 1986) flown aboard a dedicated mission of Earth Radiation Budget Satellite, and the NOAA-9 and -10 operational meteorological spacecraft (Barkstrom and Smith, 1986). The radiometers first began providing Earth radiation budget data in November 1984 and have remained operational, providing a record of nearly 8 years of data to date for researchers. Although they do not produce measurements with the resolution given by the scanning radiometers, the results from the non-scanning radiometers are extremely useful for climate research involving long-term radiation data sets. This paper discusses the non-scanning radiometers, their stability, the method of analyzing the data, and brief scientific results from the data

Bess, T. Dale

Green, Richard N.

Lee, Robert B., III

Rutan, David

Smith, G. Louis

English

NASA Technical Reports Server

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NASA-T_-112534
Non-scanning Radiometer Results for Earlh Radiation Budget Invesligations
by
G. Louis Smith, Richard N. Green, Robert B. Lee, III and T. Dale Bess
Atmospheric Sciences Division
8SMOI FA I4.2
,- _/// .; ._ /"
Langley Research Center
llamplon, Virginia 23681-001
'J I : G'
and
David Rutan
Lockheed Engineering and Sciences Co.
144 Research Road
Hampton, Virginia 23666
Presenled a!
t
I
Eighth Symposium on Meteorological Observations and Instrumentation
Anaheim, California
January 17-22, 1992
https://ntrs.nasa.gov/search.jsp?R=19980023499 2020-06-16T00:53:37+00:00Z
Non.ScanningRadiometerResultsfor Earth Radiation
Budget Investigations
by
G. Louis Smith I, Richard N. Green 1, Robert B. Lee Ill 1, T. Dale Bess l, and David Rutan 2
1. Atmospheric Sciences Division, NASA, Langley Research Center, Hampton, VA 23681-0001
2. Lockheed Engineering and Sciences Co., 144 Research Road, Hampton, VA 23666
1. Introduction
The Earth Radiation Budget Experiment (ERBE)
included non-scanning radiometers (Luther, 1986)
flown aboard a dedicated mission of Earth Radiation
Budget Satellite, and the NOAA-9 and -10 opera-
tional meteorological spacecraft (Barkstrom and
Smith, 1986). The radiometers first began providing
Earth radiation budget data in November 1984 and
have remained operational, providing a record of
nearly 8 years of data to date for researchers.
Although they do not produce measurements with the
resolution given by the scanning radiometers, the
results from the non-scanning radiometers are
extremely useful for climate research involving long-
term radiation data sets. This paper discusses the non-
scanning radiometers, their stability, the method of
analyzing the data, and brief scientific results from the
data.
2. Description of Instruments
The non-scanning package, shown in Fig. l, con-
sists of a pair of wide field-of-view (WFOV) radiome-
ters, a pair of medium field-of-view (MFOV)
radiometers, all of which were Earth-viewing, and a
solar monitor. Each of these is an active cavity radi-
ometer. Each pair of Earth-viewing channels has a
total channel and a shortwave channel. The WFOV
p,i _ ta at _ ,
-_ oo*_ 1n,l,_ hl_,lll_ COVll I15
IAmX_ Vq_W
Figure 1. Non-scanning radiometer instrument.
channels view the Earth from one limb to the other.
The MFOV channels each have an aperture which
restricted its FOV to a circle with a 10 ° great circle
diameter. Figure 2 shows the sensor design for the
WFOV shortwave channel. In order to maintain the
accuracy of their calibration while in orbit, the Earth-
viewing channels are mounted on a beam which could
rotate so that the instruments can look at internal
blackbodies or through viewing ports at the Sun, as a
high intensity well-known source, and at space for a
near-zero radiance source. Thomas et al. (1992) have
shown that the non-scanning radiometers have been
quite stable, the shortwave channels having a gradual
degradation which can be accounted for to a fraction
of a percent by use of the on-board calibration system.
Figure 3 shows the change with time in mean outgo-
ing radiative flux at the top of the atmosphere (TOA)
averaged over the portion of the Earth which is
observed by the I_OV and WFOV non-scanning
radiometers and the scanning radiometer. It is seen
that with degradation accounted for by use of the on-
board calibration systems, the change is on the order
of 1 W/m 2.
RIFIRIN_
I t | VA'h_I _AT _NK
/ , \ \,,,,,,,,,
_ _[ t () t t_Jt"llt,lO O_,4t
Figure 2. Wide field-of-View shortwave
channel.
ERBE scanning and non-scanning radiometer
packages were placed aboard three spacecraft. In
October 1984, the Earth Radiation Budget Satellite
was launched into an orbit with an inclination of 57 °
and an altitude of 610 krn, so that it precesses around
the Equator every 72 days. The NOAA-9 operational
spacecraft was placed into a near-polar (99 ° inclina-
tion) Sun-synchronous orbit with an altitude of 812
tc
8
[
/
#e ," _.j/ _.
.i.
_ SCANNER _ WkW _ MLW J
i
Figure 3. 12 months running mean at top of atmo-
sphere for outgoing longwave radiation for ERBS.
ien and an Equator crossing ume of 1430 hours
(ascending node). The NOAA-10 operational space-
craft was launched in October 1986 into a similar
orbit (830-kin altitude and 990 inclination) except for
the Equator crossing time of 0730 hours. All three
sets of non-scanning radiometers have operated con-
tinuously. At the time of this writing, all three space-
craft are operating well.
Figure 4 shows the availability of radiation mea-
surements over the years. ERBE was proceeded by
the Nimbus 6 and 7 ERB WFOV radiometers, which
gave a low-resolution data record from ]975 through
1987 (Kyle et al., 1985), so that a i 7-year record of
low-resolution radiation data is available at present
for studies, e.g., of interannual variations. The Clouds
and Earth Radiant Energy System (CERES) aboard
the Tropical Rainfall Measurement Mission (TRMM)
is scheduled to begin providing scanning radiometer
R1),fllU$ "/S_,X'R_.R
NIMBUS 6 I. ? WI_V
hvO_, 9 SCAt_F..R
I_OAA 10 $_
_fJI[ I',ION SCA_I'F_S
$C._It_J
! I
Figure 4.
y f.,AIt
Time line of Earth radiation budget mea-
surements.
data in 1997 over the Tropics, and the CERES aboard
the first EOS platform should begin operation in 1998.
In the meantime, the Franco-German-Russian SCAn-
ning RAdiation Budget (SCARAB) radiometer (Kan-
del et al., 1992) is scheduled to fly aboard a Russian
METEOR spacecraft in 1993 to flu the scanning-radi-
ometer gap. However, most of the long-term radia-
tion data record is from non-scanning radiometers.
3. Earth Viewing Data Analysis
The measurement equation for the non-scanning
radiometers is
m(rs) = _ Lcosadfl
F V
where m (r) is the measurement at the spacecraft
s
position r , L (r s, v) is the radiance at the space-s
craft from all directions within the FOV, tx _s the
nadir angle of the incoming radiance, and dfl is the
differential solid angle. For the W'FOV radiometers,
the FOV is the entire Earth scene, from limb to limb.
For the MFOV radiometers, the FOV is limited by an
aperture to a circle with a I 0° diameter great circle arc
at the "IDA. The Iongwave measurement is the total
radiometer measurement minus the shortwave radi-
ometer measurement. The radiance is related to the
flux M(r) at TOA by the bidirectional reflectance
function for shortwave and the limb-darkening func-
tion for longwave. These functions vary with the
scene type and view angles and are denoted here as
R(r,u), where u denotes the unit vector from r to h.
The measurement equation can thus be written as
m(rs) =n-1 f M(r) R(r,u)cosotdt"i
F O V
For the shortwave case, the TOA flux is given in
terms ofalbedo a (r) , so that the measurement
equation for the shortwave case becomes
ms(rs) = n-lSF_Va(r)R(r'u)C°SqC°sadf'
where S is the solar oulput, i.e., the total solar irradi-
ance normalized to mean Earth-Sun distance, q is the
solarzenith angle at r, and the integration is over the
sunlit FOV. The data analysis problem is to compute
M(r) from m (r°) for thelongwave caseand a (r)
for the shortwav_ case. For ERBE, the Earth viewing
non.scanning radiometer data have been analyzed
using two methods: shape factor and numerical filter.
3.1 Shape Factor Method:
The shape factor method assumes that M(r) is con-
stant over the FOV for the longwave case and that
a (r) is constant for the shortwave case. For the
longwave case, the measurement equation simplifies
to
m L(r s) = SFLM(rs)
= n-IM (rs) _ R (r, u) cosctdf2F V
and for the shortwave case the measurement equation
becomes
m s(r s) = SFsa(r s)
= n-ISa (rs) I R (r, u) cosqcosadxq
FOV
where SF L and SF s are the longwave and shortwave
shape factors. For the WFOV, if R (r, u) is inde-
pendent of location, the longwave shape factor is the
inverse of the square of the ratio of the orbit radius to
the Earth's radius. During the period of time for
which the scanning radiometers operated, scene iden-
tification data from the scanning radiometers
(Wielicki and Green, 1989) were used to determine
the appropriate R (r, u) for computation of the
shape factors. The longwave shape factor computed
in this manner varied by only .25%, so the longwave
shape factors were taken to be constants for data pro-
cessing.
Studies were conducted by Green and Smith
(1991) in which scenes of various types but constant
over the FOV were assumed for the computation of
the shortwave shape factors. Shape factors computed
using the bidirectional reflectance function for mostly
cloudy scene over ocean were found to produce short-
wave fluxes at TOA that agreed with .scanning radi-
ometer results with a bias less than 1 W/rn 2 and with
standard deviations comparable to those obtained
with the scene-dependent shape factor. This result is
reasonable since the Earth's surface is three fourth
ocean and since clouds do most of the reflecting on a
global-wide basis. Non-scanning radiometer results
have thus been processed in this manner after the ces-
sation of the scanning radiometer on each spacecraft.
The difference between the monthly mean shortwave
maps (at 10%!0 ° resolution) using the mostly-
cloudy-over-ocean scene identification rather than the
full scene identification algorithm is less than 1 W/m 2
for most of the globe, with a'few regions for which the
difference is 1 to 2 W/m2; there are three regions in
which the difference is 4 W/m 2 for the medium FOV
radiometers. The WFOV results are less sensitive. A
study by Green (1983) showed that the resolution of a
WFOV radiometer at an altitude of 820 km corre-
sponds to a circle with a radius of 4o. Extensive vali-
dation studies were conducted for the scanning and
non-scanning radiometers by Green et at. (1990).
3.2 Numerical Filler Method:
The numerical filter method is a one-dimensional
resolution enhancement technique conceived by
House and Jafolla (1980) as an improvement over the
inherently low resolution of the shape factor
approach. This approach treats the measurement
equation as an integral equation to be solved for M(r)
and a (r) . Because data are only available along the
orbit track for periods of less than one orbit and no
data are available in the cross track direction, the
equations are considered to be one- dimensional and
the solutions for M(r) and a (r) are expressed as
integrals over the orbit track. These integrals are dis-
cretized and stabilized, thereby producing the numeri-
cal fihcrs, which are written in the form
N
Mj = E _imj+i
i=-N
Here, Mj is the retrieved instantaneous flux at TOA,
mi, j are the measurements and the m. are filter
weights. For ERBE, N = 6. 'measurements are
taken to be 32-second averages of the radiometer out-
put, which is sampled every 0.8 seconds. The effects
of bidirectional reflectance function dependence on
scene identification for these filters woe studied as
for the shape factor case, with similar results. The
instantaneous TOA longwave fluxes and albedos from
the shape factor and numerical filter analyses are
azchived for scientific investigations on magnetic
tapes, denoted as S-7 data products.
4. Monthly MeanMaps
The TOA fluxes and albedo values obtained by
each technique are compiled for each calender month,
and the monthly mean outgoing longwave radiation
and albcdo are computed over the Earth, as described
by Brooks et al. (1986), with modifications which
were developed as a result of extensive validation
investigations. The resulting monthly mean maps of
absorbed solar radiation, based on these albedos, have
been compared with scanning radiometer results by
Rutan and Smith (1991). Rutan and Smith (1991)
found that the accuracy of the monthly mean map
could be improved by reconstructing the map with
only the lowest 12 Fourier waves in longitude, This
result is due to the sampling pattern in space and time
for a non-scanning radiometer, which causes the
major error source for a monthly map to be time vari-
ations rather than the lack of resolution. The sam-
pling problem for a wide FOV radiometer has been
studied by Salby (1988a and b)o The radiation field
for a day has a number of short-range transient fea-
tures, whereas the monthly mean radiation field of the
Earth is fairly smooth. Moreover, these transients
move primarily in a west-cast or east-west direction,
so that longitudinal variations tend to be small (there
are obvious exceptions in the major Equatorial con-
vective regions and near coastlines). The RMS
50
4O
E
m2o
10'
--- l)eco=tvolvcd
......... Numerical Filter
--- NF Wave O-It
GlobM RMS
t 0.3 _T/J_ =
1.1.2 /mr
9,6 W/m
I;
• ...\
i'\.
O-gO" '' " " ' "' " ' ..... : '" "-at o a'o 6o oo
Latitude
Figure 5. Zonal comparison of shortwave zonal
averages.
accm'acy of the non-scanner results vary with Sun-
scene-satellite geometry for shortwave radiation, and
its variation with latitude is shown in Fig. 5. Globally,
with the Fourier filter eliminating longitudinal wave-
numbers higher than 11, the RMS deviation of the
numerical filter results from the scanninj_ radiometer
results at 5°x5 ° resolution was 9.6 W/re'= and the
RMS deviation of the zonal means was 3.5 W/m 2.
The algorithm far the monthly mean maps has been
modified and now places the derived fluxes not only
in the 5°x5 a grid-box containing the subsatellite point,
but also in the adjacent grid-boxes in the crosstrack
direction. These monthly maps are archived on mag-
netic tapes as S-10 data products.
5. Solar Output Measurements
The ERBE solar monitors have been shown to have
an accuracy of 0.2%, with a precision better than
0.02% (Lee et al., 1991). Figure 6 shows the solar
irTadiance decreasing from 1984, when ERBE began
producing data, until 1986, which was the minimum
sunspot number and solar magnetic activity for sun-
spot cycle number 2 I. There was an increasing trend
in solar irradiance from 1986 through 1989, when the
maximum magnetic activity and sunspot numbers
occurred for sunspot cycle number 22. The variation
of solar irradiance over the measurement period is
0.1%.
736J
r_ 136'E
LOt 366
U
1365
iv- 1564
rr"
_ 363
o
U3 t 362
1361
o
0%. ° o0._,:=._'_%-" *o" , "
i o
G
0
J OJ A JOJ AJ OJA JOJ AJ OJ AJ OJ AJOjAJ OJAJ
84 '85 186 '8'7 '88 'SQ 'QO '91 '92
Figure 6. Solar irradiance based on ERBS
solar monitor.
5. Conclusions
The ERBE non-scanning radiometers have pro-
vided a 7-year record of Earth radiation and solar out-
put measurements thus far. The response of these
radiometers are very stable. The monthly mean maps
which are produced from the data have been shown to
agree with the scanning radiometer results to 9.6 W/
m 2 for 5°x5 ° resolution and to 3.5 W/m 2 for zonal
means.Thetime period of this record includes two
ENSO events and a volcano to date. Measurements
of solar output cover much of one solar cycle and
agree very well with other measurements.
6. Data Availability
The scientific data products for the non-scanning
radiometers, which are designated S-2, S-7 and S-10,
are available from the National Space Science Data
Center at the Goddard Space Flight Center of NASA,
Greenbelt, Maryland, 20771.
References
Barkstrom, B. R. and G. L. Smith, 1986: The Earth
Radiation Budget Experiment: Science and imple-
mentation, Rev. of Geophys., 24,379-390.
Brooks, D. R., E. F. Harrison, P. Minnis, J. T.
Suttles and R. S. Kandel, 1986: Development of
algorithms for understanding the temporal and spa-
tial variability of Earth radiation balance, Rev.
Geophys., 24, 422-438.
Green, R. N., 1983: Accuracy and resolution of Earth
radiation budget estimates, J. Atmos. Sci., 40,977-
-985.
Green, R. N. and G. L. Smith, 1991: Shortwave shape
factor inversion of Earth radiation budget observa-
tions, J. Atmos. Sci., 48,390-402.
Green, R. N., E B. House, P. W. Stackhouse, X. Wu,
S. A. Ackermann, W. L. Smith and M. J. Johnson,
1990: Intercomparison of scanner and nonscanner
measurements for the Earth Radiation Budget Ex-
periment (ERBE), J. Geophys. Res., 95, 11,785-
11,798.
House, E B. and J. C. Jafolla, 1980: One-dimensional
technique for enhancing Earth radiation budget
observations from Nimbus 7 satellite, 1980 Inter-
national Radiation Symposium, Fort Collins, Colo-
rado.
Kandel, R. S., J. L. Monge, M. Viollier, L.A. Pakho
mov, V. 1. Adasko, R. G. Reitenbach and E. H.
Raschke, 1992: The SCARAB Project: Earth Radi-
ation Budget observations from the METEOR sat-
ellites, Presented at 29th COSPAR Plenary Meet-
ing, Washington, D. C., U.S.A., August/September.
Kyle, H. L., P. E. Ardanuy and E. J. Hurley, 1985:
The status of the Nimbus 7 Earth radiation budget
data set, Bull. Amer. Met. Soc., 66, 1378-1388.
Lee, R. B., III, M. A. Gibson, S. Thomas, J. R.
Mahan, J. L. Meekins and N. E. Tira, 1990: Earth
Radiation Budget Experiment scanner radiometric
calibration results. Long-Term Monitoring of the
Earth's Radiation Budget, 1299, pp 80-91, Soc.
Photo-optical Instrum. Eng., April 17 -18, Orlando,
Florida.
Lee, R. B. IIl, M. A. Gibson, N. Shivakumar, R. Wil-
son, H. L. Kyle and A. T. Mecherikunnel, 1991:
Solar irradiance measurements: Minimum through
maximum solar activity, Metrologla, 265-268.
Luther, M. R., J. E. Cooper and G. R. Taylor, 1986:
The Earth Radiation Budget Experiment nonscan-
ning instrument, Rev. of Geophys., 24,391-399.
Rutan, D. and G. L. Smith, 1991: Shortwave wide
field-of-view results from the Earth Radiation Bud-
get Experiment, Proc. Seventh Syrup. Met. Obs. &
lnstr., pp. 160-165, New Orleans, Louisiana, Jan.
13-15.
Salby, M., 1988a: Asynoptic sampling considerations
for wide field-of-view radiometer measurements of
outgoing longwave radiation. Part I: Spatial and
temporal resolution, J. Atmos. Sci., 45, 1176-
I1183.
Salby, M., 1988b: Asynoptic sampling considerations
for wide field-of-view radio meter measurements of
outgoing Iongwave radiation. Part II: Diurnal and
random space-time variability, J. Atmos. Sci., 45,
1184-1204.
Thomas, S., R. B. Lee, M. A. Gibson, R. S. Wilson
and W. C. Bolden, 1992: In-flight shortwave cali-
bration of the active cavity radiometers using tung-
sten lamps. Instrumentation for Planetary and Ter-
restrial Atmospheric Remote Sensing, 1745, pp
227-234, Soc. Photo.optical Instrum. Eng., July
23-24, 1992, San Diego, California.
Wielicki, B. A. and R. N. Green, 1989: Scene identi-
fication for ERBE radiative flux retrieval, J. Appl.
Met., 28, 1133-1146.


Non-Scanning Radiometer Results for Earth Radiation Budget Investigations

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