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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 |
This symposium was one of more than seventy held during the 31st Scientific Assembly of COSPAR that took place in Birmingham, England. Other sessions included C1.2 (`Horizontal and vertical small scale structures in the middle atmosphere'), C4.3 (`CIRA Part III Reference Atmospheres -Trace Constituent Models- Comparison with Latest Data'), and sessions on measurement techniques, retrieval methods and instrument calibration. The Assembly was held over six days in July1996. Being more compressed than the previous two week assemblies, there were inevitably more parallel sessions, and session C1.2 unfortunately took place at the same time as session A1.1. A total of 35 oral papers and 11 posters were scheduled, although a small number were withdrawn.
The session was dominated by UARS, with the Upper Atmosphere Research Satellite data probably accounting for more than half of those shown. C.Jackman gave an overview of the UARS data, adding to material which he had already presented in Session C4.3. Several authors gave papers about specific UARS instruments and their data sets, and for a number of studies data sets from various instruments were combined with each other and with other analyses such as from the NMC. Now that the UARS record exceeds five years, questions of year-to-year variability can be seriously addressed, and the first year which was affected by Mt.Pinatubo aerosol can be contrasted with the later years. Many of us never expected to see any meaningful temporal trends in the UARS data, but the increase of HCl and HF are clearly seen in the HALOE data.
The wind data, which are directly measured from Doppler shifts by WINDII and HRDI, are impressive, and values are obtained continuously from 10 to 200 km altitude apart from a gap between about 42 and 50km. Such features as the Quasi-Biennial Oscillation are clearly revealed, and it seems likely that these measurements will be particularly valuable in the tropics where derivation of dynamical quantities from the temperature and geopotential height is inaccurate. Several instruments are showing high quality tracer latitude/height cross sections that point to patterns of meridional circulation indicating upwelling in the tropics. These patterns were seen in methane and nitrous oxide in the Nimbus7 SAMS data 15 years ago as single or double humped structures that peaked in the tropics and varied with season. They are now seen much more clearly and also in CFC12 (from CLAES) and water vapour (from several instruments), and subtle variations in the pattern (`rabbit ears', sub-tropical steepening of the latitudinal gradients, etc.) are observed and found to depend on the phase of the QBO.
The behaviour of the winter vortex, particularly in relation to ozone depletion is still a major issue with most attention now being focused on the Northern Hemisphere. L.Hood (using TOMS and MSU data) and S.Pawson addressed the question of to what extent stratospheric ozone changes are induced by changes in the dynamics, and it appears that in the tropics dynamics could account for all of the changes observed, with dynamics also being a major cause at mid latitudes.
R.Rood described the assimilation work of the NASA/GSFC Data Assimilation Office. He showed comparisons between his own and the UK Meteorological Office assimilation for identical data input. They showed near perfect agreement in the troposphere where the data was a strong constraint, but much more marked differences in the stratosphere. He showed results for an event on 10September1992 when a tongue of tracers in the tropical stratosphere was seen to be extruded towards the Southern Polar vortex (data for this event were also shown in a number of other papers). In this case he found that using just HALOE data for a few days (i.e. with very few profiles, mainly in the tropics) gave very good results, apparently implying that the structures were mainly generated by the dynamics and that a low resolution tracer field was adequate to give the initial field. A simple form of assimilation was used for validation of the ERS2 GOME total column ozone data by A.Piters and P.Levelt from KNMI in the Netherlands. They advected column amounts as if at a single level using 200hPa ECMWF winds (this level was found to give the best results) as a way of mapping the column amounts for internal consistency checks and comparison with other measurements.
S.Pawson described the SPARC GRIPS modelling intercomparison project in some detail and went on to give some results of comparisons with the Berlin data sets. He also expressed a considered desire for observations of temperature, height, winds and ozone at 500kmx500kmx2km resolution over a decade or more for long term studies, and better for process studies. J.Gille described the HIRDLS instrument due for launch on EOS-Chem that goes some way to addressing these needs.
Several papers (by Y.J.Orsolini, T.Hirooka, D.Ortland) were about free travelling Rossby waves, such as the 2-day and the 5-day waves, again using mainly UARS data. Orsolini's waves (seen in the UK Met. Office UARS assimilation fields for January1992) seemed to be in lower latitudes than previous studies; it was concluded that they were caused by inertial instabilities. Hirooka saw 5-day waves in ISAMS data with for example a global structure in May1992 at about 1hPa.
The UARS orbit is not sun synchronous, which allows solar tides to be studied provided their effects can be separated from other, often much bigger, temporal changes. M.Gelman studied differences between 1hPa MLS and NMC temperatures and found patterns that varied over the orbit precession cycle in a repeatable manner that appeared to be tidal. This demonstrated that diurnal variations need to be allowed for, certainly in the upper stratosphere and above, when data for difference local times are gridded or merged. Differences were up to about 4K. Similar results were reported by P.Keckhut when comparing between MLS and French lidars.
D.Rees described the ALOMAR lidar being installed at Andoya in northern Norway. It uses two steerable 2m diameter telescopes and is able to measure winds, temperature and PSC/aerosol reflections both by day and night. He made the point that the presence of polar stratospheric clouds or noctilucent clouds enabled much more accurate wind measurements.
A number of other papers addressed PSCs. A.Lambert showed how UARS ISAMS data could be used to map PSCs and determine their composition from measurements at different infra red wavelengths, while S.Pawson presented a study of northern winter temperatures (which are critical to PSC formation) as obtained from the SSU operational sounders over the last 17 years and from Berlin radiosonde analyses over the last 40years.
M.Lopez-Valverde presented CO measurements from ISAMS and compared them with data from the March/April1992 Space Shuttle flight of the ATMOS solar occultation interferometer. Although both are infra red measurements, they rely on different physics (ATMOS is not affected by non LTE; ISAMS relies on NLTE effects). They give similar results below 40km, but at 60km ATMOS is lower by two orders of magnitude, and then displays a very rapid increase to come back into agreement. The speaker believed that the ATMOS results were too low. M.Summers reported OH measurements from the MAHRSI instrument carried on the ATLAS-3 Shuttle mission along with MAS and CRISTA (CRISTA papers which were presented in a parallel session will be the subject of an article in the SPARC Newsletter8). The measurements are consistent with models in the upper mesosphere but less with models in the lower mesosphere. He suggested that this is part of the larger problem whereby theory underpredicts ozone at 50km. Given that they have simultaneous CRISTA data for ozone and other species they now have a good chance of making some progress.
M.Coffey showed some results from four aircraft campaigns (three in the Arctic, one in the Antarctic) which measured column amounts of stratospheric species using a Fourier Transform Spectrometer looking up at the Sun. He addressed the problem of how to inter-relate them and found that they were well correlated with potential vorticity derived from meteorological analyses, such that PV fields could possibly be used to extrapolate the results of individual flights to much larger regions.
The session included papers on variety of other topics including the mesosphere and thermosphere (notably from HRDI), on comparison with CIRA temperature and constituent climatologies, and calculation of UVB dose at the surface from stratospheric ozone fields.
Overall the session was well attended with an audience of typically about 40 despite overlapping sessions, and despite the last session being held on a Saturday morning during uncharacteristically hot and sunny weather!
John J. Barnett