The APE-THESEO Tropical Campaign: An Overview

Journal of Atmospheric Chemistry 48: 1–33, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

1

The APE-THESEO Tropical Campaign:An Overview

L. STEFANUTTI 1, A. R. MACKENZIE 2, V. SANTACESARIA 1,A. ADRIANI 3, STEFANO BALESTRI 4, S. BORRMANN 5, V. KHATTATOV 6,P. MAZZINGHI 13, V. MITEV 7, V. RUDAKOV 4, C. SCHILLER 9, G. TOCI 10,C. M. VOLK 12, V. YUSHKOV 6, H. FLENTJE 14, C. KIEMLE 14,G. REDAELLI 15, K. S. CARSLAW 16, K. NOONE 17 and TH. PETER 8

1Geophysica-EEIG, Polo Scientifico, Campus CNR, via Madonno del Piano, Sesto Fiorentino,Firenze, Italy, e-mail: [email protected] of Lancaster, Department of Environmental Science, Lancaster, U.K.3Istituto per la Scienza dell’Atmosfera e del Clima-CNR, via del Fosso del Cavaliere, Roma, Italy4APE-Srl, via J. Nardi 63, Firenze, Italy5University of Mainz, Germany6Central Aerological Observatory, Dolgoprudny, Moscow Region, Russian Federation7Observatoire Neuchatel, Switzerland8MPI-Chemie, Mainz, Germany. Now with ETH, Zurich, Switzerland9Forschungszentrum Jülich GmbH, Institute I: Stratosphere (ICG-I), Germany10Quantum Electronics Institute, National Research Council, Firenze, Italy11Istituto di Elettronica Quantistica-CNR, via Pancaldo, Firenze, Italy12Universität Frankfurt, Germany13Istituto Nazionale di Ottica Applicata, Florence, Italy14Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany15Department of Physics, University of L’Aquila, Italy16The Environment Centre, University of Leeds, U.K.17Department of Meteorology, Stockholm University, Sweden

(Received: 26 March 2002; accepted: 3 February 2004)

Abstract. The APE-THESEO campaign was held from 15 February to 15 March 1999 from theSeychelles in the western Indian Ocean. APE-THESEO stands for ‘Airborne Platform for Earthobservation – (contribution to) the Third European Stratospheric Experiment on Ozone’. The cam-paign aimed to study processes controlling the low water content of the stratosphere, including themechanisms of cloud formation in the tropical tropopause region, and transport processes, studiedusing measurements of long-lived trace gases and ozone.

Two aircraft were used: the high-altitude research aircraft, M-55 Geophysica, which can fly upto 21 km, and the Falcon of the Deutsches Zentrum für Luft- und Raumfahrt, a tropospheric aircraft.Seven flights were performed, including the first simultaneous in situ and remote sensing of sub-visible cirrus clouds, the first interception of sub-visible cirrus using in-flight guidance from a path-finding aircraft, and guided descent of a high-altitude research aircraft into the anvil cloud at thetop of a tropical cyclone. In this paper we describe the payload used, the objectives for each flight,and the meteorological conditions encountered. First results show that a new type of cloud, whichwe have dubbed ultra-thin tropical tropopause cloud (UTTC), has been observed frequently, andobserved to cover areas of 105 km2. The frequent coincidence of the tropopause and hygropause

2 L. STEFANUTTI ET AL.

implies that the western Indian Ocean played an important role in the dehydration of the lowertropical stratosphere during the season of the mission. UTTCs, sub-visible cirrus, and visible cirrus,have all been implicated in the observed dehydration. Tracer measurements indicate little mixing ofstratospheric air into the upper tropical troposphere.

Key words: tropical tropopause, cirrus, Geophysica, Meteosat 5, atmospheric water vapour.

1. Introduction

Air enters the stratospheric ‘overworld’ in the tropics (Brewer, 1949; Holton etal., 1995). The air entering the overworld has well-defined concentrations of gaseswith long chemical lifetimes in the troposphere – the chlorofluorocarbons (CFCs),for example – but much less well-defined concentrations of short-lived species.Among the most important of these short-lived gases are the non-N2O nitrogenoxides. It has also been demonstrated (Wittrock et al., 2000) that iodine andbromine compounds could reach the stratosphere, where they take part in ozone-destroying catalytic cycles. The concentrations of these short-lived gases enteringthe stratosphere, as well as the concentrations of water vapour and long-lived tracegases, depend critically on the mechanisms by which air crosses the tropopause,and on the properties and structure of the tropopause itself. Recently, the specialcharacter of the altitude range a few kilometres below the tropical tropopause,which is only rarely penetrated by deep convection, has been emphasised by nam-ing this region ‘substratosphere’ (Thuburn and Craig, 2000) or ‘tropical tropopauselayer’ (TTL) (Folkins et al., 1999; Sherwood and Dessler, 2000).

Seasonal oscillations in tropopause temperatures leave an imprint on strato-spheric water vapour – the so-called atmospheric tape recorder (Mote et al., 1996).A favoured region for this troposphere-stratosphere exchange may be the maritimecontinent, because of the persistently low tropopause temperatures there (the so-called ‘stratospheric fountain’ (Newell and Gould-Stewart, 1981)). Net downwardtransport in the stratosphere above the maritime continent has been diagnosed fromradiosonde ascents (Sherwood, 2000). This need not preclude the region from be-ing important for dehydration. It is likely that transport through the TTL involvessignificant horizontal motions, as well as vertical motions, so that the region of themaritime continent can still act as a ‘cold trap’ for air that eventually moves intothe stratosphere (Holton and Gettelman, 2001). Other regions of the tropics mayalso be important, depending on the contribution of overshooting cumulonimbi towater vapour transport across the tropopause (Danielsen, 1982, 1993; Holton et al.,1995), and depending on our definition of entry-level water vapour mixing ratios(Dessler and Kim, 1999).

In the lower stratosphere itself, the meridional transport of air is poorly un-derstood. Certainly, the propagation of the tape-recorder signal in water vapourimplies that mixing between tropics and middle latitudes is limited (Mote et al.,1996). Of particular interest is the steep change in the NOy/ozone ratios between

OVERVIEW OF THE APE-THESEO CAMPAIGN 3

the tropical and mid-latitude stratosphere (Fahey et al., 1996), which gave rise tothe term ‘sub-tropical mixing barrier’. The existence of a mixing barrier impliesthat emissions, from aircraft, say, released or transported into the tropical lowerstratosphere, are likely to move slowly upwards carrying with them the potentialfor ozone destruction. Volk et al. (1996) showed that measured NOy/ozone ratiosin the tropical lower stratosphere were consistent with the tropics being relativelyisolated from the middle latitudes, but that transport into the tropics from the mid-dle latitudes did take place. Indeed, up to half the air at 21 km in the tropics wascalculated to come from the middle latitudes.

A number of measurement campaigns have used an enhanced ozone sonde net-work (e.g., Thompson et al., 2002), often with accompanying hygrosonde launches(Kley et al., 1997; Fujiwara et al., 2001), to study the behaviour of the tropi-cal tropopause region. The only previous aircraft campaigns that were explicitlydesigned to test mechanisms for the entry of water vapour into the stratospherewere the Water Vapour Exchange Experiment (WVEE) in Panama in 1980 (Kleyet al., 1982), and the STEP Tropical campaign in Darwin in 1987 (Russell etal., 1993). WVEE investigated cross-tropopause transport during Northern Hemi-sphere summer/autumn (September), and found a water vapour minimum 3 km(i.e., 80 K in potential temperature) above the local tropopause. STEP Tropi-cal investigated cross-tropopause transport during Northern Hemisphere winter(January–February), and found that the water vapour minimum coincided with thelocal tropopause. This seasonal motion of the water vapour minimum relative tothe tropopause has been reconciled by the ‘tape recorder’ hypothesis (Holton etal., 1995; Mote et al., 1996). The STEP measurements over the maritime conti-nent, from Darwin, have been interpreted as showing that overshooting convectiveturrets and tropical cyclone upwelling contribute significantly to the transport ofdry air into the stratosphere (Danielsen, 1993). However, aircraft measurementsof tropical water vapour and clouds away from regions of intense convection dur-ing the Northern Hemisphere winter have not been made, so that it has not beenpossible to say conclusively that the maritime continent is a favoured location fortroposphere-to-stratosphere transport.

The APE-THESEO tropical campaign was held from 15 February to 15 March1999, and was based at Mahé International Airport (4.6◦ S, 55.5◦ E), Seychelles,in the Indian Ocean. During the campaign two aircraft were used simultaneously:the high-altitude research aircraft, M-55 Geophysica, and the tropospheric aircraft,DLR Falcon of the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The Geo-physica has unique capabilities that make it ideal for atmospheric observationsboth in the turbulent regions around cumulonimbus clouds as well as the lowerstratosphere up to 21.5 km (Stefanutti et al., 1999a). During APE-THESEO, theGeophysica was manoeuvred into the cirrus decks or other areas of scientific inter-est, according to the instructions given by the lidar scientists on board the Falcon,leading to the first successful vertical path-finding in the tropical region.


Seven flights in different directions over the western Indian Ocean, flying as farsouth as 20◦ S (near Mauritius) and as far north as 10◦ N, were performed during theperiod on the following dates: 19, 24, 27 February and 04, 06, 09, 11 March. Belowwe give an overview of the campaign, describe the measurements made, providea meteorological context for those measurements, and highlight key discoveries.We begin with a description of data sources – aircraft, sonde, and satellite – thenprovide a short rationale for each of the mission sorties. This is followed by adiscussion of key findings.

2. Data Sources

The scientific payload of the Falcon belonging to Deutsches Zentrum fuer Luftund Raumfahrt (DLR), consisted of the OLEX (Wirth and Renger, 1996) lidar anda suite of radiometers. (Note: in general we treat instrument acronyms as propernames below, and so do not spell-out the acronyms, since the salient points ofthe instrument are described in the text). The vertical aerosol distribution above theaircraft could be monitored in real time, while ozone profiles could be obtained justa few minutes after the measurement. OLEX, the DLR ozone Lidar experiment, isan airborne 4-wavelength lidar, mounted zenith-pointing. It emits at 1064, 532,354, and 308 nm. The 354 and 308 nm signals are used for ozone measurements.An additional depolarisation channel at 532 nm complements the aerosol detectioncapabilities. Narrow-band detection filters permit daylight measurements in thetropics with small solar zenith angles and yet low noise. A highly sophisticateddata controller enables real-time monitoring of atmospheric backscatter.

The M-55 Geophysica Scientific payload is listed in Table I, and discussedbriefly, below. See also Stefanutti et al. (1999a, b).

ABLE is a high-power lidar emitting at 532 and 355 nm. It can sound the airvolume vertically either above or below the aircraft: the firing direction is decidedbefore each flight. The system is able to detect aerosols from 1 km to 10 km fromthe aircraft (Fiocco et al., 1999; Pace et al., 2003a).

MAL is a low-power (microjoule) lidar (Stefanutti and Borrmann, 2000) emit-ting a linearly polarised pulse at 532 nm, and measuring both the parallel andperpendicular polarisation backscattered signals. The system measures aerosols inthe first kilometre above the aircraft.

MAS is a miniaturised aerosol sampler, which operates like a backscattersonde(Adriani et al., 1999). It has laser transmitters operating at several wavelengths.During daytime, only the 532-nm detection channel has a sufficient S/N ratio topermit detection of the backscattered signal. Both direct and cross polarisation aremeasured.

The FSSP-300 is a forward scattering probe to measure aerosol size distributionin the range 0.1–10 µm (Baumgardner et al., 1992; Borrmann et al., 1997; Thomaset al., 2002). During APE-THESEO, sampling periods of 60 s were commonlyrequired to accumulate statistically significant measurements of particle number


Table I. Geophysica payload during APE-THESEO

Instrument Measured Principle Technique Averaging Accuracy Precisionparameter investigator time

ECOC O3 V. Rudakov, Electro-chemical 5 s 5% 2%CAO cell (ECC)

FOZAN O3 V. Yushkov, Dye chemi- 1 s 0.01 ppmv 8%CAO luminescenceF. Ravegnani,CNR

FISH H2O C. Schiller Lyman-α photo- 1 s 0.2 ppmv 4%(total) FZJ fragment

fluorescence

FLASH H2O V. Yushkov, Lyman-α 8 s 0.2 ppm 6%(gas phase) CAO

ACH Dew point, M. Mezrin, Cooled mirror, >300 s for 0.5 ◦C 3%frost point CAO detection of optical T < −60 ◦C

scattering

HAGAR SF6 M. Volk, GC/ECD 45 s 1% 3%N2O, CFC 11, CFC 12, Uni Frankfurt GC/ECD 90 s 1% 1%Halon 1211 GC/ECD 90 s 1% 3%CO2 IR absorption 23 s 1% 0.2 ppm

COPAS Condensation nuclei S. Borrmann, 2-channel CN 1 s 10% 5%(CN-total, CN-non- Uni. Mainz counter,volatile) one inlet heated

CVI-H2O Condensed-phase H2O K. Noone, Lyman-α N/r a N/r a N/r a

MISU absorption

CVI-TDL Condensed-phase P. Mazzinghi, Tunable diode laser 2.5 s Unknown b 9%HNO3 INOA spectroscopy

FSSP300 Size speciated aerosols S. Borrmann, Laser-particle 20 s 20% 10%(0.4–40 µm) Uni. Mainz spectrometer

MAS Aerosol optical A. Adriani,c Multi-wavelength 10 s 5% 5%properties IFA-CNR scattering

MAL 1 & 2 Remote Aerosol Profile V. Mitev, Microjoule-lidar 30–120 s 10% 10%(2 km from aircraft Obs. Neuchatelaltitude)

ABLE Remote aerosol G. Fiocco, Backscattering lidar Variable N/r a N/r a

profile Uni Roma

GASCOD O3, NO2-column and G. Giovanelli, DOAS Variable 5% 5%quasi-in situ FISBAT-CNR 2π optical heads4π Spectral irradiance

SORAD Net short-wave V. Yushkov, Integrating 40 s 5% 5%radiation CAO pyranometer

a Not reported.b The accuracy of the measurement depends on the chracterisation of the CVI probe, which has not yet been finalised.c PI at the time of APE-THESEO. The PI for MAS is currently F. Cairo.


densities. Particle size retrievals were done by Mie theory in cloud-free air, and bythe T-matrix method within clouds (Borrmann et al., 2000).

Mini Copas is a two-channel condensation nucleus counter connected to anisokinetic inlet. An air sample flow containing the aerosol particles is first ductedthrough a saturator unit, where the air is supersaturated with butanol. The air passesthen through a condenser unit, where the air is cooled, the supersaturated butanolvapour condenses onto the aerosol particles, and the aerosol particles attain sizeslarge enough for counting with optical detectors (Wilson et al., 1983; Brock et al.,1995).

CVI stands for Counterflow Virtual Impactor (Noone et al., 1988). This is asubisokinetic inlet, which admits only particles having kinetic energies larger thana critical value. If particle density is constant, the inlet admits only particles hav-ing diameters larger than a critical value. A carefully adjusted counterflow, withinthe inlet, is used to effect the separation of larger particles from smaller particlesand the ambient air. The particles that are admitted into the instrument pass intoa stream of clean air and are heated as they decelerate, causing any volatiles toevaporate. The sample, containing volatilised gases and residual particles, is thensplit: part is sent to a Lyman-α water vapour instrument, part to a tunable diodelaser spectrometer (TDL), and part to a condensation nuclei particle counter (CO-PAS). The TDL was originally designed to measure nitric acid and subsequentlymodified to measure simultaneously, water vapour and nitric acid (Toci et al., 2000,2002). COPAS counts residual, involatile, particles using the same technique asMini COPAS.

FLASH and FISH are both Lyman-α instruments. FLASH measures the gaseouswater vapour content (Yushkov et al., 1998; Merkulov and Yushkov, 1999), whilstFISH measures total water (gas phase + condensed phase, (Zöger et al., 1999)).The forward facing inlet from FISH samples the air anisokinetically, having ahigher sensitivity for water in the condensed phase. The overall accuracy of thishygrometer is 6% (i.e., about 0.3 ppmv in the tropical upper troposphere). FISHmeasurements are consistent with other stratospheric water vapour measurements(Kley et al., 2000). In the presence of clouds, FISH measures total water, with anover-sampling of cloud elements (Schiller et al., 1999). The best estimate of thisover-sampling factor, for typical Geophysica altitudes and speeds, and for micron-sized particles, is 5. Thus FISH measurements are very sensitive to cloud watercontent.

ACH is a frost point hygrometer (Mezrin and Starokoltsev, 2000). It is based ona standard cooled-mirror technique with multi-angle optical scattering detection.The instrument has sophisticated control electronics and a temperature-dependentresistor to allow condensate growth rates to increase as the absolute humidity de-creases. ACH has a lower measurement limit of –85 ◦C (equivalent to a saturationmixing ratio of 2.3 ppmv at 100 mbar) and a time constant of between 200 s and300 s (Mezrin and Starokoltsev, 2001).


ECOC and FOZAN are two ozone sensors: ECOC is a modified ECC sonde(Kyrö et al., 2000), FOZAN is a chemiluminescent ozone sensor (Yushkov et al.,1999) based on a solid-phase dye. FOZAN can detect fast ozone variation, butneeds frequent calibration. ECOC may be used as a reference instrument for theFOZAN.

HAGAR consists of a 2-channel gas chromatograph (GC/ECD) along with aCO2-sensor based on IR absorption. During APE-THESEO the GC was configuredto measure N2O, CFC-12, CFC-11, and halon-1211 every 90 s, and SF6 every45 s, yielding a precision of generally better than 1%, except about 3% for halon-1211 and SF6. The CO2-sensor (a modified LI-COR 6251) was flown for the firsttime during APE-THESEO and achieved a precision of 0.2 ppm for data averagedover 23 s. In-flight calibrations are performed every 7.5 minutes using two cali-bration gases that are intercalibrated to within 1% with a gravimetric standard ofNOAA/CMDL.

GASCOD-A is an UV-visible spectrometer designed to measure ozone and NO2

vertical columns and quasi-in situ (Petritoli et al., 2002), along with actinic fluxes(Kostadinov et al., 2000). The instrument is a spectrally resolved radiometer oper-ating from the near UV to a few micrometres wavelength. It can measure both theupward and downward radiation fluxes.

SORAD measures the integrated solar radiation above and below the aircraftfrom the near UV to the near IR (Krasnova et al., 1997) using standard M-80Mpyranometers of the Meteorological Service of Russia.

Table II lists, flight by flight, the status of each scientific instrument. Only twoinstruments, COPAS and Mini COPAS, suffered from problems that could not besolved during the campaign. Due to freezing of the primary mirror, ABLE did notcollect good data for some time periods. The CVI instrument had problems withthe Lyman-α lamps, which had to be rebuilt during the campaign in the laboratoryin Sweden.

In addition to the aircraft, a ground-based lidar operated at night from Mahé air-port throughout the campaign (Pace et al., 2003b). The Seychelles MeteorologicalService launched standard meteorological radiosondes twice per day throughoutthe campaign. Satellite images of the region were obtained every 30 minutes fromMeteosat V. This satellite was relocated to 65◦ E to support the INDOEX campaign(Lelieveld et al., 2001), an intensive field phase of which ran concurrently withAPE-THESEO. Meteorological forecast products, including fields of convergenceand humidity, were provided by the U.K. Meteorological Office. These fields wereused to identify the approximate position of the Inter-tropical Convergence Zone(ITCZ). High-resolution trajectory forecast maps of a potential-vorticity-basedtracer, based on ECMWF data, were provided by the university of l’Aquila (cf.Dragani et al., 2002).


Table II. Instrument operation during APE-THESEO. See Table I and the main textfor short descriptions of the instruments. The symbol ‘�’ signifies nominal operationof the instrument. The symbol ‘×’ signifies instrument malfunction or data loss

19/02 24/02 27/02 04/03 06/03 09/03 11/03

ABLE × × × × × � ×MAL × � � � � � ×MAS � � � � � � �FSSP × � � × � × ×SORAD � � � � � � �Mini COPAS × × × × × × ×COPAS × × × × × × ×CVI-Lyman α × × × × × � ×HNO3/H2O TDL × � a � � × � ×GASCOD-A × × × × � � ×FLASH � � × � � � �FISH � � � � � � �ACH × � � � � � �FOZAN � � � � � � �ECOC � � � � � � ×HAGAR × � � � � � �OLEX � � � – b � � �

a Tuned for HNO3 only.b Did not fly.

3. The Flights

A series of flight templates for cloud encounters were drafted ahead of the mission(Figure 1). The first two templates were designed to investigate flights above, andclose to, cumulonimbus clouds (Cb), while the third template was designed toinvestigate high cirrus, possibly formed in the upper troposphere from the decayof Cb or by other mechanisms, such as wave-induced cooling. Of the seven flightsaccomplished, 2 were based on template A, 1 on template B, and 2 on template C.The 2 remaining flights were long-duration transects of the ITCZ, with somewhatsimpler flight paths. The lack of flights modelled on template B was due to thescarcity of convection reaching the tropopause, and due to a reassessment of ob-jectives following the discovery of ultra-thin cirrus at the local tropopause (seebelow). Meteosat data were used to tune flight plans, particularly with respect tothe coordinates of convective structures, until shortly before take-off. The verticalprofile flown by the Geophysica was determined by real-time lidar images from theFalcon (see below). Only for flight 6, to the tropical cyclone Davina, could plansbe started early, as it took several days for the cyclone to come within range of theaircraft.


Generally the Falcon flew 30 minutes ahead of the Geophysica. The OLEXlidar was switched on immediately after the Falcon took-off. A mission scientistresponsible for each specific flight flew on board the Falcon. The mission scientistwould direct the Geophysica to the areas of maximum interest, trying to fly theGeophysica inside high cirrus or in the anvil of Cb. In some of the last flights,because the speed of the Falcon was a little higher than that of the Geophysica, theorder of take-off was reversed, so that, at times, the two aircraft were flying almostone over the other.

Take-off normally took place just before sunrise, in order not to overheat theaircraft and the scientific instrumentation on the airport taxiway under the tropicalsun. Daylight was required for the measurements from GASCOD and SORAD,and also for flight safety, so that night flights were not possible. Except for the firstflight, take-off took place at night or just at sunrise, while the landing was in thelate morning. From the routine observation of METEOSAT images, it was notedthat the maritime tropical convection present around the Seychelles was often mostintense in the evening and night. Figure 2 shows this, in terms of average cloudcoverage (for cloud tops colder than –70 ◦C) in the region within flying range ofMahé. The mean area covered by cold clouds has a maximum between 1400 and1600 UTC (18:00 to 20:00 hours, local time), and a secondary peak between 2000and 2200 UTC (24:00 to 02:00 hours, local time).

In Table III, the seven scientific flights are summarised. During each flightthere was a stratospheric component. During all flights there was at least onedive through the upper troposphere (to 14 km, or about 170 mbar). The maximumpressure altitude reached was close to 22 km. Measurements taken during take-off,landing, and dives provide profiles through the troposphere. All flights, except theone of 4 March, were conducted together by the two aircraft.

3.1. FLIGHT NUMBER 1, ON 19 FEBRUARY 1999

Figure 3 shows an IR Meteosat 5 image, taken during the flight, together with theGeophysica flight path (the Falcon flew precisely the same path, 30 to 60 minutesahead of the Geophysica). The contours used in Figure 3 represent the cloud-topbrightness temperature. Two different convective regions can be identified. Themore intense and extensive system is located west-south-west of the Seychelles,and has a cloud top temperature around –65 ◦C, corresponding to roughly 2 kmbelow the tropopause measured along the flight path. Another system is locatednorth of the Seychelles, characterised by a cluster of isolated Cb with top cloudbrightness temperatures warmer than –50 ◦C. The flight path adopted that daytook the two aircraft downwind of the more moderate Cb region north of theSeychelles. Even though the satellite cloud-top altitude for the convective cloudswas some kilometres below the local tropopause, a cirrus cloud was observed atthe tropopause. The cirrus had an optical depth of about 0.1 at 532 nm, and wastherefore on the threshold of visibility. A detailed analysis of this high cloud, using


Figures 1(a, b).

in situ and remote measurements, has shown that the high cloud was not anvilcloud from the local convection, nor anvil cloud from the convection to the south-west. Rather, the cloud appears to have formed by turbulent water vapour transportresulting from the breaking of gravity waves emanating from the local convection(Santacesaria et al., 2003).


Figure 1(c).

Figure 1. Flight plan templates drawn up for use in APE-THESEO. (a) Survey of the tropicalUTLS above convection that does not penetrate far into the ‘substratosphere’ region. (b) Profilethrough anvil cirrus at the tropopause and survey in the lower stratosphere above. (c) Profilethrough detached cirrus in the ‘substratosphere’ or at the tropopause, including subvisible andvisible cloud.

Figure 2. Average percentage of the area shown in Figures 3–9 (i.e., 10◦ N–20◦ ,S, and40◦ E–70◦ E) that is covered by clouds with tops colder than –70 ◦C, as a function of hourof the day, for the period of the APE-THESEO campaign. Local time in the Seychelles isUTC+4.


Table III. Details of the APE-THESEO mission flight schedule

Flight data, Take-off Duration Turning point(s), Main objective

1999 time (UTC) (h) degrees

Falcon,

Geophysica

19 February 9:30 4 1.2◦ N, 48.6◦ E High cirrus cloud

9:50 studies (template C)

24 February 02:45 5 8.4◦ S, 46.3◦ E Study of the UT/LS

02:58 8.2◦ S, 51.1◦ E structure above a mesoscale

6.9◦ S, 51.2◦ E Cb activity (template A)

7.1◦ S, 54.1◦ E

27 February 02:55 4.5 7.5◦ S, 54.5◦ E High cirrus cloud

02:40 7.7◦ S, 54.8◦ E studies (template C)

8.5◦ S, 55.0◦ E

8.7◦ S, 55.8◦ E

10.0◦ S, 55.8◦ E

9.7◦ S, 59.4◦ E

12.9◦ S, 50.2◦ E

12.2◦ S, 50.8◦ E

6.0◦ S, 52.2◦ E

04 March Falcon did not 5 6.2◦ S, 57.7◦ E Study of the UT/LS

fly 2.1◦ S, 57.4◦ E structure (template A)

01:05 2.3◦ S, 53.4◦ E

6.0◦ S, 53.5◦ E

6.4◦ S, 54.5◦ E

6.2◦ S, 57.7◦ E

3.5◦ S, 54.0◦ E

06 March 02:10 6 19.0◦ S, 56.0◦ E Long South-North transect

02:00 to 20◦ S

09 March 01:35 6 19.1◦ S, 60.3◦ E Study of the UT/LS

01:05 structure above a tropical

cyclone (template B)

11 March 02:46 5.7 15.1◦ S, 59.5◦ E Long transect across the ITCZ

02:40 0.7◦ S, 54.1◦ E


Figure 3. Infra-red Meteosat 5 image for 11:16 UTC on 19 February 1999. Part of equatorialeast Africa can be seen in the top left of the image. The APE-THESEO base, Mahe, is in thecentre of the image. The Geophysica flight path is shown in black. The three contour lines arefor brightness temperatures of –50 ◦C, –65 ◦C, and –80 ◦C.


This flight, and that of 9 March, were the only flights in which the aircraft flewdirectly into convection. The main goal of the flight on 24 February was the studyof a mesoscale convective system west of the Seychelles, in order to detect changesin cloud particle number density, total water, and cloud scattering properties as afunction of distance downstream of a hot tower. A meteosat IR image along withthe flight path followed by the Geophysica is reported in Figure 4.

The two aircraft took off around 03 UTC and flew to the west towards themesoscale structure. The temperature measured at the top of the convective clouds,at an altitude of about 14 km, was between –65 and –70 ◦C according to the tem-perature profile recorded by the Geophysica on-board sensors. This is consistentwith the cloud-top temperature for this mesoscale system, shown in Figure 4. In-situ measurements showed wavelike structures in the temperature and in the zonal


Figure 4. As Figure 3, but for 3:46 UTC on 24 February 1999.

wind. The lidar system installed on board the Falcon, and the in-situ instrumentson the Geophysica, detected an ultrathin tropical tropopause cloud (UTTC) at thetropopause above and detached from the Cb during this flight (Thomas et al., 2002;Peter et al., 2003). We define UTTCs as geometrically thin (200–300 m) and op-tically thin (optical depth, τ < 10−3) clouds that may cover large areas (105 km2)and that are clearly separated from convection. The only likely mechanism forthe generation of UTTCs, which contain only around 2% of the available watervapour, is stabilisation of a nearly monodisperse cloud particle population by alaminar upwelling of some millimetres per second (see Luo et al. (2003a), where anumber of other possible mechanisms are discounted). The source of such a largeupwelling, in the absence of convection that can reach the altitude of the UTTC, isnot currently known, although it does appear that upwelling of this scale is presentin the meteorological analyses.


Figure 5. As Figure 3, but for 3:46 UTC on 27 February 1999.


The flight performed on the 27th of February aimed at in-situ analysis of UTTCs.An IR meteosat image acquired during the flight with the path followed by the M55Geophysica is reported in Figure 5. There were clusters of Cb located south andwest of the western-most parts of the flight. The instruments on board the Falconand the Geophysica during this flight detected a UTTC – above, and detached from,the Cb. The UTTC was observed in combination with a more diffuse layer ofmarginally enhanced aerosol backscatter at altitudes about 1 km lower than theUTTC. This diffuse layer is likely a layer of aerosol that has grown by the uptakeof water sedimented from above by thicker cirrus that pre-dates the UTTC (Luo etal., 2003a).

3.4. FLIGHT NUMBER 4, ON 4 MARCH 1999

This is the only scientific flight during which the Falcon did not support theGeophysica. The Meteosat picture reported in Figure 6 indicates that no deep


Figure 6. As Figure 3, but for 3:46 UTC on 4 March 1999.

convection was present in the area of the sortie. The main aim of this flight wasto study the structure of the UTLS (upper troposphere and lower stratosphere)along the flight and to probe thin cirrus layers at 16 km or above. Some aerosolinstruments installed on board of the Geophysica (MAL and CVI) again detectedthe presence of an extremely thin aerosol/cloud layer at the tropopause.


On the 6th of March, the Geophysica and the Falcon performed a meridional tran-sect of the tropopause and of the lower stratosphere, which included a sounding bythe Geophysica to 14 km, at a point remote from Mahé. The main objective wasto study tracer gradients, across the southern tropics, as indicated in domain-fillingtrajectory analyses. In Figure 7 an IR Meteosat image acquired during the flightwith the path followed by the Geophysica is reported.

On this day the ITCZ was not well defined, but meteorological forecasts in-dicated that it should have been located south of the Seychelles. Little convective



activity was expected, and this was indeed the case (Figure 7). This flight was there-fore primarily dedicated to the chemical characterisation of the low stratosphere.The Geophysica flew over some low-level convection located around 10◦ S 56◦ E,during both the outward and inward legs.

The OLEX lidar installed on board the Falcon did not observe any cirrus atthe local tropopause over the convection. However, the in-situ and remote aerosolsensors installed on board the Geophysica did observe an extremely faint struc-ture (backscatter coefficient = 1.2) located at the local tropopause. Furthermore,a wave-like perturbation of the lower stratosphere was detected in the in-situ me-teorological measurements made above the Cb area (see below). The Meteosatpicture in this area shows only the convective cloud, whose brightness temperaturewas around –65 ◦C corresponding to an altitude of 13 km.




Special attention was given during APE-THESEO to the investigation of the trop-ical cyclone, Davina. The cyclone was followed from its formation south east ofIndia almost ten days in advance. Figure 8 shows the Meteosat IR image nearest intime to the encounter of the Falcon and the Geophysica with Davina.

The eye of the cyclone is clearly shown in the image. For safety reasons, themission was designed to probe only the outer edge of the cyclone, with measure-ments of the outer cirrus deck and eventually of the Cb structures. The two aircraftproceeded close to the one another at their respective cruise altitudes. Approachingthe cyclone, the Falcon weather radar monitored no particularly strong turbulentareas, and a break in the cyclone wall was detected. The Falcon then flew directlyinto the eye of Davina and the Geophysica entered the cyclone in different areas,directly sampling clouds and trace gases present in Davina (see below).




This was the last scientific flight of the campaign. The main objective was toperform a cross-ITCZ transect at upper tropospheric and stratospheric altitudes,measuring water vapour, ozone tracers and state variables at the tropopause. Alsoduring this flight remote and in-situ measurements of cirrus cloud at the tropopausewere performed. In Figure 9, a Meteosat IR image for the time of the flight isreported. The ITCZ is particularly well-defined by the convective cloud field, andwas clearly identifiable in the forecast convergence fields (not shown). A mesoscaleCb cluster reaching the tropopause region was present on that day close to theSeychelles. The flight path was deliberately located between Cb clusters for safetyreasons. Relatively thick cirrus cloud, and decaying Cb, were still present on theflight path, and were crossed several times by the Geophysica.


4. Scientific Issues Emerging from the APE-THESEO Campaign

4.1. STRUCTURE AND VARIABILITY OF THE TROPICAL TROPOPAUSE

It is not always easy to define the instantaneous altitude of the tropical tropopause,because different definitions give different altitudes (e.g., Highwood and Hoskins,1998). Very often, a complex structure in the temperature profiles around thetropical tropopause has been observed. Besides the main objectives reported inthe Table III, each flight provided useful in-situ information on the tropopausestructure by means of the temperature, pressure, ozone, and water vapour instru-ments. MacKenzie et al. (the tropopause and hygropause in the equatorial IndianOcean during February and March 1999, manuscript submitted to the Journal ofthe Atmospheric Sciences, hereafter referred to as ‘MacKenzie et al., submitted’)focus on water vapour and temperature profiles. Their conclusions are summarisedbelow, along with further analysis on satellite data, ozone, and long-lived trace gasdistributions.

On average, the mean tropopause temperature (defined as the coldest pointin the temperature vertical profile) was 192 K (–81 ◦C) observed at a meanpressure-altitude of 17 km, which is equivalent to a potential temperature of 380 K(MacKenzie et al., submitted). On average, the water vapour mixing ratio at thehygropause was between 2 and 2.5 ppmv, as measured by FISH and FLASH,and located close to the temperature minimum. All this indicates that the localtropopause was colder and drier than expected from climatology. If compared withthe data of Kelly et al. (1993), relative to the measurements carried out by theER-2 over Panama and Darwin, the water vapour mixing ratios were even lowerthan those measured over Darwin, and generally, as in Darwin, hygropause andtropopause were very close together. However, unlike the Darwin measurements,there were rather few convective clouds reaching tropopause altitudes. Figure 10shows the frequency of cloud top temperatures for the rectangular area 10◦ N,40◦ E to 20◦ S, 70◦ E (i.e., as shown in Figure 3), measured by Meteosat 5. Thetemperatures thresholds between –50 ◦C to –80 ◦C, shown in Figure 10, correspondroughly to altitudes of 11, 13, 14, and 15.5 km. Clearly, cloud tops below –80 ◦Cwere very uncommon, and occurred in measurable quantities only at the end of thecampaign.

Furthermore, temperature fluctuations in the upper troposphere and lowerstratosphere region have been observed when the aircraft was flying above convec-tive clouds during flights of 24, 27 February and 6 March 1999. Even though thisphenomenon is still under investigation, it seems likely that it is due to upward-propagating gravity waves generated by the Cb. As an example, in Figure 11,meteorological variables (temperature, pressure, and wind speed) recorded by theGeophysica are reported, together with the IR brightness temperature retrieved byMeteosat V along the path of the aircraft, for the period when the aircraft flewover the Cb during the inward leg of the flight performed on 6 March 1999. Theflight altitude was above the local tropopause and was increasing slowly until just


Figure 10. Percentage of the area shown in Figures 3–9 (i.e., 10◦ N–20◦ S, and 40◦ E–70◦ E)with cloud top temperatures lower than –50 ◦C, –60 ◦C, –70 ◦C, and –80 ◦C, for each day ofthe mission. Days are numbered from 1 January 1999, so the figure runs from 11 February to11 March.

below 21 km. Thus the aircraft was flying in the lower tropical stratosphere. Whenthe aircraft flew above regions with high brightness temperature retrieved by theMeteosat 5 (around 20 ◦C), indicating that no convective clouds were present, thetemperature measured by the sensors on board increased linearly with increasingaltitude. This was due to the fact that the aircraft was climbing into the stratosphere.When the Geophysica was over the convection at a constant altitude, a wavelikestructure appears in the temperature and in the wind speed, in this case with anamplitude (in temperature) of roughly 1.5 K. During this part of the flight, temper-atures were too high, even with the wave-induced oscillation, for cloud formation,and so MAS data, for example, show no significant backscatter above instrumentnoise. However, the basic mechanism of wave-induced cooling is shown.

4.2. CHARACTERISTICS OF TROPICAL TROPOPAUSE CIRRUS

One of the major objectives of the campaign was the in-situ analysis of thin cirrusat the tropopause. The morphology and occurrence of thin cirrus observed from theair are described in Peter et al. (2003). Ground-based observations are described inPace et al. (2003b). At altitudes between 380 and 410 K, thin clouds were observedduring almost every flight. Figure 12 shows the mean scattering ratio as measuredboth by MAS and MAL over the whole campaign, versus potential temperature.The bars indicate the variability. Above 400 K, both instruments indicate a molec-ular scattering ratio and a very small variability, denoting the absence of clouds. Forpotential temperatures up to 370 K, the average value of the scattering ratio is muchgreater than for purely molecular scattering, and there is a large variability. This


Figure 11. A comparison of Meteosat 5 brightness temperature, interpolated along the Geo-physica flight path, with in-situ measurements of aircraft altitude, zonal wind, and temperaturemade by the Geophysica on 6 March 1999. Note the oscillations in wind and temperature,coincident with the decreased brightness temperature, between 22500 s and 23500 s.

indicates that relatively thick clouds are often observed up to those altitudes. Thereis also variability that cannot be neglected between 370 K and 400 K, denoting theoccasional presence of (thin) clouds up to that altitude.

Figure 13 shows an example of the lidar return from the upper troposphericclouds observed on 24 February. The OLEX lidar detected two very thin layersbetween 16.5 and 17.5 km. The scattering ratio at 1.064 µm was between 2.7and 4, and was not measurable at 0.53 µm and 0.35 µm. The scattering ratiois defined as the ratio between total, i.e., aerosol plus molecular scattering, overpurely molecular scattering. The temperature profile from the PTU sonde of Mahéindicated two minima at the tropopause, which could explain the double cloudlayer. The layer extended for several hundred km and the minimum temperaturewas between –85 ◦C (for the lower cloud layer) and –80 ◦C. We call such cloudsultra-thin tropical tropopause clouds (UTTCs).

UTTCs occur at the cold point, when the cold-point temperature is low: between–90 ◦C and –73 ◦C. UTTCs are very thin, with a geometrical thickness of only afew hundred meters, but they are extensive in the horizontal, covering thousandsof square kilometres. UTTCs depolarise the backscattered signal, measured byMAS for example, with an average aerosol depolarisation between 15% and 20%.


Figure 12. Scattering ratio (left panel) and depolarisation (right panel) measured in the UTLSby MAS and MAL during APE-THESEO. Lines show the average values for all measure-ments, including clear sky, and bars show maximum values, for each 5 K potential temperaturebin.

Low backscattering ratios associated with high depolarisation ratios indicate thepresence of few, large, solid particles.

Thomas et al. (2002) give a detailed description of aerosol and cloud numberdensities, based mainly on data from the FSSP-300. The size distributions of all thesub-visible cirrus measured have a mode around 10 µm in diameter. For diametersbelow 1 µm, the distribution is similar inside and outside clouds.

In-situ measurements of condensed phase water and HNO3, performed by theCVI, excluded an early hypothesis that UTTC particles are composed of nitricacid hydrates (Toci et al., 2000). In order to explain the maintenance of UTTCs,Luo et al. (2003a) have suggested a special mechanism, based on the presence ofstrong upwelling (about 5 mm s−1) and a pronounced maximum in the relativehumidity above the clouds. The upwelling and humidity profile stabilise particlesof a given size in a shallow layer of the atmosphere. The UTTCs formed by thismechanism may not have a high impact on the radiation budget because of theiroptical thinness, but they may be of major importance in dehydration (Luo et al.,2003b). Since the amount of water vapour in the stratosphere determines the po-tential amount of HOx available to take part in ozone-destroying catalytic cycles,UTTCs may, therefore, be important in ozone chemistry.

4.3. TRANSPORT PROCESSES NEAR THE TROPICAL TROPOPAUSE

The Geophysica measurements of long-lived tracers, water vapour/humidity, andozone, provide information on transport into the study region. Two flights werespecifically dedicated to the study of the distribution of stratospheric tracers: the


Figure 13. OLEX backscatter ratios at 1064 nm (top), 532 nm (middle), and 354 nm (bottom)for a section of the 24 February 1999 flight. A thin double layer is noticeable in the 1064 nmchannel near 17 km, but not in the other two channels. The left-hand side of the plots refer tonorthbound flight from 7.7◦ S. The aircraft turned at 06:00 UTC, at a latitude of 4.7◦ S. Theright-hand side of the plots refer to southbound flight, reaching 6.9◦ S.


Figure 14. Trace gas measurements from HAGAR on 6 March 1999. (A): pressure-altitude(dashed line, kilometres, left-hand scale) and potential temperature (solid line, Kelvin,right-hand scale) of the Geophysica. Other panels: mixing ratio of N2O (B), CFC-11 (C),CFC-12 (D), and SF6 (E). The aircraft crossed isentropes, during constant-altitude flight, after22,000 seconds UTC.

flights of 6 and 11 March (see Figures 6 and 8). Vertical profiles of tracer mixingratios, for these and other flights, show no systematic decrease in the TTL, indicat-ing that mixing of stratospheric air into this region – as suggested by Tuck et al.(1997) – is insignificant at least at the time and place of these observations.

Looking more closely at the flight of 6 March: a North–South transect wasflown. The aircraft were flown first straight south – from Mahé to 20 ◦S, closeto Mauritius – and then back to Mahé. Although not well defined in the UKMOforecast, the ICTZ was probably in a region between 10 and 15◦ S. The Geophysicaslowly climbed, during the southward leg, from 16 to about 19 km. It then dived to13 km, turned back and climbed to maximum altitude, reaching 21 km. Figure 14shows the HAGAR data for this flight. It shows that, until 12000 s UTC, the flightwas in the upper troposphere: N2O, CFC-11, and CFC-12 are practically constant.From 12000 s UTC to 14400 s UTC there is a decrease in the CFC-12, N2O, andSF6 mixing ratios, and a stronger variability of CFC-11 (Volk et al., 2000).


Between 14400 s and 16000 s there is a clear step decrease of all long-livedtrace gases. The aircraft is in the stratosphere during this period. Then there is thedive to 13 km, during which all the trace gases increase again to the values typicalof the troposphere. The aircraft turns back northward and climbs to maximumaltitude. Between 22000 s and 25000 s there is a clear decrease of all long-livedtrace gases, as the aircraft returns towards the equator at constant altitude. Thetrend is consistent with a transient wave response to the presence of the ITCZ,i.e., the near-constant pressure-altitude cruise of the aircraft intersects isentropeswhich are sloping due to a large-scale wave. The ozone vertical distribution, mea-sured by OLEX, and the in-situ ozone measured by ECOC and FOZAN are alsoconsistent with such an explanation. The time-slices 14400–16000 s and 22000–25000 s give similar signals in the long-lived trace gases, but the thermodynamicdata demonstrate that these similar signals can be generated in quite different ways.

Another interesting feature in the trace gas distribution is related to the mini-mum in the ozone profile detected at the tropopause during some flights. Two typesof profile were observed. Figure 15 shows one of these cases observed during thethird flight on 27 February 1999, discussed above. The aerosol backscatter profile(left panel) indicates the presence of a thin aerosol/cloud layer at the tropopause,and the ozone profile (right panel) indicates a corresponding minimum in thetropopause region. The apparent ozone deficit in this minimum is about 20 ppb, or25% of the ozone mixing ratio immediately above and below the cloud. A similarobservation was made during the first flight on 19 February 1999, and is discussedby Santacesaria et al. (2003). The most likely explanation for the profile on 19February is turbulent transport of higher water vapour mixing ratios and lowerozone mixing ratios from the bottom of the TTL to the cold-point tropopause.This turbulent mixing is likely due to the breaking of convectively-induced gravitywaves. Although the Geophysica did not have a turbulence probe on-board, its in-situ measurements – and the remote measurements from the Falcon – do showa large amount of small-scale wave activity that could reasonably be attributedto convectively-induced gravity waves. However, wave activity notwithstanding,chemical depletion of ozone cannot be excluded as the reason for the apparentozone deficit in cirrus at the tropopause (cf., Reichardt et al., 1996; Roumeau etal., 2000).

A different behaviour has been observed in a case during which very strongconvection was encountered. Figure 16 shows the OLEX lidar cross section of thetropical cyclone Davina on 9 March 1999, showing the distribution of the cloudsaround the eye. The thick cloud layer between 14 and 16 km was crossed twice bythe Geophysica during both the outward and inward legs. The TDL associated withthe CVI measured a consistent amount of HNO3 (2.5 ppb), which is likely to bea consequence of NOx lighting production inside the strong Cb (Toci et al., 2000,2001).

Figure 17 shows the ozone mixing ratio versus altitude, together with the aerosolbackscattering ratio, temperature, potential temperature, and the water vapour mix-


Figure 15. Vertical profiles of backscatter ratio (left panel), temperature (middle panel), andpotential temperature (line) and ozone (dots) (right panel), for slow ascent through a thin cirrusat approximately 0515 UTC, 58◦ E and 10◦ S, on 27 February 1999. The altitude scale is theaircraft pressure-altitude.

ing ratio, measured in situ by the Geophysica during a dive through the tropicalcyclone on 9 March 1999. The left-hand panel reports the aerosol backscatterand the temperature profiles, indicating the presence of thick cirrus between 14and 15.5 km just below an inversion layer in the temperature profile. The right-hand panel shows that the ozone profile (like the potential temperature) is almostconstant up to the altitude where the inversion layer is located, and is characterisedby very low values typical of the lower part of the troposphere. Only above 15.5 kmthe ozone sharply increases. A strong gradient at this altitude is also observed inthe water vapour profile. This is a clear example of rapid vertical transport throughthe troposphere, at least as far as the cloud-top at 15.5 km, and is associated withthe strong convective activity in the tropical cyclone, as seen in the Darwin expe-dition (Danielsen, 1993). The lack of short-lived tracers of lower tropospheric airon board the Geophysica during APE-THESEO makes it difficult to observe anytroposphere-to-stratosphere transport occurring due to the cyclone, as observed inDarwin (Danielsen, 1993).


Figure 16. OLEX backscatter ratio at 1064 nm within the tropical cyclone Davina on 9 March1999. The aircraft turned 180◦ at the 400-km point on the plot. The shading on the figure isindicative only, not grayscale: The backscatter ratio for the edge of the cloud is about 1.5,whilst for the highly reflective region shown in black the backscatter ratio is 300.

5. Summary and Synthesising Comments

APE-THESEO was based on the Seychelles, in the Indian Ocean, and tookplace in February and March 1999. During the campaign two aircraft were usedsimultaneously: The high-altitude research aircraft, M-55 Geophysica, and thetropospheric aircraft, DLR Falcon. The Geophysica carried a payload designedto characterise the long-lived-trace-gas composition of the atmosphere and theatmospheric aerosol. The Falcon carried an ozone-and-aerosol lidar and radiome-ters. The aircraft measurements were supported by satellite data from Meteosat V,ground-based lidar in the Seychelles, and regular radiosonde launches. DuringAPE-THESEO, the Geophysica was manoeuvred into the cirrus decks or otherareas of scientific interest, according to the instructions given by the lidar scien-tists on board the Falcon, leading to the first successful vertical path-finding inthe tropical region. Seven flights, in different directions over the western IndianOcean, flying as far south as 20◦ S (near Mauritius) and as far north as 10◦ N, wereperformed.

APE-THESEO observed a very cold tropopause (compared to the existing cli-matologies). The coincidence of the tropopause, hygropause, and high clouds,


Figure 17. Vertical profiles of backscatter ratio and FISH total water (left panel), temperature(middle panel), and potential temperature, and ozone (right panel), at about 18◦ S on 9 March1999. The potential temperature values have had 350 K subtracted. The altitude scale is theaircraft pressure-altitude.

lead us to believe that this area was actively involved in the dehydration of thelower tropical stratosphere during the campaign (MacKenzie et al., submitted). Theparticle instruments show occasional cloud particle signatures up to 405–410 K.

The frequent occurrence of thin tropopause clouds may be related to dehydra-tion. Such clouds do not seem generally to be connected with the remnants ofanvils of Cb (Thomas et al., 2002; Santacesaria et al., 2003; Peter et al., 2003). Itis important now to investigate if the transformation of one type of high cloud toanother occurs regularly in this and other regions. In order to explain the mainte-nance and stability of UTTCs at the tropopause, Luo et al. (2003a) have suggesteda special mechanism for the cloud maintenance based on the presence of strongupwelling (about 5 mm s−1) about 1 km below the tropopause, driven by anas-yet-undiagnosed dynamics.

Measurements of long-lived trace gases show little evidence for intrusion of‘aged’ stratospheric air (i.e., air with reduced mixing ratios of long-lived gases)into the upper troposphere. Similarly, only occasionally – most dramatically duringthe passage of a tropical cyclone – did convection reach sufficiently high that ozone


mixing ratios indicative of the marine boundary layer were observed close to thetropopause.

Acknowledgements

The authors wish to express their gratitude to the crews of the Geophysica and theFalcon, and to all the technicians and co-workers who contributed to the successfulexecution of the APE-THESEO campaign. The authors thank the authorities of theRepublic of Seychelles, particularly the Directorate for Civil Aviation. An impor-tant contribution of manpower and equipment was made to the campaign by theU.K. Meteorological Office, for which we express our gratitude. This project waspart of the European Union’s Third European Stratospheric Experiment on Ozone,and was funded by the European Commission under contract ENV5 CT95-0533and INTAS YSF00-231. Funding was also provided by the Italian Space Agency(ASI), by the European Science Foundation (ESF), and by the German AerospaceResearch Centre (DLR). The authors are very grateful for the guidance given bythe Scientific Steering Group of the ESF APE programme (including amongst itsnon-APE members Prof. Adrian Tuck, Dr Rod Jones, and Dr Geir Braathen). Notleast, we gratefully acknowledge the constructive criticisms and patience of thereviewers and editorial team in seeing this paper through to publication.

References

Adriani, A., Viterbini, M., Cairo, F., Mandolini, S., and Di Donfrancesco, G., 1999: Multi Wave-length Aerosol Scatterometer for airborne experiments to study the stratospheric particle opticalproperties, J. Atmos. Oceanic Technol. 16, 1328–1335.

Baumgardner, D., Dye, J. E., Gandrud, B. W., and Knollenberg, R. G., 1992: Interpretation of mea-surements made by the Forward Scattering Spectrometer Probe (FSSP-300) during the AirborneArctic Stratospheric Expedition, J. Geophys. Res. 97, 8035–8046.

Borrmann, S., Solomon, S., Dye, J. E., and Luo, B., 1997: The potential of cirrus clouds forheterogeneous chlorine activation, Geophys. Res. Lett. 102, 3639–3648.

Borrmann, S., Luo, B.-P., and Mishchenko, M., 2000: The application of the T-matrix method to themeasurement of aspherical ellipsoidal particles with forward scattering optical particle counters,J. Aer. Sci. 31, 789–799.

Brewer, A. W., 1949: Evidence for a world circulation provided by the measurements of Helium andwater vapour distribution in the stratosphere, Quart. J. Roy. Meteorol. Soc. 75, 351–363.

Brock, C. A., Hamill, P., Wilson, J. C., Jonsson, H. H., and Chan, K. R., 1995: Particle formationin the upper tropical troposphere: A source of nuclei for the stratospheric aerosol, Science 270,1650–1653.

Danielsen, E. F., 1993: In situ evidence of rapid, vertical, irreversible transport of lower troposphericair into the lower tropical stratosphere, J. Geophys. Res. 98, 8665–8681.

Dessler, A. E. and Kim, H., 1999: Determination of the amount of water vapour entering thestratosphere based on the Halogen Occultation Experiment (HALOE) data, J. Geophys. Res.104, 30,605–30,607.

Dragani, R., Redaelli, G., Visconti, G., Mariotti, A., Rudakov, V., MacKenzie, A. R., and Stefanutti,L., 2002: High resolution stratospheric tracer fields reconstructed with lagrangian techniques: Acomparative analysis of predictive skill, J. Atmos. Sci. 59, 1943–1958.


Fahey, D. W., Donnelly, S. G., Keim, E. R., Gao, R. S., Wamsley, R. C., DelNegro, L. A., Wood-bridge, E. L., Proffitt, M. H., Rosenlof, K. H., Ko, M. K. W., Weisenstein, D. K., Scott, C. J.,Nevison, C., Solomon, S., and Chan, K. R., 1996: In situ observations of NOy , O3, and theNOy /O3 ratio in the lower stratosphere, Geophys. Res. Lett. 23, 1653–1656.

Fiocco, G., Calisse, P., Cacciani, M., Casadio, S., Pace, G., and Fua, D., 1999: ABLE: Developmentof an Airborne Lidar, J. Atmos. Oceanic Technol. 16, 1336–1343.

Folkins, I., Loewenstein, M., Podolske, J., Oltmans, S. J., and Proffitt, M., 1999: A barrier to ver-tical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements, J.Geophys. Res. 104, 22095–22102.

Fujiwara, M., Hasebe, F., Shiotani, M., Nishi, N., Vömel, H., and Oltmans, S. J., 2001: Water vapourcontrol at the tropopause by equatorial Kelvin waves observed at the Galapagos, Geophys. Res.Lett. 28, 3143–3146.

Highwood, E. J. and Hoskins, B. J., 1998: The tropical tropopause, Quart. J. Roy. Meteorol. Soc.124, 1579–1604.

Holton, J. R., Haynes, P. H., McIntyre, M. E., Carr, E. S., Douglass, A. R., Rood, R. B., and Pfister,L., 1995: Stratosphere-troposphere exchange, Revs. Geophys. 33, 403–439.

Kley, D., Schmeltekopf, A. L., Kelly, K., Winkler, R. H., Thompson, T. L., and MacFarland, M.,1982: Transport of water through the tropical tropopause, Geophys. Res. Lett. 9, 617–620.

Kley, D., Smit, H. G. J., Vömel, H., Grassl, H., Ramanathan, V., Crutzen, P. J., Williams, S., Meyw-erk, J., and Oltmans, S. J., 1997: Tropospheric water-vapour and ozone cross-sections in a zonalplane over the central Pacific Ocean, Quart. J. Roy. Meteorol. Soc. 123, 2009–2040.

Kostadinov, I., Giovanelli, G., Ravegnani, F., Bortoli, D., Petrioli, A., Bonafé, U., Rastello, M. L.,and Pisoni, P., 2000: UV-VIS spectroradiometric system for actinic measurements on boardof geophysica aircraft, in M. Colacino and G. Giovanelli (eds), Conference Proceedings, 8thWorkshop Italian Research on Antarctic Atmosphere, ISBN 88-7794-247-9, Società Italiana diFisica, pp. 293–303.

Krasnova, T. M., Skouratov, S. N., and Vinnichenko, N. K., 1997: Height dependent absorption ofsolar radiation in the Arctic atmosphere, Atmos. Res. 44, 61–71.

Kyrö, E., Kivi, R., Turunen, T., Aulamo, H., Rudakov, V. V., Khattatov, V. V., MacKenzie, A. R.,Chipperfield, M. P., Lee, A. M., Stefanutti, L., and Ravegnani, F., 2000: Ozone measurementsduring the Airborne Polar Experiment: Aircraft instrument validation; isentropic trends; andhemispheric fields prior to the 1997 Arctic ozone depletion, J. Geophys. Res. 105, 14599–14611.

Lelieveld, J. and 26 others, 2001: The Indian Ocean Experiment: Widespread air pollution fromSouth and Southeast Asia, Science 291, 1031–1036.

Luo, B. P. and 26 others, 2003a: Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilisationmechanisms, Atmos. Chem. Phys. 3, 1093–1100.

Luo, B. P. and 27 others, 2003b: Dehydration potential of ultrathin clouds at the tropical tropopause,Geophys. Res. Lett. 30 (11), 1557, doi:10.1029/2002GL016737.

Merkulov, S. and Yushkov, V., 1999: Some results of aircraft stratospheric water vapour measurementin the arctic polar vortex (in Russian), Meteorol. Hydrol. 7, 107–113.

Mezrin, M. Yu. and Starokoltsev, E. V., 2000: Aircraft Condensational Hygrometer, in Proceedingsof the 13th International Conference on Clouds and Precipitation, 14–18 August 2000, Reno,Nevada, Vol. 1, pp. 202–205.

Mezrin, M. Yu. and Starokoltsev, E. V., 2001: Aircraft condensation hygrometer and some results ofmeasuring humidity in the zone of the equatorial tropopause, Atmos. Res. 59, 331–341.

Mote, P. W., Rosenlof, K. H., McIntyre, M. E., Carr, E. S., Gille, J. C., Holton, J. R., Kinnersley,J. S., Pumphrey, H. C., Russell III, J. M., and Waters, J. W., 1996: An atmospheric tape recorder:The imprint of tropical tropopause temperatures on stratospheric water vapour, J. Geophys. Res.101, 3989–4006.

Newell, R. E. and Gould-Stewart, S., 1981: A stratospheric fountain?, J. Atmos. Sci. 38, 2789–2796.


Noone, K. J., Ogren, J. A., Heinzenberger, J., Charlson, R. J., Covert, D. S., 1988: Design andcalibration of a counterflow virtual impactor for sampling atmospheric fog and cloud droplets,Aerosol Sci. Tech. 8, 235–244.

Pace, G., Cacciani, M., Calisse, P. G., di Sarra, A., Fiocco, G., Fua’, D., Rinaldi, L., and Casadio, S.,2003a: Observation of polar stratospheric clouds with the able lidar during the APE-POLECATflight of 9 January 1997, J. Aer. Sci. 34, 801–814.

Pace, G., Cacciani, M., di Sarra, A., Fiocco, G., and Fua’, D., 2003b: LIDAR observations ofequatorial cirrus clouds at Mahé Seychelles, J. Geophys. Res. 108, doi 10.1029/2002JD002710.

Peter, Th. and 26 others, 2003: Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphologyand occurrence, Atmos. Chem. Phys. 3, 1083–1091.

Petritoli, A., Ravegnani, F., Giovanelli, G., Bortoli, D., Bonafé, U., Kostadinov, I., and Oulanovsky,A., 2002: Off-axis measurements of atmospheric trace gases by use of an airborne ultraviolet-visible spectrometer, Appl. Optics 41, 5593–5599.

Reichardt, J., Ansmann, A., Serwazi, M., Weitkamp, C., and Michaelis, W., 1996: Unexpectedly lowozone concentration in midlatitude tropospheric ice clouds – a case-study, Geophys. Res. Lett.23, 1929–1932.

Rosenfield, J. E., Considine, D. B., Schoeberl, M. R., and Browell, E. V., 1998: The impact ofsubvisible cirrus clouds near the tropical tropopause on stratospheric water vapour, Geophys.Res. Lett. 25, 1883–1886.

Roumeau, S., Brémaud, P., Rivière, E., Baldy, S., and Baray, J. L., 2000: Tropical cirrus clouds: Apossible sink for ozone, Geophys. Res. Lett. 27, 2233–2236.

Russell, P. B., Pfister, L., and Selkirk, H. B., 1993: The Tropical Experiment of the Stratosphere-Troposphere Exchange Project (STEP): Science objectives, operations, and summary findings, J.Geophys. Res. 98, 8563–8589.

Santacesaria, V., Carlá, R., MacKenzie, A. R., Adriani, A., Cairo, F., Didonfrancesco, G., Kiemle, C.,Redaelli, G., Schiller, C., Peter, T., Ravegnani, F., Oulanovsky, A., Yushkov, V., and Stefanutti,L., 2003: Clouds at the tropical tropopause: A case study during the APE-THESEO campaignover the western Indian Ocean, J. Geophys. Res. 108 (D2), 4044, doi: 10.129/2002JD002166.

Sherwood, S. C. and Dessler, A. E., 2000: On the control of stratospheric humidity, Geophys. Res.Lett. 27, 2513–2516.

Stefanutti, L. and Borrmann, S., 2000: Airborne instrument for aerosol measurements, in R. A.Meyers (ed.), Encyclopaedia of Analytical Chemistry, Wiley and Sons Ltd, Chichester, pp. 1892–1914.

Stefanutti, L., Sokolov, L., MacKenzie, A. R., Balestri, S., and Khattatov, V., 1999a: The M-55Geophysica as a platform for the Airborne Polar Experiment, J. Atmos. Oceanic Technol. 16,1303–1312.

Stefanutti, L., MacKenzie, A. R., Balestri, S., Khattatov, V., Fiocco, G., Kyrö, E., and Peter, T.,1999b: Airborne Polar Experiment-Polar Ozone, Leewaves, Chemistry, and Transport (APE-POLECAT): Rationale, Road Map and Summary of measurements, J. Geophys. Res. 104, 23941–23959.

Thomas, A., Borrmann, S., Kiemle, C., Cairo, F., Volk, M., Beuermann, J., Lepouchov, B., San-tacesaria, V., Matthey, R., Yushkov, V., MacKenzie, A. R., and Stefanutti, L., 2002: In situmeasurements of background aerosol and subvisible cirrus in the tropical tropopause region,J. Geophys. Res. 107 (D24), 4763, doi:10.1292001JD001385.

Thompson, A. M. and 15 others, 2003: Southern Hemisphere additional ozonesondes (SHADOZ)1998–2000 tropical ozone climatology, 1. Comparison with Total Ozone Mapping Spectrometer(TOMS) and ground-based measurements, J. Geophys. Res. doi: 10.129/2001JD000967.

Thuburn, J. and Craig, G. C., 2000: Stratospheric influence on tropopause height: The radiativeconstraint, J. Atmos. Sci. 57, 17–28.

Toci, G., Mazzinghi, P., Mielke, B., and Stefanutti, L., 2000: Condensed H2O and HNO3 concentra-tion measurements by the CVI payload in the APE-THESEO tropical campaign, in N. R. P.


Harris, M. Guirlet, and G. T. Amanatidis (eds), Stratospheric Ozone 1999, Proc. 5th Euro-pean Workshop on Stratospheric Ozone, St. Jean de Luz, France, Air Pollut. Res. Report 73,EUR19340, pp. 657–660.

Toci, G., Mazzinghi, P., Mielke, B., and Stefanutti, L., 2002: An airborne diode laser spectrometer forthe simultaneous measurement of H2O and HNO3 content of stratospheric cirrus clouds, OpticsLaser Engineer. 37, 459–480.

Tuck, A. F. and 15 others, 1997: The Brewer–Dobson circulation in the light of high altitude in situaircraft observations, Quart. J. Roy. Meteorol. Soc. 124, 1–70.

Volk, C. M. and 11 others, 1996: Quantifying transport between the tropical and mid-latitude lowerstratosphere, Science 272, 1763–1768.

Volk, C. M., Riediger, O., Strunk, M., Schmidt, U., Ravegnani, F., Ulanovsky, A., and Rudakov, V.,2000: In situ tracer measurements in the tropical tropopause region during APE-THESEO, inN. R. P. Harris, M. Guirlet, and G. T. Amanatidis (eds), Stratospheric Ozone 1999, Proc. 5thEuropean Workshop on Stratospheric Ozone, St. Jean de Luz, France, Air Pollut. Res. Report 73,EUR19340, pp. 661–664.

Wilson, J. C., Huyn, J. H., and Blackshear, E. D., 1983: The function and response of an improvedstratospheric condensation nucleus counter, J. Geophys. Res. 88, 6781–6785.

Wirth, M. and Renger, W., 1996: Evidence of large-scale ozone depletion within the arctic polarvortex 94/95 based on airborne LIDAR measurements, Geophys. Res. Lett. 23, 813–816.

Wittrock, F., Muller, R., Richter, A., Bovensmann, H., and Burrows, J. P., 2000: Measurements ofIodine monoxide (IO) above Spitsbergen, Geophys. Res. Lett. 27, 1471–1474.

Yushkov, V., Merkulov, S., and Astakhov, V., 1998: Optical balloon hygrometer for upper troposphereand stratosphere water vapour measurements, in J. Wang, B. Wu, T. Ogawa, and Z-H. Guans(eds), Optical Remote Sensing of the Atmosphere and Clouds, Proc. SPIE, Vol. 3501, pp. 439–445.

Yuskhov, V., Oulanovsky, A., Lechenuk, N., Roudakov, I., Arshinov, K., Tikhonov, F., Stefanutti,L., Ravegnani, F., Bonafé, U., and Georgiadis, T., 1999: A chemiluminescent ozone analyser forstratospheric measurements of the ozone concentration (FOZAN), J. Atmos. Ocean Technol. 16,1345–1350.

Zöger, M. and 11 others 1999: Fast in situ hygrometers: A new family of balloonborne and airborneLyman-α photofragment fluorescence hygrometers, J. Geophys. Res. 104, 1807–1816.

Documents

The APE-THESEO Tropical Campaign: An Overview