Measurements of longwave radiation from the Earth Radiation Budget (ERB) experiment aboard the Nimbus 6 spacecraft have been analyzed to show the annual cycle of the distribution of Earth emitted radiation

Bess, T. D.

Smith, G. L.

English

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Paper No. 49
THE ANNUAL CYCLE OF EARTH EMITTED RADIATION DISTRIBUTION
T. D. Bess and G. L. Smith, Langley Research Center, Hampton, Virginia
ABSTRACT
Measurements of longwave radiation from the Earth
Radiation Budget (ERB) experiment aboard the Nimbus 6
spacecraft have been analyzed to show the annual cycle
of the distribution of Earth emitted radiation
INTRODUCTION
The Earth Radiation Budget (ERB) instrument on the Nimbus 6
and Nimbus 7 spacecraft has provided a wealth of information
about the Earth's radiation field (ref. 1). The instrument has
wide-field-of-view (WFOV) radiometers for measuring total radia-
tion leaving the Earth and shortwave (solar) radiation reflected
by the Earth (ref. 2), so that longwave radiation emitted by the
Earth can be determined. In this paper, results are presented
for the annual cycle of longwave radiation distribution. The
radiation field q(0,_,t) at the top of the atmosphere may be
described in the form
q(e,d_,t) =Z Chin(t)Ynm(e'qb)
m,n
where @ is the colatitude, _ is the longitude of a point
t is time. The coefficients C_(t) serve as a method of
describing the time dependent distribution of emitted radiation.
.... r+_ have been caicuiated from the ERB WFOV data.
Because the WFOV radiometers measure radiation flux
incident from all directions onto a flat plate, the problem of
relating these measurements to the radiation at the top of the
atmosphere is one of resolution enhancement, or deconvolution.
The theory of deconvolution has been developed for Earth emitted
radiation (refs. 3 and 4) and has been applied to Nimbus 6 ERB
data (refs. 5 through 7). The longwave deconvolution technique
provides the C_ directly. For the results presented here
the C_ were computed using a nominal limb darkening function.
297
https://ntrs.nasa.gov/search.jsp?R=19790012511 2020-03-20T18:30:59+00:00Z
RESULTS
The C_{t) have been computed for 1 month averages for 1
year of Nimbus 6 ERB data, from July 1975 through June 1976. The
axisymmetric terms {m = 0) dominate up to n = 5, beyond which
the pattern appears random. The magnitudes of C_(t) decrease
rapidly with increasing n. The results are shown in figures 1
and 2 for C_(t) for n = 0 through 12.
Figure 1 shows the yearly cycle of C_(t), C;(t) and C_(t).
The C_(t) term is the global average emitted radiation. The
C_(t) ter_ is a measure of hemispherical differences, or pole to
pole differences. Its annual cycle is seen to have a nearly
perfect sine shape, with a total range between its minimum and
maximum values of 20 W/m 2. It does not oscillate about zero, but
has a bias of 2-1/2 W/m 2. If the Earth were symmetric about the
o
equator, one would expect the Cn terms for odd n values to
have time histories_which are symmetric about the time axis.
Thus, the 2ol/2 W/m 2 bias in the annual cycle of C_ is due to
land/ocean distribution differences between the Northern and
Southern Hemispheres. The C_(t) term may be considered to be a
measure of equator to pole gradient. It is seen to have an
average value of approximately - 26 W/m 2, with a small variation
which appears to be nearly semiannual.
The C_(t) for n = 3 through 12 are shown in figure 2.
The C_ is nearly sinusoidal, wi_h a total variation of 15 W/m 2
and a mean of approximately 4 W/m _. As with C_, this bias is a
result of hemispheric differences of land/ocean distribution.
The C_ term has a mean of approximately - 7 W/m 2 with variation
of 5 W_m2. Its shape is not so sinusoidal as the C_ or C_
histories. The C_ through C_o terms each have a significant
o O O
annual sine component, and the C6 and C8 terms ha_e a bias
such that they do not change signs. The maximum absolute value
decreases with increasing n until C_1 and C_2, which are
small and show little discernible pattern. Previous studies by
Green and Smith (ref. 6) have indicated that twelfth degree is
the limit of deconvolution for the orbit altitude of Nimbus 6.
Whether the lack of pattern in the computed values of C_I and
C_2 is due to the nature of the atmosphere or due to the
limitations of sampling and analysis is unclear at present.
CONCLUDING REMARKS
Analysis of I year of longwave data from the ERB instrument
aboard Nimbus 6 shows a surprisingly simple variation with time
of the longwave distribution. Analysis of other years of data
will help to define the average annual cycle better, and to
define the interannual variations. In order to complete the
description of the radiation budget, work is ongoing at Langley
Research Center to develop a technique for deconvolutiong short-
wave WFOV data.
298
ACKNOWLEDGEMENT
The authors are grateful to Dr. Herb Jacobowitz and other
members of the Meteorological Satellite Laboratory of NOAA for
providing us with ERB data and for discussions with respect to
the data.
REFERENCES
Io Jacobowitz, H.; Smith, W. L.; Howell, H. B.; Nagle, F. W.;
and Hickey, J. R.: The First Eighteen Months of Planetary
Radiation Budget Measurements from the Nimbus-6 ERB Experi-
ment. Third Conference on Atmospheric Radiation, American
Meteorological Society, Davis, CA, June 28-30, 1978.
2. Smith, W. L.; Hickey, J.; Howell, H. B.; Jacobowitz, H.;
Hilleary, D. T.; and Drummond, A. J.: Nimbus-6 Earth Radi-
ation Budget Experiment. Applied Optics, vol. 16, No. 2,
Feb. 1977, pp. 306-318.
3. Smith, G. Louis and Green, Richard N.: A Technique for
Analysis of Low Resolution Measurements of Earth Radiation
Budget. Second Conference on Atmospheric Radiation, Ameri-
can Meteorological Society, Oct. 1975, pp. 111-114.
4. Smith, G. Louis and Green, Richard N.: Theoretical Analysis
of Wide Field of View Radiometer Measurements of Earth En-
ergy Budget. NASA paper presented at Fifth Annual Remote
Sensing of Earth Resources Conference, Tullahoma, TN,
March 29-31, 1976.
S. Green, R. N. and Smith, G. L.: Deconvolution of Earth Radi-
ation Budget Data. Third National Aeronautics and Space
Administration Weather and Climate Program Science Review,
NASA CP-2029, 1977, pp. 299-303.
6, Green, R. N. and Smith, G. L.: Deconvolution Estimation
Theory Applied to Nimbus-6 ERB Data. Third Conference on
Atmospheric Radiation, American Meteorological Society,
1978, pp. 376-379.
7. Green, R. N. and Smith, G. L.: Parameter Estimation Applied
to Nimbus-6 Wide-Ankle Longwave Radiation Measurements,
NASA TP-1307, Dec. 1978.
299
240 -
230
220
0
-5
-10
-15
-20
-25
-30
JULY
AUG
m
15-
10-
5
1 I I I
SEPT NOV JAN MAR MAY
OCT DEC FEB APR JUNE
Figure 1. C_ histories for n = O, 1, 2, for July 1975
through June 1976
3OO
15
I0 , -.-0
5
0
-5 CO
-10 "I_
-5 0
-i0 _
-10 C_
lO
0
-5 0 0
5 C9 Cll
-5 I I
JULY SEPT NOV JAN MAR MAY
AUG OCT DEC FEB APR JUNE
2. _ histories for n = 3 through 12 for July 1975Figure
through June 1976.
301


The annual cycle of Earth emitted radiation distribution

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