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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
The session on stratosphere-troposphere exchange (STE) held at the recent EGS Assembly attracted scientists working on a whole range of problems in dynamics, transport, radiation and chemistry in the upper troposphere and lower stratosphere. This wide interest reflects the perceived scientific importance of the STE problem (see reports on the SPARC SSG meeting and on the Pointe-du-Lac meeting both in Newsletter-6).
At a large meeting such as EGS there is a danger that a session on an 'interdisciplinary' subject such as STE will fragment, as specialists in different areas are attracted away to parallel sessions in their own speciality. In the event, there seemed to be a substantial core of participants who stayed for most of the session. Over 45 papers and posters were presented during the session and there is space here to record only some of them.
Dynamics and transport (extratropics). J. Thuburn (Univ. of Reading) began the session by describing his work with G. Craig on a GCM investigation of the factors determining the height of the tropopause. This has been identified as one of the key issues in STE, given that the characteristics of the 'reservoir' in the lowermost stratosphere depend on the position of the tropopause, its time variation, and its effectiveness as a transport barrier. This theme continued as first M. Ambaum (KNMI) and then P. Haynes (Univ. of Cambridge) described investigations of the formation and maintenance of the tropopause by the isentropic stirring action of baroclinic eddies. C. Appenzeller (Univ. of Washington) described the observed seasonal evolution of the reservoir and showed that it was consistent with the springtime maximum in radioactive debris observed during the period of stratospheric bomb tests.
Dynamics and transport (tropics). Moving the focus to low latitude, P. Mote (Univ. of Edinburgh) discussed satellite observations of the annual cycle in water vapour and comparison with observational estimates of upwelling velocities. This work has been important in supporting the idea of a tropical tape-recorder that leads to the upward propagation of the annual water vapour signal from the tropopause into the middle stratosphere. E. Highwood (Univ. of Reading) discussed the basic question of defining the tropopause at low latitudes. Certainly there seems to be a clear distinction between the height to which convection penetrates and that, for example, at which the temperature is minimum. D. Jackson (Imperial College) showed the upper tropospheric/lower stratospheric water vapour distribution revealed by the HALOE instrument, with, for example, striking differences at low latitudes between Northern Hemisphere summer and Northern Hemisphere winter. G. Reid (NOAA, Boulder) discussed further the observed annual cycle in tropopause temperatures, noting a number of aspects that still require explanation. Transport out of low latitudes (another key STE issue) was considered by H. Rogers (Univ. of Oxford), using ISAMS aerosol observations. Associated transport calculations using observed winds highlighted interesting sensitivities to the particular wind dataset used.
Radiation and chemistry. K. Shine (Univ. of Reading) reminded the session of the radiative processes that are important in the neighbourhood of the tropopause and the implications of changes in this region for e.g. tropospheric temperatures. He noted in particular the large trend in lower stratospheric ozone and its implications for the radiative forcing of the climate system. B. Christiansen (Danish Met. Inst.) described a GCM investigation of the effect of idealised perturbations in ozone on the circulation. J. Lelieveld (Univ. of Wageningen) then reviewed the chemistry of the lowermost stratosphere, identifying a number of important general issues before focusing on recent observations of carbon monoxide and other trace gases during the STREAM campaign.
Observations. G. Vaughan (Univ. College of Wales, Aberystwyth) discussed many recent observations from the TOASTE-B campaign, including the identification of air with stratospheric chemical characteristics, but tropospheric dynamical characteristics (i.e. PV values). Trajectory calculations supported the idea that this air had entered the troposphere many days earlier. A. Tuck (NOAA, Boulder) noted a number of features seen in the recent ASHOE/MAESA campaigns that remind us that the division between troposphere and stratosphere is, at best, only a convenient conceptual simplification. Air with high water vapour content had been observed above the sub-tropical tropopause, whilst there was evidence from CFCs of air of stratospheric origin in the tropical upper troposphere. B. Belloul (Service d'Aéronomie-CNRS) described a new method for satellite temperature measurement using radio occultation. The combination of good vertical resolution (about 1 km) and global coverage gives this technique considerable potential for observing the tropopause region. A. Hertzog (Service d'Aéronomie-CNRS) presented an approach for distinguishing, in vertical profiles of ozone observed using lidar, between vertical structure due to gravity wave activity and vertical structure due to filamentation (i.e. differential advection by the large-scale flow). F. Bertin (CETP, St Maur) discussed the capability of ST radars to estimate the vertical diffusivities, via measurements of the turbulent energy dissipation rate, and reminded the audience of the range of very different physical models on which such estimates have been based.
Mesoscale and small scale. U. Schumann (DLR Oberpfaffenhofen) reviewed the observational constraints on small-scale turbulence in the tropopause region. He noted the inconsistency of assumptions concerning inertial ranges and gave observational evidence that, at least over oceans, small-scale turbulence may often be entirely absent. He also noted a possible association between the occurrence of cirrus and turbulence in the tropopause region. J. F. Lamarque (NCAR) described a numerical investigation of mixing between tropospheric and stratospheric air due to topographic waves, motivated by DIAL observations in the neighbourhood of Fritts Peak. H. Davies (ETH Zurich) reviewed observations of mesoscale features at tropopause levels and concluded that the tropopause was 'awash' with such features. He discussed the various different models that might capture some of the relevant dynamics and described numerical investigation of the evolution of simple vortex structures within a primitive-equation model. H. Wernli (ETH Zurich) continued by describing a quantitative analysis, based on high resolution ECMWF data, of the contribution of different types of feature such as streamers and cut-offs to STE. V. Wirth (Univ. Munchen) noted the need for a theoretical model to predict the quantitative relation between water vapour satellite images on one hand and PV maps on the other hand and presented a first attempt at such a model. M. Bithell (Rutherford Appleton Lab.) described simulations of tropopause folds and the occurrence of tube-like PV structures in a high resolution GCM and discussed the role of model dissipation in the evolution of such structures. Finally, Y. Yamazaki (Univ. of Toronto) described very-high resolution simulations of tropopause folds in a non-hydrostatic model and discussed sensitivity to resolution.
Peter Haynes, Volkmar Wirth
Figure caption:
Annual variation of the zonal and monthly mean relative pressure on the tropopause (defined by the PV=2PVU surface) for the years 1992 and 1993, derived from UKMO analyses. The pressure is given relative to the annual zonal mean, with a contour interval of 10 hPa, negative values dashed, zero line dash-dotted. The see-saw nature of the pattern in pressure in the SH means that the annual variation in the hemispheric average pressure is far smaller in the SH than in the NH. This leads to corresponding inter-hemispheric differences in the annual variation of the mass flux across the tropopause (Figure supplied by Dr. C. Appenzeller, Univ. of Washington).
An open session on the Middle Atmosphere was organised by Dr. M. Dameris and Prof. Dr. P. Fabian. The session was very well attended. 38 oral and 14 poster papers were presented during this two day event covering various aspects of Middle Atmosphere dynamics and chemistry, investigations of atmospheric waves, new measurements of chemical tracers, aerosols, etc. in the whole middle atmosphere (10-120 km), modelling studies of atmospheric processes, comparison of model results and observations.
The abstracts are published in Annales Geophysicae, Supplement III to Volume 14, C701-C713, 1996.
For more information, contact martin.dameris@dlr.de.
Martin Dameris