The role of space in sparc research

~

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Adv. Space Res. Vol. 27, No. 8, pp. 1445-1456, 200i © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/01 $20.00 + 0.00 PII: S0273-1177(01)00213-7

THE ROLE OF SPACE IN SPARC RESEARCH C. Phillips 1, M.-L. Chanin2 and M. Geller3

ISPAR C Office, BP 3, 91371 Ferri&es-le-Buisson, France. 2Service D'A3ronomie, CNRS, BP3, 91371 Ferri&es-le-Buisson, France. 3State University of New York at Stony Brook, Stony Brook, NY 11794-5000, USA

ABSTRACT The research project on Stratospheric Processes And their Role in Climate (SPARC), was set up in 1992 by the World Climate Research Programme (WCRP) to help the stratospheric research community focus on the issues of importance for climate. In this paper, we present the atmospheric data requirements of the SPARC research community, in particular the needs for monitoring of climate and for understanding stratospheric processes. We present briefly the work carried out by the SPARC project and illustrate the role of space in this work using recent results from research carried out by SPARC working groups and the.stratospheric research community. Advantages of space-based data are presented, as are their weaknesses, the lessons learned when using such data for monitoring purposes and the challenges posed to future generations of space-based instruments. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION Stratospheric processes play a significant role in the earth's climate. The absorption of solar radiation in the stratosphere by ozone modulates the solar forcing of climate. The concentrations of some stratospheric gases, principally ozone, carbon dioxide and water vapour, determine significant radiative forcing terms, and there is two-way interaction between stratospheric and tropospheric dynamics. Recognising the importance of these processes for the climate system, the World Climate Research Programme (WCRP) set up, in 1992, a research project to study Stratospheric Processes and their Role in Climate (SPARC). The principal objective of this project is to help the stratospheric research community focus on the issues of particular interest to climate. The Scientific Steering Group (SSG) of the SPARC project work towards this, with the help of the SPARC office, through the SPARC newsletter, SPARC meetings and the preparation of SPARC reports, as well as the promotion of needed field measurement programs. This permanent focus provided by the project on the role of stratospheric processes in climate has proved to be very useful to a research community which must report regularly, through bodies such as the WMO and UNEP and through the Intergovernmental Panel on Climate Change (IPCC) of WMO/UNEP, to the Parties of the Montreal Protocol and the United Nations Conventions on Climate Change. The scientific objectives of the project are to improve: the understanding of stratospheric processes, the detection and attribution of any stratospheric trends which could affect, or be attributable to climate change, the modelling of stratospheric processes and trends and of their effects on climate. 1445

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The data requirements of research on stratospheric processes and their role in climate are very diverse. They have to be met by a great number of different technologies and platforms, including laboratory studies, ground-based remote sensing, in situ or remote sensing instruments on balloons or on aircraft, and space-based remote sensing instruments. In this paper, we will discuss the general data requirements for research on stratospheric processes and their role in climate. Then, after a brief introduction to the initiatives taken by SPARC, we will highlight some of the areas where space-based instruments have proven to be of fundamental importance to the project. Finally, we will summarise some of the lessons learned during this work regarding space-based instruments and the future needs for space-based instruments by the SPARC community.

THE DATA REQUIREMENTS FOR RESEARCH ON STRATOSPHERIC PROCESSES AND THEIR ROLE IN CLIMATE

The general data requirements of research for stratospheric processes and their role in climate are summarised in tables 1 and 2. These tables have been compiled to advise programmes such as the Global Climate Observing System (GCOS) and the Integrated Global Observing Strategy (IGOS), who are currently implementing global measurement systems. The rms accuracy at which these parameters should be measured is indicated, as is the maximum bias tolerated. The scientific motivation for measuring each parameter is indicated briefly in the column of remarks. Table 1 lists the parameters which need to be monitored over the long-term for the following: • the assessment of climate forcing due to the stratosphere, its variability and any trends in this forcing; • monitoring stratospheric climate, its variability and any long-term changes; • monitoring the attributed causes of stratospheric climate forcing and stratospheric climate; • the validation of models, enabling the prediction of the role of the stratosphere in determining the future climate. To better assess climate forcing due to the stratosphere, the atmospheric constituents involved in radiative exchange: 03, H20, N20, CH4, CO2 and the most abundant CFCs (F11, F12 and F22), must be measured, as must the solar forcing and aerosol properties (albedo, optical depth, concentration). For those constituents which are not well mixed horizontally (all except CO2), good global coverage is essential. In order to detect any changes in stratospheric forcing of climate due to either natural (e.g., solar cycle) or anthropogenic causes (e.g., greenhouse gases) the parameters must be measured over periods of time much longer than known cycles (e.g., 11 years for the solar cycle). High quality measurements of temperature and wind are essential to characterise the stratospheric climate, its current variability due either to internal modes of the climate system such as the quasi-biennial oscillation (QBO) or due to its response to a climate forcing. Finally, atmospheric data are essential for modelling studies, for the forcing, validation and testing of models, and they provide climate reference data and the initial conditions for forward projections. Table 2 lists the parameters required for studying stratospheric processes such as: • transport and mixing • chemistry • microphysics. These parameters are essential for modelling studies. They need not all be measured over the long-term, some need only be measured during intensive campaigns where many parameters are measured simultaneously and at high resolution. Also, models depend on parameterisations, and there is an additional need for data sets to develop and test these parameterisations. Table 2 does not include the measurements required for improving knowledge and the parameterisation of gravity waves processes, an

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important aspect o f S P A R C which is currently compiling a global climatology o f gravity wave parameters in the lower stratosphere using very high-resolution operational radio-sonde data (-50m), and is involved in planning an intensive field campaign to characterise gravity wave fields generated by tropospheric moist convection in the tropics, the Convective Excitation o f Gravity Waves Experiment (CEGWE) which is planned to take place in October-November 2001 or 2002. The table also does not include the laboratory measurements required for improving our knowledge o f the chemistry and microphysics of the atmosphere.

Table 1. Long-term monitoring requirements o f research on stratospheric processes and their role in climate. ' Requirement

Altitude

Accuracy

region

Rms

Accuracy] b Bias*

i

Remarks L

Temperature profile

1000 - 500 hPa

0.1K

500-100hPa

O.1K

I

100-10hPa

O.1K

t

< 10hPa

0.5K

i

1000- 10hPa

3ms -n

[

< 10 hPa

3ms -I

I

To assess the changes in stratospheric climate and the anthropogenic influence on the vertical profile of temperature trend in the atmosphere. High resolution at the tropopause is desirable to follow any trend in tropopause height

0.1

P

High priority for more measurements in the southern hemisphere.

I I

wind profile

6ms "l

To assess changes in stratospheric circulation due to climate change, and which could change the distribution of stratospheric forcing. High priority for more measurements in the tropics (QBO)

F

1

ozone profile

(03)

1000-10hPa

5%

! J I

Clouds

2%

] Stratospheric forcing of climate. i High priority for more measurements in the tropics. ! ', High vertical resolution is desirable near the hygropause.

i

< 10hPa

5%

1000- 10 hPa

1%

< 10hPa

5%

F

l

F i,

Frequency of occurrence of high altitude clouds (cirrus, contrails, PSCs)

aerosol profile I 1000 - 500 hPa (optical depth, 500 - 100 hPa particle size, 100 - 10 hPa composition)] < 10hPa COS

P To assess the changing effect on stratospheric i forcing of climate. L I High resolution at the tropopause is desirable for I ii climate forcing issues. iI High priority for more measurements in the tropics.

i J

i

water vapour i profile (H20)

1%

ii

i

Fb

i

i

10% 10%

20%

10% 10%

ground level

! .i

J

SO2

I

all

Stratospheric forcing of climate. To determine the background level of aerosol.

10%

20%

Precursor gas for background aerosol : indicates probability of trend. Aerosol precursor.

C. Phillips et al.

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Table 1 continued. Altitude

Accuracy

Accuracy

Requirement

region

Rms

Bias*

Remarks

N20 profile

all

10%

20%

Radiative forcing. To constrain the total amount of NOy.

CH4 profile

all

10%

20%

Radiative forcing. HOy chemistry, interpretation of H20 trend if measured simultaneously with H20

CO2 profile

all

Stratospheric forcing on climate.

F l l , F12, F22 profiles

all

HCI**

all

10%

20%

Cly chemistry (reservoir species)

CIONO2**

all

10%

20%

Cly and NOy chemistry : the role of heterogeneous processes in the stratosphere (Cly reservoir)

OC10

all

10%

20%

Clx and Brx chemistry

CIO

all

10%

20%

Clx chemistry

NO2 and/or NO

all

10%

20%

NOx chemistry

HNO3

all

10%

20%

NOy chemistry, tracer

BrO, OBrO

all

10%

20%

Brx chemistry

IO

all

10%

20%

Ix chemistry

OH, HO2

all

10%

20%

HOx chemistry

Source gases

all

Ozone depletion, tracers

CH3Br

ground level

most abundant source gas for Bry

Radiative forcing.. Most abundant CFCs : lead to estimates of total inorganic chlorine and total available chlorine.

Solar Constant Solar Flux Spectrum

Total solar flux will indicate changes in the solar constant I

Top of the atmosphere

0.1%

0.2%

to understand the observed correlation with atmospheric climate variables

i

(200-40Onto) I * must all be measured with continuity **rem : both of the most abundant Cly reservoirs, HC1 and C1ONO2 are needed. They may vary over time (season) and location Horizontal Resolution: 500km Vertical Resolution: 0.5 - 2 km Observation cycle: 0.5 - 3 days

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Table 2. Data requirements of research on stratospheric processes. Altitude

Accuracy

Accuracy

Requirement

region

Rms

bias

Temperature profile

1000 - 10 hPa

0.5K

0.2K with continuity

< 10hPa

[

ozone profile

1000 - 10 hPa

"

1%

i

(03)

< 10 hPa

'q

1%

i

wind profile

1000 - 10 hPa

Remarks

. . . . 2%

tracer of transport and mixing in the lower stratosphere and upper troposphere (with H20)

I

2 - 3 ms "l i

6ms't

I

< 10hPa aerosol profile

1000 - 10 hPa

I i l

< lOhPa

i

I

[

!i l

i

!i i

[

l

] frequency of occurrence and mechanism of formation

clouds polar stratospheric clouds all

water vapour isotopes

all

I

I

5%

all

I

] I I [

10%

:

10%

OCIO

all

[

10%

all

i

all

l

10%

HNO4

all

[

10%

HNO3

i

all

I

10%

N20

!

all

l

10%

SO2

;

all

[

10%

BrO

I

all

I

all

CH4

i

all

Source gases

J

i

all

OH,

NO3

HO2

I i

!

i

cirrus in the upper troposphere, sub-visible stratospheric cirrus clouds, contrails

i

[ potential to study the processes affecting water vapour ]l distributions in the upper trop. and lower strat. 20% 20%

[

20%

i Cly and NOy chemistry : the role of heterogeneous processes in the stratosphere (Cly reservoir) Clx and Brx chemistry !: J

Clx chemistry

,

NOy chemistry

~,

NOy chemistry

20%

'

NOy chemistry, tracer

20%

,i

to constrain the total amount of NOy

20%

[

aerosol precursor

20%

i

Brx chemistry

10%

20%

[

HOx chemistry

10%

20%

i

HOy chemistry, tracer

~i I

Ozone depletion, tracers transport "age" of stratospheric air

j

',

transport "age" of stratospheric air

'

i

I I

!

i i

'

20%

Cly chemistry (reservoir species)

i

20% l

tracer of transport and mixing in the lower stratosphere and upper troposphere (with 03)

20%

i

i

!

I

I

isotopes of CO2 SF6

]

10%

;

N205,

[

] i

,

i i j'

all

CIO

2%

,

C1ONO2 *

NO2,

. Optical depth, particle size and composition required

l

water vapour profile (HzO) [

HC1 *

10%

~E k [ 20%

10%

l

!

i

i Top of the 0.1% 0.2% i in order to understand the observed correlation with Solar Flux i atmosphere ! atmospheric climate variables Spectrum (200 - 300 nm) *rem : both of the most abundant Cly reservoirs, HCI and CIONO2 are needed. They may vary over time (season) and location Horizontal Resolution: 100km. Vertical Resolution: 0.5 km. Observation cycle: 1 minute - 1 day

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THE INITIATIVES OF THE SPARC RESEARCH PROJECT The objectives of SPARC are described in detail in the SPARC Implementation Plan (WCRP report No.105, 1998). The SPARC project builds on the existing work of the stratospheric research community, a very diverse community of numerous different scientific disciplines using varied research tools. The SSG identifies the areas which need extra focused attention, builds links between the different disciplines, where necessary, and encourages new areas of research of fundamental importance for climate, as needed. To date, the SPARC project has taken initiatives (set up working groups, organised workshops, or written review articles or reports) on the following themes: 1) detection of stratospheric trends which indicate climate change or could affect climate : • detection of stratospheric temperature trends • detection of trends in the vertical distribution of ozone • compilation of a water vapour climatology and detection of long-term changes • detection of trends in the dynamical activity in the stratosphere 2) understanding stratospheric processes and their relation to climate : • dynamics and transport in the lower stratosphere and upper troposphere • chemistry and microphysics in the lower stratosphere and upper troposphere • gravity wave processes • the quasi-biennial oscillation and its possible role in coupling the stratosphere and troposphere 3) modelling stratospheric processes and trends and their effects on climate : • modelling of observed stratospheric temperature trends • improving the parameterisation of gravity wave processes • improving climate-middle atmosphere modelling • compilation of a stratospheric reference climatology against which model results can be compared using global satellite data. Finally, a new initiative taken by the SPARC project in 1997 is to provide, for the climate modelling community, the current best estimates of stratospheric parameters which play a role in climate forcing, for example of the variation over time of the forcing due to the changes in stratospheric ozone and aerosol. The WCRP has also asked the SPARC project to help define the solar radiance forcing to be used in climate models, an activity to be carried out jointly with the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP). It is also envisaged, to work with the International Global Atmospheric Chemistry (IGAC) project and the WMO Global Atmosphere Watch (GAW) to improve knowledge of the penetration of UV radiation in the lower stratosphere and troposphere. EXAMPLES OF THE ROLE SPACE HAS PLAYED IN SPARC RESEARCH In this section, we will firstly describe the role space-based instruments have played in monitoring stratospheric temperature and the vertical distribution of ozone, then we will illustrate briefly the role space measurements have played in the understanding of stratospheric processes. Monitoring Two important assessments have been carried out recently by the SPARC project: an assessment of trends in the vertical distribution of ozone (SPARC report no. 1, 1998) and an assessment of trends in stratospheric temperature (to be published in 1999). The work carried out for these reports has contributed to the 1998 WMO/UNEP Assessment of Ozone. We will highlight below only those aspects

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of the assessments which deal with the use of space-based instruments, and any lessons learned for future monitoring. Assessment of trends in the vertical distribution of ozone. SPARC, together with the International Ozone Commission (IOC) and the WMO/GAW, has conducted an assessment of trends in the vertical distribution of ozone (SPARC report no. 1, 1998) using an ensemble of ground-based (Umkehr, ozonesondes) and space-based (SAGE, SBUV) measurement systems. The assessment addressed the following three issues: • the existing data sets were reanalysed with particular attention paid to the inherent errors in the trends determined by each measurement system and their associated algorithms; • the stability of the data sets used for the determination of trends was tested by comparing them with other available data sets: ground-based lidar measurements and space-based measurements made by the HALogen Occultation Experiment (HALOE) and the Microwave Limb Sounder (MLS), both on board the Upper Atmosphere Research Satellite (UARS). State of the art validation techniques were used and the stability of the long-term data sets was shown to be coherent with the inherent errors predicted by the analysis of the measurement systems; • different trend calculations were compared and the sensitivity of the resulting trend to, for example, gaps in the data and natural variability such as the QBO and solar cycle was tested. Of particular scientific interest to the SPARC project was the determination of the trend in the lower stratosphere. In a previous assessment of ozone trends (WMO/UNEP, 1995), a major discrepancy had been found between the trends obtained at northern mid-latitudes using the SAGE instruments (-20+8 %/decade) and ozone sondes (-7+3 %/decade). Since model studies show that any changes in ozone in the lower stratosphere/upper troposphere are crucial for climate, it was considered essential to fully understand the cause of this discrepancy. Only two long-term space-based data sets are available to derive trends in the vertical distribution of ozone: that from the Stratospheric Aerosol and Gas Experiment (SAGE I and SAGE II), and that from the Solar Backscattered UltraViolet instruments (SBUV on board Nimbus-7 and SBUV2 on board NOAA11). The SAGE instruments are limb-scanning UV-visible spectrometers. The SAGE I data set extends from 1979-81, and the SAGE II data set from 1984 onwards. Wang et al. (1996) showed that there is an important systematic error in the SAGE I data above 20km. Since, to date, there has been no new retrieval of the SAGE I data to account for this error, an ad-hoc correction was used during the assessment. The SAGE II data used in the assessment were retrieved using version 5.96 of the algorithm, which included, among other improvements, a better removal of aerosol influences (see assessment for details). The SBUV instruments are nadir-viewing spectrometers working in the ultraviolet. The SBUV data set extends from November 1978 to May 1990, and the SBUV2 data set from January 1989 to October 1994. Both data sets were processed using the Version 6 BUV algorithm (Bhartia et al., 1996). Great care was taken to resolve calibration issues in these data sets, and additional corrections were brought to both. The SBUV data include a calibration which corrects for time-dependent instrument changes. The SBUV2 data were reprocessed using updated calibrations and instrument behaviour characterisations and an algorithm change to correct for grating position errors in the latter part of the record. The absolute calibration was adjusted to match the Shuttle SBUV (SSBUV) calibration and a time-dependent calibration was maintained using an on-board calibration lamp-system and verified through comparisons with SSBUV measurements. Finally, empirical adjustment factors were used to combine the two data sets (see report for details).

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C. Phillips et al.

The trends obtained at northern mid-latitudes using the four long-term measurement systems available are illustrated in Figure 1. General agreement is found between all the trends, even in the lower stratosphere, where the large discrepancy observed previously between the trends obtained using SAGE I&II and using ozonesondes is now resolved. The trends now obtained at 20 km using these two systems are respectively -4.90_+2.84 %/decade and -5.99_+1.07 %/decade. However, despite this improved agreement in the lower stratosphere at northern mid-latitudes, confidence in the trends obtained in the tropics and southern midlatitudes is still not high as only one measurement system is available there, the SAGE data set. In order to have confidence in the trends obtained at these latitudes, at least two independent measurement systems are necessary. Since ground-based data are limited in their geographical coverage (very concentrated at northern midlatitudes), space-based data have the advantage of near-global coverage. To date they have, however, been limited to altitudes above the tropopause, at most latitudes, and are heavily perturbed by the presence of volcanic aerosol after major volcanic eruptions, as was the case after the eruptions of El Chichon in April 1982 and Mount Pinatubo in June 1991. The SAGE II data were contaminated for approximately 2 years after the Pinatubo eruption, despite the improvements in the algorithm designed to minimise these perturbations. Since the error bars reflected this contamination, only data with error bars less than 12% were used for trend analyses. Similarly, the SBUV and SBUV2 data equatorward of 40 ° were invalidated for one year after the two volcanic eruptions.

Umkehr ( 2 - s i g m a )

50

I/II

SAGE

(Z-sigma) [

I I I

50

4O

4O

30

~0 30

.e

zo

~ 2o

I I I I

10

I I I I I I 0 -15.0-12.5-10.0-7.5-5.0-2.5 0.0 T r e n d (PC/decade)

10 i 2.5

0

I

I

I

I

I

-15.0-12.5-10.0-T.6-5.0-2.5

5.0

I

0.0

2.5

5.0

Trend (Z/decade)

SBUV (2-slgma)

Sonde (Z-sigma)

50

50

40

40 ,ti

• 30

30 "0

"0

= 20

~ ~o

=.

10 ,

-15.0-12.5-10.0-7.5-5.0-2.5

Trend (Z/decade)

0.0

2.5

0

5.0

I

I

I

I

I

~

-15.0-12.5-10.0-7.5-5.0-2.5 0.0 Trend (Z/decade)

I

2.5

5.0

Fig. 1. Estimates of the mean trend and combined statistical and instrument drift uncertainty at northern midlatitudes for a) Umkehr, b) SAGE I/II, c) SBUV/SBUV2, and d) ozonesondes. Estimates were made at 2.5 km intervals from 2.5 to 20 km and 5 km intervals from 20 to 50 km. Uncertainties shown are 2a. Note that the sonde results were obtained as variance-weighted means. In the troposphere, there is an additional unquantified uncertainty resulting from the representativeness of the small number of stations. (SPARC/IOC/GAW Assessment of trends in the vertical distribution of ozone, 1998).

Role o f Space in SPARC Research

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Assessment of trends in stratospheric temperature. A SPARC working group is currently carrying out a thorough evaluation of the temperature trends in the stratosphere using and intercomparing all available sources of data from ground-based instruments (radiosonde, rocketsonde and lidar), satellite instruments (MSU and SSU) and reanalyses. Satellite-based measurements of stratospheric temperature have been available since 1979 using two nadir-viewing instruments: the Microwave Sounding Unit (MSU) which observes microwave emissions from atmospheric oxygen and the Stratospheric Sounding Unit (SSU) which observes emissions in the thermal infrared. These instruments offer global coverage with a vertical resolution larger than 10 km. The trends in the temperature profile at ~30°N obtained by the different data sets from 1979-1994 are illustrated in Figure 2. At 30°N, cooling is observed at all altitudes from 15 to 50 km (100 to 1 hPa). The trends deduced from the space-based instruments are shown in Figure 2(c). The vertical resolution of the space-based measurements is shown by the vertical bars. In the lower stratosphere, the peak in the emission for the SSU Channel 15X comes from 50 hPa, while for the MSU Channel 4, it is centred around 90 hPa. It is, therefore, difficult to directly compare the trends measured by these two systems and to compare the satellite data sets with the other data sets. Therefore, although the satellite instruments offer global coverage, because of their low vertical resolution, for a complete picture of temperature trends, ground-based data are essential, especially in the lower stratosphere/upper troposphere where a vertical resolution of the order of 1 km would be optimal. At tropical latitudes, for example, a substantial part of the lower stratospheric satellite signal originate from the upper troposphere, posing a problem for the interpretation of the actual trend.

(a) 19;~-{MTempe~a~ " [ h ~ ''1''''1

I

(b)

28-38N

1979-g4Tempen~umTmn~

(c)

28- 38N

197g-84Tempendum1'ranch

28-38N

II

O.3

0.3

55

0.3 1

1

45 4O

3

.g

~

10 20 80

3

11°

i,°

3O

80

7O

70 100

15

mRocket 2.ilPl a Rocket 34N -3.0

-2.0 -i .0 O Trend (deom/d~ade)

10 1.0

100

~

• Bedln

~

-3.0

-2.0 -1.0 0 "rrond {doom/de(~e)

~

~

L

-115

o CPC

j

~I

I 10 1.0

-3.0

-2.0 *I.0 0 Trend {degree/decade)

1.0

Fig. 2. Vertical profiles of trends in temperature from 1979 to 1994 at -30°N from different data sets. Horizontal bars denote statistical significance at the 26 level while vertical bars denote the approximate altitude range "sensed" by the satellites (courtesy of the SPARC Stratospheric temperature trends assessment). Since these temperature trends are derived from multiple instruments, be it satellite instruments or radiosondes, which were designed for the needs of meteorology and not for the long-term monitoring of very small changes, the interpretation of the data has to take into consideration problems related to data quality and continuity. The time continuity, instrument calibration and orbital drift of the satellite data pose particular problems. Wentz and Schabel (1998) have shown recently that decay over time of the satellite orbital height (due to the drag of the atmosphere on the satellite) has an effect on the temperature trends derived from MSU off-nadir channels. This has a significant effect on trends derived in the lower troposphere, but at the time of writing this paper it is not yet clear to what extent trends derived from the

1454

C. Phillips et al.

off-nadir SSU channel 15X are affected. MSU Channel 4 and SSU channels 25, 26 and 27 are all nadir channels, so the trends deduced from these measurements are not likely to be greatly affected. The interpretation of the SSU data is also complicated by the fact that there are discontinuities in the time series owing to the measurements being made by different satellites since 1979. Stratospheric Processes (Chemistry_, Transport and Mixing) Space-based measurements of chemical constituents have greatly improved our understanding of the chemistry of the atmosphere. Most recently, high-quality global and simultaneous measurements of many constituents by instruments onboard UARS, have provided invaluable data sets for testing our knowledge of chemical processes, as have instruments which have flown onboard the Space Shuttle (e.g., the ATMOS missions). Measurements of chemical constituents are also used to diagnose mixing and transport processes. The data available from various instruments onboard UARS have been of great use, since the spatial resolution and duration of the data permit quantitative analyses which can be compared with models. A striking example was the demonstration by Mote et al. (1995) of the vertical propagation of the seasonal cycle in water vapour in the tropics using data from the Microwave Limb Sounder (MLS), later confirmed using data from HALOE, SAGE II and the Cryogenic Limb Array Etalon Spectrometer (CLAES) (Mote et al., 1996). Another instrument which promises to be of great value is the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA), flown on the Space Shuttle. This infrared limb sounder is designed to measure the vertical distribution of up to 20 atmospheric trace gases with a high temporal and spatial distribution (<500km x 600km) (Grossmann et al., 1994). The aim of the experiment is to show to what extent small and medium scale dynamical features influence the distribution of trace gases and how frequently these features are observed. Results of the CRISTA-2 mission, for example, have confirmed that around 30 km altitude, horizontal transport of stratospheric trace gases from the tropics to mid-latitudes takes place in the form of streamers or "tongues" as proposed by Chen et al. (1994). THE FUTURE ROLE OF SPACE IN SPARC RESEARCH For future research on stratospheric processes and their role in climate, two different types of instrument will be required, operational monitoring systems and research satellites designed for process studies. Also, since some of the space-based data requirements of SPARC research are not yet met using current technologies, new technologies will have to be developed. Climate Monitoring The SPARC assessments of trends in the vertical distribution of ozone and of stratospheric temperature have underlined the importance of ensuring, in future climate monitoring systems, the quality, continuity and homogeneity of data over the long term. Only if these are guaranteed will it be possible to continue extracting small long-term signals for parameters which have relatively large natural variability. Ensuring the quality of the data from future space-based instruments will require ensuring the calibration and validation of the data, and recording the knowledge of the history of the instrument and platform. Ensuring the continuity of data will minimise biases introduced by changing instruments and/or instrument algorithms. The homogeneity of the data (i.e., an even sampling rate and global spatial distribution) are essential when studying processes which act on a range of space and time scales. Finally, the performance of a climate monitoring system must be regularly re-evaluated if the data are to be of real value for climate studies.

Role of Space in SPARC Research

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For the purposes of monitoring atmospheric temperature, a new nadir-viewing instrttment, the Advanced Microwave Sounding Unit (AMSU), is due to be launched in 1998 to replace the MSU and SSU instruments. Let us underline here the importance of operating the two systems in parallel for a while to eliminate any biases in future derivation of temperature trends. Currently, temperature measurements with high vertical resolution are made using the limb-scanning instruments flown with SAGE II, and onboard UARS. Future limb scanning instruments are planned which will continue to measure temperature at high vertical resolution (e.g., GOMOS on Envisat, SAGE III), however there is as yet no plan to develop an operational monitoring system with a high vertical resolution (see below for the possibilities offered by new technologies). Numerous instruments are planned to measure stratospheric ozone, but the data sets produced will only be of using for monitoring purposes if great care is taken to ensure their quality, calibration and continuity. The CEOS is currently working to develop an operational monitoring system for atmospheric ozone. Understanding Stratospheric Processes Limb sounding instruments such as SCIAMACHY, GOMOS, MIPAS, SAGE III, MLS and HIRDLS are due to be launched in the coming years and should contribute to our understanding of stratospheric processes. Comprehension of the chemistry and microphysics in the lower stratosphere/upper troposphere and of the complex transport and mixing processes across the tropopause requires highresolution measurements of numerous physical and chemical parameters and poses a particular challenge to future generations of space-borne instruments, the quality of current measurements at this level being limited. New technologies New technologies which show promise for space-based stratospheric research include radio occultation and lidar technologies. In the past few years a new potential for measuring temperature profiles has been demonstrated using radio occultation (monitoring the phase changes in radio signals between satellites). This technique was developed using high performance radio transmitters on high orbit satellites of the Global Navigation Satellite Systems, GNSS, (the US Global Positioning System, GPS, and the Russian GLObal Navigation Satellite System, GLONASS) and receivers on low-earth orbiting platforms (Kursinski et al., 1996). Using the radio occultation technique, self-calibrated high-resolution global temperature measurements can be made between 5 and 50 km with sufficient accuracy for climate monitoring. Operational instruments planned include the Global Navigation satellite system Receiver for Atmospheric Sounding (GRAS) on the ESA/Eumestat Metop satellite and other similar instruments may be embarked on other European and Non-European platforms (e.g., the Earth Explorers and NPOESS). Both ESA and NASA are pursuing the development of space-borne lidar instruments. These instruments will be able to measure physical parameters or chemical constituents at high vertical resolution (-50m or less). Mie scattering lidars, designed for cloud studies, can also provide vertical profiles of stratospheric aerosol (and, if the platform is in polar orbit, of polar stratospheric clouds). To date two cloud-top lidars have flown: the Lidar In-space Technology Experiment (LITE) on board the space shuttle (Winker et al., 1996) and ALISSA on board MIR (Chanin et al., 1998). The PICASSO-CENA instrument, a joint project of NASA and CNES, is due to fly in 2003. A new generation of lidars, space-borne Doppler wind lidars, is currently being studied: the Atmospheric LAser Doppler INstrument, ALADIN, planned by ESA for the international space station, and several projects of coherent and incoherent wind lidars are being considered by NASA. These instruments will principally measure tropospheric winds, though they may also provide measurements in the lower stratosphere. As yet, there is no extension of these techniques to space-based measurements of ozone or water vapour.

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Conclusion Space-based instruments have been a great source of quality data for SPARC research and should continue to be so if the criteria described above are met. A review of the space-based instruments planned for the coming decades can be found in the 1997 CEOS yearbook (CEOS, 1997). It will, of course, always be necessary to complement space-based instruments to form an integrated global observing system with air-borne and ground-based observing systems such as the ground-based monitoring systems set up by the Network for the Detection of Stratospheric Change (NDSC) and WMO/GAW. SPARC works closely with measurement programmes such as the Global Climate Observing System (GCOS) and the Integrated Global Observing Strategy (IGOS) towards this goal. REFERENCES Bhartia, P.K., R.D. McPeters, C.L. Mateer, L.E. Flynn, and C.G. Wellemeyer, Algorithm for the Estimation of Vertical Ozone Profiles from the Backscattered Ultraviolet technique, Journal of Geophysical Research, 101, 18793-18806 (1996). CEOS, Towards an Integrated Global Strategy, 1997 CEOS Yearbook, ESA, Smith System Engineering Limited, UK (1997). Chanin, M-L , A Hauchecorne, C Malique, D Nedeljkovic, J-E Blamont, M Desbois, G Tulinov, V Melnikov, Premiers rrsultats du lidar ALISSA embarqu6 g bord de la station MIR. First results of the ALISSA lidar on board the MIR Platform. (submitted to CRAS, 1998). Chen, P., J.R.Holton, A.O'Neill and R.Swinbank, Isentropic mass exchange between the tropics and extratropics in the stratosphere, Journal of Atmospheric Science, 51, 3006 (1994). Grossmann,K.U., D.Offermann, P.Barthol and R.Trant, The CRISTA project, Society of Photo-Optical Instrumentation Engineers (SPIE), 2209, 50 (1994). Kursinski, E.R., G.A.Hajj, W.I.Bertiger, S.S.Leroy, T.K.Meehan, L.J.Romans, J.T.Schofield, D.J.McCleese, W.G.Melbourne, C.L.Thornton, T.P.Yunck, J.R.Eyre and R.N.Nagatani, Initial Results of Radio Occultation Observations of Earth's Atmosphere using The Global Positioning System, Science, 271, 1107 (1996). Mote P.W., K.H.Rosenlof, J.R.Holton, R.S.Harwood and J.W.Waters, Seasonal variations of water vapor in the tropical lower stratosphere, Geophysical Research Letters, 22, 1093 (1995). Mote P.W., K.H.Rosenlof, M.E.Mclntyre, E.S.Carr, J.C.Gille, J.R.Holton, J.S.Kinnersley, H.C.Pumphrey, J.M.Russel III, and J.W.Waters, An atmospheric tape recorder: The imprint of trpoical tropopause temperatures on stratospheric water vapor, Journal of Geophysical Research, 101, 3989 (1996). SPARC Implementation Plan, WCRP- 105, WMO/TD-No.914 (1998). SPARC/IOC/GAW Assessment of trends in the vertical distribution of ozone, Eds. N.Harris, R.Hudson, C.Phillips, SPARC Report No. 1, WMO/GO3RMPNo. 43 (1998). Wang, H.J., D.M.Cunnold and X.Bao, A critical analysis of Stratospheric Aerosol and Gas Experiment ozone trends, Journal of Geophysical Research, 101, 12495-12514 (1996). Wentz, F.J., and M.Schabel, Effects of orbital decay on satellite-derived lower-tropospheric temperature trends, Nature, 394, 661 (1998). Winker D. M., Couch R.H. Mc Cormick M.P.. An overview of LITE : NASA's Lidar In-space Technology Experiment, Proceedings of the 1EEE, 84, 164-180 (1996). WMO/UNEP, Scientific Assessment of Ozone Depletion: 1994, Global Ozone and Monitoring Network, WMO Report No. 3 7, Geneva (1995).