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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 Middle Atmosphere Sciences Symposium (MASS) was held in conjunction with the IUGG XXI General Assembly in Boulder, Colorado, during the first two weeks of July, 1995. Over 250 papers, invited and contributed, were presented at the Symposium. Oral and poster sessions were held throughout the first week of the Assembly, and on Monday and Tuesday of the second week. The MASS included four topical symposia : Dynamics, Chemistry, Radiation ; and Transport ; Solar-Terrestrial Interactions ; and Polar Mesosphere. Symposium participants also had the opportunity to attend a Union lecture on Ozone Depletion and Global Change, symposia on Equatorial Atmosphere-Ionosphere Interactions and Effects of the Mt. Pinatubo Eruption, and workshops on Solar Cycle Effects, Gravity Waves, and Middle Atmosphere Models. The very large number of topics covered by the MASS precludes a complete discussion of all of the sessions ; thus, this summary provides only a partial account of the many interesting papers presented at the Symposium.
The sessions on Dynamics of the Middle Atmosphere covered many topics of current interest, including tropical dynamics, planetary and gravity waves, tides, and numerical modelling. M. Salby (U. of Colorado, US) showed how data from the International Satellite Cloud Climatology Project (ISCCP) can be used to infer the spectrum of equatorial wave activity that forced by deep convection in the tropics. His results suggest that a very rich spectrum is excited, with substantial power at wavenumbers and frequencies beyond the sampling capabilities of polar orbiting satellites. Studies by F. Sassi (NCAR, US) and L. Pfister (NASA/Ames Research Center, US) indicate that some of these smaller-scale, high-frequency waves may play important roles in the forcing of the tropical quasi-biennial and semiannual oscillations.
Substantial progress has also been made in observations and modelling of the small-scale gravity waves that are thought to be important in the momentum budget of the middle atmosphere. O. Andreassen (NDRE, Norway) and T. Palmer (U. of Colorado, US) presented three-dimensional simulations of gravity wave breaking. They showed that cross-stream instabilities can develop from small, random perturbations in the flow field of an initially two-dimensional gravity wave, altering its subsequent breakdown. D. Durran (U. of Washington, US) showed how the use of wave pseudomomentum allows a much more concise and physically meaningful description of the interaction between a breaking wave and the background flow than the conventional Eulerian zonal-mean formulation. J. Prusa (Iowa State U., US) discussed a set of numerical calculations illustrating how gravity wave packets forced by localised sources in the troposphere undergo substantial dispersion as they propagate to the mesosphere. As a consequence, the mesospheric wave field is dominated by monochromatic wavetrains, much as envisaged by parameterizations of gravity wave breaking.
Observational evidence of the interaction between small-scale gravity waves and the circulation of the middle atmosphere was presented by A. Smith (NCAR, US), who used data from the High Resolution Doppler Imager (HRDI) on UARS to illustrate how the stratospheric flow field apparently filters the spectrum of upward-propagating gravity waves to produce a mesospheric circulation which shows a strong negative correlation with stratospheric winds. UARS/HRDI observations were also employed by R. Liebermann (U. of Michigan, US) to deduce the mean circulation at altitudes between 70 and 90 km. Other interesting phenomena of the mesosphere/lower thermosphere region were studied by J.A. Whiteway (York U., Canada) and J. Meriwether (Clemson U., US), who documented the occurrence of mesospheric inversion layers and speculated on possible mechanisms for their formation. Numerical modelling by T. Leblanc (Service d'Aéronomie, CNRS, France) suggests that gravity wave breaking may be involved in the formation of the inversion layers.
A thorough review of the status of numerical modelling of the middle atmosphere was presented by K. Hamilton (Geophysical Fluid Dynamics Laboratory, US). He pointed out that, despite substantial advances in the field, comprehensive models of the middle atmosphere are still subject to uncertainties arising from the need to parameterise subgrid-scale processes. J. Kinnersley (Edingburgh U., UK) showed that it is possible to produce a realistic middle atmosphere climatology in a quasi-linear model wherein the mean state is influenced by suitably parameterised gravity and planetary wave driving. R. Roble (NCAR, US) reported on the use of comprehensive three-dimension models to study the behaviour of tides in the mesosphere and lower thermosphere.
These sessions included the largest number of papers contributed to the Symposium, an indication of the rapid growth of the field in the years since the discovery of the Antarctic ozone hole. Advances in numerical modelling of stratospheric chemistry were highlighted in papers by M. Chipperfield (Cambridge U., UK), who studied the behaviour of chlorine species and their impact on polar and midlatitude ozone, and by F. Lefevre (Météo-France, CNRM, Toulouse), who demonstrated the ability to simulate realistically the behaviour of chemical species during Arctic and Antarctic winters.
Two full days were devoted to the presentation of observations of chemical constituents and their long-term trends. The papers by A. Tuck (NOAA, Aeronomy Laboratory, US), on dehydration of the polar lower stratosphere, J. Remedios (Oxford U., UK), on UARS/ISAMS observations of NO2 in the upper stratosphere and mesosphere, S. Ruth (Rutherford-Appleton Laboratory, UK), on temporal variability of methane and nitrous oxide, and J. Harries (Imperial College, London, UK), on the validation of UARS/HALOE observations of water vapour, are representative of the many interesting contributions in the area of new chemical observations. The subject of long-term trends has gained importance in recent years because of the need to document changes that may be brought about by human activities and to distinguish these from natural variability. N. Harris (UK) reviewed global ozone trends, while Y. Yung (California Institute of Technology, US) discussed the decadal evolution of the Antarctic ozone hole. J. Russell (NASA/Langley Research Center, US) used HCl and HF observations from UARS/HALOE to provide the most convincing demonstration to date that chlorine derived from CFCs is responsible for the observed trend in the stratospheric trend of this species.
Several sessions addressed observational, laboratory and modelling studies of heterogeneous processes and their impact on stratospheric ozone. A.R. Ravishankara (NOAA, Aeronomy Laboratory, US) presented a review of the heterogeneous chemistry of species containing chlorine, bromine and iodine, and their possible role in the ozone budget of the lower stratosphere. Y. Kondo (Nagoya U., Japan) discussed measurements of nitrogen compounds and their implications for heterogeneous chemistry and ozone depletion. The effect of the eruption of Mt. Pinatubo on stratospheric ozone was reviewed by W. Randel (NCAR, US), while G. Wetzel (U. Karlsruhe, Germany) and A. Roche (Lockheed/Palo Alto Research Laboratory, US) presented evidence for perturbations in nitrogen chemistry attributable to the enhanced aerosol concentration produced by Mt. Pinatubo.
Radiative processes and their relation to stratospheric chemistry and climate were the subject of a review talk by V. Ramaswamy (GFDL, US). M. Schwarzkopf (GFDL, US) discussed calculations made with the GFDL "SKYHI" model, showing the impact of known and expected changes in the stratospheric concentration of N2O, methane, CFCs, carbon dioxide and ozone. Recent observations from UARS were used by M. Mlynczak (NASA/Langley Research Center, US) to calculate the radiative budget of the middle atmosphere and by M. Lopez-Puertas (Instituto de Astrofisica de Andalucia, Spain) to study non-LTE emission from the upper stratosphere and mesosphere.
The impact of transport processes on chemical species in the middle atmosphere was the subject of several sessions (well-attended, despite being held on a Saturday!) Constituent transport by the quasi-biennial oscillation was discussed by K.-K. Tung (U. of Washington, US), who documented the relationship between the tropical QBO and its manifestations at higher latitudes. J. Austin (Meteorological Office, UK) discussed how the QBO influences the severity of the Antarctic ozone hole. The effect of planetary waves was addressed in talks by A. Plumb, on polar vortex dynamics, W. Grose (NASA/Langley Research Center, US), on tracer correlations, and M. Hitchman (U. of Wisconsin, US), on longitudinal variations of transport across the tropical stratosphere. A. Plumb (Massachusetts Institute of Technology, US) discussed the concept of the "tropical pipe", wherein tropospheric air enters the stratosphere in the tropics and remains substantially unmixed with midlatitude air. In the extratropical "surf-zone", on the other hand, constituents are thoroughly mixed by planetary wave breaking.
The concept of the tropical pipe was supported by the work of Mote and co-workers, who showed that water vapour mixing ratios in the tropical lower stratosphere bear the imprint of the seasonal cycle in upper tropospheric temperatures. This signal persists as the tropical branch of the mean meridional circulation transports nearly unmixed air to the middle stratosphere. Transport by the mean meridional circulation was also highlighted by M. Lopez-Puertas, who showed that CO mixing ratios in the high latitude stratosphere are influenced by descent of CO-rich air from the mesosphere. These results are consistent with those of D. Siskind (Naval Research Laboratory, US), who presented evidence for the transport NO-rich air from the lower thermosphere to the upper stratosphere.
The last two days of the Symposium were devoted to sessions on Solar-Terrestrial Interactions and the Polar Mesosphere, respectively. T. Huang (NCAR, US) reviewed modelling work on the response of the middle atmosphere to solar variability. G. Rottman (NCAR, US) presented observations of solar UV variance made during solar cycle 22 by the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) on UARS. The results indicate variations of nearly 10% at 180 nm, but less than 1% longward of 300 nm. G. Thomas (U. of Colorado, US) used water vapour measurements from UARS/HALOE and irradiance measurements from UARS/SOLSTICE to document an inverse relationship between Lyman-alpha intensity and water vapour abundance. L. Callis (NASA/Langley Research Center, US) discussed the production of NOy by relativistic electron precipitation (using data from the Solar Anomalous and Magnetospheric Particle Explorer on UARS). He showed that this source of NOy can be significant, even with respect to oxidation of N2O, and that large increases in NO observed by HALOE are apparently due to electron precipitation.
The sessions on the Polar Mesosphere focused primarily on observations of small-scale dynamical processes, and on modelling and observational studies of noctilucent clouds, and of polar mesospheric summer echoes (PMSE). However, a detailed review of the large-scale circulation and energetics of the region was presented by R. Portmann (NOAA, Aeronomy Laboratory, US), who also addressed the question of climate change in the middle atmosphere induced by increases in the concentration of carbon dioxide.
E. Thrane (NDRE, Norway) discussed rocketsonde, lidar and radar observations of mesospheric dynamics. He pointed out that small-scale waves appear to play a fundamental role in the formation of noctilucent clouds and of PMSE. G. Witt (Sweden) reviewed current knowledge of noctilucent clouds, including nucleation and growth mechanisms, influence of gravity waves, and the possible role of particle charging in the production of PMSE. L. Thomas (U. of Wales, UK) discussed radar observations of PMSE and noted that lidar measurements recorded the presence of ice crystals simultaneously with PMSE. T. Blix (NDRE, Norway) presented rocketsonde observations that document the presence of negatively charged aerosols during occurrences of PMSE, and of positively charged aerosols in connection with a noctilucent cloud layer.
Rolando Garcia
This session was part of MASS (already reported briefly by R. Garcia) ; it consisted of 4 invited talks, approximately 14 contributed talks, and 19 poster or abstract presentations. The morning session, chaired by C. Jackman and J. Lastovicka, began with an invited review of two-dimensional models of the middle atmospheric response to solar radiative flux variations. The paper was presented by G. Brasseur of NCAR (substituting for T. Huang). In a contributed paper, G. Rottman and T. Woods of NCAR/HAO summarised UARS SOLSTICE measurements of solar ultraviolet flux variations during the decline of solar cycle 22. In an invited paper, S. Chandra of GSFC reviewed recent observations of the effects on middle atmospheric ozone and temperature of solar UV variations occurring mainly on the solar cycle time scale. In a miss-scheduled paper, C. A. Reddy of GSFC described rocket measurements of the zonal wind SAO in the equatorial stratosphere. M. Kirkland and B. Tinsley of the University of Texas at Dallas discussed correlative evidence linking variations in GCR flux with tropospheric dynamics in winter as represented by the 500 mb vorticity area index. R. Donnelly of Solar Radiation Research in Boulder applied the 10.7 cm radio flux together with satellite measurements of the Mg II core-to-wing ratio to estimate solar UV fluxes for the 1947-1978 time period. J. McCormack and L. Hood of the University of Arizona presented a study of the solar cycle variation of upper stratospheric ozone and temperature using Nimbus 7 SBUV data, NMC data, and radiative model calculations. G. Thomas of the University of Colorado (with L. Chen, J. Russell, and G. Rottman) described UARS HALOE measurements of mesospheric water vapour variability in the tropics and its relationship to solar Lyman-alpha variations.
The afternoon session, chaired by L. Callis and S. Chandra, began with an invited paper by J. D. Winningham and R. Link of Southwest Research Institute in Texas on middle atmospheric effects of solar particle events observed by the UARS PEM instrument. This instrument primarily measures energetic electrons over the energy range from 5 eV to 5 MeV and protons from 1 eV to 150 MeV. In an invited paper, J. Lastovicka of the Institute of Atmospheric Physics in Prague (with E. S. Kazimirovsky) reviewed the influence of geomagnetic activity and energetic electrons on the dynamics, composition, and ionisation of the upper middle atmosphere. L. Callis of Langley Research Center (with R. Boughner, D. Baker, R. Mewaldt, J. Blake, and M. Natarajan) used the relativistic electron data from SAMPEX to estimate long-term changes in middle atmospheric energy deposition. They suggested that the middle atmosphere NOy production due to precipitation of relativistic electrons can be significant compared to the oxidation of N2O. M. Cordescu of the University of Colorado (with T. Fuller-Rowell, R. Roble, and D. Evans) discussed the effects of medium-energy (30 keV to 2.5 MeV) particles on the mesosphere and the lower thermosphere based on the model results from the NCAR TIME-GCM. W. Swider of Hanscom Air Force Base in Massachusetts suggested that D region ionisation sets limits on NO amounts in the mesosphere. He derived NO profiles from the electron density ([e]) distribution assuming [NO] = 20 e2. E. A. Martinez of Ciudad University in Buenos Aires (with S. Duhau) presented the results of cross-correlation studies of geomagnetic activity and column ozone during the 1989 solar and geomagnetic disturbances. W. Mende of the Free University of Berlin suggested that the northern hemisphere temperature record has a high degree of coherency with the Gleissberg cycle (88 year period) as inferred from the Carbon 14 record.
A number of related papers were presented as posters or as published abstracts. S. Perov of the Central Aerological Observatory, Moscow, analysed statistically the relationship between equatorial total ozone oscillations and solar activity. G. Petrova and colleagues of the Polar Geophysical institute, Murmansk, modelled the ion composition in the D region during solar proton events of 1989. D. Ponyavin of the Institute of Physics in St. Petersburg suggested evidence for a correlation between air surface temperature variations and solar and geomagnetic activity for the period since 1775. J. Sharber and colleagues of Southwest Research Institute in San Antonio estimated the input of magnetospheric particle energy into the middle atmosphere using UARS PEM data. G. Teptin of the State University of Kazan in Russia discussed a model for the non-linear interaction in the lower ionosphere between the solar daily tidal variations and seasonal atmospheric oscillations. E. Terez and G. Terez of Simferopol State University, Ukraine, discussed evidence for 27-day and solar cycle periods in ground-based ozone records in the latitude range from 43N to 60N. W. K. Tobiska of JPL and colleagues presented new preliminary results on the solar cycle variability of Lyman alpha and EUV irradiance based on a series of satellite records. T. Valchuk of Izmiran in Russia discussed possible connections between geomagnetic activity and climate changes. Q. Zhang and colleagues of Lanzhou Institute in China discussed the relation between precipitation anomalies over China and solar variability as modulated by the QBO. G. Zhrebtsov and colleagues of the Institute of Solar-Terrestrial Physics in Irkutsk, Russia, discussed some features of long-term variations of cosmic ray intensity and atmospheric radiation monitored at stations in eastern Siberia. B. A. de la Morena of the Atmospheric Sounding Station in Spain and colleagues presented experimental evidence for the influence of solar activity and geomagnetic variations on lower ionospheric radio absorption. W. Ding-Wen of the Institute of Atmospheric Physics in Beijing analysed statistically polar ozone data sets to investigate the possible role of the charged particle deposition in producing ozone at high latitudes. E. Kazatkina of the High-latitude Geophysical Lab in St. Petersburg and colleagues investigated possible evidence for a stratospheric aerosol enhancement following solar proton events. A. Krivolutsky of the Central Aerological Observatory, Russia, and colleagues discussed the possible influence on the ionospheric D region of the solar cycle variation of ozone. L. Makarova and A. Shirochkov of the Arctic and Antarctic research institute in St. Petersburg searched for a possible influence of the solar wind kinetic pressure on the polar middle atmosphere. M. Miah of the University of Arkansas and colleagues investigated spatial and temporal features of energetic particle precipitation using data from the Japanese EXOS-C satellite. M. Shaltout of the Institute of Astronomy and Geophysics in Cairo performed a correlation analysis between solar EUV radiation, Penticton 10.7 cm radio flux, and solar X-radiation. A. Akchurin of Kazan University in Russia and colleagues analysed effects of the semidiurnal tide on the formation of the sporadic E layer. J. Krzyscin of the Institute of Geophysics in Poland presented a statistical analysis of the quasi-decadal variation in the north polar stratospheric temperature. J. Olivero of Embry-Riddle University in Florida and colleagues presented a preliminary analysis of polar mesospheric clouds seen in solar occultation by POAM II.
Lon Hood
Dave Fritts