- Email: [email protected]
Performance and early results from the stratospheric and mesospheric sounder (SAMS) on Nimbus 7
Adv. Space Rca. © COSPAR, 1981.
26l—265. Vol. 1, pp. Printed in Great Britain.
02731177/81/03010261$05.00/0
PERFORMANCE AND EARLY RESULTS FROM THE STRATOSPHERIC AND MESOSPHERIC SOUNDER (SAMS) ON NIMBUS 7 F.W. Taylor, J.J. Barnett, I. Colbeck, R.L. Jones, C.D. Rodgers, M.J. Wale and E.J. Williamson Department of Atmospheric Physics, Oxford University, England ABSTRACT The design and performance of SANS, an infrared limb—scanning instrument for sounding the temperature and composition of the atmosphere from 15 to 150 km alti— tude, are reviewed. Some examples of preliminary results on temperature and water vapour and nitrous oxide abundance versus latitude and height are presented. INTRODUCTION The Stratospheric and Mesospheric Sounder (SANS) is an infrared radiometer on the Nimbus 7 experimental meteorological satellite. It was designed to measure temperature and chemical composition over a wide range of altitudes, using the pressure modulation technique for high selectivity and sensitivit 1 to the constituents of interest. This technique, first utilized on Nimbus 6 0-i uses a cell containing the gas being observed as a filter and chopper, which gives very high effective spectral resolution. The latter is of the order of a line width under the conditions of temperature and pressure which apply in the cell. The molecular species and spectral bands selected for SANS are summarized in Table 1, which also lists the height ranges over which measurements typically can be made in each case. The instrument is described in further detail in a recent paper c2) The instrument measures vertical profiles by scanning the atmospheric limb, and therefore has stringent requirements for accurate calibration and characterization of the field of view, radiometric offsets etc., as well as a requirement for pointing knowledge to better than 0.003°. The actual stability of the Nimbus spacecraft is not better than 1°, and so a method has been evolved to use the SANS itself to measure the level viewed to the desired accuracy. This is done over a limited range of heights by comparing the measurements from broad band and pressure modulator channels measuring CO2 emission over different spectral intervals. When the atmosphere, as viewed through the modulator, is nearly opaque, that channel gives the temperature almost independently of the viewing level knowing the temperature the other channel chosen to be much more sensitive to the pressure at the tangent point, can give that pressure to an accuracy equivalent to 0.2 km. Once this is done for one point in the scan the pressure at other points can be calculated. The CO2 radiance measurements can then be used to determine the temperature profile and finally the measurements of other channels can be used to obtain concentration profiles. A similar technique, using filters in strongly and
261
262
F.W. Taylor et al. TABLE
Species
1
SANS Measurements
Spectral Band
Carbon dioxide
Water vapour
Height range covered
4.3
pm
15—120 km
15
jim
15—150 km
2.7
jim
80—110 km
25 to 100
pm
15—80
km
Carbon monoxide
4.7
pm
15-50
km
Nitrous oxide
7.7
pm
20-50
km
Methane
7.7
pm
15—50
km
Nitric oxide
5.3
pm
15-50
km
C3]
weakly absorbing spectral bands, has been derived by Bailey and Gille and applied to the LIMS experiment on the sane spacecraft as well as the earlier Nimbus 6 LRIR. RESULTS Initial data analysis has concentrated on calibrating the instrument and perfecting the attitude and temperature measurements. Some preliminary composition profiles have been produced for comparison with models and other measurements, prior to commencing routine production of gridded parameters on a daily basis. The status of this work at the time this review was prepared, and some preliminary examples of retrieved parameters, are given below. Temperature Maps of temperature on constant pressure surfaces have been produced and compared to results from the radiosonde network and from the Nimbus 7 LIMS instrument. Good general consistency is found with some interesting deviations Fig 1 shows an example of the zonal mean temperature field as retrieved from SANS radiances in the 15 pm CO 2 band. Carbon Dioxide Emission profiles have been obtained for the 4.3 pm and 15 pm bands of CO2 at levels well above those where non—LTE processes become important.The possibility that CO emission due to resonance fluorescence is being observed at altitudes from l0~to 150 km is under investigation. Water Vapour Water vapour has been observed both in emission and fluorescence. Fig. 2 shows examples of measured thermal radiances in the far infrared (25 to 100 pm) channel. These provide a good example of the usefulness of pressure modulator radiometry to obtain signals to much higher levels in the atmosphere than is possible using filter (broad band) radiometry. Eventually, water vapour abundances up to about 80 km are expected from SANS thermal emission data, with the measurements from the fluorescence channel extending this to perhaps 110 km on the sunlit hemisphere. Prelimix)ary ~nve~sio~s of the radiance data show the water vapour mixing ratio increasing with neignt in the stratosphere and generally decreasing with latitude.
Performance and Early Results from SAMS
~
~
0.0w
0.01
-
-
-.
—
4 11’
- --
-~ -
- -
263
-
- -
-
1
- -
~~2~—~TTN
-
~
~0
_____ -~ -
10
~
~
~-~L~-~<-
~
100
~------
-
S
~ SOS
lOS
~3S
20t
lOS
0
iON
20N
30N
4Ci
5&~ ~JN
70N
Zonal mean temperature. Dau 24 1979 Fig. 1 Zonal mean temperature cross section from 5O°S to 67.5ON and 100 mb to 0.001 mb for 24 January 1979. Carbon Monoxide Difficulties with detector cooling in this channel have resulted in poorer signal to noise ratios than intended. As a result, only globally averaged profiles are produced at present. These agree with model predictions. Nitrous Oxide and Methane Profiles at selected latitudes are generally in agreement with previous measure— ments where overlap exists. Qualitative agreement exists with models for height and latitude variability, but absolute amounts differ significantly. Fig. 3 presents examples of retrieved profiles. These have less vertical coverage than can be expected ultimately, because only one modulator setting (high pressure) is represented. The low pressure and broad band modes, when analysed, will extend the profiles upward and downward respectively (c.f. Fig.2). NO A problem with chemical production of N2O in the modulator cell has delayed analysis of the NO channel radiances Profiles show substantial emission at high levels and day—night variability but quantitative analysis is likely to take some time.
264
F.W. Taylor at al. 100.0
~
~
r
1
—
I
~O.O
‘!
~~~~1
-
1-----——rT’~
4
(b)_
~!ld~hond Pce~1~6
(c)_
~ceu1~~~5
“~
I :::~_ -~
_(c)
.
0 -
0
—
.
-
~o.o
-
—
-c .-‘
0
30.0
20.0
-
-
-
-
(a)
—
10.0
0.8o
0.1
0.2 0.5
0.4
0.5
0.~ 0.7
O.~ 0.9
1.~
Rod,.once unLt.~ (BB ot. 29~K = 1) Fig.2 Radiances vs. height for three of the SANS far-infrared (25 to 100 pin) water vapour channels. ODN~WSION With the exception of problems with radiative cooling of one detector, and chemical contamination of the nitric oxide gas in one modulator cell, the SANS instrument is performing according to expectations. Good data has been received for nearly two years of daily operation and is expected to continue for at least two more. Data retrieval algorithms have been tested and optimised for temperature and most species with full—scale gridding and analysis due to begin shortly. REFERENCES
1.
P.D. Curtis, J.T. Houghton, G.D. Peskett and C.D. Rodgers. Proc.R.Soc.Lond. A337, 135—150 (1974).
2.
J.R. Druminond, J.T. Houghton, G.D.Peskett, C.D. Rodgers, M.J. Wale, .3. Whitney and E.J. Williamson. Phil.Trans.R.Soc.Lond. A296,219—241 (1980).
3.
P.L. Bailey and J.C. Gille, in ~Remote Sensing of the Atmosphere: Inversion Methods and Applications’, ed. AL. Fymat and V.E. Zuev, p.115,
Performance
50.3
\
and Early Results
‘----—-------~-
\
‘
1
/0 N~I
k
;J~soo~
1 1
\
\\~tor
-~
20.0
—9.00
‘~
0O°N—~
30.0
10.0
265
—~--~----------~---
‘
10 0
“
from SAMS
—8.00
loSio(volune
1~20
—7.00
S
.-~
.—~--
—6.~0
—5.00
mLxLn
9 ratio)
CH4 AND N20 PROFILES, 29th MAY/2nd JUNE. 1980 Fig. 3 Examples of preliminary retrievals~of composition profiles from SANS data.
26l—265. Vol. 1, pp. Printed in Great Britain.
02731177/81/03010261$05.00/0
PERFORMANCE AND EARLY RESULTS FROM THE STRATOSPHERIC AND MESOSPHERIC SOUNDER (SAMS) ON NIMBUS 7 F.W. Taylor, J.J. Barnett, I. Colbeck, R.L. Jones, C.D. Rodgers, M.J. Wale and E.J. Williamson Department of Atmospheric Physics, Oxford University, England ABSTRACT The design and performance of SANS, an infrared limb—scanning instrument for sounding the temperature and composition of the atmosphere from 15 to 150 km alti— tude, are reviewed. Some examples of preliminary results on temperature and water vapour and nitrous oxide abundance versus latitude and height are presented. INTRODUCTION The Stratospheric and Mesospheric Sounder (SANS) is an infrared radiometer on the Nimbus 7 experimental meteorological satellite. It was designed to measure temperature and chemical composition over a wide range of altitudes, using the pressure modulation technique for high selectivity and sensitivit 1 to the constituents of interest. This technique, first utilized on Nimbus 6 0-i uses a cell containing the gas being observed as a filter and chopper, which gives very high effective spectral resolution. The latter is of the order of a line width under the conditions of temperature and pressure which apply in the cell. The molecular species and spectral bands selected for SANS are summarized in Table 1, which also lists the height ranges over which measurements typically can be made in each case. The instrument is described in further detail in a recent paper c2) The instrument measures vertical profiles by scanning the atmospheric limb, and therefore has stringent requirements for accurate calibration and characterization of the field of view, radiometric offsets etc., as well as a requirement for pointing knowledge to better than 0.003°. The actual stability of the Nimbus spacecraft is not better than 1°, and so a method has been evolved to use the SANS itself to measure the level viewed to the desired accuracy. This is done over a limited range of heights by comparing the measurements from broad band and pressure modulator channels measuring CO2 emission over different spectral intervals. When the atmosphere, as viewed through the modulator, is nearly opaque, that channel gives the temperature almost independently of the viewing level knowing the temperature the other channel chosen to be much more sensitive to the pressure at the tangent point, can give that pressure to an accuracy equivalent to 0.2 km. Once this is done for one point in the scan the pressure at other points can be calculated. The CO2 radiance measurements can then be used to determine the temperature profile and finally the measurements of other channels can be used to obtain concentration profiles. A similar technique, using filters in strongly and
261
262
F.W. Taylor et al. TABLE
Species
1
SANS Measurements
Spectral Band
Carbon dioxide
Water vapour
Height range covered
4.3
pm
15—120 km
15
jim
15—150 km
2.7
jim
80—110 km
25 to 100
pm
15—80
km
Carbon monoxide
4.7
pm
15-50
km
Nitrous oxide
7.7
pm
20-50
km
Methane
7.7
pm
15—50
km
Nitric oxide
5.3
pm
15-50
km
C3]
weakly absorbing spectral bands, has been derived by Bailey and Gille and applied to the LIMS experiment on the sane spacecraft as well as the earlier Nimbus 6 LRIR. RESULTS Initial data analysis has concentrated on calibrating the instrument and perfecting the attitude and temperature measurements. Some preliminary composition profiles have been produced for comparison with models and other measurements, prior to commencing routine production of gridded parameters on a daily basis. The status of this work at the time this review was prepared, and some preliminary examples of retrieved parameters, are given below. Temperature Maps of temperature on constant pressure surfaces have been produced and compared to results from the radiosonde network and from the Nimbus 7 LIMS instrument. Good general consistency is found with some interesting deviations Fig 1 shows an example of the zonal mean temperature field as retrieved from SANS radiances in the 15 pm CO 2 band. Carbon Dioxide Emission profiles have been obtained for the 4.3 pm and 15 pm bands of CO2 at levels well above those where non—LTE processes become important.The possibility that CO emission due to resonance fluorescence is being observed at altitudes from l0~to 150 km is under investigation. Water Vapour Water vapour has been observed both in emission and fluorescence. Fig. 2 shows examples of measured thermal radiances in the far infrared (25 to 100 pm) channel. These provide a good example of the usefulness of pressure modulator radiometry to obtain signals to much higher levels in the atmosphere than is possible using filter (broad band) radiometry. Eventually, water vapour abundances up to about 80 km are expected from SANS thermal emission data, with the measurements from the fluorescence channel extending this to perhaps 110 km on the sunlit hemisphere. Prelimix)ary ~nve~sio~s of the radiance data show the water vapour mixing ratio increasing with neignt in the stratosphere and generally decreasing with latitude.
Performance and Early Results from SAMS
~
~
0.0w
0.01
-
-
-.
—
4 11’
- --
-~ -
- -
263
-
- -
-
1
- -
~~2~—~TTN
-
~
~0
_____ -~ -
10
~
~
~-~L~-~<-
~
100
~------
-
S
~ SOS
lOS
~3S
20t
lOS
0
iON
20N
30N
4Ci
5&~ ~JN
70N
Zonal mean temperature. Dau 24 1979 Fig. 1 Zonal mean temperature cross section from 5O°S to 67.5ON and 100 mb to 0.001 mb for 24 January 1979. Carbon Monoxide Difficulties with detector cooling in this channel have resulted in poorer signal to noise ratios than intended. As a result, only globally averaged profiles are produced at present. These agree with model predictions. Nitrous Oxide and Methane Profiles at selected latitudes are generally in agreement with previous measure— ments where overlap exists. Qualitative agreement exists with models for height and latitude variability, but absolute amounts differ significantly. Fig. 3 presents examples of retrieved profiles. These have less vertical coverage than can be expected ultimately, because only one modulator setting (high pressure) is represented. The low pressure and broad band modes, when analysed, will extend the profiles upward and downward respectively (c.f. Fig.2). NO A problem with chemical production of N2O in the modulator cell has delayed analysis of the NO channel radiances Profiles show substantial emission at high levels and day—night variability but quantitative analysis is likely to take some time.
264
F.W. Taylor at al. 100.0
~
~
r
1
—
I
~O.O
‘!
~~~~1
-
1-----——rT’~
4
(b)_
~!ld~hond Pce~1~6
(c)_
~ceu1~~~5
“~
I :::~_ -~
_(c)
.
0 -
0
—
.
-
~o.o
-
—
-c .-‘
0
30.0
20.0
-
-
-
-
(a)
—
10.0
0.8o
0.1
0.2 0.5
0.4
0.5
0.~ 0.7
O.~ 0.9
1.~
Rod,.once unLt.~ (BB ot. 29~K = 1) Fig.2 Radiances vs. height for three of the SANS far-infrared (25 to 100 pin) water vapour channels. ODN~WSION With the exception of problems with radiative cooling of one detector, and chemical contamination of the nitric oxide gas in one modulator cell, the SANS instrument is performing according to expectations. Good data has been received for nearly two years of daily operation and is expected to continue for at least two more. Data retrieval algorithms have been tested and optimised for temperature and most species with full—scale gridding and analysis due to begin shortly. REFERENCES
1.
P.D. Curtis, J.T. Houghton, G.D. Peskett and C.D. Rodgers. Proc.R.Soc.Lond. A337, 135—150 (1974).
2.
J.R. Druminond, J.T. Houghton, G.D.Peskett, C.D. Rodgers, M.J. Wale, .3. Whitney and E.J. Williamson. Phil.Trans.R.Soc.Lond. A296,219—241 (1980).
3.
P.L. Bailey and J.C. Gille, in ~Remote Sensing of the Atmosphere: Inversion Methods and Applications’, ed. AL. Fymat and V.E. Zuev, p.115,
Performance
50.3
\
and Early Results
‘----—-------~-
\
‘
1
/0 N~I
k
;J~soo~
1 1
\
\\~tor
-~
20.0
—9.00
‘~
0O°N—~
30.0
10.0
265
—~--~----------~---
‘
10 0
“
from SAMS
—8.00
loSio(volune
1~20
—7.00
S
.-~
.—~--
—6.~0
—5.00
mLxLn
9 ratio)
CH4 AND N20 PROFILES, 29th MAY/2nd JUNE. 1980 Fig. 3 Examples of preliminary retrievals~of composition profiles from SANS data.