Quantifying transport into the lowermost stratosphere

  • Since the discovery of the ozone hole [Farman et al., 1985], the dynamics of the stratosphere and the transport of anthropogenic trace gases from the surface to the higher atmosphere has come into the focus of interest. In the tropics, air rises high into the stratosphere and is transported poleward. Trace gases like the CFCs1, photochemically stable in the troposphere, are thus transported into regions where they are photolyzed. The products of the photolysis reactions (eg. Cl, Br) interact in the catalytic ozone cycles and lead to enhanced ozone depletion. Regarding the transport of trace gases, the so-called lowermost stratosphere (LMS) is a very interesting region, where the troposphere and the stratosphere directly interact and air masses out of both regions are mixed. It is the lowest part of the stratosphere between the tropopause and the 380 K isentrope. Tropospheric air can enter this region directly via isentropic transport across the extra-tropical tropopause whereas stratospheric air descends across the 380 K isentrope via the mean meridional circulation. Stratosphere-troposphere exchange (STE) controls the chemical composition of the LMS as well as of the tropopause region and thus has an important effect on the radiative and chemical balance of these regions and the climate system. STE exhibits a strong seasonality [Holton et al., 1995]. While downwelling of stratospheric air across the 380 K isentrope is the dominant process in winter, troposphere-to-stratosphere transport (TST) gains importance in summer, when the downwelling from the stratosphere is only weak. Isentropic transport across the extra-tropical tropopause occurs in regions where the tropopause is strongly disturbed and is connected to tropopause folds, streamer events, frontal zones, polar and subtropical jets, warm conveyor belts and cut-off low systems. A short introduction into STE, the LMS region, and methods to study atmospheric transport is given in Chapter 1. One useful tool to analyse the motions of air and transport processes are longlived trace gases. Since the lifetimes of these tracers are longer than the time scale of the transport processes they are involved in, the distribution of tracers in the atmosphere is mostly determined by dynamics. In the context of this thesis, measurements of such long-lived tracers were performed and used to study transport into the LMS region in the northern hemisphere. During the Vintersol/EuPLEx and ENVISAT validation campaigns in winter 2003, long-lived tracers such as N2O, CH4, CFC-12, CFC-11, H-1211, H2, SF6 and CO2 were measured with the High Altitude Gas Analyser (HAGAR), a two channel in-situ gas chromatograph combined with a CO2 instrument, based on nondispersive infrared absorption. Combined with measurements taken during campaigns in Forli/Italy (ENVISAT validation) in July and October 2002, tracer data were gathered from the tropopause up to altitudes around 20 km during 25 flights on board the Russian high-altitude aircraft M55 Geophysica. Thus, a substantial set of high quality tracer data has been obtained covering the polar vortex region as well as the mid latitudes of the northern hemisphere. Chapter 2 gives an overview of the HAGAR instrument and necessary improvements of the instrumental set up (implementing a CH4 channel) that were performed in the context of this thesis, and review data processing, the measurement campaigns. In order to study transport into the LMS it is assumed that air basically enters the LMS via three different pathways: a) quasi-isentropic transport from the troposphere, b) downward advection from the middle stratosphere through the 380 K surface and c) in the polar vortex region subsidence of air from of the polar vortex. Fractions of air originating in each of these source regions are determined with a simple mass balance calculation by using observations of a subset of the above species with distinct lifetimes (N2O, CH4, CFC-11, H-1211, H2 and O3) yielding complementary constraints on transport from each region. Details of the mass balance calculation and the results are presented in Chapter 3. During the mid-latitude measurement campaigns in Forlí the passing of a cut-off low system associated with an elongated streamer over Europe was observed. The impacts of this event on the trace gas mixing ratios in the LMS are examined in Chapter 4. Finally, a summary is given in Chapter 5.
  • Die unterste Stratosphäre ("lowermost stratosphere", kurz LMS) ist der Teil der Stratosphäre zwischen der Tropopause und der 380 K Isentrope, der in direktem Austausch mit der Troposphäre steht [Holton et al., 1995]. Der Eintrag von Luftmassen in die unterste Stratosphäre erfolgt in den mittleren Breiten im wesentlichen auf zwei Wegen: Zum einen durch Absinken stratosphärischer Luft von oberhalb der 380 K Isentrope im Rahmen der Brewer-Dobson Zirkulation [Brewer, 1949] und zum anderen durch quasi-isentropen Transport von troposphärischer Luft durch die Tropopause. Austauschprozesse zwischen der Troposphäre und der LMS (stratospheretroposphere-exchange, kurz STE) bestimmen in signifikantem Ausmaß die chemische Zusammensetzung [Pan et al., 1997] und die Strahlungsbilanz der LMS und der Tropopausenregion [Holton et al., 1995]. Änderungen der Strahlungsbilanz der Tropopausenregion durch Änderungen der Treibhausgaskonzentrationen sind in höchstem Maÿe relevant für das Klima der Atmosphäre [Haynes & Shepard, 2001]. So führt der Anstieg der Treibhausgaskonzentrationen zur Abkühlung der Stratosphäre und Erwärmung der Troposphäre [Pawson et al., 1998]. Im Zusammenhang mit tieferen Temperaturen in der mittleren und oberen Stratosphäre wird die Abnahme der Stabilität in der Stratosphäre [Stohl et al., 2003a] und damit Änderungen der globalen Zirkulation diskutiert. Zusammenfassend lässt sich sagen, dass das Wissen über Transportprozesse in der Tropopausenregion und der LMS elementar ist für das Verständnis von Spurengasbudgets (z.B. Ozon) und dem Klimawandel in der Atmosphäre. Diese Studie liefert einen Beitrag zur Untersuchung der Dynamik im Bereich der LMS. Anhand von flugzeuggetragenen in-situ Messungen der langlebigen Spurengase N2O, CFC-12, CFC-11, H-1211, CH4, SF6, H2 und CO2 wurde der Transport in der und in die LMS untersucht. ...

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Author:Anja Sabine WernerGND
Referee:Ulrich Schmidt, Thomas RöckmannORCiDGND
Document Type:Doctoral Thesis
Date of Publication (online):2008/02/11
Year of first Publication:2006
Publishing Institution:Universitätsbibliothek Johann Christian Senckenberg
Granting Institution:Johann Wolfgang Goethe-Universität
Date of final exam:2007/03/05
Release Date:2008/02/11
Institutes:Geowissenschaften / Geographie / Geowissenschaften
Dewey Decimal Classification:5 Naturwissenschaften und Mathematik / 55 Geowissenschaften, Geologie / 550 Geowissenschaften
Licence (German):License LogoDeutsches Urheberrecht