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Stratospheric methane and nitrous oxide
Adv. Space Re:. Vol. 13, No. 1, pp. (1)31—(1)44, 1993 Printed in Great Britain. All rights reserved.
0273-1177193 $15.00 Copyright ~ 1992 COSPAR
STRATOSPHERIC METHANE AND NITROUS OXIDE F. W. Taylor Department ofAtmospheric, Oceanic and Planetary Phsycis, Oxford University, Clarendon Laboratory, Oxford OXJ 3PU, U. K
ABSTRACT
A model of the abundances of nitrous oxide and methane in the stratosphere has been produced using data from the Stratospheric and Mesospheric Sounder on Nimbus 7, and reported in the Handbook for Map, Vol.31 (in press).
This paper describes the
data which went into the model, its limitations, and the general behaviour of methane and nitrous oxide in the middle atmosphere. Plans for preparing an improved model from new data expected from the Upper Atmosphere Research Satellite program beginning in late 1991 are outlined.
(1)3 1
(1)32
F. W. Taylor
INTRODUCTION
A model of the abundances of nitrous oxide and methane in the stratosphere has been reported in the Handbook for Map, Vol.31
/1/.
The data for this came primarily from the Stratospheric
and Mesospheric Sounder on Nimbus 7 /2/. Rodgers
Taylor,
Dudhia &
(1986) described the model in an earlier paper /3/,
while Taylor and Dudhia /4/ performed an error analysis and analysed three and five year data sets for trends in the abundance of either species at any level in the range 20 to 0.2 mb pressure (25 to 62 kin altitude).
No trend was detected, but
this is not surprising since the errors in the observing technique are quite large and the changes which would be expected, even over a five year span, are only a few percent at most.
New measurements with better accuracy and precision are planned for the Upper Atmosphere Research Satellite program beginning in late 1991.
With these, it will be possible to produce an
improved model with better global and height coverage, as well as smaller errors.
There will be a much better chance of
observing trends in the abundances, especially if the UARS measurements are extended beyond their initial 1.5 year goal, and in any case the UARS data set can be compared to the Nimbus observations from ten to fifteen years earlier.
Finally, it is
to be hoped that improvements in both satellite and in-situ techniques will lead to some resolution of the discrepancies with balloon data noted previously /4/.
Stratospheric CH
4 and N20
STRATOSPHERIC
(1)33
METHANE
All of the methane in the atmosphere originates
at the surface,
mostly in plant matter in humid regions (especially bogs and marshes) and in animal interiors, especially ruminants.
It is
estimated /5/ that 443 to 850 megatons is produced annually by these biological sources.
In addition, there is an estimated 16
to 110 megatons from non-biological (chiefly industrial) sources.
The total atmospheric burden of methane is about 4,600
megatons.
Removal is by reactions with hydroxyl, atomic oxygen, or atomic chlorine, or by photolysis, and the mean lifetime is around seven years.
The main product of methane destruction is water
vapour, but carbon monoxide (as an intermediate product, which finally itself is oxidised by OH to form carbon dioxide), and molecular hydrogen, are produced as well. showed that the sum
[2 CH4
+
H2O]
Jones et al /6/
is approximately conserved
aver a large range of latitudes and heights in the stratosphere, showing that the proportion of H2 produced is relatively small, as expected.
Methane is a strong infrared absorber in spectral regions where the atmosphere would otherwise be relatively transparent, and is therefore
an important component of the atmospheric
‘greenhouse’, coming after CO2 and H20.
Because the column
abundance of CH4 is small compared to the latter two gases, the
(1)34
F. W. Taylor
transmittances in the methane bands are not in general in the saturated part of the curve of growth /7/ and increases in the methane burden therefore have a proportionately larger incremental greenhouse contribution.
Thus, the fact that
methane is observed to be increasing near the surface by more than 1% per annum is of particular interest, as is its fate in the middle atmosphere.
Methane in the stratosphere exhibits a considerable natural variability, as we would expect from its injection from the troposphere at preferred places and times and its subsequent redistribution by the prevailing motions in the stratosphere. This, together with its intermediate lifetime (long enough to survive in parcels of air following global-scale trajectories, but not so long that the mixing ratio tends to become constant everywhere), make methane a good dynamical tracer/5/.
Nitrous oxide is also a species of intermediate lifetime (several decades in the stratosphere) which originates at the surface in a variety of natural and anthropogenic processes (fixation of nitrogen by bacteria, lightening, artificial fertilisers,
etc.).
Destruction in this case is mainly by
photolysis to form nitrogen and atomic oxygen.
Stratospheric CH
(1)35
4 and N20
GLOBAL
OBSERVATIONS
NITROUS
OF
STRATOSPHERIC
METHANE AND
OXIDE
The only global observations to date are from the from the Stratospheric and Mesospheric Sounder (SANS) on Nimbus 7 /2/. The SANS data extend from 50°Sto 70°Nin latitude, and have useful accuracy for a vertical range from 20 mb ( 0.2 mb
(
60 kin) for CH4
and 0.6 mb (
25 km) up to
53 kin) for N20.
The
accuracy, according to Jones and Pyle /8/, who combined conservative estimates of all of the known sources of error including spectroscopic and retrieval uncertainties, and noise due to instrumental sources and spacecraft jitter, is at best 20% for CH4 and 25% for N20. The corresponding precision is -3% for CH4 and -6% for N2O.
Comparisons have been made between SANS retrievals and other measurements, particularly from balloons. The in-situ and satellite data agree quite well near the top of the region of overlap (about 10 mb or 30 kin) but lower down discrepancies occur with the SANS amounts being systematically higher. The reason for this are not likely to be known until new observations are forthcoming/4/.
IMPLICATIONS
FOR
DYNAMICS
The dynamical implications of the SANS-observed fields of both methane and nitrous oxide have been discussed in general terms
(1)36
F. W. Taylor
by Taylor and Dudhia /9/
and Taylor /2/.
The former includes
colour maps of the residual distribution of methane (that is, the deviation from the mean value) as a function of height, latitude and season for zonal and time averaged observations by SANS, which are useful for understanding the following summary. The overall appearance of the plots for CH4
and N2O
is quite
similar, tending to confirm that their distributions are controlled more by dynamics than chemistry.
The general characteristics of the distributions feature negative gradients with latitude and with height, so that the general appearance is of a bulge over low latitudes. Following the monthly changes, the most noticeable trend is for the bulge to migrate towards the Summer hemisphere. This is due to the seasonal dependence of the mean circulation, a feature of which must be more rapid vertical transport in the summer than in the winter.
The total amounts of methane and nitrous oxide, and
hence presumably of all minor constituents having their sources in the troposphere, in each hemisphere rises and falls with the seasons.
Local maxima
relative to the mean occur at about 0.6
mb and 20°latitude, towards the end of Summer, i.e. in September/October in the Northern Hemisphere and March/April in the Southern (Plate 2 of ref. 9).
Minima occur at the same
latitudes six months later in each case. The
late summer low-
latitude maxima (in the N hemisphere at least) are accompanied by higher-latitude minima, and vice-versa. The 60°N minimum in
-
L
_‘
-
~
Stratospheric CH
~~-r
4 and N20
0.3~
(1)37
8
5 -
30
-
—
50S
u
r
40$
30$
~
~ i
20S
los
r
o
r
U
iON
20N
3aM
40N
50N
i
60N
3 70N
lat.i,tude
Fig..1
Mean latitudinal distribution of stratospheric methane
(ppmv) for March (solid line) and September (dashed line) /2/.
CH4
concentration could be produced by a mass of descending
air, which has spent an extended period of time in the stratosphere and so become depleted of CH4 by reaction with OH and other radicals. A single circulation cell with maximum upwelling at 200 and maximum downwelling at about 60°, building up in intensity during the summer and declining in the winter, would then be implied.
The reversal of the abundance maxima and
minima during late winter does not imply necessarily that the circulation reverses, since these are maxima and minima relative to the mean values at that particular latitude. In fact, the absolute amounts of methane and nitrous oxide are always lower at 60°N than at 200 N. Thus,the sense of the circulation is
(1)38
F. W. Taylor
always from equator to pole, only its strength changing with the solar energy available
to drive it.
The data also show that at
any given latitude, the zonal mean abundances tend to peak earlier at higher altitudes,
the opposite to the behaviour to be
expected if material from the troposphere was simply being advected vertically. There has therefore to be strong horizontal, poleward, advection as well.
This is qualitatively
consistent with theoretical expectations (c.f. fig.2).
l~6
L......—’
~
~
E~(O~
I
:!--
~--:.-~-
--
~
36
16
\~.
~
60
SUMMER
Fig.2
Streamlines
30
0
‘~::
30
LATITUDE (degrees)
_~: -
60
90
WINTER
(heavy lines) and net diabatic heating and
cooling, from the model of Garcia and Solomon /10/
Stratospheric CH
4 and N20
OUTSTANDING
(1)39
QUESTIONS
Next we summarize the main uncertainties which exist in the seasonal variation of of CH4 and N20 abundances, and the scientific questions which are raised thereby
The first three
arise from prominent features of the SANS fields, which are observed but remain only very marginally understood.
(1)
The
‘double peaked’ structure.
This is strongest in April
/8/ but can also be seen in other months (cf. fig.1). It appears to be due to downward velocities over the equator which occur as a consequence of the semi-annual oscillation in tropical temperature and zonal wind. /11,12/
(ii)
Other long-period oscillations.
At high altitudes (near
the 0.2 mb level) there is a pronounced semi-annual oscillation in the which is not directly related to the semi-annual oscillation in temperature at tropical latitudes.
The methane
SAO occurs at higher latitudes than the temperature SAO and is much more prominent in the Southern than the Northern Hemisphere.
(iii) Non-seasonal trends.
At
the highest levels at which CH4
was observed, i.e. 0.2 mb or around 60 kin,
all latitudes in
both hemispheres tend to have maxima around September, and minima around March. This effect may be associated with the eccentricity of the Earth’s orbit, and if so it suggests that increased advection of ex-tropospheric air upwards due to
(1)40
F. W. Taylor
stronger heating
dominates increased photolytic
destruction,
since the solar intensity is greatest in the Northern Spring.
A number of other issues, not arising directly from SANS findings but relevant to the improvements to be desired in future reference models,
(iv)
and
Contribution N2O
can be listed as follows.
to greenhouse
effect.
The precise role of CH4
in determining the temperature structure of the
atmosphere through their contribution to heating and cooling rates by radiative transfer needs clarifying.
(v)
Trends in abundances.
The surface production rates of both
gases are thought to be increasing, although the existing global data is not precise enough to detect stratospheric changes /4/ and local measurements cannot adequately distinguish trends from seasonal and other fluctuations.
(vi)
Chemical budgets.
Much remains to be done to show that
the oxidation of methane is in detailed balance with the stratospheric water and carbon monoxide budgets, and the oxidation of nitrous oxide with nitric oxide and other species involved in ozone chemistry. determine,
for example,
It would be very valuable to
how much free hydrogen is produced in
the methane oxidation cycle, and how much N2O is changed by photolysis back to N2 after reaching the stratosphere.
Stratospheric CH
4 and N20
(vii)
Polar Dynamics.
(1)41
There is almost endless scope for
improving the use of CH4 and N20
as dynamical tracers by
obtaining higher precision and, particularly, higher spatial resolution measurements.
A particular priority must be to get
good coverage of the polar regions, in order to understand exchange processes going on in major phenomena like sudden warmings in northern winters and the onset and decay of the Antarctic ozone hole.
(viii)
Stratospheric-tropospheric exchange.
Water vapour has
traditionally been the focus for observations of species which allow inferences to be drawn about the mechanism whereby tropospheric air enters the stratosphere and vice-versa. Information will also be present in studies of methane and nitrous oxide gradients near the tropopause.
FUTURE
OBSERVATIONS
It is likely that all of the issues discussed above will be resolved, or at least addressed, by new data which will be obtained from satellite and other measurement programmes presently under development.
One of the most important, and
most imminent, is the Upper Atmosphere Research Satellite, which will carry an improved version of SANS (ISAMS) into orbit in August 1991.
The following table shows how the basic parameters
of ISANS compare to SANS.
Note in particular the improvement in
vertical resolution, in sensitivity, and in global coverage. see in more detail
how the expected instrumental
improvements
To
(1)42
affect
F. W. Taylor
the vertical
coverage and precision
of the CR4
and N20
measurements, test retrievals incorporating estimates of all known error sources have been carried out (C.J. Marks, private communication).
The results suggest that reliable profiles can
be expected from 15 to 50 km for N20 methane will be forthcoming.
and 15 to 60 km for
Within this range, the principal
source of error is likely to be uncertainties in the spectroscopic line parameters, which limit the precision to the region of 10%. ________________________
SANS
ISAMS
Vertical Resolution
10 km
2.4 km
Integration time (typical, for a signal to noise ratio of 100:1)
20 sec
2 sec
6
32
CO2. H~O,CH4, CO, N20
CO2. H20, CH4, CO, N20, NO, NO2, N205, H~JO3, 03, window
Number of detectors Species observed
Latitude coverage
Table 1.
50°S
-
70°N
80°S
-
80°N
Comparison between SANS and ISANS parameters.
For measurements close to the tropopause, channels chosen for their relative transparency, while the remaining opacity continues to be dominated by methane or nitrous oxide, need to be used.
The High-resolution Infra Red Dynamics Limb Sounder
(HIRDLS) intended for the Eos-A polar platform in 1997 includes this facility.
It has been calculated that the HIRIJLS CH4
Stratospheric CH
4 and N20
channel,
(1)43
for example, will measure down to heights as low as 7
to 8 kin above the surface under cloud-free conditions.
REFERENCES 1 (in press). 1.
Handbook for Map, Vol.3
2.
F.W. Taylor. Remote sounding of the middle atmosphere from
satellites: the Stratospheric and Mesospheric Sounder experiment on Nimbus 7.
3.
Surveys in Geophysics,
9, 123-148 (1987).
F.W. Taylor, Dudhia, A., and Rodgers, C.D. Reference models
for CH 4 49-62
4.
and N20 in the middle atmosphere. Adv. Sp. Res., 7, 9, (1987).
F.W. Taylor and A. Dudhia.
and Trends.
5.
‘Adv. Space Research, 10, 6, 65-70 (1990).
G. Brasseur and S. Solomon.
Atmosphere.
6.
Reference Model for CH4 and N20
D. Reidel, Dortrecht (1986).
R.L.Jones, J.A.Pyle, J.E.Harries, A.M.Zavody, J.M.Russell,
and J.C.Gille.
The water vapour budget of the stratosphere
studied using LIMS and SANS data. 112,
Aeronomy of the Middle
1127-1143
(1986).
Quart.J. Roy. Meteor. Soc.,
(1)44
7.
F. W. Taylor
R.M. Goody and Y.L.Yung.
Atmospheric Radiation. Theoretical
Basis. Oxford University Press (1989).
8.
R.L.Jones and J.A.Pyle.
observations
of CR4
and N20
by
the Nimbus 7 SANS: A comparison with in situ data and twodimensional model calculations. J. Geophys. Res. 5279,
9.
(1985)
89, 5263
-
-
Taylor, F.W., and A.Dudhia. Satellite measurements of middle
atmosphere composition. 576,
,
Phil Trans Roy Soc Lond.,
A323, 567-
(1987)
10. R. Garcia and S. Solomon.
A numerical model of the zonally-
averaged dynamical and chemical structure of the middle atmosphere. J. Geophys. Res.,88, 1379,
11.
Gray, L.D. and
(1983).
J.A.Pyle. The semi-annual oscillation and
equatorial tracer distributions. Quart.J. Roy. Meteor. Soc., 116, 387-407
12.
(1986).
S. Solomon, J.T. Kiehl, R.R. Garcia, and W. Grose. Tracer
transport by the diabatic circulation deduced from satellite observations. J. Atmos.
Sci.,
43, 1603-1617
(1986).
0273-1177193 $15.00 Copyright ~ 1992 COSPAR
STRATOSPHERIC METHANE AND NITROUS OXIDE F. W. Taylor Department ofAtmospheric, Oceanic and Planetary Phsycis, Oxford University, Clarendon Laboratory, Oxford OXJ 3PU, U. K
ABSTRACT
A model of the abundances of nitrous oxide and methane in the stratosphere has been produced using data from the Stratospheric and Mesospheric Sounder on Nimbus 7, and reported in the Handbook for Map, Vol.31 (in press).
This paper describes the
data which went into the model, its limitations, and the general behaviour of methane and nitrous oxide in the middle atmosphere. Plans for preparing an improved model from new data expected from the Upper Atmosphere Research Satellite program beginning in late 1991 are outlined.
(1)3 1
(1)32
F. W. Taylor
INTRODUCTION
A model of the abundances of nitrous oxide and methane in the stratosphere has been reported in the Handbook for Map, Vol.31
/1/.
The data for this came primarily from the Stratospheric
and Mesospheric Sounder on Nimbus 7 /2/. Rodgers
Taylor,
Dudhia &
(1986) described the model in an earlier paper /3/,
while Taylor and Dudhia /4/ performed an error analysis and analysed three and five year data sets for trends in the abundance of either species at any level in the range 20 to 0.2 mb pressure (25 to 62 kin altitude).
No trend was detected, but
this is not surprising since the errors in the observing technique are quite large and the changes which would be expected, even over a five year span, are only a few percent at most.
New measurements with better accuracy and precision are planned for the Upper Atmosphere Research Satellite program beginning in late 1991.
With these, it will be possible to produce an
improved model with better global and height coverage, as well as smaller errors.
There will be a much better chance of
observing trends in the abundances, especially if the UARS measurements are extended beyond their initial 1.5 year goal, and in any case the UARS data set can be compared to the Nimbus observations from ten to fifteen years earlier.
Finally, it is
to be hoped that improvements in both satellite and in-situ techniques will lead to some resolution of the discrepancies with balloon data noted previously /4/.
Stratospheric CH
4 and N20
STRATOSPHERIC
(1)33
METHANE
All of the methane in the atmosphere originates
at the surface,
mostly in plant matter in humid regions (especially bogs and marshes) and in animal interiors, especially ruminants.
It is
estimated /5/ that 443 to 850 megatons is produced annually by these biological sources.
In addition, there is an estimated 16
to 110 megatons from non-biological (chiefly industrial) sources.
The total atmospheric burden of methane is about 4,600
megatons.
Removal is by reactions with hydroxyl, atomic oxygen, or atomic chlorine, or by photolysis, and the mean lifetime is around seven years.
The main product of methane destruction is water
vapour, but carbon monoxide (as an intermediate product, which finally itself is oxidised by OH to form carbon dioxide), and molecular hydrogen, are produced as well. showed that the sum
[2 CH4
+
H2O]
Jones et al /6/
is approximately conserved
aver a large range of latitudes and heights in the stratosphere, showing that the proportion of H2 produced is relatively small, as expected.
Methane is a strong infrared absorber in spectral regions where the atmosphere would otherwise be relatively transparent, and is therefore
an important component of the atmospheric
‘greenhouse’, coming after CO2 and H20.
Because the column
abundance of CH4 is small compared to the latter two gases, the
(1)34
F. W. Taylor
transmittances in the methane bands are not in general in the saturated part of the curve of growth /7/ and increases in the methane burden therefore have a proportionately larger incremental greenhouse contribution.
Thus, the fact that
methane is observed to be increasing near the surface by more than 1% per annum is of particular interest, as is its fate in the middle atmosphere.
Methane in the stratosphere exhibits a considerable natural variability, as we would expect from its injection from the troposphere at preferred places and times and its subsequent redistribution by the prevailing motions in the stratosphere. This, together with its intermediate lifetime (long enough to survive in parcels of air following global-scale trajectories, but not so long that the mixing ratio tends to become constant everywhere), make methane a good dynamical tracer/5/.
Nitrous oxide is also a species of intermediate lifetime (several decades in the stratosphere) which originates at the surface in a variety of natural and anthropogenic processes (fixation of nitrogen by bacteria, lightening, artificial fertilisers,
etc.).
Destruction in this case is mainly by
photolysis to form nitrogen and atomic oxygen.
Stratospheric CH
(1)35
4 and N20
GLOBAL
OBSERVATIONS
NITROUS
OF
STRATOSPHERIC
METHANE AND
OXIDE
The only global observations to date are from the from the Stratospheric and Mesospheric Sounder (SANS) on Nimbus 7 /2/. The SANS data extend from 50°Sto 70°Nin latitude, and have useful accuracy for a vertical range from 20 mb ( 0.2 mb
(
60 kin) for CH4
and 0.6 mb (
25 km) up to
53 kin) for N20.
The
accuracy, according to Jones and Pyle /8/, who combined conservative estimates of all of the known sources of error including spectroscopic and retrieval uncertainties, and noise due to instrumental sources and spacecraft jitter, is at best 20% for CH4 and 25% for N20. The corresponding precision is -3% for CH4 and -6% for N2O.
Comparisons have been made between SANS retrievals and other measurements, particularly from balloons. The in-situ and satellite data agree quite well near the top of the region of overlap (about 10 mb or 30 kin) but lower down discrepancies occur with the SANS amounts being systematically higher. The reason for this are not likely to be known until new observations are forthcoming/4/.
IMPLICATIONS
FOR
DYNAMICS
The dynamical implications of the SANS-observed fields of both methane and nitrous oxide have been discussed in general terms
(1)36
F. W. Taylor
by Taylor and Dudhia /9/
and Taylor /2/.
The former includes
colour maps of the residual distribution of methane (that is, the deviation from the mean value) as a function of height, latitude and season for zonal and time averaged observations by SANS, which are useful for understanding the following summary. The overall appearance of the plots for CH4
and N2O
is quite
similar, tending to confirm that their distributions are controlled more by dynamics than chemistry.
The general characteristics of the distributions feature negative gradients with latitude and with height, so that the general appearance is of a bulge over low latitudes. Following the monthly changes, the most noticeable trend is for the bulge to migrate towards the Summer hemisphere. This is due to the seasonal dependence of the mean circulation, a feature of which must be more rapid vertical transport in the summer than in the winter.
The total amounts of methane and nitrous oxide, and
hence presumably of all minor constituents having their sources in the troposphere, in each hemisphere rises and falls with the seasons.
Local maxima
relative to the mean occur at about 0.6
mb and 20°latitude, towards the end of Summer, i.e. in September/October in the Northern Hemisphere and March/April in the Southern (Plate 2 of ref. 9).
Minima occur at the same
latitudes six months later in each case. The
late summer low-
latitude maxima (in the N hemisphere at least) are accompanied by higher-latitude minima, and vice-versa. The 60°N minimum in
-
L
_‘
-
~
Stratospheric CH
~~-r
4 and N20
0.3~
(1)37
8
5 -
30
-
—
50S
u
r
40$
30$
~
~ i
20S
los
r
o
r
U
iON
20N
3aM
40N
50N
i
60N
3 70N
lat.i,tude
Fig..1
Mean latitudinal distribution of stratospheric methane
(ppmv) for March (solid line) and September (dashed line) /2/.
CH4
concentration could be produced by a mass of descending
air, which has spent an extended period of time in the stratosphere and so become depleted of CH4 by reaction with OH and other radicals. A single circulation cell with maximum upwelling at 200 and maximum downwelling at about 60°, building up in intensity during the summer and declining in the winter, would then be implied.
The reversal of the abundance maxima and
minima during late winter does not imply necessarily that the circulation reverses, since these are maxima and minima relative to the mean values at that particular latitude. In fact, the absolute amounts of methane and nitrous oxide are always lower at 60°N than at 200 N. Thus,the sense of the circulation is
(1)38
F. W. Taylor
always from equator to pole, only its strength changing with the solar energy available
to drive it.
The data also show that at
any given latitude, the zonal mean abundances tend to peak earlier at higher altitudes,
the opposite to the behaviour to be
expected if material from the troposphere was simply being advected vertically. There has therefore to be strong horizontal, poleward, advection as well.
This is qualitatively
consistent with theoretical expectations (c.f. fig.2).
l~6
L......—’
~
~
E~(O~
I
:!--
~--:.-~-
--
~
36
16
\~.
~
60
SUMMER
Fig.2
Streamlines
30
0
‘~::
30
LATITUDE (degrees)
_~: -
60
90
WINTER
(heavy lines) and net diabatic heating and
cooling, from the model of Garcia and Solomon /10/
Stratospheric CH
4 and N20
OUTSTANDING
(1)39
QUESTIONS
Next we summarize the main uncertainties which exist in the seasonal variation of of CH4 and N20 abundances, and the scientific questions which are raised thereby
The first three
arise from prominent features of the SANS fields, which are observed but remain only very marginally understood.
(1)
The
‘double peaked’ structure.
This is strongest in April
/8/ but can also be seen in other months (cf. fig.1). It appears to be due to downward velocities over the equator which occur as a consequence of the semi-annual oscillation in tropical temperature and zonal wind. /11,12/
(ii)
Other long-period oscillations.
At high altitudes (near
the 0.2 mb level) there is a pronounced semi-annual oscillation in the which is not directly related to the semi-annual oscillation in temperature at tropical latitudes.
The methane
SAO occurs at higher latitudes than the temperature SAO and is much more prominent in the Southern than the Northern Hemisphere.
(iii) Non-seasonal trends.
At
the highest levels at which CH4
was observed, i.e. 0.2 mb or around 60 kin,
all latitudes in
both hemispheres tend to have maxima around September, and minima around March. This effect may be associated with the eccentricity of the Earth’s orbit, and if so it suggests that increased advection of ex-tropospheric air upwards due to
(1)40
F. W. Taylor
stronger heating
dominates increased photolytic
destruction,
since the solar intensity is greatest in the Northern Spring.
A number of other issues, not arising directly from SANS findings but relevant to the improvements to be desired in future reference models,
(iv)
and
Contribution N2O
can be listed as follows.
to greenhouse
effect.
The precise role of CH4
in determining the temperature structure of the
atmosphere through their contribution to heating and cooling rates by radiative transfer needs clarifying.
(v)
Trends in abundances.
The surface production rates of both
gases are thought to be increasing, although the existing global data is not precise enough to detect stratospheric changes /4/ and local measurements cannot adequately distinguish trends from seasonal and other fluctuations.
(vi)
Chemical budgets.
Much remains to be done to show that
the oxidation of methane is in detailed balance with the stratospheric water and carbon monoxide budgets, and the oxidation of nitrous oxide with nitric oxide and other species involved in ozone chemistry. determine,
for example,
It would be very valuable to
how much free hydrogen is produced in
the methane oxidation cycle, and how much N2O is changed by photolysis back to N2 after reaching the stratosphere.
Stratospheric CH
4 and N20
(vii)
Polar Dynamics.
(1)41
There is almost endless scope for
improving the use of CH4 and N20
as dynamical tracers by
obtaining higher precision and, particularly, higher spatial resolution measurements.
A particular priority must be to get
good coverage of the polar regions, in order to understand exchange processes going on in major phenomena like sudden warmings in northern winters and the onset and decay of the Antarctic ozone hole.
(viii)
Stratospheric-tropospheric exchange.
Water vapour has
traditionally been the focus for observations of species which allow inferences to be drawn about the mechanism whereby tropospheric air enters the stratosphere and vice-versa. Information will also be present in studies of methane and nitrous oxide gradients near the tropopause.
FUTURE
OBSERVATIONS
It is likely that all of the issues discussed above will be resolved, or at least addressed, by new data which will be obtained from satellite and other measurement programmes presently under development.
One of the most important, and
most imminent, is the Upper Atmosphere Research Satellite, which will carry an improved version of SANS (ISAMS) into orbit in August 1991.
The following table shows how the basic parameters
of ISANS compare to SANS.
Note in particular the improvement in
vertical resolution, in sensitivity, and in global coverage. see in more detail
how the expected instrumental
improvements
To
(1)42
affect
F. W. Taylor
the vertical
coverage and precision
of the CR4
and N20
measurements, test retrievals incorporating estimates of all known error sources have been carried out (C.J. Marks, private communication).
The results suggest that reliable profiles can
be expected from 15 to 50 km for N20 methane will be forthcoming.
and 15 to 60 km for
Within this range, the principal
source of error is likely to be uncertainties in the spectroscopic line parameters, which limit the precision to the region of 10%. ________________________
SANS
ISAMS
Vertical Resolution
10 km
2.4 km
Integration time (typical, for a signal to noise ratio of 100:1)
20 sec
2 sec
6
32
CO2. H~O,CH4, CO, N20
CO2. H20, CH4, CO, N20, NO, NO2, N205, H~JO3, 03, window
Number of detectors Species observed
Latitude coverage
Table 1.
50°S
-
70°N
80°S
-
80°N
Comparison between SANS and ISANS parameters.
For measurements close to the tropopause, channels chosen for their relative transparency, while the remaining opacity continues to be dominated by methane or nitrous oxide, need to be used.
The High-resolution Infra Red Dynamics Limb Sounder
(HIRDLS) intended for the Eos-A polar platform in 1997 includes this facility.
It has been calculated that the HIRIJLS CH4
Stratospheric CH
4 and N20
channel,
(1)43
for example, will measure down to heights as low as 7
to 8 kin above the surface under cloud-free conditions.
REFERENCES 1 (in press). 1.
Handbook for Map, Vol.3
2.
F.W. Taylor. Remote sounding of the middle atmosphere from
satellites: the Stratospheric and Mesospheric Sounder experiment on Nimbus 7.
3.
Surveys in Geophysics,
9, 123-148 (1987).
F.W. Taylor, Dudhia, A., and Rodgers, C.D. Reference models
for CH 4 49-62
4.
and N20 in the middle atmosphere. Adv. Sp. Res., 7, 9, (1987).
F.W. Taylor and A. Dudhia.
and Trends.
5.
‘Adv. Space Research, 10, 6, 65-70 (1990).
G. Brasseur and S. Solomon.
Atmosphere.
6.
Reference Model for CH4 and N20
D. Reidel, Dortrecht (1986).
R.L.Jones, J.A.Pyle, J.E.Harries, A.M.Zavody, J.M.Russell,
and J.C.Gille.
The water vapour budget of the stratosphere
studied using LIMS and SANS data. 112,
Aeronomy of the Middle
1127-1143
(1986).
Quart.J. Roy. Meteor. Soc.,
(1)44
7.
F. W. Taylor
R.M. Goody and Y.L.Yung.
Atmospheric Radiation. Theoretical
Basis. Oxford University Press (1989).
8.
R.L.Jones and J.A.Pyle.
observations
of CR4
and N20
by
the Nimbus 7 SANS: A comparison with in situ data and twodimensional model calculations. J. Geophys. Res. 5279,
9.
(1985)
89, 5263
-
-
Taylor, F.W., and A.Dudhia. Satellite measurements of middle
atmosphere composition. 576,
,
Phil Trans Roy Soc Lond.,
A323, 567-
(1987)
10. R. Garcia and S. Solomon.
A numerical model of the zonally-
averaged dynamical and chemical structure of the middle atmosphere. J. Geophys. Res.,88, 1379,
11.
Gray, L.D. and
(1983).
J.A.Pyle. The semi-annual oscillation and
equatorial tracer distributions. Quart.J. Roy. Meteor. Soc., 116, 387-407
12.
(1986).
S. Solomon, J.T. Kiehl, R.R. Garcia, and W. Grose. Tracer
transport by the diabatic circulation deduced from satellite observations. J. Atmos.
Sci.,
43, 1603-1617
(1986).