ISAMS is a limb sounding radiometer flying on the UARS, and designed to measure temperature, pressure, O3, CO, NO, NO2, N2O5, HNO3, CH4, H2O, N2O, and aerosol. Its capabilities are described, together with the present status of validation of its data products, and plans for future improvement

Barnett, J. J.

Corney, Marie

Dudhia, Anu

Lopez-Valverde, M. A.

Marks, C. J.

Morris, P.

Nightingale, T.

Remedios, John J.

Rodgers, Clive D.

Taylor, Fredric W.

English

NASA Technical Reports Server

N95-11008
MEASUREMENTS OF STRATOSPHERIC CONSTITUENTS BY ISAMS.
C.D. Rodgera, F.W. Taylor, J.J. Barnett, M. Corney, A. Dudhia, M.A. Lopez-Valverde,
C.J. Marks, P. MorriJ, T. Nightingale, J. Remedio_, D. RoiJin, R.J. Welb
Atmospheric, Oceanic and Planetary Physics,
Clarendon Laboratory, Parks Road, Oxford, England
J. Ballad, B.J. Kerridge, R.]. Knight,
Rutherford Appleton Laboratory, Chilton, Didcot, England
A. Chu,
GATS Inc, Hampton, VA
B.J. Connor,
NASA Langley Research Center, Harupton, VA
C. Scheuer,
Science and Technology Corp., Hampton, VA
ABSTRACT
ISAMS is a limb sounding radiometer flyingon the
UARS, and designed to measure temperature, pressure,
03, CO, NO, NO2, N2Os, HNOs, CH4, H20, N20 and
aerosol. Its capabilitiesare described, together with the
present status ofvalidationofitsdata products, and plans
for future improvements.
1. INTRODUCTION
The Improved Stratospheric and Mesospheric
Sounder (ISAMS) is a limb sounding infrared pressure
modulator radiometer, which is a part of an integrated
payload to study the energetics, chemistry and dynam-
ics of the Middle Atmosphere, the Upper Atmosphere
Research Satellite (UARS). The main objectives of the
ISAMS project arc to study: (1) the circulation and dy-
namics of the Middle Atmosphere, using measurements of
thermal structure and tracers (CH4, N20, H20); (2) the
middle at,nosphere water budget and associated chem-
istry, using measurements of H20, CH4 and CO and the
circulation; (3) the ,fitrogeu chemistry and ozone deple-
tion, using measurements of N2 O, NO, NO2, HNOs, N20s
and Os, together with chemical models. Other objectives
include the study of aerosols, using data from spectral
window regions and a range of other topics involving data
from more than one UARS instrument, and from correl-
ative measurements.
2. THE INSTRUMENT
Measurements are made of thermal emission in in-
frared spectralvibrationbands of the individualmolecular
constituents,using limb scanning geometry. Radiation in
each channel isspectrallyselectedby an interferencefilter,
providing a 'wideband' signal,and then further selected
using a pressure modulator cell(PMC) to selectradiation
from just the individual finesof the target species (the
'pressure modulated' signal). Thus each detector pro-
vides two signals. The optics allows the fieldof view to
be scanned both vertically,in steps of 1.18 or 2.36 km
on the limb, and horizontally,to compensate for doppler
shiftof the atmospheric linesrelativeto the spacecraft.
Each channel uses an array of four detectors,each 2.36
km wide when projected onto the limb, and separated by
one detector width. The detectors are cooled to 80-90 K
by means of a miniature Stiflingcycle cooler developed
specificallyfor ISAMS. (Taylor et al, 1992)
The nominal verticalresolutionof the instrument is
2.36 kin, and the horizontal resolution,which is deter-
mined by the scan pattern, is about 200 km along the
view point (tangent) track and about 200-400 km along
the lineof sight.Detailsof the spectralintervalsused and
the altitude ranges of each measurement are given in the
table.
The spacecraft is in a 57 ° inclination orbit, at all
altitude of 585 kin. Consequently the limb view at 90 ° to
the orbit track gives a view point track extending from
444
https://ntrs.nasa.gov/search.jsp?R=19950004595 2020-06-16T11:34:12+00:00Z
Channel Wxvenumber Objective Interfering Species
0
1
2
3.0
3.1
3.2
3.3
4
5
6
7.0
7.1
7.2
7.3
2139-2221
1425-1513
1250-1290
626-660
605-625
860-900
990-1010
1839-1941
1580-1612
1320-1380
626-660
605-625
1230-1250
821-836
CO
HsO
NsO
COs
COs
HNOs
03
NO
NO2
CH,
C02
COs
NsOs
aerosol
COs, Os, NsO, HsO
Os, CH4
CH4, HsO, COs, HNOs
Os, H=O
Os, HsO, NsO
COs, Os, HsO, Fll, F12
COs, HsO
HsO, COs, Os
HsO, 0=, CH4, N=O
HsO, COs, NsO, HNOs
03, HsO
Os, HsO, NsO
CH4, NsO, COs, HsO, O,
Os, COs, HsO, Fll
34 ° in one hemisphere to 80 ° in the other. ISAMS nor-
mMly views on the side of the spacecraft away from the
sun, but when the spacecraft is in darkness it can view
the sun-side. This allows coverage from 80 ° N to 80 ° S st
favourable parts of the orbital precession cycle.
3. DATA PROCESSING
Data is processed along with all the other spacecraft
data at the UARS Central Data Handling Facility, using
software developed and maintained by the ISAMS team.
The retrieval technique uses a sequential optimal estima-
tor, or Kalman Smoother, to obtain distributions of tem-
perature, pressure and constituents from the measured
radiances. (Marks and Rodgers, 1992)
A critical component of the retrieval method is the
forward model. This calculates the radiance that would
be seen by the instrument for a given atmospheric profile.
The optimal estimator adjusts the profile until the calcu-
lated radiance matches the measured radiance to within
experimental error. The forward model does a radiative
transfer calculation for the spherical geometry of the limb
geometry, including refraction and line-of-sight variation,
using all the emitting gases and aerosol that are relevant
in each channel. The estimator also requires derivatives
of the radiance with respect to the temperature and con-
stituent profiles.
After data quality checks and radiometric calibra-
tion, radiances from the CO2 15 /_m band are used to
retrieve the temperature distribution. Subsequently, the
temperature profile and radiances from each of the con-
stituent channels in turn are used to retrieve the corre-
sponding constituent. The order of retrieval is significant,
as emission from more than one gas is present in most of
the spectral regions, and for example the previously re-
trieved concentration of water is required as a correction
in the NO2 retrieval.
4. CURRENT VALIDATION STATUS
To date the ISAMS team has been involved in check-
ing out the instrument and in validating the data process-
ing software and retrieval techniques. Except for a period
from January 18 to March 27 1992, when there was a
hardware problem with the chopper, the instrument has
performed extremely well, and good quality data is being
received.
Details of this process for some channels will be found
in companion papers presented at this meeting, on ozone
(Connor et al, 1992), aerosol (Lambert et al, 1992) and
NO= (Kerridge et al, 1992). The current status is that
the data are mainly of such a quality that they can be
released to the rest of the UARS Science Team for further
evaluation and intercomparison, but are not yet suitable
for general scientific use in the refereed literature. There
are some known deficiencies that are still being worked on
by the ISAMS team.
5. TEMPERATURE MEASUREMENTS
Temperature is obtained from the COs 15 #m band
emission, as measured in two separate groups of detec-
tors. The forward model CO2 emission includes the effects
of gradients of temperature along the line of sight, and
emission by ozone and aerosol as spectral contaminants.
The ozone effect is small, and is currently neglected. Due
to the eruption of Mt Pinatubo, the aerosol effect is ex-
tremely significant, especially below 10 rob, and work is
continuing on the best approach to allowing for it. For
example a preliminary aerosol retrieval may be carried
out using climatological or NMC temperatures, The pre-
liminary aerosol retrieval then being used to allow for the
aerosol emission in the CO2 channels when carrying out
the temperature retrieval.
The accuracy of the temperature retrieval is impor-
tant not only for dynamical studies, but also for the re-
trieval of constituents. Consequently, validation of tem-
perature retrieval is critical to the overall success of
ISAMS.
Figure 1 shows averaging kernels (Rodgers, 1990) for
a typical temperature retrieval. These indicate the ver-
tical resolution of the retrieved temperature and its sen-
sitivity to the atmospheric temperature. Figures 2 show
some typical retrieved profiles, compared with coincident
lidar data from the site at Obervatoire Haute-Provence.
These indicate the extent to which ISAMS can retrieve
the small scale wave motions seen by the lidar. Figure
3 shows a cross section along a view point track through
a recent stratospheric warming event, showing lhe famil-
iar pattern of cooling in the mesosphere corresponding to
warming in the stratosphere.
445
15AMS Tern eroture Avero in Kernels
Integcptld Response Rtv/A-Pr Error 100
.16 ............. ._ 90
'_ _"........... "'" .. 80
-E5 / / " $
- 70
-2o i
_" :: 50
-- t51
!
--¢--- .i,o ]- ,o
!
x ...... 30
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Response [d_v/dTi]
6. AEROSOLS
Emission from aerosol from the Mt Pinatubo erup-
tion is present in all channels up to about 10 rob, so it is
important to understand this well. Retrieval of the opti-
cal depth in the window channel at 12.1 pm gives a good
indication of the amount present, Mthough conversion of
optical depth to e.g. mass loading is problematical. To
allow for aerosol emission in other channels, it is neces-
sary to know the emission spectrum relative to 12.1 pro.
This varies with time and position, as it depends on the
nature of the aerosol and its drop-size distribution, and
so cannot be precomputed exactly. However retrieval of
aerosol optical depth in other bands, assuming that the
constituents mixing ratios have their climatological val-
ues, gives an empirical estimate of the spectrum (Lam-
bert et al, this meeting), so that approximate corrections
can be made. The accuracy of corrections made by this
method are still being studied, and retrieved constituents
below about 10 mb are suspect at present.
Figure 2. ISAMS temperature profile for s
22.41 UT on 10 December 1991 at 43°N, 4.3°E,
compared with a Lidar profile taken _;t Observa-
toire Haute Provence (44°N, 6°E). In the right 4
hand panel, the lidar profile has been smoothed
using averaging kernels similar to those of figure
1, in order to make a more direct comparison.
Figure 1. A selection of averaging kernels for an ISAMS
temperature retrieval. The solid line is the derivative of
the retrieved profile with respect to the true profile. The
width of the peak indicates the vertical resolution of the
retrieval, and its area (the dotted line) indicates qualita-
tively the partition of the retrieved profile between the
true profile and the a priori. The dashed line is the for-
mal error on the retrieval divided by the a priori standard
deviation.
7. GASEOUS CONSTITUENTS
Ozone is measured in a wideband channel only, as
it is too reactive to contain in a pressure modulator cell.
The only significant spectral contaminant is aerosol. The
current status of Ozone retrieval validation is described
by Connor et al (this meeting).
Of the nitrogen species, ISAMS measures NO, NO2,
N2Os, N20 and HNO3. The validation status of the first
three of these gases is described by Kerridge et al (this
meeting).
CH4 and N20 are retrieved jointly since each is the
major contaminant in the signal measured by ISAMS for
the other gas. Since both the CH4 and N20 channels
contain PMCs, there are four channels (two PM and two
wideband) which may be used to provide radiances for the
joint retrieval. However, the channels must also provide
consistent information and due to aerosol contamination
in the wideband signal, so far only the two PM channels
ha're been used. Much of the data appears to be of good
quality and produces retrieved CH4 and N20 similar to
Direct corn Corn
0.001
160 180 200 220 240 260
: O.Olo
zso
1.000
• ._10.000
,,l,,_l,,,If ,I,,,I,
160 180 200 220 240 260 280
Ttmper-_ure/K Ternpef-_'o/K
Shaded region = Lidar +/- 1SD. Solid Lines = ISAMS. Dotted lines = ISAMS +/- 1SD
446
3O
18 35 53
Lotitude
7O 79 67 5O 33 16
0
Z
o
3
V)
n_
<
25-
20-
15-
10
1O0
20 40 60 80 1O0
Profile No.
Figure 3. A retrieved temperature section along the view point track on 20 Decem-
ber 1991, passing through the warm and cold areas of a stratospheric warming.
that from SAMS (Jones and Pyle ). Some days in January
are a tittle anomalous and these are under investigation.
Nitric acid, like ozone, is a wideband channel only.
Its signal, however, is much weaker. Furthermore it is
concentrated in the lower stratosphere. For both of these
reasons, the effects of the aerosol contamination on the
HNOs channel has been severe. We have therefore de-
ferred any attempt at correcting the aerosol effects until
they are better understood through work on other chan-
nels.
Nitric oxide is proving to be dimcult to calibrate at
present, owing to thermospheric emission which shows
large variation with solar activity (Ballaxd et al, this meet-
ing).
Retrieval of CO is particularly complex, because of
departures from local thermodynamic equilibrium, and
in tile daytime, resonance fluorescence scattering sun-
light. The latter allows us to see emission from CO up
to about 100 km in the daytime, but makes the valida-
tion of the retrieval difficult. Qualitatively reasonable re-
suits have been obtained, showing the expected increase
in CO with height into the upper mersosphere, and an in-
crease towards high latitudes in the winter, as predicted
by Solomon et al (1985).
ACKNOWLEDGEMENT: Our thanks are due to Dr
M.L. Chanin for permission to show the OHP lidar data.
REFERENCES
Ballard, J. et al., Evidence from ISAMS data of nitric ox-
ide production in the earths thermosphere following
solar flare activity, this meeting.
Connor, B.J. et al., 1992, Validation of ISAMS Measure-
ments of Ozone, this meeting.
Kerridge, B.J. et al., 1992, Preliminary results concerning
validation of ISAMS measurements of NO, NO2 and
N205, this meeting.
Lambert, A. et al., 1992, ISAMS Observations of Strato-
spheric Aerosol, this meeting.
Marks C.J. and C.D. Rodgers, 1992. A retrieval method
for atmospheric composition from limb emission mea-,
surements. Submitted to J. Geophys. Res.
Rodgers, C.D., 1990, Characterisation and Error Analy-
sis of Profiles Retrieved from Remote Sounding Mea-
surements, J. Geophys. Res. 95 pp5587-5595.
Solomon S. et al., 1985, Photochemistry and transport
of Carbon Monoxide in the Middle Atmosphere, J.
Atmos. Sci., 42, p1072-1081.
Taylor, F.W., C.D. Rodgers, J.G. Whitney, S.T.Werrett,
J.J. Barnett, G.D. Peskett, P. Venters, J. Ballard,
C.W.P. Palmer, R.J. Knight, P. Morris, T. Nightin-
gale, A. Dudhia, 1992. Remote sensing of atmo-
spheric structure and composition by pressure modu-
lator radiometry from space: the ISAMS experiment
on UARS. Submitted to J Geophys. Res.
447


Measurements of stratospheric constituents by ISAMS

Measurements of stratospheric constituents by ISAMS

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