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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 |
This workshop began with a discussion by J. Haigh of Imperial College, London, of a modelling study of solar variability and atmospheric change. She reported results of several experiments using a 3-D model in which solar forcing was simulated by changes in both the incoming solar radiation and stratospheric ozone concentrations. A second invited paper by M. Salby of the University of Colorado addressed statistical issues that must be considered in assessing the influence of solar variability on the middle atmosphere in the presence of the QBO. A paper by L. Hood of the University of Arizona summarised evidence for a solar cycle variation of lower stratospheric ozone and temperature using satellite measurements covering > 16 years. It was concluded that the solar cycle variation of total ozone occurs in the lower stratosphere and is forced primarily by changes in dynamical transport. K. Labitzke of the Free University of Berlin and H. van Loon reviewed correlative evidence for a 10-12 year oscillation of lower stratospheric geopotential height based on Berlin analyses of radiosonde data extending over 4 decades. (Figure 3) The observed variation is largest near 30?N and is strongest in spring, summer, and autumn. D. Rind and N. Balachandran of the Goddard Institute for Space Studies in New York presented results of numerical experiments with the GISS GCM to investigate the effect of UV variability on the stratosphere and troposphere. Finally, C. Jackman of GSFC and colleagues reviewed the status of current research into the effects of energetic particle precipitation on stratospheric odd Nitrogen and ozone on the solar cycle time scale.
The workshop was well attended and was marked by discussions and numerous questions and comments from the participants.
Lon Hood.
The workshop included eight invited papers and about 30 posters, several of which were related to the GRIPS initiative. The invited papers reviewed progress and presented new results in the areas of dynamics, radiation transfer and chemistry of the middle atmosphere.
The workshop began with four papers about the current ability of GCMs to simulate the dynamics of the middle atmosphere. U. Langematz (FU Berlin, Germany) gave a historical overview of the modelling of sudden warmings in comprehensive models, beginning with the pioneering work in the 1970s and ending with recent results from several contemporary models. Current models are capable of simulating minor and major midwinter warmings, although in the latter stages of the modelled events the polar vortex often does not break down fully and the radiative recovery back to the wintertime state is too slow. The dynamical coupling between the troposphere and stratosphere in several GCMs was discussed by K. Kodera (MRI, Tsukuba, Japan) : the results were intriguing, in that the various models showed different "coupled" modes of variability which were related to SST anomalies in rather complex ways. K. Hamilton (GFDL, USA) addressed the current status of modelling the tropical lower stratosphere, which is dominated by the quasi-biennial oscillation. This feature is absent from current GCMs, even though a realistic spectrum of tropical waves is simulated. One reason could be the need to resolve strong vertical shears but even GCMs with increased vertical resolution seem incapable of representing the oscillation. Detailed analysis of the momentum budget suggests possible reasons for the absence of the oscillation. The role of dissipative mechanisms in the GCMs needs further study, as mechanistic-type models can support QBO-like oscillations. N. McFarlane (CCC, Canada) discussed the importance of the gravity wave drag to the middle atmospheric circulation. The non-local effects of the drag and the problems associated with calculating the propagation of waves from their poorly-defined sources was emphasised. N. McFarlane also introduced the idea that hydraulic jumps across mountain ranges might be an important process to parameterise for surface-atmosphere interactions. While it is clear that gravity wave drag schemes have a strong positive effect on the model climate, critical tests to verify the mechanisms of the schemes do not exist.
The topic of exchange across the tropopause, which is important because of the climatological relevance of many trace gases in the lower stratosphere, was addressed by J. F. Lamarque (NCAR, USA). Since the dynamics of cross-tropopause transport cannot be captured by GCMs with limited spatial resolution, mesoscale models are often applied. Problems of exchange in both directions were discussed using state-of-the-art modelling. Topics discussed included the role of transport in the tropics and the sub-tropical barrier, the role of convection in mid-latitude stratosphere-troposphere exchange and the water vapour budget in the lower stratosphere. Stratosphere-troposphere exchange has taken on new significance because of the international assessments of the environmental impact of aircraft flying in the lower stratosphere.
Chemical and radiative effects of stratospheric trace gases were also discussed. A. Douglass (NASA GSFC, USA) discussed the current state of atmospheric chemistry models; it was particularly clear how the use of recent satellite data has helped improve the modelling capabilities. The recent results from HALOE data that suggest that the 40 km ozone problem was solved were the subject of lively discussion that was not resolved until after the meeting. Focusing on transport-related issues, the sensitivity of chemical processes to the model's ability to represent the long-lived trace gases was discussed. For instance, the same dynamical shortcomings that lead to the "cold-pole" problem lead to inadequate wintertime descent of chlorine compounds necessary to represent polar chemistry. Furthermore, the temperature biases strongly impact the ability to simulate heterogeneous chemistry. R. Orris (GFDL, USA) presented calculations which addressed the consistency of current knowledge about middle atmospheric trace gas and temperature distributions -- this is an important theme since many current GCMs use prescribed ozone distributions. This is a long-standing problem and she used several new data sets in the study. The use of a fixed dynamical heating model suggests that the middle atmospheric temperature can be modelled correctly to within +/-5K in the middle and upper stratosphere, which is within the range of observational uncertainty. The uncertainties were slightly larger at higher levels.
The increased availability of satellite observations of middle atmospheric trace species, thermal structure, and even horizontal wind velocities has been accompanied by the development of new data analysis techniques. Data assimilation is an effective method of combining observational data sampling into numerical models, ultimately enabling more sophisticated diagnosis than by traditional methods and constraining the models to behave like the atmosphere. R. Swinbank (UKMO, UK) showed how the data assimilation has proven quite successful in providing winds to study transport and to integrate tracer measurements for chemical studies. However, significant problems remain in the representation of tropical fields where the data are not sufficient to constrain the model.
The invited papers were supplemented by the posters; these ranged from basic descriptions of current middle atmosphere-climate models and their performance, through studies of their sensitivity as well as contributions about specific aspects of the middle atmospheric circulation, composition and chemistry. They illustrated diverse applications of current circulation models which include the middle atmosphere, as well as highlighting several improvements in recent years as well as remaining limitations of our ability to simulate the structure and composition of the middle atmosphere.
A lively discussion session ended the meeting, when several people were able to present their views and air opinions about the current state of middle atmospheric modelling.
Steven Pawson, Richard Rood
This 1 1/2 day workshop during the Boulder IUGG General Assembly in July 1995 attracted approximately 40 oral and poster presentations. Much activity addressed gravity wave sources, both from observational and numerical perspectives. Modelling studies addressed convective, orographic, and shear excitation and the potential effects of such waves at greater altitudes. Observational studies examined the possible influences of various sources and their climatologies. Discussion focused on the possible viability of various sources and their contributions geographically, seasonally, and statistically. Other efforts examined the evolution of the atmospheric wave spectrum with altitude, analytically, numerically, and observationally. Discussion here concerned the importance of various wave processes and interactions in accounting for spectral evolution, wave saturation, turbulence, and diffusion.
A number of presentations also discussed gravity wave parameterizations having various emphases. Several addressed specific sources and provided comparisons with observations which were very encouraging. Others described the implementation, testing, and effects of existing schemes in large-scale models. For the most part, these showed the newer schemes to behave reasonably and to have results consistent with large-scale observations when properly tuned. Also noted, however, were areas in which present schemes remained unable to account fully for observed features of the atmospheric circulation.
Overall, workshop participants appeared pleased with the interactions and the apparent progress that was being made both in quantifying the spatial and temporal variations of the gravity wave spectrum throughout the atmosphere and in describing their effects increasingly well in large-scale models.
Dave Fritts