J.-L Baray
email: baray@univ-reunion.fr

J Leveau

S Baldy

J Jouzel

P Keckhut

G Bergametti

G Ancellet

H Bencherif

B Cadet

M Carleer

C David

M De Mazie

D Faduilhe

S Godin Beekmann

P Goloub

F Goutail

J M Metzger

B Morel

J P Pommereau

J Porteneuve

T Portafaix

F Posny

L Robert

M Van Roozendael

An instrumented station for the survey of ozone and climate change in the southern tropicsw

) On the quasi-horizontal scale across the southern subtropical barrier separating the tropical stratospheric reservoir from mid-and high latitudes.

Introduction

In the last 25 years, research in atmospheric chemistry and physics has been marked by the destruction of the stratospheric ozone layer in Antarctica [START_REF] Wmo | Scientific assessment of ozone depletion[END_REF] and the impact of increasing levels of carbon dioxide and other greenhouse gases on global climate. [START_REF]Climate change: the IPCC scientific assessment[END_REF][START_REF] Wuebbles | Global change: state of the science[END_REF] The discovery of the sensitivity of the global atmosphere to growing emissions of anthropogenic substances put in evidence the lack of adequate observations for understanding and thus possibly predicting the global evolution of the composition of the atmosphere (Fig. 1). The NDSC (Network for Detection of Stratospheric Change) was set up rapidly at the end of the eighties based on the use of the most recent research instruments which could be deployed at stations distributed in latitude for starting the mandatory long term monitoring of the composition of the stratosphere. [START_REF] Kurylo | Network for the Detection of Stratospheric Change: a status and implementation report[END_REF] Largely based on already existing stations at mid-and high latitudes of the Northern Hemisphere, the challenge was to extend the network to other latitudes, particularly to the Southern Hemisphere and the tropics where almost nothing was available. In 2006, to better reflect the free tropospheric and stratospheric coverage of NDSC, measurement, analyses, and modeling activities, as well as to convey the linkage to climate change, the steering committee voted to change the name of the network to NDACC (Network for the Detection of Atmospheric Composition Change). It is indeed now well-known that the UTLS (Upper Troposphere-Lower Stratosphere) is the atmospheric layer where the effects of the climate changes should be particularly marked. For example, climate models predict a cooling in the stratosphere up to 7 K, and a warming in the troposphere of about 2 K. [START_REF] Royer | Simulation des changements climatiques au cours du XXI e`me sie`cle incluant l'ozone stratosphe´rique[END_REF] Thus, it is of primary importance to be able to detect and quantify such changes and to connect these changes into the different atmospheric compartments, to identify the dynamical links and the strength of their exchanges.

Especially, tropical latitude bands are of special interest since, in these regions, the upper troposphere is affected by specific dynamical processes like the subtropical jet stream and the tropical convection. A program like SHADOZ (Southern Hemisphere ADditional OZonesondes) focuses on the tropospheric ozone balance in these regions. [START_REF] Thompson | The 1998-2000 SHADOZ (Southern Hemisphere ADditional OZonesondes) tropical ozone climatology. 1. Comparisons with TOMS and ground-based measurements[END_REF] In the stratosphere, tropical waves, meridional transfers of air masses and tidal perturbations of the middle atmosphere are the main drivers of the dynamics. Reunion Island is a tropical island located in the south-western part of the Indian Ocean at 20.81 S and 55.51 E. Due to its location, Reunion Island is seasonally submitted to biomass burning plumes transported from the subcontinent of Southern Africa, which can significantly affect the free tropospheric concentrations of ozone and other pollutants like CO. Moreover, it is affected by the dynamical influence of the subtropical jet stream and the tropical convection. In the stratosphere, the island is located near the subtropical barrier and special patterns of stratospheric ozone transport can be observed.

The understanding of the barrier effect and dynamical exchanges between the tropical reservoir and midlatitudes, and vertically between the troposphere and the stratosphere is of great interest to document climate change and Reunion Island is ideally located to make measurements in order to document these themes (Fig. 2). Hence, in response to the request of the network NDACC, the Service d'Ae´ronomie and the University of Reunion Island decided in 1991 to start building a stratospheric observing capacity in the southern tropics in the Indian Ocean.

The paper is organized as follow: firstly an overview of the instruments and facilities currently available at Reunion Island is presented. Then some important scientific results already achieved based on these measurements are given, and finally the plans for further improving and developing the station are displayed.

Instrumentation

The instrumentation operating presently at Reunion Island has been developed gradually. In collaboration with the Service d'Ae´ronomie (SA/CNRS) and the Institut Pierre Simon Laplace (IPSL), measurements of ozone, temperature and humidity profiles by radio soundings started in 1992 (Fig. 3). A SAOZ UV-visible spectrometer was installed nearly one year later, in 1993 (SAOZ, Syste`me d'Analyse par Observation Ze´nitale). Based on the Rayleigh-Mie techniques to retrieve temperature and aerosol profiles, the first LiDAR (Light Detection And Ranging) experiments started by early 1994, thanks to the established expertise of the SA/CNRS at the Observatoire de Haute Provence and Dumont D'Urville. Then, in parallel to the continuous improvement of the existing instruments, several other instruments have been developed and implemented at Reunion station, as a set of complementary instruments, in order to: (1) provide high quality datasets that meet the NDACC quality standard, (2) contribute to validation of a number of global atmospheric experiments on space shuttles or satellites (e.g., TOMS, GOME, ENVISAT, AURA and CALIPSO), (3) survey and report on original geophysical events, such as sudden stratospheric warming, total ozone reduction, biomass burning and stratosphere-troposphere exchanges, (4) improve and assess models outputs at various scales.

In the light of these developments and requirements, the Observatoire de Physique de l'Atmosphe`re de la Re´union (OPAR) was officially formed in February 2003. It is a French consortium based on four partners: (1) University of Reunion Island, (2) University of Versailles-Saint-Quentin, (3) Reunion Regional Council (Conseil Re´gional de La Re´union), and (4) CNRS, the French national research centre. The description of the instruments is organized as follow: ozone and water vapor probes, and SAOZ and MAXDOAS UV-visible spectrometers, sun-photometer, FTIR, and finally LiDAR systems. The pictures of each instrument are given in Fig. 4. In order to show how routinely each instrument operates, the annual frequency of measurements is given in Table 1.

Ozone probes

The ozone probe program started in 1992 using ECC (Electro-Chemical Cells) and Vaisala RS80 radio probes providing temperature, pressure and humidity every 7 s during the ascent of the balloon at 5 m s À1 up to the burst altitude at 30-35 km. In 2006, the interface change led to a measurement rate of 2 s which had improved the resolution, which is about 50 m. The precision of the ozone concentration is estimated to 5% in the stratosphere and 10% in the troposphere. [START_REF] Thompson | The 1998-2000 SHADOZ (Southern Hemisphere ADditional OZonesondes) tropical ozone climatology. 1. Comparisons with TOMS and ground-based measurements[END_REF] According to the manufacturer, the precision of the temperature of the RS80 is 0.5 1C, while humidity readings are accurate only in the lower troposphere, up to 5-8 km, but less significant above.

The ozone probe program began with bi-monthly measurements from September 1992 to December 1998, further enhanced to weekly probes in January 1999 when the station became part of the SHADOZ network. [START_REF] Thompson | The 1998-2000 SHADOZ (Southern Hemisphere ADditional OZonesondes) tropical ozone climatology. 1. Comparisons with TOMS and ground-based measurements[END_REF] The objective of this network is to document the tropospheric ozone concentrations to better assess the estimation of tropical tropospheric ozone budget and to participate in satellite validation. To fulfill these objectives, an ozone probe is launched every week from 12 stations of the southern subtropics. Within this framework, the Reunion Island ozone probe system was involved in the JOSIE 2000 project, where the ECC probes and the methods of preparation used at SHADOZ sites were tested and qualified. [START_REF] Smit | Assessment of the performance of ECC-ozonesondes under quasi-flight conditions in the environmental simulation chamber: Insights from the Ju¨lich Ozone Sonde Intercomparison Experiment (JOSIE)[END_REF] In addition to the routine program, some radiosounding specific campaigns are organized, for studying mesoscale dynamical events in the troposphere and stratosphere-troposphere exchange induced by the subtropical jet stream (15 additional soundings in July 1998), [START_REF] Baray | Planetary-scale tropopause folds in the southern subtropics[END_REF] or by the tropical convection (8 additional soundings in February 2002) or for validating remote sensing measurements from long duration balloons passing close to the station (2 soundings in February 2001 and 2 in February 2004). [START_REF] Borchi | Evaluation of SHADOZ sondes, HALOE and SAGE II ozone profiles at the tropics from SAOZ UV-Vis remote measurements onboard long duration balloons[END_REF] In order to assess the water vapor distribution in the tropical UTLS and in the framework of the validation of satellites AQUA and AURA, some specific radiosoundings have been performed using a dedicated sensor developed by NOAA at the University of Colorado 10 allowing water vapor measurements in the local UTLS. Two specific campaigns were performed in March 2004 and February 2005.

SAOZ UV-visible spectrometer

The SAOZ (Syste`me d'Analyse par Observation Ze´nitale) is an automated UV-visible spectrometer developed by the Service d'Ae´ronomie at the end of the eighties for studying the polar stratosphere. [START_REF] Pommereau | O 3 and NO 2 ground-based measurements by visible spectrometry during Arctic winter and spring 1988[END_REF] It is a UV-visible spectrometer of 1 nm resolution looking at the sunlight scattered at zenith in the 300-630 nm spectral range. The spectrometer, housed in a waterproof container with a quartz window to enable measurements at zenith within a 101 conical field of view, is installed on the roof of the laboratory building. Total ozone and NO 2 columns are measured twice a day at sunrise and sunset between 861-911 solar zenith angle with an accuracy of, respectively, 3 and 5%.

The first SAOZ equipped with a 512 pixel detector was installed at Reunion Island in 1993. The detector was replaced by one with 1024 pixels in 1997 after a detector failure. The SAOZ instrument has been further updated from the Hewlett-Packard to a PC version in 2002. Between 250 and 350 days of measurements are available every year archived in the NDSC database.

Fig. 5 shows the series of data retrieved since 1993. Both ozone and NO 2 show a seasonal cycle of, respectively 24 DU (Dobson units) and 1.17 Â 10 15 mol cm À2 average amplitude, with a late autumn-winter minimum and a spring maximum. Additional noise can be seen on NO 2 : it is due to urban pollution since the instrument is located in the city of Saint Denis. A multi-regression analysis has been performed on these data showing a significant quasi-biennial oscillation modulation of 17 DU for ozone as well as some influence of magnetic activity of 8 DU, while the largest modulation on NO 2 after the seasonal cycle is the ENSO with an average reduction of 0.45 Â 10 15 mol cm À2 during the El Nino event of 1998-99. After removing those contributions no significant trend could be observed since 1993. Together with those of the other SAOZ stations, the Reunion Island data are being extensively used for the validation of TOMS, GOME and more recently SCIAMACHY and OMI ozone and NO 2 space instruments, for which very limited observations from the southern tropics are available. [START_REF] Lambert | Combined characterisation of GOME and TOMS total ozone measurements from space using ground-based observations from the NDSC[END_REF]

MAXDOAS UV-visible spectrometer

The Multi-AXis DOAS (MAXDOAS) instrument is another UV-visible spectrometer designed to measure stratospheric and/or tropospheric columns of a variety of atmospheric trace gases (NO 2 , BrO, HCHO, O 3 , SO 2 , IO, O 4 ). [START_REF] Theys | Determination of tropical stratospheric and tropospheric BrO columns from multi-axis DOAS measurements at Reunion Island[END_REF] Assembled at BIRA-IASB (Institut d'Ae´ronomie Spatiale de Belgique), the instrument has been installed at the University of Saint-Denis in July 2004 during the second campaign of the FTIR instrument (described in the following section), and after that continuously operated during one full year except for the period from March 25 until April 28, 2005, when it had to been dismantled due to a storm. Longer-term operation is foreseen from 2007 onwards.

In its current configuration, the MAXDOAS instrument consists of a grating spectrometer from ARC (Acton Research Corporation SpectraPro 275) equipped with a grating of 600 grooves mm À1 that covers the spectral range from 300 to 450 nm with a resolution of approximately 0.75 nm FWHM. The detector is a two-dimensional back-illuminated CCD array from Roper Scientific (NTE/CCD-400EB) operated at the nominal temperature of À40 1C. A fiber optic bundle mounted on a telescope and coupled to a rotating mirror is used to collect the skylight scattered from a series of elevation angles (typically 31, 61, 101, 181 and zenith) within a field of view of approximately one degree. The whole system is placed inside a thermally regulated container that is directed towards the north. The data acquisition is controlled automatically via a computer and an internet connection allows for remote processing of the recorded spectra.

The MAXDOAS observations are intended to be complementary to the SAOZ total ozone and NO 2 measurements already conducted on the site since 1993 by allowing the detection of more trace gases as well as enabling a more accurate separation of their tropospheric and stratospheric contents. In the current stage of the data evaluation, the capability of the system to detect spectral absorption features of all primary target gases (NO 2 , O 3 , HCHO, SO 2 and BrO) has been demonstrated. More specifically the analysis of BrO data has received a special attention. A new retrieval scheme has been designed to infer the tropospheric and stratospheric BrO columns based on an analysis of the complete diurnal evolution of the measured column. The evaluation makes uses of a multiple-scattering radiative transfer code coupled to a chemical box model that properly accounts for the large gradients in the stratospheric BrO field at twilight. [START_REF] Lambert | Combined characterisation of GOME and TOMS total ozone measurements from space using ground-based observations from the NDSC[END_REF] Results confirm the presence of a free-tropospheric BrO background of approximately 1 Â 10 13 mol cm À2 corresponding to approximately 1-2 pptv at 6 km altitude. The inferred total columns are consistent with simultaneous observations from the SCIAMACHY instrument on board ENVISAT.

FTIR spectrometer

The FTIR (Fourier transform infra-red) instrument is a high spectral resolution Fourier transform spectrometer able to provide quasi simultaneously total column and/or vertical profiles of about 20 atmospheric species among which are ozone, CO, N 2 O, OCS, HF, HCl and HNO 3 .

Up to now, FTIR solar absorption measurements have been made using commercially available Bruker 120 M spectrometers. The observations are not running in routine mode but are performed on a campaign basis in the framework of a collaboration between the Universite´de la Re´union and the Belgian institutes, ULB (Universite´Libre de Bruxelles) and BIRA-IASB. Two FTIR measurement campaigns have been performed. A first campaign was organized in September-October 2002 during which measurements have been performed from two sites at very different altitudes (Universityof Saint Denis, 50 m asl and Piton Maı¨do, 2200 m asl, Fig. 3b). The data obtained at Piton Maı¨do at 2200 m were of very good quality. At the sea level site (University of Saint Denis), because of too high H 2 O vapor absorptions, a few species could not be observed, like NO, or showed larger uncertainties, like is the case for HF. But the overall quality of the data was good and reliable data retrievals could be performed. For logistical reasons, the second campaign (July to end of October 2004) took place at the university site. These campaigns have demonstrated the feasibility of a long-term monitoring of the atmospheric composition based on FTIR solar absorption observations, especially if they are performed at a high altitude station.

During these campaigns, the following observations have been made. In 2002, the differential column of tropospheric species such as N 2 O and C 2 H 2 in the 2150 m thick layer between the altitudes of both observation sites has been observed. In 2004, the seasonal variation of CO in the local spring period has been in good agreement with correlative MOPITT data. The comparisons between the FTIR retrieved profiles for HNO 3 and O 3 and overpass data of the ACE/ SciSat-1 satellite instrument and local O 3 probe measurements showed a good agreement confirming the reliability of the FTIR data.

Further data analysis of the 2004 campaign is still ongoing. Among others, we are investigating the information content of the FTIR spectra regarding water vapor in the upper troposphere/lower stratosphere. The FTIR experiment has been run using BARCOS, the Bruker Automation and Remote Control System, developed at IASB-BIRA for enabling the operation of the FTIR instrument in an automatic or remotely-controlled way. [START_REF] Neefs | The BARCOS system for automatic and remote control of a Bruker FTS for solar absorption measurements from ground[END_REF][START_REF] Neefs | BARCOS: a system for making atmospheric observations with a Bruker FTS in an automatic or remotely controlled way[END_REF] We intend to use this system in any future campaign as well as for the permanent operations in the future NDACC infrastructure.

Sun-photometer

A CE 318 automatic sun tracking photometer designed and realized by CIMEL has been installed in 1997, in collaboration with the LOA (Laboratoire d'Optique Atmosphe´rique) and NASA. This instrument is the standard instrument used in the AERONET (AErosol RObotic NETwork) international network which provides most of aerosol optical measurements used to characterize the aerosol properties and to validate satellite data. [START_REF] Holben | AERONET-A federated instrument network and data archive for aerosol characterization[END_REF] The CIMEL 318 measures the solar radiance in 6 spectral bands: 440, 670, 870, 936, and 1020 nm. The spectral band of 870 nm is measured from 3 polarized channels of 120 degrees. This instrument gives the integrated optical thickness of atmospheric aerosols, the volume size distribution, and the Angstro¨m coefficient.

Two measurement campaigns have first been organized: the first one was performed at Pas de Bellecombe (June 1997-June 1998), near the volcano Piton de la Fournaise, while the second campaign (March 1999-May 2000) took place at St Pierre, in the south west of Reunion Island (Fig. 3b). The instrument works again routinely since January 2004 at the University (Saint Denis). In addition, a second instrument (type CE 312) has been installed since January 2006 in order to provide measurements in the infra-red spectral band simultaneously with LiDAR measurements of clouds and aerosols. Data are treated by PHOTONS-AERONET and are available on internet (web links in appendix).

LiDAR measurements

The principle of the LiDAR is based on the active emission of laser pulses in the atmosphere. [START_REF] Hauchecorne | Density and temperature profiles obtained by lidar between 35 and 70 km[END_REF] Backscattered photons by atmospheric layers result from an elastic or inelastic diffusion. Depending on the emission/detection characteristics, the return signals are processed in order to derive accordingly, on a daily basis, atmospheric parameters. Two LiDAR systems are operating, one for aerosol, temperature, water vapor and tropospheric ozone profiles, and a second for stratospheric ozone profiles.

Temperature and aerosol profiles

The first LiDAR operating at Reunion observatory was a Rayleigh-Mie one. It has been devoted to retrieve temperature and aerosol profiles by the use of two channels in the 30-75 km and 12-35 km height ranges, respectively. The initial laser was a Nd:YAG laser, doubled by a KDP crystal, in order to emit at 532 nm, with a repetition rate of 10 pulses per second, and delivering 300 mJ per pulse. In 1996, the laser source was changed. The operating laser is a Nd:YAG emitting with a higher repetition rate (30 Hz) and delivering 500 mJ pulse À1 . The reception system is composed by a mosaic of 4 parabolic mirrors, with a diameter of 500 mm each; the backscattered signal is transmitted using optical fibers to photomultiplier tubes (Hamamatsu R1477S). The electronic chain is similar to that operating at the Observatoire de Haute Provence in the South of France. 18,[START_REF] Hauchecorne | LiDAR monitoring of the temperature in the middle and lower atmosphere[END_REF] Due to the aerosol distribution, temperature retrieval from a Rayleigh-Mie LiDAR is possible only for altitudes higher than 30 km (where aerosol contribution to the LiDAR signals is negligible). In order to extend temperature measurements by the LiDAR downward, a Raman-N 2 setup has been implemented in 1999. [START_REF] Faduilhe | Stratospheric temperature monitoring using a vibrational Raman LiDAR. Part 1: Aerosols and ozone interferences[END_REF] The Raman-N 2 method is based on the Raman shift induced by the nitrogen molecules.

Clouds, aerosol and water vapor profiles

Aerosol profiles obtained from the Rayleigh-Mie channels do not cover all tropospheric heights. Those channels are also dedicated to detect and characterize the seasonal and vertical distribution of the tropical cirrus clouds over the Reunion site. In November 1995, the LiDAR was improved by adding new channels using a smaller telescope (diameter 200 mm), to extend the aerosol altitude range downwards. The installation of a dioptric afocal optical system allows the reduction of the emitted laser beam divergence. Polarized channels have also been implemented, especially to study cirrus clouds. In 1997, six simultaneous counting channels were installed, using the infra-red fundamental Nd:YAG emission. Raman channels allow to retrieve tropospheric temperature [START_REF] Faduilhe | Stratospheric temperature monitoring using a vibrational Raman LiDAR. Part 1: Aerosols and ozone interferences[END_REF] and water vapor concentration. [START_REF] Robert | Development of Raman water-vapor LiDAR at a NDSC complementary station in the southern tropics[END_REF] The technology used is presented by Whiteman et al. [START_REF] Whiteman | Raman LiDAR system for the measurement of water vapour and aerosols in the earth atmosphere[END_REF] The detection of Raman signals is obtained by a Czerny-Turner spectrometer using a holographic grating. The water vapor mixing ratio is deduced from the ratio between the shifted Raman backscattered photons and the Raman nitrogen backscattered photons. This second signal is used as a reference and allows to derive directly the water vapor mixing ratio with a limited sensitivity to instrumental parameters. However, modifications of the filtering devices and detectors are specific to each channel. Today some calibrations of water vapor measurements remain to be realized and hence this channel is still in a validation phase.

Tropospheric ozone profiles

In 1998, a differential LiDAR system has been implemented in order to measure tropospheric ozone. The method is based on the differential absorption between two UV wavelengths (289 nm and 316 nm), generated in a high pressure deuterium cell. Because of difference in beam emission divergence (between visible and UV beams) and in expected vertical ranges (troposphere for ozone, stratosphere for temperature), the use of the fibers of the temperature-aerosol system was not appropriate and an original system of double fibers has been developed, constituting the originality of this system. [START_REF] Baray | Description and evaluation of a tropospheric ozone lidar implemented on an existing lidar in the southern subtropics[END_REF] The spectral separation of both wavelength of the ozone system is obtained by a Czerny-Turner spectrometer using a holographic grating. The reception system is common with the temperature Rayleigh/Raman system.

Stratospheric ozone profiles

Another LiDAR system, built by Geneva University (Switzerland) in collaboration with the Service d'Ae´ronomie has been implemented at Reunion Island in June 2000. The principle is the same as for the LiDAR used for tropospheric ozone measurements. However, to observe the stratosphere, it is necessary to use wavelengths less absorbed by ozone and the emitted wavelengths are shifted towards the near UV. [START_REF] Godin-Beekmann | Systematic DIAL ozone measurements at Observatoire de Haute-Provence[END_REF] Thus, the technical choices for the beam generation are different: two lasers are used, one Nd:YAG tripled producing the 355 nm wavelength at 30 Hz and 150 mJ per pulse and one Excimer laser producing the 308 nm wavelength at 40 Hz and 200 mJ per pulse.

The reception system is similar to the other LiDAR system (same mosaic of 4 parabolic mirrors and transmission of signal by optical fibers). To avoid the saturation of the photomultiplier tubes, a mechanical chopper rotating in vacuum at 800 Hz is used. A separation lens after the spectrometer block is also used to separate the return signal in two channels in the proportions of 8% for the lower stratospheric channel and of 92% for the upper stratospheric channel. In addition, electronic gating is used for upper stratospheric channels.

With a time integration of two hours, the statistical error linked to the signal a˜N ˜noise ratio is less than 10% between 18 and 38 km, and less than 5% between 20 and 35 km. This instrument is well adapted to the survey of stratospheric ozone over Reunion Island. On 20 July 2000, both tropospheric and stratospheric ozone DIAL systems operated simultaneously, and an ozone probe was launched. As depicted on Fig. 6, corresponding ozone profiles show a good agreement. When the conditions of observations are good, ozone measurements combining the tropospheric and stratospheric systems are available from 3 to 45 km.

Scientific studies

Some examples of scientific studies at Reunion Island are now given, to illustrate how continuous data can be used to improve our knowledge of atmospheric processes.

Aerosols and tropospheric ozone balance

Measurements provided by ozone soundings have allowed the characterization of the seasonal variability of tropospheric ozone. It has been shown that this variability results from the coupling between photochemical and dynamical processes: [START_REF] Baldy | Field observations of the vertical distribution of tropospheric ozone at the Island of La Reunion[END_REF] the photochemical influence of biomass burning from South Africa and Madagascar, [START_REF] Randriambelo | An improved detection and characterization of active fires and smoke plumes in Southeastern Africa and Madagascar[END_REF][START_REF] Taupin | Ozone generation over the Indian Ocean during the South African biomass-burning period: case study of October 1992[END_REF] the ozone diurnal cycle in the boundary layer [START_REF] Bremaud | Ozone nighttime recovery in the marine boundary layer: Measurement and simulation of the ozone diurnal cycle at Reunion Island[END_REF] and the ozone loss due to cirrus clouds. [START_REF] Roumeau | Tropical cirrus clouds: a possible sink for ozone[END_REF][START_REF] Cadet | A sub-tropical cirrus clouds climatology from Reunion Island (211S, 551E) lidar data set[END_REF] Moreover, at the end of the biomass burning season, the daily variability of the dynamical context can induce an important daily variability of the tropospheric ozone profile. [START_REF] Randriambelo | Investigation of the short-time variability of tropical tropospheric ozone[END_REF] Troposphere-stratosphere transport, dynamics of tropical cyclones

The exchanges between stratosphere and troposphere have been especially examined and the relative roles of processes like tropopause fold, [START_REF] Baray | Planetary-scale tropopause folds in the southern subtropics[END_REF] Rossby wave breaking and tropical cutoff lows [START_REF] Baray | Dynamical study of a tropical cut-off low over South Africa and its impact on tropospheric ozone[END_REF] or tropical convection [START_REF] Baray | Tropical cyclone Marlene and stratosphere-troposphere exchange[END_REF] have been quantified. The localization of Reunion Island makes it ideal for the observation of cyclones during the austral summer. Therefore tropical cyclones have been the subject of many studies such as Doppler radar observations of tropical cyclone Dina, [START_REF] Roux | Doppler radar observations of tropical cyclone Dina as it passed near La Re´union on 22[END_REF] mesoscale modelisation of gravity waves generated by cyclone Hudah, [START_REF] Chane Ming | Gravity wave characteristics over Tromelin Island during the passage of cyclone Hudah[END_REF] and of stratosphere-troposphere exchange generated by cyclone Marlene [START_REF] Leclair De Bellevue | Stratosphere-troposphere exchange near tropical convection: observations, mesoscale and global analyses[END_REF] or the effects of Rossby wave breaking on cyclogenesis, intensity and trajectories of cyclones.

Presently, we are working mainly on the use of dynamical tools such as Reverse Domain Filling techniques, Flexpart and mesoscale models (meso-nh and MM5) to obtain more insight to these processes.

Atmospheric dynamics

It is well established that the subtropical barrier, a dynamical border that separates the tropical stratospheric reservoir from mid-latitudes, as for the polar vortex, controls and limits the meridional exchanges. Indeed, the efficiency of the confinement of the tropical stratospheric reservoir depends on the strength of the subtropical barriers and on their variability. [START_REF] Portafaix | Fine scale study of a thick stratospheric ozone lamina at the edge of the southern subtropical barrier[END_REF] Recent studies conducted at Reunion University, combining ground-based and satellite measurements together with global modeling, showed that developments of laminae nearby the southern subtropical barrier might be associated with the anomalous growth and break of gravity waves and planetary waves that propagate from the troposphere into the stratosphere during winter. [START_REF] Bencherif | Lidar observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier[END_REF][START_REF] Morel | Fine-scale study of a thick stratospheric ozone lamina at the edge of the southern subtropical barrier. Part II: Numerical simulations with coupled dynamics models[END_REF] In fact, with regard to its geographical position in the southern tropics, the Reunion site might be under influences of several transient and/or seasonal dynamical processes, such as ITCZ, convective and cyclone developments, jet-streams, or subtropical barriers.

Up to now, the number of ground-based instruments running in the Tropics has still been limited. We thus have taken the advantage of the variety of qualified atmospheric datasets offered at Reunion Island, to contribute to several topics related to dynamical processes in the stratosphere-upper troposphere region:

(1) Atmospheric tides and gravity waves characteristics by the use of daily LiDAR temperature profiles; [START_REF] Morel | Evidence of tidal perturbations in the middle atmosphere over Southern Tropics as observed by Rayleigh Lidar[END_REF][START_REF] Morel | Investigation of the tidal variations in a 3-D dynamics-chemistry-transport model of the middle atmosphere[END_REF][START_REF] Chane Ming | Vertical short-scale structures in the upper tropospheric-lower stratospheric temperature and ozone at[END_REF] (2) Survey and variability of isentropic transport and planetary waves propagation nearby the southern subtropical barrier, by the use of ozone profiles obtained by LiDAR and radio probes and daily ozone columns recorded by SAOZ; [START_REF] Portafaix | Fine scale study of a thick stratospheric ozone lamina at the edge of the southern subtropical barrier[END_REF] (3) Stratospheric ozone trend estimation using satellite (SAGE II, HALOE) and radio probe data;

(4) Tropical tropopause characteristics; [START_REF] Sivakumar | Tropopause characteristics over a southern sub-tropical site: Reunion Island (211 S, 551 E): using Radiosonde/Ozonesonde data[END_REF] (5) Climatology and variability of stratospheric parameters: temperature, ozone and aerosols. 18,[START_REF] Faduilhe | Stratospheric temperature monitoring using a vibrational Raman LiDAR. Part 1: Aerosols and ozone interferences[END_REF][START_REF] Sivakumar | Lidar observations of middle atmospheric gravity wave activity over a low latitude[END_REF] In fact the LiDAR technique, as well as radiosounding, are well adapted to study the dynamic and variability of the stratosphere-upper troposphere region at different timescales, through regular measurements of atmospheric key parameters: temperature, ozone, water vapour, and aerosol contents.

Conclusion and future plans

In order to provide significant improvement of our understanding of the dynamic of the tropical troposphere and stratosphere, scientific studies have to be based on the complementarity of several different instruments, and of multiple approaches (ground based observations, satellite data, and models). These studies contribute to the support of the decisions taken in international agreements like the Montreal protocol, relative to the production of CFCs and halogenated species, or like the Kyoto protocol, relative to emissions of greenhouse gases.

The atmospheric survey is, then, important all over the globe, and especially in the tropical region, which is a key region for many mechanisms playing a role in climate change. Reunion Island is an exceptional site for the study of atmospheric dynamics, and participates in this international effort. The instrumentation of this station uses recent technologies, and studies based on the recorded data are of a special interest for understanding processes, climatologies and trends.

However, measurement techniques are still lacking, especially for dynamical observations and the measurement of water vapor in the UTLS is an actual instrumental challenge. Then, in the near future, in order to improve the contribution of Reunion Island in this regard, some additional instruments will be implemented:

-A Doppler LiDAR giving horizontal components of the wind from 5 to 50 km, for the study of gravity wave activity. [START_REF] Souprayen | Rayleigh-Mie Doppler wind LiDAR for atmospheric measurements. I. Instrumental setup validation and first climatological results[END_REF] A micro-wave radiometer with one large (8-13 m) and three thick (8.2-9.2 m/10.3-11.3 m/11.5-12.5 m) spectral bands in the infrared in order to assess the tropical cirrus cloud microphysics, such as the size of the crystals.

A radiometer giving the water vapor profile from 15 to 60 km, with a vertical resolution of 5-15 km, for the survey of the tropical UTLS variability.

The last plan for the future of this site is to build an altitude station at Piton Maı¨do, located in the western part of the island, 2200 m asl (Fig. 3b). The building of the altitude station is expected by 2010. The functioning of many instruments (LiDAR, FTIR and radiometers especially) will be improved, avoiding the perturbation by the humidity contained in the lower layer of the tropical troposphere. This new altitude station will be a platform to receive other instruments on a temporary basis for international campaigns. Moreover, in the future, the station could be included in the OARA structure (Observatoire Atmosphe´rique des terres australes) grouping the activities of all the southern hemisphere French atmospheric stations (Reunion, Crozet, Saint Paul, Kerguelen and Amsterdam islands) and new instrumentation onboard the research vessel Marion Dufresne.

Fig. 2

 2 Fig. 2 Latitude-altitude cross-section schematic representation showing the different compartments of the atmosphere. The letters represent the mechanisms studied in Section 3. A: Tropopause folds, cut-off lows, Rossby wave breaking and filamentation of the tropopause induced by the subtropical jet stream. B: Mesoscale modeling of the tropical convection (cyclone, convective depressions), gravity waves induced by the convection. C: Stratosphere-troposphere induced by convection, variability of ozone and water vapor in the tropical UTLS region. D: Stratospheric ozone lamina, meridian exchanges, gravity and Kelvin waves in the tropical stratosphere. As function of the season, Reunion Island can be affected by these mechanisms.

Fig. 1

 1 Fig.1The primary objective of the NDSC, the survey of the stratosphere, is complementary to the other scientific environmental issues, like pollution, climate change, or atmospheric ozone balance.

Fig. 3

 3 Fig. 3 (a) Parameters (bottom x axis) measured by the instrumental set (top of x axis) developed at Reunion Island, or at the planning stage. (b) Map showing the locations of the instruments on Reunion Island: The university is located at Saint Denis, radio soundings are performed from Gillot airport, and the future altitude station will be located at Piton Maı¨do.

Fig. 4

 4 Fig. 4 Photos of the instruments: (a) ozone probe; (b) SAOZ; (c) Sun-photometer; (d) FTIR container at Piton Maı¨do; (e) FTIR instrument at the university; (f) image of the stratospheric ozone LiDAR with the different component labelled (1: laser beam, 2: parabolic mirrors, 3: excimer laser, 4: Nd:YAG laser, 5: optical fibers, 6: spectrometer bloc and photomultiplier tubes, 7: mechanical chopper, 8: electronic chain); (g) laser beam emitted by the Rayleigh-Mie-Raman LiDAR.

Fig. 5

 5 Ozone (top) and nitrogen dioxide total columns obtained from 1993 to 2005 by the SAOZ spectrometer installed at Reunion Island.

Fig. 6

 6 Fig. 6 Ozone profiles by stratospheric DIAL (bold), tropospheric DIAL (dotted) and ozone probe (thin) on 20 June 2000.

Table 1

 1 Number of times each routine instrument was operated per year

		T-aerosol LiDAR	Strato-O 3 DIAL a	Tropo-O 3 DIAL a	SAOZ	O 3 probes
	1992					13
	1993				94	22
	1994	54			341	16
	1995	90			321	18
	1996	130			181	17
	1997	102			116	25
	1998	54		36	304	35
	1999	99		32	320	49
	2000	83	28	17	314	39
	2001	59	8	4	217	28
	2002	88	8	30	111	30
	2003	93	0	24	326	21
	2004	115	14	52	255	41
	2005	121	10	61	261	43

a DIAL = DIfferential Absorption Lidar.

Acknowledgements

Ge´rard Me´gie ) was an internationally recognized scientist and played a leading role in setting up the Network for the Detection of Stratospheric Changes. The development of an atmospheric station at Reunion Island has been made possible thanks to his action at the Service d'Ae´ronomie, then at CNRS. We acknowledge the work of the technical staff of Reunion Island and of the collaborating laboratories, with a special attention to Guy Bain and Ste´phane Richard. The MAXDOAS and FTIR campaigns (IASB-BIRA and ULB) are funded by the Belgian Science Policy through the IASI and ACE ESA-Prodex arrangements and the national project ESACII. The station is supported by French regional, national (PNCA, INSU, CNRS), and international (EQUAL, NASA/ GFSC) fundings.

Appendix

Web links of data networks AERONET, http://aeronet.gsfc.nasa.gov/; PHOTONS, http:// www-loa.univ-lille1.fr/photons/; NDSC (NDACC), http:// www.ndsc.ws/; SHADOZ, http://croc.gsfc.nasa.gov/shadoz/.


H Bencherif
email: hassan.bencherif@univ-reunion.fr

T Portafaix

J L Baray

B Morel

S Baldy

J Leveau

A Hauchecorne

P Keckhut

A Moorgawa

M M Michaelis

R Diab

LIDAR observations of lower stratospheric aerosols over South Africa linked to large scale transport across the southern subtropical barrier

Keywords: LIDAR, Aerosols, Isentropic transport, Stratosphere, Subtropical barrier, PV-contour advection

The study of the variability of stratospheric aerosols and the transfer between the di erent atmospheric regions improves our understanding of dynamical processes involved in isentropic exchanges that take place episodically in the lower stratosphere through the subtropical barrier. One useful approach consists in combining in situ ground-based and global measurements with numerical analyses. The present paper reports on a case study of a horizontal transfer evidenced ÿrst by Rayleigh-Mie LIDAR observations over Durban (29:9 • S, 31:0 • E, South Africa). Additional data from MeteoSat and SAGE-2 experiments, and from ECMWF meteorological analysis have been used in this study. Contour advection maps of potential vorticity from the MIMOSA model derived from ECMWF ÿelds, were also used.

By the end of April, 1999, LIDAR observations showed that aerosol extinction, in the lower stratosphere, has increased signiÿcantly and abnormally in comparison with other LIDAR and SAGE-2 observations recorded for the period from April 20 to June 14, 1999. The dynamical context of this case study seems to exclude the possibility of a local in uence of the subtropical jet stream or tropical convection, which could inject air masses enriched with tropospheric aerosols into the stratosphere. On the contrary, a high-resolution model based on PV advection calculations and ECMWF meteorological analyses shows that air masses are isentropically advected from the equatorial zone close to Brazil. They cross the southern barrier of the tropical reservoir due to laminae stretching and reach the southern subcontinent of Africa 5-6 days later.

Introduction

Aerosols have been shown to have a signiÿcant in uence on radiative forcing and hence temperature [START_REF] Robock | Winter warming from large volcanic eruption[END_REF][START_REF] Labitzki | Stratospheric temperature increases due to Pinatubo aerosols[END_REF][START_REF] Kinninson | The chemical and radiative e ects of the Mount Pinatubo eruption[END_REF]. Furthermore, a survey of aerosols from Mount Pinatubo indicated that aerosol loading has important chemical e ects by enhancing heterogeneous reactions [START_REF] Brasseur | Mount Pinatubo aerosols, chloro uorocarbons and ozone depletion[END_REF] and can modify ozone concentrations [START_REF] Solomon | The role of aerosol variations in antropogenic ozone depletion in northern mid-latitudes[END_REF][START_REF] Roumeau | Tropical cirrus clouds: a possible sink for ozone[END_REF]. Knowledge of mass exchange between di erent parts of the atmosphere is essential in understanding the transport and vertical and spatial distribution of aerosols. Eddy transport barriers separate the tropical stratospheric reservoir from the mid-latitude stratosphere [START_REF] Holton | Stratosphere-troposphere exchange[END_REF][START_REF] Haynes | E ective di usivity as a diagnostic of atmospheric transport[END_REF]. Steep aerosol and trace gas gradients clearly characterize the edges of that reservoir [START_REF] Grant | Use of volcanic aerosols to study the tropical reservoir[END_REF]. Sporadic disruptions of those barriers occur due to planetary wave breaking and the generation of eddies that enhance transport out of the tropics [START_REF] Waugh | Subtropical stratospheric mixing linked to disturbance of the polar vortices[END_REF][START_REF] Waugh | Seasonal variation of isentropic transport out of the tropical stratosphere[END_REF][START_REF] Chen | Isentropic mass exchange between the tropics and extratropics in the stratosphere[END_REF][START_REF] Horinouchi | Synoptic-scale Rossby waves and the geographic distribution of lateral transport routes between the tropics and extratropics in the lower stratosphere[END_REF]. Since tropospheric air enters the stratosphere mostly through the tropical tropopause, exchanges between the tropics and mid-latitudes have a large in uence on the distribution of many long-lived stratospheric constituents.

The transport of aerosols from the tropics to mid-latitudes is well documented in a number of publications. [START_REF] Randel | Stratospheric transport from tropics to middle latitudes by planetary wave mixing[END_REF] using global maps obtained from satellite-borne instruments showed that aerosols can be transported through eddy mixing. [START_REF] Waugh | Contour advection with surgery: a technique for investigating ÿne-scale structure in tracer transfer[END_REF] using meteorological data sets showed that aerosols can also be transported through isentropic advection. However, most dynamical studies and co-ordinated campaigns focus on the northern hemisphere or on the southern polar region. In the southern hemisphere, dynamical processes are not similar to those studied or observed in the northern hemisphere. Besides, the height-latitude structure of dynamical ÿelds and of disruption mechanisms controlling the release of material between the tropics and mid-latitudes, is still under study. One of the useful approaches consists in combining in situ ground-based and global measurements and modeling.

We report here a case study of an observed tropical/extratropical aerosol loading event detected by LIDAR in the lower southern stratosphere. Analyses take advantage of the high vertical resolution of LIDAR proÿles. Developments of large-scale "tongues" through the surf zone are simultaneously surveyed by the MIMOSA high resolution contour advection model based on ECMWF data sets and by water vapor infrared MeteoSat images.

LIDAR instrumentation, aerosol observations and comparisons with SAGE 2

South African LIDAR system

The present study has been undertaken using LIDAR observations obtained at Durban (30:7 • S, 31:0 • E), South Africa. The system was implemented on the campus of the University of Natal within the framework of a bi-lateral Franco-South African cooperation (Service d'Aà eronomie of CNRS, Laboratoire de Physique de l'Atmosph ere of Reunion Island University and University of Natal).

The LIDAR system operating at Durban is of the Rayleigh-Mie type. It uses a Nd:YAG pulsed laser with a repetition rate of 10 Hz and transmits in the green at 532 nm. The receiver consists of two separate telescopic systems and two photomultiplier detectors. LIDAR observations are thus made through two separate-but instrumentally identical-channels. One channel is allocated to probe the troposphere. It uses the return signal collected by the smaller telescope whose total receiving surface is about 0:03 m 2 . The other channel has a total receiving surface of about 0:31 m 2 , and its observation ÿeld is directed to receive photons backscattered from stratospheric layers.

Assuming that the observed atmospheric layers behave as a perfect gas in hydrostatic equilibrium, aerosol and temperature proÿles can be derived from the return signal backscattered by successive atmospheric layers in the local troposphere and stratosphere [START_REF] Hauchecorne | Density and temperature proÿles obtained by LIDAR between 35 and 70 km[END_REF][START_REF] Chazette | Comparative LIDAR study of optical, geometrical, and dynamical propreties of stratospheric post-volcanic aerosols, following the eruptions of El-Chichon and Mount Pinatubo[END_REF]. More information about the instrumental details and temperature proÿle retrieval method and validation are given by [START_REF] Bencherif | Observation and ÿrst validation of stratospheric temperature proÿles obtained by a Rayleigh-Mie LIDAR over Durban, South Africa[END_REF]. The aerosol proÿles are derived from the LIDAR equation using the Klett procedure [START_REF] Klett | Stable analytical inversion solution for processing LIDAR returns[END_REF][START_REF] Klett | LIDAR inversion with variable backscatter/ extinction ratios[END_REF] that gives a convergent solution to the total volumetric backscatter coe cient ÿ(z) = ÿ aerosol (z) + ÿ molecular (z). The backscatter ratio R(z) (total to molecular volume backscatter coe cient ratio) is then calculated from

R(z) = ÿ aerosol (z) + ÿ molecular (z) ÿ molecular (z) = 1 + ÿ aerosol (z) ÿ molecular (z) ¿ 1:
Values of R(z) depend on the choice of Rayleigh parameters (molecular contribution) which should be as realistic as possible. For the present study, molecular parameters are computed on the basis of ECMWF daily ÿelds. It allows a better determination of the molecular density proÿle than that of a climatological model [START_REF] Keckhut | A critical review of the database acquired for the long-term surveillance of the middle atmosphere by the french Rayleigh LIDARs[END_REF]. Moreover, the determination of the total volumetric backscatter coe cient from the LIDAR equation requires quantitative knowledge of a relationship between the aerosol extinction coe cient aerosol (z) and the aerosol backscatter coe cient ÿ aerosol (z), usually the ratio aerosol (z)=ÿ aerosol (z) [START_REF] Klett | Stable analytical inversion solution for processing LIDAR returns[END_REF]. That ratio depends strongly on the size, shape and refractive index of the aerosols. Accounting for micro-physical properties of stratospheric aerosols, [START_REF] Wang | Interference of stratospheric aerosol composition and size distribution from SAGE II satellite measurement[END_REF] estimated the value of this ratio for particles to be 30 sr around 15 km, increasing with altitude up to 50 sr at 28 km. Here, the ratio of extinction to back-scattering coe cient of aerosols is considered to be 50 sr throughout the whole stratospheric altitude range. The Klett procedure also requires a boundary condition, i.e., the knowledge of ÿ aerosol at a reference altitude z ref , chosen above the aerosol layer: at and above this reference altitude, the particle content is negligible and scattering is only due to the molecular atmosphere, assuming that R(z) = 1:0. Typically, we choose

z ref ≈ 40 km.
Uncertainties in the determination of the total volumetric backscatter coe cient ÿ and in the aerosol extinction coefÿcient aerosol and in the backscatter ratio R are due to three main causes: (1) statistical uctuations of the measured signal associated with random detection processes, (2) the presence of particles at and above the chosen reference altitude and (3) the subsequent uncertainty on the value of R(z ref ), and the value of the ratio of extinction to back scattering coe cient of aerosols and its altitude dependence. The resulting total uncertainty is less than 7%, with the signal being signiÿcant up to 40 km [START_REF] Chazette | Comparative LIDAR study of optical, geometrical, and dynamical propreties of stratospheric post-volcanic aerosols, following the eruptions of El-Chichon and Mount Pinatubo[END_REF].

The algorithm used here is similar to the one used to retrieve aerosol LIDAR optical properties from the NDSC primary stations of the Observatoire de Haute-Provence in Southern France [START_REF] Chazette | Comparative LIDAR study of optical, geometrical, and dynamical propreties of stratospheric post-volcanic aerosols, following the eruptions of El-Chichon and Mount Pinatubo[END_REF] and Dumont d'Urville in Antarctica [START_REF] David | Polar stratospheric clouds climatology over Dumont d'Urville Between 1989 and 1993 and the in uence of volcanic aerosols on their formation[END_REF]. It has thus been tested within the framework of the NDSC comparison of the processing algorithms for stratospheric aerosols [START_REF] Steinbrecht | NDSC intercomparison of stratospheric aerosol processing algorithms[END_REF].

Data

The LIDAR data set used in this study consists of daily records of back scattered photons obtained over Durban during the period from April 21 to June 14, 1999 (22 proÿles). Daily LIDAR measurements are time-integrated over about 3:5 h on average.

In order to validate aerosol extinction proÿles derived from LIDAR observations, we use the 5.99 version SAGE-2 (Stratospheric Aerosol and Gas Experiment) measurements. Indeed, the long life (since 1984) and the calibration-independent nature of the SAGE-2 instrument [START_REF] Mccormick | SAGE II: an overview[END_REF][START_REF] Thomason | A global climatology of stratospheric aerosol surface area density deduced from Stratospheric Aerosol and Gas Experiment II measurements: 1984 -1994[END_REF]) make it well suited for observations of the long-term variability of aerosols and for comparison with other instruments such as LIDARs. For the present work, SAGE-2 data are derived from the 525-nm wavelength channel and are selected depending on:

• the geographical position, i.e., a SAGE-2 proÿle is selected if its latitude and longitude correspond to the LIDAR position ±6 • of latitude and ±10 • of longitude, • the time acquisition, i.e., a SAGE-2 proÿle is selected if it was recorded on the same day as one of the 22 LIDAR proÿles ±12 h.

Taking into account the duration of the study, the LIDAR dependency on meteorological conditions and the SAGE-2 operating mode [START_REF] Chu | SAGE II inversion algorithm[END_REF] which is very di erent in comparison with the LIDAR's, we expect a very small number of coincidences on the basis of the selection criteria mentioned above. Only one coincident SAGE-2 extinction proÿle has thus been selected. It was recorded on June 8, 1999, sunrise, at 24:8 • S and 38:4 • E. The corresponding LIDAR proÿle was recorded on the same day, at Durban, and was time integrated from 16:30 to 19:55 UTC.

In the local stratosphere, the coincident SAGE-2 and LIDAR aerosol extinction proÿles, superposed on Fig. 1, show closely similar vertical distribution. Despite the fact that both proÿles' error bars overlap in the whole studied altitude range, the observed discrepancy in the 17-20 km layer is mainly due to the operating modes: SAGE-2 measurements are obtained by looking through a horizontal path of a few hundred kilometres (solar occultation) and thus correspond to a horizontally averaged atmosphere, while LIDAR measurements are obtained vertically and are time-integrated over a few hours. It is important to note that other e ects can contribute to the mentioned discrepancy: the di erence in wavelength between SAGE-2 and LIDAR, the validity of hypotheses used for the inversion of the LIDAR equation such as the height independence of the aerosol extinction to back-scattering ratio, the spatial inhomogeneity of aerosol distribution and the instrument geometry.

Knowing that all LIDAR proÿles used in the present work have been retrieved using the same inversion method, this comparison, even though it is made with one coincidence, shows that the quality of aerosol proÿles obtained by LIDAR over Durban is consistent. These observations can be used consequently for climatological studies as well as for the purpose of a case study, within the framework of co-ordinated campaigns or for validation and inter-comparisons.

Results

From the LIDAR data set described above, an averaged aerosol extinction proÿle has been derived (see the solid thick line showed in Fig. 2). As expected, note that extinction values of the stratospheric background aerosols decrease rapidly with increasing altitude up to the 28-30 km altitude range. Beyond that layer, which can be called the local stratospheric aerosol top layer, the observed atmosphere can be considered as typically molecular, i.e., free of aerosols. Moreover taking into account the extinction mean values the daily LIDAR proÿles distribution, illustrated by superposition of all LIDAR proÿles in Fig. 2, highlights a high variability of the observed stratospheric aerosols. This variability can be associated to the fact that, during the studied period, the transition of the stratospheric zonal-mean winds to the westerly regime was occurring, as shown by [START_REF] Bencherif | Observation and ÿrst validation of stratospheric temperature proÿles obtained by a Rayleigh-Mie LIDAR over Durban, South Africa[END_REF].

However, if one considers the 29th of April proÿle for stratospheric heights under 27 km, the observed extinction values are signiÿcantly higher than those of the mean proÿle and higher too than all other values from all LIDAR proÿles. The aerosol increase observed on April 29 is therefore unusual and suggests that, by the end of April, 1999, aerosol extinction has increased signiÿcantly and abnormally in the lower stratosphere, as a result of an aerosol loading through a transport process.

For a discussion of the origin of the stratospheric aerosol loading observed over Durban two possible origins are investigated; the ÿrst is related to vertical transport from the troposphere to the stratosphere, the second to horizontal transfer between the tropics and extra-tropics.

Discussion on the origin of the aerosol loading over South Africa

Dynamical characteristics of the stratospheric air mass over South Africa on April 29

Potential vorticity (PV) contours for the 400 K potential temperature level, in the lower stratosphere, have been computed from ECMWF data for the whole studied period. ECMWF data used in this study are re-analysed data, with a horizontal resolution of 1 • by 1 • , 15 vertical levels between 1000 and 10 hPa, and with 4 ÿelds per day. These data have already been used for tropospheric-stratospheric dynamical studies in the tropical southern hemisphere [START_REF] Baray | Planetary scale tropopause folds in the southern subtropics[END_REF].

Fig. 3 shows the latitude vs. longitude PV ÿeld for the 400 K isentropic surface, averaged for the period April 18 to May 10, 1999, that corresponds to the aerosol loading event date ±10 days. The mean value of PV over the LIDAR site of Durban lies between 7 and 8 PVU. The PV ÿeld of April 29 is presented in Fig. 3b

A local tropospheric origin?

The dynamic mechanisms which can induce a transfer from the upper troposphere to the lower stratosphere are tropical convection and jet-streams (polar and subtropical). An examination of the Meteosat image for April 29 (Fig. 4) shows that there was virtually no cloud cover over Durban on April 29, thus precluding the possibility of locally strong convective activity capable of injecting an air mass enriched in tropospheric aerosols into the lower stratosphere.

Tropospheric jets/frontal systems are a very perturbed zone and can induce areas of convergence and subsidence, in association with horizontal and vertical ageostrophic circulations. They can result either in strong subsiding motions in tropopause folds, or in strong ascending motion, generally on the anticyclonic side near the entrance zone of the jet stream [START_REF] Shapiro | Research aircraft measurements of jet stream geostrophic and ageostrophic winds[END_REF][START_REF] Holton | Stratosphere-troposphere exchange[END_REF]. In our case study, the upper level front associated with the subtropical jet stream is clearly visible on the Meteosat image (Fig. 4) and is localised in the middle of the Atlantic Ocean, far from the south-western coast of Africa. Within the upper troposphere, winds over Durban are weak, and no PV signature is visible (ECMWF data, not shown). Hence we can conclude that the lower stratosphere over Durban has not been in uenced by a local tropospheric source, either by convective activity or by the jet stream.

A long-range equatorial origin?

Having excluded the possibility of a short-range troposphere-stratosphere transport, we now consider the hypothesis of long-range horizontal transport. In order to check for the origin of the aerosol increase, associated with a low-PV decrease, the high resolution advection MIMOSA model [START_REF] Hauchecorne | Quantiÿcation of the transport of chemical constituents from the polar vortex to middle latitudes in the lower stratosphere using the high-resolution advection model MIMOSA and e ective di usivity[END_REF] has been used to produce a continuous evolution of PV ÿelds for the period from April 21 to 30. The MIMOSA model is an enhancement of the original contour advection algorithm of [START_REF] Norton | Breaking Rossby waves in a model stratosphere diagnosed by vortex-following coordinate system and a contour advection technique[END_REF] and [START_REF] Waugh | Contour advection with surgery: a technique for investigating ÿne-scale structure in tracer transfer[END_REF]. In the lower stratosphere PV is assumed to be conserved on isentropic surfaces for periods of 1 week [START_REF] Orsolini | On the formation ozone laminae at the edge of the Arctic polar vortex[END_REF]. For periods longer than 1 week, the diabatic transport across isentropic surfaces due to radiative cooling and warming of air as well as the diabatic advection term in the PV equation have to be taken into account. See [START_REF] Haynes | On the conservation and impermeability theorems for potential vorticity[END_REF] for further discussion on PV conservation. The information on diabatic changes in the PV ÿeld, at least for larger scales, can be extracted from ECMWF ÿelds. This is made by applying to the MIMOSA PV ÿeld a relaxation toward the ECMWF PV ÿeld with a time constant of 10 days. In order to preserve the ÿlamentation structure, the relaxation term is only applied to scales larger than 300 km. To do that, both MIMOSA and ECMWF ÿelds are smoothed to the same resolution and the di erence between the two ÿelds is used to compute the relaxation term.

MIMOSA has been used for real time 5 day forecasts of the PV ÿelds and has been found useful for the detection of sub-tropical and polar ÿlaments and alert release in the frame of the METRO project (MEridional TRansport of Ozone in the lower stratosphere, a research project included in the European campaign THESEO [START_REF] Heese | Forcast and simulation of stratospheric ozone ÿlaments: a validation of a high-resolution potential vorticity advection model by airborne ozone lidar measurements in winter 1998-1999[END_REF]).

Four advected-PV maps, derived for the 400 K level from the MIMOSA model are presented in Fig. 5. A low-PV air mass of tropical origin (over the South American continent) stretches latitudinally in the form of a "tongue" towards the southern mid-latitudes. The tropical air mass was localised close to Brazil on April 23 (see Fig. 5a), and moved south-eastwards in the days thereafter. On April 25, the forward edge of the event is positioned in the middle of the Atlantic Ocean around 30 • S and has a south-easterly movement (see Fig. 5b). On April 28, the forward edge is situated near the west coast of southern Africa, extending towards the north (Fig. 5c). The tropical low-PV ÿlament has elongated and covers nearly all of South Africa by April 29 and 30 (Fig. 5d). Moreover, kinematic back-trajectories calculated from ECMWF (not shown) are in a close agreement with the advection contour model, and show the same origin for the stratospheric air mass over Durban.

Conclusion

The central line of this work is the use of both LIDAR measurements and modelling in order to analyse an isentropic transport event through the southern subtropical barrier. We ÿrst focused on the use of aerosol proÿles obtained by LIDAR over Durban, South Africa. Because of their life expectancy, stratospheric aerosols provide a good tracer. Aerosols can thus be used to study stratospheric dynamics and notably to study transport from the tropical stratospheric reservoir to mid-latitudes. As reported by several authors [START_REF] Trepte | Tropical stratospheric circulation deduced from satellite aerosol data[END_REF][START_REF] Mccormick | SAGE II measurements of early Pinatubo aerosols[END_REF], the tropical stratospheric reservoir has an air mass that is distinctly di erent from that in the mid-latitude stratosphere.

That is evidenced in Fig. 6, which shows a tropical and a subtropical height vs. time cross-section of aerosol extinction as recorded by the 525-nm wavelength channel of the SAGE-2 experiment, from 1994 to 1999. Whatever the period of observation is, one can observe that aerosol distributions in the lower stratosphere are di erent in the tropical zone and the subtropics. During the period from April to June 1999 (indicated with two vertical heavy lines), a 2 km-layer with a maximum extinction value of ≈ 1:7 km -1 was evident in the tropical lower stratosphere; while in the subtropical stratosphere the maximum extinction value only reached ≈ 0:6 km -1 . This indicates that the observed aerosol loading event over Durban can correspond to a transport process of air mass from tropics.

Examination of MIMOSA output PV-maps at 380 and 420 K potential temperature levels (not shown) show almost the same ÿlamentary development, as for the 400 K level (see Fig. 5), with the same trajectory and during the same period. The detected horizontal transport event in the lower stratosphere concerns several isentropic levels in the lower stratosphere, in agreement with LIDAR observations over the Durban site (see Fig. 2).

All these results, LIDAR observation together with SAGE-2 measurements, the PV-maps, and the MIMOSA model retrieval of ÿlamentary structures derived from ECMWF ÿelds, agree with each other supporting the hypotheses of a horizontal transport in the lower stratosphere of air mass from tropics towards the subtropics and mid-latitudes.

Moreover, it is well known that the winter stratosphere is disrupted by wave activity, chie y gravity and planetary waves. These waves are mainly generated in the troposphere and propagate with the westerly winds in winter through the middle atmosphere. It is also known that horizontal air mass exchanges between the tropical stratospheric reservoir and mid-latitudes are episodic and take place through the subtropical barrier due to planetary wave breaking. Indeed, it is the large-scale Rossby waves in the westerlies that pull air masses and tracers from the tropics into mid-latitudes in the form of large-scale ÿlaments through the surf zone [START_REF] Leovy | Transport of ozone in the middle stratosphere: evidence for planetary wave breaking[END_REF][START_REF] Mcintyre | The "surf zone" in the stratosphere[END_REF].

Using the same LIDAR data base recorded over Durban during the same period as in the present study, [START_REF] Bencherif | Observation and ÿrst validation of stratospheric temperature proÿles obtained by a Rayleigh-Mie LIDAR over Durban, South Africa[END_REF] have shown that the daily variability of the local thermal structure was getting large, as the zonal wind component was becoming westerly and Rossby waves could propagate upward. Furthermore, in recent years and mostly in the northern hemisphere, there has been much interest in understanding the e ects of mixing barriers on either side of the tropical stratosphere. With regard to the latitudinal extent of the air mass transport, coupling of observations and modelling is therefore essential in studying large-scale events.

In order to understand and quantify the meridional exchange events through the southern subtropical barrier, we carry on monitoring the LIDAR over Durban, in parallel with similar observations over Reunion Island (20:8 • S, 55:5 • E).

  Fig. 1. LIDAR and SAGE-2 aerosol extinction proÿles recorded quasi-simultaneously, on June 8, 1999 (see legend). The horizontal lines represent the SAGE-2 error bars. The LIDAR proÿle is framed by the aerosol extinction proÿles at ± the total uncertainty.

Fig. 2 .

 2 Fig. 2. Aerosol extinction proÿles recorded by LIDAR over Durban from April 20 to June 14, 1999. The corresponding averaged proÿle is shown by the solid thick line (see legend).

  and shows a zone of low-PV (2 6 PV 6 5 PVU) lying between 25 • S and 35 • S and 20 • E and 45 • E. With the exception of the equatorial zone, the lowest PV values (PV 6 3 PVU) appear around the 27 • S and 39 • E position. This low-PV air mass either has a local upper-tropospheric origin or has originated from long range isentropic quasi-horizontal transport.

Fig. 3 .

 3 Fig. 3. Potential vorticity analyses derived from ECMWF ÿelds, for the 400 K potential temperature level. (a) PV mean ÿelds from April 18 to May 10, 1999, and (b) PV ÿeld on April 29, 1999.

Fig. 4 .

 4 Fig. 4. Water vapour and cloud distributions in the troposphere derived from MeteoSat satellite for April 29, 1999.

Fig. 5 .

 5 Fig. 5. Advected-PV contours as obtained with the MIMOSA model for April (a) 23, (b) 25, (c) 28 and (d) 30, 1999. The four PV contours are for the 400 K potential temperature level.

Fig. 6 .

 6 Fig. 6. Time vs. height cross-sections of aerosol extinction derived from SAGE-2 proÿles obtained from the 525-nm wavelength channel, for the period from January 1994 to December 1999. The upper and lower plates present the 0-15 • S and 20-40 • S latitudinal bands, respectively. The heavy red solid lines frame the period from April to June 1999.

Acknowledgements

The authors wish to acknowledge the South African Weather Service and ECMWF for providing meteorological data and the G.R.T.R. (University of Strasbourg, France) for providing the Meteosat data. We also want to thank the French Embassy at Pretoria, the National Research Foundation (South African agency), le Minist ere de l'Education Nationale de l'Enseignement Supà erieur et de la Recherche, le Minist ere des A aires Etrang eres et de la Coopà eration and le Conseil Gà enà eral de la Rà eunion, for their ÿnancial support.