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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 |
Report on the 2nd International Limb Workshop
Stockholm, Sweden, October 11-14, 2004
Jörg Gumbel (gumbel@misu.su.se), and Jacek Stegman (jacek@misu.su.se), Dept. of Meteorology, Stockholm Univ., Stockholm, Sweden
Christian von Savigny, Univ. of Bremen, Germany (csavigny@iup.physik.uni-bremen.de)
Martin G. Mlynczak, NASA Langley Research Center, Hampton, VA, USA, (m.g.mlynczak@nasa.gov)
Introduction
During recent years, satellite instruments measuring scattered and emitted radiation from the Earth's limb have become more and more central for studies of the middle atmosphere. The 2nd International Limb Workshop brought together 46 scientists from various satellite communities to discuss limb instruments, retrievals and results. The workshop was hosted by the Dept. of Meteorology at Stockholm University (Sweden) under the auspices of SPARC and IGAC. Financial support was available from the International Meteorological Institute in Stockholm. For the detailed workshop programme and the list of participants see http://www.misu.su.se/limb2004.html.
While the 1st International Limb Workshop (Bremen, Germany, 2003) focused on UV/Vis limb scattering, the scope of the 2nd Workshop was broadened to include limb measurements of infrared and microwave emissions. This opened new perspectives for future collaborations on atmospheric results. Nevertheless, the main focus of the workshop continued to be on the important first steps of data analysis: instrumental issues, retrieval algorithms and data validation.
The following satellite instruments were represented at the workshop: OSIRIS and SMR (on Odin), SCIAMACHY, MIPAS and GOMOS (on ENVISAT), SABER (on TIMED), ILAS-II (on ADEOS-II), SAGE III (on Meteor-3M), ACE (on SciSat), as well as SOLSE and LORE (on the Space Shuttle). In addition, exciting outlooks were given towards new limb missions ranging from the ultraviolet to the microwave part of the spectrum.
Limb Techniques in the UV, Visible and Near-Infrared
Similar to the 1st Limb Workshop, the majority of presentations concerned techniques and results related to the limb detection of scattered sunlight in the ultraviolet and visible. Efforts have focused in particular on the co-analysis of various species and their interaction in stratospheric chemistry. The set of stratospheric species retrieved from SCIAMACHY, OSIRIS, GOMOS, SAGE-III and ILAS-II is comprised of O3, NO2, N2O, OClO, BrO and aerosols. Presentations on these observations were given by C. S. Haley, A. Rozanov, C. Sioris, S. M. Brohede, A. Seppälä, D. F. Rault, E. Kyrölä, and F. Khosrawi. The solar proton event of October/November 2003, presented by A. Seppälä and G. J. Rohen, was of particular interest for satellite-borne limb observations. Enhanced NO2 chemistry led to long-lasting effects on ozone depletion in the upper stratosphere.
D. F. Rault and E. Kyrölä presented limb scatter results from instruments that were designed primarily for occultation measurements, such as SAGE-III and GOMOS. Instrumental issues like out-of-field scatter are particular challenges for the latter. Their analysis can provide important input for the general understanding and development of new limb techniques. Stratospheric limb measurements were also presented from SOLSE and LORE onboard STS107 by D.E. Flittner and R. Loughman. These experiments serve as important steps towards the next generation of U.S. instrumentation for ozone-related studies.
From the OSIRIS data, exciting perspectives were presented by E. J. Llewellyn to retrieve mesospheric OH and possibly water vapour from the OH (A-X) solar fluorescence at 308 nm. First highlights were also presented by J. Sloan and G. Witt from the ACE instrument onboard the Canadian SciSat platform.
Limb scatter techniques have become increasingly important tools for the detection and characterization of middle atmospheric aerosol layers. Near infrared measurements along the limb path provide the necessary sensitivity and signal-to-background ratio to detect cirrus and subvisual cirrus clouds, and polar stratospheric clouds in the tropopause region and the lower stratosphere. Presentations regarding these issues were given by A. Bourassa, J. Sloan, and C. von Savigny.
Progress was also reported on mesospheric clouds in a noctilucent cloud (NLC) climatology and particle characterization by B. Karlsson, C. von Savigny, and S. Petelina. Comprehensive NLC data sets are now available from OSIRIS and SCIAMACHY, but a number of challenges need to be overcome in the study of these thin inhomogeneous cloud layers.
Infrared and Microwave Limb Techniques
Techniques based on infrared and sub-mm emissions were a new feature of the 2nd Limb Workshop. Presentations about these limb techniques readily showed the synergies with data retrieved from limb scattered UV/Vis/IR radiation. A major purpose of the workshop was to provide a forum to prepare future co-analysis of these data sets.
On the infrared side, comprehensive non-LTE (local thermo-equilibrium) data sets are available from TIMED/SABER, presented by M. Mlynczak and C. Mertens, and ENVISAT/MIPAS presented by B. Funke. These missions provide comprehensive information on the energy budget, chemistry, and exchange processes in the mesosphere and lower thermosphere. C. von Savigny presented the retrieval of mesospheric temperatures from the OH (3-1) Meinel band, demonstrating the ability of SCIAMACHY to perform limb analysis of infrared emissions.
Unique limb measurements of trace gases related to chemistry and transport processes were also reported from the Odin/SMR sub-mm instrument by J. Urban, N. Lautié, and E. Dupuy. Species presented include ClO, N2O, HNO3, O3 and H2O in the stratosphere, as well as H2O and CO in the mesosphere. The large potential of sub-mm techniques has also been suggested for future missions such as STEAM/SWIFT (D. Murtagh, and P. Eriksson) and, JEM/SMILES (Y. Kasai).
Model Studies
Presentations about numerical tools were an important complement to instrument and data contributions. W. Lahoz, S. Hassinen, and J. Rösevall presented methods for chemical data assimilation, which have in recent years provided new tools for the analysis of limb data in the stratosphere. Presentations on tools for radiative transfer modelling and limb retrievals were given by J. Kaiser, J. I. van Gent, H. Walter, A. Doicu, and P. Eriksson, and included both new theoretical concepts and operational codes.
Data Comparison and Technical Collaboration
Data comparison projects are ongoing for a number of stratospheric species involving several of the satellite instruments represented at the workshop. In order to facilitate data comparisons in the future, it has been suggested that a number of case studies with co-located measurements be defined. Further details about these efforts will be made available on the Odin/OSIRIS website (http://osirus.usask.ca). Evidently, co-location is a very limiting requirement when it comes to the comparison of more than two satellites. In a long-term perspective, chemical data assimilation will need to provide a more solid basis for the joint analysis of individual satellite data sets.
When it comes to mesospheric limb data, comparisons between various satellite missions are still at a more basic stage. There is a large potential for co-analysis concerning the chemistry and transport of tracers H2O, O3, and CO primarily from the IR and microwave limb instruments. A prime candidate for joint mesospheric analysis efforts in the UV/Vis is the climatology and properties of noctilucent clouds.
As instrumental issues and retrieval techniques are of central importance for this workshop series, a number of areas of collaboration have been defined. In order to further improve collaborations, it was agreed that a data base of technical notes be created. The following topics have been defined as issues of particular interest: pointing accuracy and altitude reconstruction, baffle effects and their quantification (out-of-field effects), quantification of albedo and multiple scattering (in-field effects), inhomogeneity along the line-of-sight (terminator problem etc.), and topographic techniques.
We invite everybody to share experience on these issues by submitting technical notes to a common data base. The data base will be hosted by the Odin/OSIRIS website (http://osirus.usask.ca). Please contact N. Lloyd (nick.lloyd@usask.ca) with any contributions or suggestions.
Conclusions
During recent years, satellite instruments scanning the Earth's limb have provided a quickly growing data base on middle atmospheric chemistry and dynamics. Efficient networks exist for the analysis of stratospheric data, and similar developments are under way for the mesosphere. In the years to come, we can also expect a downward extension of limb techniques, contributing to important scientific issues in the Upper Troposphere and Lower Stratosphere. In this region, rapid progress in new microwave limb techniques will play a particular role.
The 2nd International Limb Workshop extended its focus towards data comparisons over a spectral range from the UV to the microwave. Nevertheless, the major objective continued to be to provide a forum for the detailed exchange of ideas on measurement issues and limb retrieval techniques. The refinement of retrieval algorithms and validation efforts are in many cases still ongoing. Both of these developments benefit greatly from collaboration between individual missions. This was clearly confirmed by the fruitful exchange of ideas during the workshop.
The instrumental and retrieval issues, and the focus on final data products, make the limb workshop series a valuable complement to large-scale conferences. The 3rd International Limb Workshop be held in Montreal, Canada, in the spring of 2006.
E. J. Llewellyn from the University of Saskatchewan, Saskatoon (Canada) volunteered to organise this next meeting.