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Stratospheric Processes And their Role in Climate
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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 |
Convener: Martin Dameris, DLR Oberpfaffenhofen, Wessling, Germany (martin.dameris@dlr.de)
This year, the open session had two main topics, the variability of the stratosphere, and the assimilation of chemical data. For example, N. Butchart presented results from the middle atmosphere version of the UKMO Unified Model of two 60 year model runs (1992-2051), assuming a plausible scenario for the future change in the concentrations of well-mixed greenhouse gases. In particular it was shown that changes in the frequency of occurrences of stratospheric warmings was unpredictable because of internally generated decadal variability.
Some new results were presented with respect to the in-situ simulation of the QBO in GCMs. The QBO has been successfully simulated in the UKMO Unified Model employing the Hines gravity wave drag scheme (D. Stainforth). K. Weber studied the influence of the QBO on the polar vortex during northern winter in the Berlin CMAM GCM, by imposing westerly and easterly winds in the equatorial lower stratosphere. The model results show significant differences at high latitudes in most dynamical parameters. D. Klasen showed that the 4-dimensional variational data assimilation represents a well-suited method to incorporate the mostly asynoptic data of all available measurements into a chemistry-transport model. She showed some interesting new results of the CTM COMMA of the application of CRISTA measurements. The assimilation of temperature data from the UARS-MLS instrument into the UKMO assimilation system was also presented. V. Asenek showed results of two model experiments: one incorporated UKMO operational observations in addition to MLS data while the other did not include the MLS data. To maximise the benefits offered by the high resolution of the MLS data, it was necessary to retune some of the assimilation parameters.
As every year, this open session will be held again next year during the XXVI Assembly of EGS.
Martin Dameris
Conveners: V. Wirth, Meteorology Institute, Munich, Germany (volkmar.wirth@Meteorologie.Physik.Uni-Muenchen.dbp.de) and Peter H. Haynes, DAMTP, Cambridge, UK (p.h.haynes@damtp.cam.ac.uk)
This session attracted some 30 oral contributions plus 6 posters. Several contributions presented first results from a major EU-funded field campaign APE-THESEO focussing on the tropical tropopause region and other campaigns focussing on the subtropical tropopause and on the upper troposphere/lower stratosphere (UT/LS) region at higher latitudes. We now have better knowledge of the variation of chemical constituents in the region immediately above the extra-tropical tropopause, where the air tends to have a character intermediate between tropospheric and stratospheric.
In quantification of STE the focus seems to have shifted away from computing fluxes across control surfaces such as certain constant-PV surfaces; instead, it seems increasingly to be based on a Lagrangian framework, for example determining histories of air parcels that arrive to a particular region.
Low latitudes: An overview paper by L. Stefanutti et al. summarised the measurements (in itu and lidar) taken at the tropical tropopause during APE-THESEO in 1999 (seven flights from two aircraft). The IR lidar identified thin subvisible cirrus in the tropopause region which could not be associated with anvil outflow from local convection. V. Santacesaria et al. tried to infer the cloud formation mechanism at the tropical tropopause. Apparently, the (relatively low) height and the (relatively high) temperature of the convective clouds during APE-THESEO are not consistent with a local freeze-drying mechanism in the light of the observed dryness of the tropopause region. However, thin cirrus clouds observed above the top of the convective clouds could be related to buoyancy waves suggesting that such waves play a role in drying the tropical UT. R. Scott and J.-P. Cammas related observations of high ozone in the tropical UT to a wave breaking event at the subtropical jet by applying the technique of contour advection with surgery to a particular case.
G. Ancellet et al. analysed ozone sonde and ozone lidar profiles taken in the subtropics; although tropopause folds are often observed and can be considered as the generic mechanism for the generation of ozone filaments in the subtropics similar as in mid-latitudes, there may be cases in which vertical motions induced by nearby synoptic systems (like tropical cyclones) considerably modify the ozone profiles in the UT. B. Hassler and M. McIntyre showed that in a simple model, which includes the tropical QBO, the Brewer-Dobson meridional circulation may have multiple equilibria; they found strong sensitivities and pointed out the need to investigate the issue with a more realistic model. B. Legras et al. reported ozone observations taken in July 1998, featuring a persistent, almost planetary scale tropopause fold associated with the SH subtropical jet. Analysis of upper level convergence and divergence fields suggests that this giant fold was associated with the descent branch of the Hadley circulation during the Asian monsoon. The degree to which this is a climatological feature has yet to be determined.
Extratropics: A. Tuck reviewed both historic and modern aircraft in-situ measurements of wind, temperature, and tracers in the tropical tropopause region. The data reveal scale invariance over a large range of scales implying power law scaling and probability density functions with long tails. The underlying mechanisms leading to this behaviour are not yet fully understood. A. Zahn analysed ozone and CO measurements taken in the UT in the context of the CARIBIC project. As a major result he noted that the ozone/CO relationship could be used as an objective definition of the tropopause, but that the strong seasonality of the relationship needed to be taken into account. H. Gouget analysed data from the TROPOZ II campaign to study STE in the SH; by combining observations with backtrajectory calculations he identified different classes of events, one of which was interpreted as mixing between stratospheric and tropospheric air. C. Kiemle showed that airborne DIAL lidar measurements of UT water vapour taken during the MAP campaign in autumn 1999 can provide realistic high resolution sections of water vapour in the tropopause region. The small-scale structure detected on those sections is broadly consistent with mesoscale modelling results. T. Wetter analysed high-precision tracer observations from the tropopause region finding a substantial amount of variability on tracer-tracer correlation plots; this was interpreted as a result of mixing associated with deep convection.
A. Dethof discussed contour advection analysis of isentropic water vapour transport from tropical UT to extratropical ST, particularly in association with the Asian summer monsoon. The transport appeared to be larger in 1998 (which was an El Niño year) than in 1997. P. Hoor showed chemical observations from the STREAM and POLSTAR campaigns. Correlation between different species indicated a distinct transition layer above the high-latitude tropopause. B. Schaeler showed LS water vapour measurements from the CRISTA satellite instrument and simultaneous aircraft. J. Kowol-Santen showed results from numerical simulations in a high vertical resolution mesoscale model of folds and intrusions near the subtropical jet. V. Wirth described a study, based on idealised dynamical models, of differences in tropopause structure between cyclones and anticyclones, with a view to explaining earlier studies of such differences. M. Juckes discussed different mathematical formulations for mean transport and implications for transport across the tropopause; this transport defined by a constant potential vorticity surface would naturally be from troposphere to stratosphere in high latitudes, consistent with earlier observational studies.
H. Wernli argued that deep exchange events, where air moved from LS to UT or vice versa, were particularly significant, e.g. for affecting chemical distributions; in the NH, such events were most common in the storm-track region in the western parts of the Atlantic and Pacific oceans. P. Konopka described a new modelling approach based on parcels on an irregular grid, with mixing between nearest neighbours allowed on a criterion based on local deformation rate of horizontal wind. M. Bourqui analysed Lagrangian behaviour in mesoscale-model simulations of baroclinic disturbances and used this as a basis for quantification of exchange. S. McCaffrey described the NARE campaign chemical observations in the UT and model simulations to investigate the bringing together of stratospheric and boundary-layer air masses as suggested by the observations. M. Greenslade discussed quasi-geostrophic baroclinic channel simulations in which distinct transport regimes were seen at upper and lower levels and argued that the transition between these regimes was a useful paradigm for the extratropical tropopause. O. Morgenstern investigated tracer transport in the wintertime subtropical tropopause region using effective diffusivity. The same quantity, namely effective diffusivity, was considered by P. Haynes and E. Shuckburgh in order to suggest an alternative definition of the tropopause. They argued that a minimum in effective diffusivity, when existing, is a more fundamental and relevant definition of the tropopause than any specific PV surface (which is the definition often used).
Global: A. Stohl presented a Lagrangian climatology of NH transport in which he identified various classes that were significant for STE. A. Bratseth discussed transport circulations and diffusivities for the troposphere and LS calculated from particle integrations based on ECMWF reanalysis data. C. Reithmeier described a new on-line transport scheme for the ECHAM4 GCM and gave results for a number of passive tracer experiments. G.-J. Roelofs used a 3D fully coupled chemistry climate model to investigate the different sources of ozone in the troposphere; the seasonality of the ozone transport from the stratosphere turns out to play a significant role. B. Bregman gave a summary of the TOPOZ-2 results in which a simplified ozone was used as a basis for intercomparison of different chemical transport models. B. Saendig described variations in the transport and mixing of tracers in a simplified GCM as the planetary wave forcing was varied.
Posters: Posters were presented both on measurements and on model-based transport studies in the tropopause region. Somewhat aside from the main focus of the STE session, but nonetheless very interesting, was a poster by K. Turnbull et al. on the development of a frost-point hygrometer with considerably increased sensitivity and much reduced response time; laboratory experiments and a balloon flight demonstrated its potential for accurate high resolution measurements in the tropopause region.
V. Wirth and P. H. Haynes
Robert Sausen, DLR, Germany (robert.sausen@dlr.de)
Peter van Velthoven, KNMI, The Netherlands (velthove@knmi.nl)
The sub-session covered 12 oral presentations (including 1 solicited paper) and 4 posters. Only 2 announced papers were not present. The audience was over 100, which led to lively discussions.
Kelder presented an overview on the topic, with emphasis on the results of the recent IPCC Special Report ‘Aviation and the Global Atmosphere’. Three papers covered in itu observations: Wohlfrom (massive chemi-ions in the wake of aircraft), Wohlfrom (sulphuric acid in the wake) and Hagen (particles from rocket emissions). Palikonda and Meyer, both studied satellite observations of contrails. Nine papers on modelling aircraft effects were presented: on plume effects (Sorokin), the impact of bromine chemistry (Hendricks), parameterising NOx from lightning (Brunner, Meijer), 3D modelling of the perturbation of atmospheric chemistry due to aviation (Berntsen, Jourdain, Hauglustaine), the impact of climate-chemistry interaction on the response to supersonic aircraft emissions (Pitari), and on the impact of aircraft powered by liquid hydrogen (Sausen). Finally, Lee discussed problems arising from controlling only aviation carbon dioxide.
Robert Sausen
Martin Dameris (martin.dameris@dlr.de)
This session was organised for the first time during an EGS-meeting. It was very well attended. At the beginning, D. Shindell gave an overview concerning global water, ozone recovery, and increasing stratospheric water. He showed that upper stratospheric temperatures indicate large decreases extending back several decades. Only by including an increase in stratospheric water vapour in climate models can these trends be reproduced. The required water vapour trend is too large to be accounted for solely by increased production within the stratosphere, suggesting that climate change may be altering input from the troposphere. A second interesting topic was the comparison of interactively coupled chemistry-climate model results. Both, a middle atmosphere version of the ECHAM/CHEM model as well as a high vertical resolution version with an upper boundary near 10 hPa indicate a general cooling of the stratosphere caused by the increased greenhouse gas concentrations, except in the northern hemisphere in late winter/early spring. This could be an indication for a general change of wave activity in this region due to a warmer troposphere (greenhouse effect). D. Hartmann's analysis of observations suggest a trend in annular modes of variability in both the surface and stratospheric climate over the past 30 years. These trends may result from a combination of changes in the chemistry of the stratosphere and increasing greenhouse gases. A further analysis of coupled models should give a clearer picture in the near future. The future development of the ozone layer (2015) was estimated using an interactively coupled chemistry climate GCM (ECHAM4.L39(DLR)/CHEM). First results indicate that in the southern hemisphere the recovery of the ozone layer is clearly delayed due to chemistry-climate interaction yielding an additional decrease of stratospheric ozone. In the northern hemisphere the recovery of ozone above 20 km is also delayed whereas a significant increase of ozone is found from the surface up to the lowermost stratosphere caused by enhanced NOx emissions, resulting in an increase of the total ozone column.
This session will be held again in two years, during the EGS 2002.
Martin Dameris
Conveners: Markus Ammann, PSI, Villigen, Switzerland (markus.ammann@psi.ch); Christian George, LACE, Lyon France (Christian.George@univ-lyon1.fr); Thorsten Hoffmann, ISAS, Germany (hoffmann@isas-dortmund.de).
This session attracted about 65 presentations focussed on heterogeneous or multiphase processes on solid or in liquid aerosols that are potentially important in determining the composition of the troposphere or affecting climate.
Emphasis has been given to four different topics which are: organic aerosols (especially formation from biogenic hydrocarbon oxidation), ageing processes of aerosols in the atmosphere and impact on their reactivity, heterogeneous chemistry on carbonaceous aerosols and finally cloud chemistry.
The session started with an overview lecture on the formation of secondary organic aerosols (SOA) given by R. Kamens from the University of North Carolina (USA). Special attention was given to model simulations showing that reasonable predictions of secondary aerosol formation are possible from both dark ozone and light/NOx reactions, exemplified for the a-pinene system. However, it was also underlined that there is little information on the quantum yields for product aldehydes which "drive" the reactivity of the NOx-light reactions. A number of other presentations also dealt with the modelling of SOA formation and the reactivity of secondary aerosols, which highlighted the importance of secondary aerosols from biogenic origin but also included studies on air quality related topics.
Aerosol modelling and characterisation requires careful analysis of the history of the particles. This point was addressed by U. Baltensperger (Paul Scherrer Institute, Switzerland) who in another invited talk focused on the modification of aerosol particle characteristics due to ageing processes in the atmosphere. This is an important issue because heterogeneous reactions have the potential to influence the gas phase chemistry of the atmosphere but also to modify the characteristics of the particles themselves. This is the case, for example, for soot particles for which adsorption or surface chemical processes are expected to drastically change the hygroscopicity by increasing the amount of water soluble material which may result in the incorporation of the particles in cloud droplets. This overview was followed by several contributions on the characterisation and reactivity of aerosol particles from different parts of the troposphere and addressing organic acids, mineral dust and sea-salt. One important topic in this session was the reactivity of soot particles. H. Saathoff (Forschungszentrum Karlsruhe, Germany) in his invited lecture, presented the AIDA soot aerosol campaign in October 1999: dynamics and optical properties of pure and mixed aerosols. This campaign with 8 German, Swiss and Austrian institutes participating was organised in October 1999 at the 84 m3 large aerosol chamber facility AIDA. The goal of the campaign was a comprehensive physical and chemical characterisation of spark generated soot (a laboratory soot source widely used in aerosol research) in comparison with real Diesel soot from a commercial engine, of ammonium sulphate aerosol, of secondary organic aerosol, and of their mixtures, as a function of ageing time. A number of other talks then presented results using novel single particle analysis instruments in field and laboratory studies, and also addressed the surface reactivity of organic compounds adsorbed on soot particles. It seems that the latter are the driving force of the particle reactivity.
Finally, the session also addressed the potential impact of cloud chemistry. This complex topic was introduced by H. Herrmann (Institute For Tropospheric research, Germany) with results from field, laboratory and modelling studies of tropospheric aerosol and cloud chemistry. This contribution and the following presentations addressed the relations between gas-phase and condensed phase chemistry (especially via OH-forming and NO3 reactions), the effects of cloud processing of aerosol particles and the microphysical effects of organic components. Especially the latter topic highlighted the need for a better understanding of multiphase organic chemistry.
Regarding the quality and quantity of contributions and the significant number of participants, it was decided to held a similar session at the forthcoming EGS general assembly in Nice in 2001.
Christian George