 |
Stratospheric Processes And their Role in Climate
|
||||||||
| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
|
Stratospheric Processes And their Role in Climate
|
||||||||
| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
In order to understand the role of the stratosphere in the climate system and in climate change, we must have a sufficient understanding of the processes that determine stratospheric structure and stratospheric interactions with other parts of the climate system. In determining the SPARC research agenda, three areas were selected by the Scientific Steering Group who considered that more focused activity was needed than existed previously : dynamics and transport in the upper troposphere/lower stratosphere, chemistry and microphysics in the upper troposphere/lower stratosphere, and gravity wave processes.
Many climate and environmental quality issues involve the interface between the stratosphere and the troposphere, known as the upper troposphere/lower stratosphere region (UT/LS). It is in this region that changes in stratospheric chemical composition have their greatest effect on surface climate. This is also the region in which aircraft exhausts enter the atmosphere. However models of this part of the atmosphere do not perform well, and our understanding of trends in radiatively important trace species, such as ozone and water vapour, is not adequate. Understanding the role of the stratosphere in climate requires the proper treatment of transport and mixing in the UT/LS (a very difficult problem involving theoretical understanding, global observations, and modelling), radiative processes, and the chemistry and microphysics of stratospheric ozone depletion, all of which are coupled since their time scales are similar. Atmospheric measurements in this region are difficult and incomplete (this is a particularly difficult region to observe globally with useful resolution using satellite instruments).
Gravity waves are known to exert major influences on the large-scale circulation and structure of the atmosphere and are responsible for much of the spatial and temporal variability in the flow above the tropopause. The divergence of the heat and momentum fluxes associated with dissipating gravity waves is one of the major driving forces of the middle atmosphere. In order to produce credible simulations of the earth's climate and predictions of climate response to anthropogenic forcing, comprehensive numerical models of the earth's atmosphere must include physically justifiable parameterisations of gravity wave momentum, heat, and constituent transport. SPARC seeks to stimulate and co-ordinate research to produce the necessary observational characterisation of gravity waves as well as the necessary theoretical and modelling developments so that proper inclusion of gravity waves exist in climate models.
The dynamical connection between the stratosphere and the troposphere has two quite distinct aspects that are relevant to climate (and to possible climate change). One aspect is the structure of the tropopause, principally its height and its latitudinal structure. This aspect controls, for example, the temperature minimum at the tropical tropopause (which strongly affects stratospheric water vapour, and thereby many aspects of stratospheric chemistry), as well as mid-latitude planetary-wave propagation properties (which strongly affect wintertime polar vortex temperatures, and thereby polar ozone chemistry). The structure of the mid-latitude tropopause also has a direct impact on planetary-wave structures in the troposphere, and thereby on regional climate perturbations. A second aspect is the transport and mixing of mass and chemical species between the stratosphere and troposphere, traditionally referred to as Stratosphere-Troposphere Exchange (STE). This aspect affects climate in numerous ways: e.g. the impact of aircraft emissions on the ozone layer, the vertical structure of greenhouse gas distributions in the upper troposphere/lower stratosphere, and mid-latitude ozone depletion.
It is fair to say that until recently, both aspects mentioned above were regarded as being controlled by tropospheric rather than by stratospheric processes. The tropopause height was usually attributed to convective adjustment in the troposphere, while STE was usually regarded as a problem of tropopause dynamics, with attention focused on the mesoscale phenomenology of strong mixing events such as mid-latitude tropopause folds and deep tropical convection. However, in recent years it has become clear that the stratosphere does not play a passive role in this respect; dynamics and transport in the vicinity of the tropopause cannot be regarded as local phenomena, but are constrained by the behaviour of the lower stratosphere as a whole. Furthermore, the structure of the tropopause is shaped in part by the same global dynamical processes -- namely the diabatic circulation, consisting of tropical upwelling and extratropical downwelling, and driven by stratospheric wave drag (the ``extratropical pump'') -- that control transport and mixing (the Brewer-Dobson circulation). In this way, the dynamical connection between the stratosphere and the troposphere is a truly global phenomenon. Thus, research priorities are increasingly focused less on the tropopause itself, but rather on the entire lower stratosphere/upper troposphere region -- hence the title of this section.
Figure 3.1. Dynamical Aspects of stratosphere-troposphere exchange.
The tropopause is shown by the thick line. Thin lines are isentropic or constant potential temperature surfaces labelled in Kelvins. Heavily shaded region is the "lowermost stratosphere" where isentropic surfaces span the tropopause and isentropic exchange by tropopause folding occurs. The region above the 380K surface is the "overworld", in which isentropes lie entirely in the stratosphere. Light shading in the overworld denotes wave-induced forcing (the extratropical "pump"). The wiggly double headed arrows denote meridional transport by eddy motions, which include tropical upper-tropospheric troughs and their cut-off cyclones as well as their mid-latitude counterparts including folds. Not all eddy transports are shown; and the wiggly arrows are not meant to imply any two-way symmetry. The broad arrows show transport by the global-scale circulation which is driven by the extratropical pump. This global scale circulation is the primary contribution to exchange across isentropic surfaces (e.g., the ~380 K surface) that are entirely in the overworld. (Holton et al., Reviews of Geophysics, 1995).
With regard to the structure of the tropopause, it is rather remarkable that such a basic property of the atmosphere remains so poorly understood. Certainly radiative-convective processes are absolutely central, and these processes themselves involve considerable uncertainties due to the role of water vapour and clouds. However, because radiative time scales are extremely long in the lower stratosphere (two weeks or so), temperatures in this region are only loosely constrained by radiative processes and are therefore comparatively easily affected by dynamical heating. (This is analogous to the situation in the stratospheric polar night.) Such dynamical heating, which is equivalent to diabatic motion, arises from stratospheric wave drag associated with breaking waves of various types, from planetary waves through synoptic-scale baroclinic disturbances (in the lowest stratosphere) to inertia-gravity waves. The effect of wave drag on the structure of the tropopause is both non-local (involving the overall diabatic circulation associated with stratospheric wave drag) and local (involving the sharpening of the potential-vorticity distribution on isentropic surfaces, which is the quasi-horizontal manifestation of the mid-latitude tropopause). Our understanding of what controls the structure and seasonal cycle of this dynamical forcing remains largely qualitative; for example, we have difficulty predicting how this dynamical forcing would change under climate change scenarios. Climate model simulations of the tropopause structure are generally quite poor, and most models have serious systematic errors in this region.
With regard to transport and mixing, recent developments have identified three important themes which are likely to characterise future research efforts. The first is the recognition that STE is just one aspect of a global picture of transport and mixing of mass and chemical species, which is constrained by the dynamics of the whole stratosphere (Holton et al. 1995). It is quite true that a considerable part of the actual mass exchange across the tropopause appears to be accomplished by the mesoscale mixing phenomena mentioned above. However, in considering the global budget of mass or long-lived chemical species, it has to be recognised that the STE occurring in the tropical and mid-latitude tropopause regions is constrained by the transport achieved in the global Brewer-Dobson circulation, because the budget must be closed. Put simply, the limiting factor in the circuit is the stratospheric component, because it is so slow compared to transport within the troposphere. (In other words, time-mean chemical fluxes through the tropopause are imposed by the global Brewer-Dobson circulation; if mixing across the tropopause were to become more (or less) intense, this would just weaken (or strengthen) the sharpness of the chemical contrast across the tropopause, not increase (or decrease) the chemical flux.) This is not true for chemical species having lower stratospheric sources and sinks, but the chemical time scales in this region are generally very long, so many chemical distributions are primarily controlled by transport. Important exceptions occur for processes involving condensed matter, including dehydration and heterogeneous chemistry (see Section 3.3), as the distribution of condensed matter can be strongly affected by the nature of the mixing processes.
Because the transport problem is global, transport time scales depend on the overall structure of the stratosphere. The classical view of stratospheric mixing and transport (as of about 10 years ago; e.g. WMO 1986 and Andrews et al. 1987) was expressed in terms of mean (Lagrangian) advection (by the Brewer-Dobson circulation) plus diffusion (mainly by planetary-wave mixing). In this view -- referred to by Plumb (1996) as the ``global diffuser'' paradigm -- tracer distributions represent a balance between the slope-steepening effects of mean vertical-meridional advection and the slope-flattening effects of diffusive mixing. However, over the past 10 years there has been increasing recognition of the heterogeneous nature of stratospheric mixing and transport, and of the existence of partial barriers to transport (often called ``transport barriers'' for convenience). The chemical isolation of the wintertime polar vortex is a well-known example of such a transport barrier; while the vortex may deform, unless it breaks it is impervious to transport on adiabatic time scales. More recently, an analogous though rather more permeable transport barrier has been identified at the edge of the tropics. These transport barriers strongly affect transport timescales (Plumb 1996). It may be noted that atmospheric models are conspicuously deficient in their ability to provide the right sort of transport timescales in the stratosphere. This failure is manifested by the fact that meridional tracer gradients in the tropics are invariably washed out in models as compared to observations, suggesting that the models do not simulate sufficiently effective subtropical transport barriers.
The lowest part of the stratosphere has also been identified as a distinguished region. In the so-called (stratospheric) ``overworld'', wherein isentropic surfaces lie entirely within the stratosphere (above about 18 km in altitude), transport timescales between the troposphere and stratosphere involve the diabatic circulation and are of the order of several years. But the so-called ``lowermost stratosphere'', equivalently the stratospheric part of the ``middleworld'', is ventilated by the troposphere along isentropic surfaces (analogous in some ways to the upper ocean, which is ventilated along outcropping isopycnals), which means that transport timescales can be much faster. The situation is depicted in figure 3.1, from Holton et al. (1995). Overall, the global picture of stratospheric mixing and transport includes the lowermost stratosphere together with these various regions of the overworld, partially isolated by transport barriers.
The second important theme in stratospheric mixing and transport is the increasing use of chemical tracer measurements. Of course the Brewer-Dobson circulation was originally inferred from measurements of ozone and water vapour, but it is only relatively recently that chemical measurements have been used to diagnose mixing and transport in a direct, quantitative fashion. Partly this is because of the recent availability, with UARS, of high-quality global satellite measurements of sufficient spatial resolution and duration to permit quantitative analyses that can be compared with models. Partly also it is because of accurate in-situ measurements, mainly from high-altitude aircraft, which when appropriately analysed (to remove natural meteorological variability) can provide chemical ``signatures'' of different transport regimes. Indeed the subtropical transport barriers were first identified from tracer measurements: aerosol distributions (Trepte and Hitchman 1992; Grant et al. 1994), chemical correlations (Murphy et al. 1993; Fahey et al. 1996), and the relatively uncontaminated vertical propagation of the seasonal cycle in water vapour (Mote et al. 1996). In fact, due to the poor quality of tropical wind analyses, chemical tracers remain the only present means of assessing mixing and transport within the tropics. (This is not the case in the wintertime polar vortex, where chemical tracer evolution can be successfully reconstructed from back-trajectories and other dynamically-based methods, using dynamically consistent winds provided by advanced data assimilation techniques.) In general, we are now moving toward a Lagrangian view of the chemical circulation of the stratosphere, which is an exciting prospect.
A third important theme is the recognition that the classical view of stratospheric mixing and transport, based as it was on a ``mixing length'' approximation (eddy length-scales much smaller than tracer-gradient length-scales), needs reconsideration when applied to the entire stratosphere. The existence of transport barriers violates the required scale separation; in such regions, eddy length-scales are actually much larger than tracer gradient length-scales, and flow properties (such as diabatic descent rates) can vary greatly over eddy length-scales. For example, sharp edges in tracer concentration can occur within one degree of latitude, yet the characteristic mixing length associated with a breaking planetary wave can be several tens of degrees of latitude. A consequence is that the problem of transport across a given control surface is not determinable solely from knowledge of the eddy structures at the surface itself together with the large-scale tracer gradient. Two relevant examples are STE and transport across the edge of the wintertime polar vortex. For STE, there has been much historical emphasis on estimates of the mass flux in individual STE events such as tropopause folds, obtained from mesoscale modelling studies or synoptic measurements. Yet it is not at all clear how such estimates are related to the global flux of chemical species through the mid-latitude lower stratosphere, such as the transport of stratospheric ozone into the upper troposphere. For mixing across the edge of the wintertime polar vortex, there has been considerable recent interest in the extent of the mass flux through the vortex edge, because of the possible connection between polar and mid-latitude ozone depletion. However, the net mass flux is evidently much less important than the flux of ozone-depleted or chemically perturbed air, which depends on the recent temperature history of the vortex. In both these examples, we see the need for more detailed Lagrangian information than is captured by local values of a mean gradient and a diffusivity.
Another limitation of the classical mixing-length approach is that it reduces mixing and transport to mean advection plus diffusion. This means that mixing occurs preferentially at small scales. However, the nature of stratospheric mixing by planetary (and in the lower stratosphere, synoptic-scale) breaking waves is much more a quasi-two-dimensional ``stirring'', which mixes tracer first at large rather than small scales (Pierrehumbert and Yang 1993). In such a situation, large-scale stirring is the source of sharp gradients, and is a precondition for small-scale homogenisation. This non-diffusive behaviour has implications for the importance of small-scale chemical structure. Because of the strongly non-linear character of many chemical reaction cycles, it makes a significant difference whether mixing occurs diffusively or non-diffusively; this affects the probability density of chemical concentration. The fact that small-scale tracer structure in the stratosphere is largely determined through stirring by large-scale advection has been convincingly demonstrated in many recent studies (e.g. Norton 1994; Waugh and Plumb 1994); this process leads to sloping sheets of tracer, whose horizontal manifestation is filaments and whose vertical manifestation is laminae (e.g. Orsolini 1995; Newman and Schoeberl 1995).
The fact that the classical approach to stratospheric mixing and transport appears to be largely inappropriate is unfortunate, because this approach at least provided a diagnostic framework within which to quantify measurements and compare them with model predictions. At the present time, it must be said that such a diagnostic framework does not exist. In fact, the recent tendency has been to consider essentially the opposite limit, treating the stratosphere as a set of well-mixed, virtually isolated air masses, which can be identified through their chemical signature. This is rather analogous to oceanographers' identification of water masses through their temperature and salinity signature. From such chemical signatures, mixing and transport pathways and timescales can be inferred (given certain assumptions). For example, Boering et al. (1994) have inferred significant detrainment of the tropical upwelling from the nature of the annual cycle of CO2 in the mid-latitude lowermost stratosphere, while Volk et al. (1996) have estimated entrainment rates into the tropical upwelling from fits to vertical profiles of species with varying chemical lifetimes. Such efforts are promising, but are clearly in their infancy.
Given the above scientific background, what are some of the key questions that presently need to be addressed with regard to dynamics and transport in the lower stratosphere/upper troposphere region? The following list provides a possible starting point, but is certainly not exhaustive.
1. How is the tropopause maintained? How does it evolve seasonally? How well do climate models represent this seasonal evolution? Appenzeller, Holton and Rosenlof (1996) have identified a qualitative difference in the nature of the seasonal mass cycle of the Northern Hemisphere versus the Southern Hemisphere lowermost stratosphere; what accounts for this difference? What can be said qualitatively, and with what confidence, about the changes in the structure of the tropopause that we might expect under various climate change scenarios? Why do models have such difficulty representing the tropopause; is it simply a case of spatial resolution?
2. Historically, STE studies have focused on the mass exchange across the tropopause as the key diagnostic quantity to determine, under the assumption that everything else (such as the ozone flux) would follow from this quantity. This might be reasonable if the mixing-length assumptions were valid, but STE represents a strong violation -- indeed the antithesis -- of the required conditions. Given this situation, what are the most appropriate measures of STE?
3. Appenzeller, Holton and Rosenlof (1996) have provided a first estimate of the seasonal mass budget of the lowermost stratosphere. Can similar budgets be obtained for chemical species such as ozone and water vapour? In other words, can we quantify the net transport (and its time variation) through the lowermost stratosphere of various chemical species? Since, in principle, every chemical tracer provides an independent constraint on mixing and transport, can this information be used to constrain mixing in the lowermost stratosphere?
4. Preliminary studies of mesoscale tracer structure in the stratosphere suggest that it is dominated by filaments and laminae generated by large-scale advection acting on sharp mean edges associated with transport barriers. What kind of statistical characterisation is possible of the mesoscale tracer structure? Are there statistical properties that can be represented in terms of properties of the large-scale flow? What is the role of inertia-gravity waves in this mesoscale tracer structure? Is there a possibility of developing parameterisations of mesoscale chemical structure for use in global chemistry-climate models? What is known about the possible effects of mesoscale temperature structure (produced for example by inertia-gravity waves) on condensed matter and thereby on chemistry?
5. It has recently been discovered that the annual cycle in tropical tropopause water vapour propagates upward, creating a vertical propagation in the location of the water vapour minimum; this phenomenon has been called the ``tropical tape recorder'' (Mote et al. 1996). The fact that the annual cycle in tropical tropopause water vapour is correlated (in phase) with the annual cycle in the tropical temperature minimum suggests that Brewer's ``cold trap'' mechanism is the controlling factor. Can this hypothesis be made quantitative? In particular, is an isolated cold region over Indonesia capable of dehydrating all the air entering the stratosphere? And is the large-scale temperature distribution sufficient to account for the dehydration, or are detailed microphysical processes critical? Yulaeva, Holton and Wallace (1994) and Rosenlof (1995) have argued that the annual cycle in the tropical temperature minimum is controlled by the annual cycle in extratropical planetary wave drag. Can this be demonstrated quantitatively? Also, how might the tropical temperature minimum be expected to change under various climate change scenarios, and how would this affect stratospheric water vapour?
6. It is generally understood that the stratospheric diabatic circulation and the Brewer-Dobson transport circulation are closely related, and are both driven by stratospheric wave drag -- referred to by Holton et al. (1995) as the ``extratropical pump''. (There is also a significant contribution to the stratospheric diabatic circulation from mesospheric wave drag in the polar night). Yet this connection has only been established under very restrictive assumptions, similar to those required for the mixing-length hypothesis. Also, tropical-to-midlatitude transport has a significant component which is not merely advective, as is evidenced by the breadth of estimated ``age spectra'' in the extratropical stratosphere (Hall and Waugh 1997), and by the striking contrast between the mean age (op. cit.) and the age produced by the (purely advective) residual circulation (Rosenlof 1995). What then is the connection between the diabatic circulation and the Brewer-Dobson transport circulation under conditions that are relevant to the real stratosphere? How much do we know about what controls the transport of ozone, its interannual variability, and possible long-term trends?
7. The classical diagnostic framework for stratospheric mixing and transport (e.g. WMO 1986; Andrews et al. 1987) was provided by the mixing-length approach. This has now been largely abandoned. More recently, the opposite limit has been considered, of well-mixed isolated air masses (including ``leaky pipes'', etc.). This approach has some merit for specific problems, such as entrainment into the tropical upwelling or polar downwelling regions, but is not sufficiently general to deal with the entire stratosphere. The truth evidently lies somewhere in between these two extremes. Our present conceptual models are essentially simple cartoons. How can we develop a comprehensive diagnostic framework within which to interpret measurements and compare them with theory and models?
8. Both free-running climate models, and chemical transport models driven by assimilated dynamical fields, have serious difficulties representing the sharp edges in tracer concentration that are observed in the subtropics. In general, the models appear to be much too diffusive. Why is it that the models represent transport so poorly in this region? Is it just a question of spatial resolution? Or is it an inadequate match between the nature of the numerical methods and the physical processes?
What are the major missing elements that presently impede progress on the research questions outlined above? As above, the following list provides a possible starting point, but is certainly not exhaustive.
1. There is a need for a much more accurate observational characterisation of the tropopause and its time variation, including long-term trends. Despite the improvements in satellite observing systems, the required vertical resolution (rather less than 1 km) remains well beyond what is possible with satellites; the only possibility is direct measurements with radiosondes or lidars, possibly coupled with data assimilation to propagate the data in time and space. With the reduced number of radiosonde stations world-wide, our capability of observing the tropopause is probably decreasing with time, which is a serious problem for climate monitoring.
2. A major uncertainty in chemical climate modelling concerns the transport of chemical species through the lower stratosphere/upper troposphere region. Ozone and water vapour are important examples. In order to provide better observational constraints on such transport, and its possible changes over time, we require an accurate and stable observational characterisation of trace species distributions in this region. It is possible that high-resolution direct measurements are not needed for those species which are largely determined by transport; in those cases, data assimilation may well be able to reconstruct the mesoscale tracer structure from the large-scale tracer distribution together with the large-scale winds (e.g. Fisher and Lary 1995), much as contour advection can reconstruct mesoscale tracer structure (Appenzeller, Davies and Norton 1996).
3. Analysed winds in the stratosphere are almost exclusively derived from measurements of temperature, together with some kind of balance condition (whether explicitly or implicitly imposed). This produces reasonably accurate winds in the extra-tropics, to the extent that much insight into polar vortex isolation and the extent of mixing has been obtained from transport studies using analysed winds, corroborated with direct measurements of chemical tracers. However this is not possible in the tropics, due to the poor quality of the analysed winds; in the tropics, the balance conditions tend to break down. Thus virtually all present knowledge about mixing and transport within the tropics has been obtained indirectly from chemical tracer measurements. A high priority should be the acquisition of reliable wind measurements in the tropics, either from direct wind measurements or, possibly, from their reconstruction via data assimilation using high-temporal-resolution chemical tracer measurements.
4. Stratospheric water vapour is crucially important for stratospheric chemistry; it affects the HOx catalytic ozone destruction cycle, as well as the distribution of condensed matter. It is therefore of major importance to clarify the processes that control the water vapour values that enter the lower stratosphere in the tropics, so that we can understand their sensitivity to possible climate change. In particular, the time is now ripe to quantify Brewer's ``cold trap'' mechanism and its seasonal cycle. Some of the relevant questions are described in point 5 of the previous subsection, and could be addressed through a focused field study in the western tropical Pacific (the region of coldest temperatures and greatest dehydration). Such a study should include a high-flying research aircraft capable of following the evolution of ice particle distributions along quasi-Lagrangian air parcel trajectories, to address the dynamics of dehydration.
5. Global atmospheric models are indispensable for at least two reasons. First, they are the primary tools for providing self-consistent predictions of climate change. Second, they are increasingly becoming part of data analysis through the techniques of data assimilation. For both reasons, it is important to have confidence in global atmospheric models and to well understand their deficiencies (and strengths). This can only be achieved through systematic comparison between models and measurements. However this is much easier said than done; useful comparison is only achieved by design. (To give a very trivial example: rather than taking isolated chemical measurements from aircraft missions, with large sampling errors, and interpolating them onto a uniform grid to make a climatological comparison with a model -- which would probably be meaningless -- it is far better to analyse the aircraft observations in a dynamically intelligent way, either by gridding them according to ``vortex latitude'' and potential temperature, or by examining chemical correlations.) With funding pressure to implement new physics or chemistry and perform new simulations, modelling groups are hard pressed to make detailed comparison with measurements, yet it is essential that they do so. There is a therefore a pressing need to facilitate such analysis.
What can SPARC do in this area? Given the wide variety of research communities involved, and the rapid pace of change at the conceptual level, a ``grand strategy'' seems premature at this time. Perhaps the greatest need is to bring the various communities closer together and working towards common goals. To this end, probably the most effective approach is to organise targeted workshops aimed at developing interdisciplinary communication, and to solicit pedagogical review-type articles (rather than literature surveys) aimed at the non-specialist, which is to say at the practising researchers in complementary fields. The one place where a targeted field experiment would seem particularly timely is with regard to the question of tropical dehydration (point 4 in the previous subsection). Comparison of models and measurements (point 5 in the previous subsection), which is so critical for assessing the validity of climate change predictions, can be pursued within the GRIPS initiative.
Andrews, D.G., Holton, J.R. and Leovy, C.B., Middle Atmosphere Dynamics. Academic Press, 489 pp., 1987.
Appenzeller, C., Davies, H.C. and Norton, W.A., Fragmentation of stratospheric intrusions. J. Geophys. Res., 101, 1435-1456. 1996.
Appenzeller, C., Holton, J.R. and Rosenlof, K.H., Seasonal variation of mass transport across the tropopause. J. Geophys. Res., 101, 15071-15078. 1996.
Boering, K.A., Daube, B.C., Wofsy, S.C., Loewenstein, M., Podolske, J.R. and Keim, E.R., Tracer-tracer relationships and lower stratospheric dynamics: CO2 and N2O correlations during SPADE. Geophys. Res. Lett. 21, 2567-2570. 1994.
Fahey, D.W. et al., In situ observation of NOy, O3, and the NOy/O3 ratio in the lower stratosphere. Geophys. Res. Lett. 23, 1653-1656. 1996.
Fisher, M. and Lary, D., Lagrangian four-dimensional variational data assimilation of chemical species, Quart. J. Roy. Met. Soc. 121, 1681-1704. 1995.
Grant, W.B. et al., Aerosol-associated changes in tropical stratospheric ozone following the eruption of Mount Pinatubo. J. Geophys. Res. 99, 8197-8211. 1994.
Hall, T.M. and Waugh, D.W., Timescales for the stratospheric circulation derived from tracers. J. Geophys. Res., 102, 8991-9001. 1997.
Holton, J.R., Haynes, P.H., McIntyre, M.E., Douglass, A.R., Rood, R.B. and Pfister, L., Stratosphere-troposphere exchange. Rev. Geophys. 33, 403-439. 1995.
Mote, P.W. et al., An atmospheric tape recorder: the imprint of tropical tropopause temperatures on stratospheric water vapor. J. Geophys. Res. 101, 3989-4006. 1996.
Murphy, D.M. et al., Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere. J. Geophys. Res. 98, 8751-8773. 1993.
Newman, P.A. and Schoeberl, M., A reinterpretation of the data from the NASA stratosphere-troposphere exchange project. Geophys. Res. Lett. 22, 2501-2504. 1995.
Norton, W.A., Breaking Rossby waves in a model stratosphere diagnosed by a vortex-following coordinate system and a technique for advecting material contours. J. Atmos. Sci., 51, 654-673. 1994.
Orsolini, Y.J., On the formation of ozone laminae at the edge of the Arctic polar vortex. Quart. J. Roy. Met. Soc. 121, 1923-1941. 1995.
Pierrehumbert, R.T. and Yang, H., Global chaotic mixing on isentropic surfaces. J. Atmos. Sci., 50, 2462-2480. 1993.
Plumb, R.A., A ``tropical pipe'' model of stratospheric transport. J. Geophys. Res. 101, 3957-3972. 1996.
Rosenlof, K.H., The seasonal cycle of the residual mean meridional circulation in the stratosphere. J. Geophys. Res. 100, 5173-5191. 1995.
Trepte, C.R. and Hitchman, M.H., Tropical stratospheric circulation deduced from satellite aerosol data. Nature 355, 626-628. 1992.
Volk, C.M. et al., Quantifying transport between the tropical and mid-latitude lower stratosphere. Science 272, 1763-1768. 1996.
Waugh, D.W. and Plumb, R.A., Contour advection with surgery: a technique for investigating fine-scale structure in tracer transport. J. Atmos. Sci., 51, 530-540. 1994.
WMO, Atmospheric Ozone 1985. Report No.20, World Meteorological Organization Global Ozone Research and Monitoring Project. 1986.
Yulaeva, E., Holton, J.R. and Wallace, J.M., On the cause of the annual cycle in the tropical lower stratospheric temperature. J. Atmos. Sci. 51, 169-174. 1994.
Ozone is a greenhouse gas . It is, however, different from most other greenhouse gases: (1) it is much shorter lived than other greenhouse gases and, consequently, not well mixed in the atmosphere, (2) it interacts with both the incoming solar and the outgoing infrared radiation, and (3) it is not directly emitted into the atmosphere, but produced in the atmosphere.
The abundance of ozone and its variation need to be measured and evaluated. This is one of the initiatives of the SPARC program. However, because ozone is created in the atmosphere - as opposed to being directly emitted into it - predicting the future impact on climate, demands that all the processes involved in the production, destruction, and redistribution of ozone be well understood. Only then can the concentration of ozone and its temporal and spatial variations be predicted. This is the main aim of this initiative.
In addition to ozone, the presence of condensed matter, which can interact with the radiation field, also can affect climate. Because these particles are present in an area sensitive to climate perturbations, it is especially important to understand the mechanisms for the production, distribution, and transport of the condensed matter in the atmosphere (aerosol). In addition to their direct effect on radiation (both incoming and outgoing), condensed matter also affect greenhouse gas concentrations by facilitating heterogeneous and multiphase reactions. Therefore, developing an understanding of the condensed phase material in UT/LS and their optical and chemical properties is another aim of this initiative.
Observations during the past few decades have shown that the abundance of ozone in the stratosphere has decreased (figure 3.2) while that in the troposphere has increased; both changes are attributed to anthropogenic activities. Most ozone depletion has been observed in the lower stratosphere (LS). There is also concern about possible ozone changes in the upper troposphere (UT). Because Earth’s climate will be sensitive to the vertical distribution of a greenhouse gas , the combination of ozone loss in the LS and increases in the UT is of concern. This concern is heightened by the knowledge that the efficiency of ozone as a greenhouse gas is highest in the UT/LS region (figure 3.3).
Figure 3.2. The vertical profile of the observed decadal ozone trends in the stratosphere taken from the SPARC-IOC ozone trends assessment. The depletion in the lower stratosphere is most significant and will affect the radiative forcing due to ozone. These decadal trends are attributed to changes in chlorine and aerosol levels in the lower stratosphere. Quantification of the trends and calculations of the future levels of ozone still requires further studies.
The lower parts of the stratosphere are distinctly different from the upper stratosphere, which for the most part is in photochemical equilibrium (see figure 3.4) and the upper troposphere is very different from the lower troposphere.
The lower stratosphere (LS) is far removed from the major ozone production regions. The LS is the region of the atmosphere where it is difficult to separate, in time scale, the chemistry and atmospheric motions. This is also the coldest region of the lower atmosphere, the pressures are still large, and condensable species are significant. Heterogeneous chemistry plays a critical role in this region. The high degree of non-linearity in the reactions in or on condensed phases also make temperature, and temperature fluctuations, very important. The LS is somewhat removed from the regions where chlorine and nitrogen oxides are released from their precursor gases. Thus, dynamics of air motion plays a major role in bringing catalysts, as well as ozone itself, to the lower stratosphere. In addition, this is the region where the chemistry of ozone is least well understood but has the most impact in terms of climate.
Figure 3.3. Expected change in temperature per Dobson unit of ozone introduced into the atmosphere at various altitudes (adopted from the work of Lacis et al. ). This picture shows that UT/LS is vulnerable to changes in ozone.
Figure 3.4 Adopted from Perliski et al. . Principal terms in the ozone balance in the stratosphere are shown at the bottom. A large fraction of the stratosphere is in photochemical balance. However, the lower stratosphere is the region where production, transport, and destruction compete to determine the local abundance of ozone.
The upper troposphere (UT) shares some of the same characteristics of the LS. The temperature is low and it is far removed from major source of trace gases. Condensed matter is present to interact with radiation and affect chemistry. Chemistry and dynamics are inseparable in time. However, in this region, water vapour can be high and there can be in-situ production of certain key ingredients such as nitrogen oxides via lightning. During the past two-three decades, it has become clear that a large fraction of tropospheric ozone is photochemically produced , in-situ, and this chemical production can be (and is) influenced by human activity. A fraction of the ozone is, no doubt, still coming from the stratosphere. This fraction is believed to be small, overall; but can be very significant in the UT. However, the fraction of ozone in the UT that comes from the LS remains to be quantified . The key ingredients for the photochemical production of ozone in the troposphere are hydrocarbons or CO, nitrogen oxides and near-UV light. In the UT, the rate of photochemical ozone production is highly uncertain because of the uncertainties in the amount of NOx. The nitrogen oxides could be locally produced, for example via lightning, or transported into this region from NOx-rich regions. Such a transport requires that the molecule(s) responsible for redistribution is (are) water-insoluble and stable on the time scale of days .
Decreasing trends of ozone in the lower stratosphere and increasing trends in the upper troposphere, both have important climate effects and are due most likely to human perturbations. The presence of the condensed matter affects the gas phase composition and the radiation; the abundance and the size distribution of the condensed matter may be affected by human activities. Understanding the UT/LS region is also of immediate practical concern for issues such as the effects of subsonic and supersonic aircraft on the atmosphere .
The UT/LS has some distinct characteristics which will influence its role in Earth’s climate. The pressures are still high enough to influence the course of reactions and photochemical processes. This region is below the ozone layer such that the available actinic radiation is limited to wavelengths greater than ~290 nm. The UT and LS contain the coldest parts of the lower atmosphere, to the extent that highly reactive particles can be produced. These particles have two consequences: (1) They facilitate heterogeneous reactions (those taking place on a solid substrate) and multiphase reactions (those taking place in a liquid droplet) and alter the composition of this region. (2) The particles, especially cirrus clouds, also directly interact with radiation . However, these are the regions where a clear separation of the timescales for chemistry and dynamical transport does not exist. Thus, chemical, microphysical, and dynamical processes all play an important role in the determination of ozone abundance and the radiative balance of the atmosphere.
One of the key components of climate research is laboratory studies of the fundamental processes that control the concentration of a greenhouse gas such as ozone. Because of the above characteristics of the UT/LS, gas phase free radical reactions and photochemical processes have to be studied at the appropriate low temperatures, 190<T<230 K and moderate pressures (20 <P<200 hPa). This is particularly true since extrapolations of higher temperature rate data to the lower temperatures may lead to errors. These studies have to encompass wavelengths above 290 nm. Lastly, heterogeneous reactions at temperatures appropriate for this region are also needed.
The upper troposphere and lower stratosphere are inescapably connected via transport of chemicals, their effect on each other is very large and significant. What happens in the LS and UT is determined to some extent by the occurrences in the lower troposphere. For example, the sources of all or most of the ingredients for photochemical production of ozone in the UT and destruction in the LS originate from the lower atmosphere and has to pass through the UT to reach the LS. Similarly, the contents of the LS are passed through the UT to be removed, and affects this area. Thus, strong interactions between stratospheric scientists and their counterparts in the troposphere is essential. This realisation has led to a co-operation between the International Global Atmospheric Chemistry (IGAC) project of the IGBP and the SPARC activity of WCRP. Some of the activities of the UT/LS initiative are the first of these joint ventures between IGAC and SPARC.
Field measurements to address the climate effects must have a coherent approach that addresses the UT/LS. Such field measurements are on-going under various other national and international projects. This initiative will facilitate such programmes by timely inputs.
The first phase of the initiative is to take stock of the current understanding of the LS/UT. Many other SPARC and IGAC activities, e.g., the SPARC-IOC ozone trends report, the SPARC Stratosphere-Troposphere Exchange activity, IGAC’s aerosol initiative, play directly into the UT/LS initiative. As a start, workshops are held to assess the state of science, bring together practitioners of various sub-fields together, and succinctly present the findings as reports and review papers. Such an activity was initiated by holding the heterogeneous chemistry workshop in Strasbourg, France, in 1996. This workshop brought together scientists engaged in laboratory studies, field measurements, and modelling and provided a venue for exchange of information and a critical evaluation of where the field stands and, more importantly, what needs to be done to achieve further progress. A similar workshop for the gas phase processes is planned jointly with IGAC in 1998.
This initiative is like the bricks of a building- essential, fundamental, and pieces that need to be assembled to get the overall picture. They are needed in all the components of SPARC and IGAC activities. Thus, the aims of this project are:
1. To act as an information exchange centre between laboratory scientists and field measurement and modelling communities.
2. Feed into assessments, such as the WMO Ozone Assessment and IPCC assessments via well timed scholarly reports.
3. Help the compilation and evaluation of the data needed by atmospheric scientists. Here identification of special areas that require attention, but are not fully covered by existing national projects, will be given precedence.
4. When necessary, bring together laboratory scientists to carry out concerted measurements and modelling to complete the understanding of a specific atmospheric chemistry problem.
5. Initiate and sustain a program which addresses measurement of the radiation field required for computation of the photodissociation processes that control the concentration of radiative gases in the LS and UT.
6. Stimulate needed field programmes (e.g. microphysical chemistry measurements in the tropics).
Albritton, D.A., G. Amanatidis, G. Angeletti, J. Crayston, D. Lister, M. McFarland, J. Miller, A.R. Ravishankara, N. Sabogal, N. Sundararaman, and H. Wesoky, Global atmospheric effects of aviation: Report of the proceeding of the symposium, 1996, National Aeronautics and Space Administration, 1996.
Chameides, W.L., and J.C.G. Walker, A photochemical theory of tropospheric ozone, J. Geophys. Res., 78, 8751-8760, 1973.
Crutzen, P.J., A discussion of the chemistry of some minor constituents in the stratosphere and troposphere, Pure Appl. Geophys., 106-108, 1385-1399, 1973.
Crutzen, P.J., Ozone in the troposphere, in Composition, Chemistry, and Climate of the Atmosphere, edited by H.B. Singh, pp. 349-393, van Nostrand Reinhold, New York, 1995.
Hansen, J., M. Sato, and R. Ruedy, Radiative forcing and climate response, J. Geophys. Res., 102, 6831-6864, 1997.
Houghton, J.T., L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg, and K. Maskell, Climate Change 1995: The science of climate change, Cambridge Univ. Press, Cambridge, 1996.
Jensen, E.J., O.B. Toon, H.B. Selkirk, and J.D. Spinhirne, On the formation and persistence of subvisible cirrus clouds near the tropical tropopause, J. Geophys. Res., 101, 21,361, 1996.
Lacis, A.A., D.J. Wuebbles, and J.A. Logan, Radiative forcing of climate by changes in the vertical distribution of ozone, J. Geophys. Res., 95, 9971-9981, 1990.
Perliski, L., S. Solomon, and J. London, On the interpretation of seasonal variations of stratospheric ozone, Planetary Space Science, 37 (12), 1527-1538, 1989.
Ravishankara, A.R., Heterogeneous and multiphase chemistry in the troposphere, Science, 276, 1058-1065, 1997.
SPARC-IOC Assessment of Trends in the Vertical Distribution of Ozone, SPARC Report No.1. 1998.
Singh, H.B., and P.L. Hanst, Peroxyacetyl nitrate (PAN) in the unpolluted atmosphere: an important reservoir for nitrogen oxides, Geophys. Res. Lett., 8, 941-944, 1981.
WMO, Scientific Assessment of Ozone Depletion: 1994, World Meteorological Organization, 1995.
One of the most fundamental problems encountered in numerical modelling of climate is the necessity to represent the effects of motions on spatial scales not explicitly resolvable in a global model. The classic example of this is the parameterisation of the unresolved convective processes in the atmosphere that is required in any comprehensive climate simulation model. In recent years, the effects of small-scale gravity waves on the global atmospheric flow has emerged as a parameterisation issue of importance that could rival that involving unresolved convective processes. Gravity waves act to transfer mean horizontal momentum between the ground and levels aloft in the atmosphere or from one layer of the atmosphere to another. Flow over topography can generate stationary gravity waves that break non-linearly in the troposphere and lower stratosphere. The waves transfer momentum from the breaking region to the earth's surface, and this process is thought to act as a significant drag on the zonal westerlies in the mid-latitude troposphere. It is known that when global atmospheric simulation models are run without any attempt to impose such a drag, they tend to produce results characterised by unrealistically intense mid-latitude surface westerlies. Other processes (such as convection, jet stream instabilities etc.) can produce gravity waves with non-zero horizontal phase speeds. These waves can act to transfer mean momentum between the troposphere and the stratosphere/mesosphere. Again, comprehensive numerical models of the atmosphere are known to produce unrealistic simulations of the extratropical stratospheric/mesospheric circulation unless some account is taken of the effects of these gravity waves. If more credible simulations of climate (and predictions of climate response to anthropogenic forcing) are to be obtained, improvements in physically justifiable parameterisations of the momentum transport due to unresolved gravity waves are required. It is also possible that the turbulent mixing resulting directly from gravity wave breaking could play a significant role in climate through the vertical transport of trace constituents, both in the middle atmosphere and across the tropopause.
The last three decades have seen impressive developments in radar and lidar techniques of remote sensing and their application to the study of the high frequency variability of the atmospheric temperature and winds at selected geographical locations. Recent work with satellite remote sensing opens the possibility of obtaining some significant information on aspects of the global gravity wave field in the middle atmosphere. Together with the historical archive of rocket soundings, observations of airglow, and in situ aircraft observations from special experiments (as well as measurements taken during the NASA Space Shuttle re-entries) these data represent an impressive accumulation of information concerning the gravity wave field in the middle atmosphere. However, none of the available data provide the complete picture that is needed for understanding (and parameterising) the dynamics of the gravity wave field. The most detailed records with the highest time resolution are those obtained with one- dimensional single station techniques, while the more global satellite observations have important limits on space and time resolution. SPARC clearly has a role in encouraging the international collaboration in the collection and analysis of observations.
A recent development of significance for SPARC is the demonstration of the potential for studying gravity waves using operational high resolution balloon soundings. Since 1991 the Australian Bureau of Meteorology has routinely recorded and archived the winds and temperatures from their operational balloon-borne radiosondes at very high vertical resolution (10-second or ~50 m spacing). The radiosondes operated by the national meteorological services of most other countries apparently have the capability of producing comparably detailed profiles. The usefulness of these data for exploring the dynamics of the upper troposphere and lower stratosphere has been shown in recent papers by Allen and Vincent (1994) and Vincent et al. (1997). SPARC has been very active in encouraging national meteorological services to archive their routine observations at very high resolution. SPARC is also leading an international project to analyse existing high- resolution radiosonde data to obtain a global climatology of gravity wave parameters in the lower stratosphere.
Encouraged by the success of the Australian experience with archiving routine radiosonde profiles at high resolution, the SPARC Gravity Wave Initiative chairs started in 1994 to determine those countries where such data were being saved and to lobby strongly for the archiving of high resolution data in other countries (see Hamilton and Vincent, 1995). The communications between the SPARC Gravity Wave Initiative and various personnel in the US National Weather Service (NWS) in late 1994 and early 1995 helped influence the decision of the NWS to begin archiving data at six second intervals starting on April 1, 1995. The data is now being archived at over 105 stations in the continental US, Alaska, Hawaii and US trust territories in the tropical Pacific. These data are now available to researchers through the US National Climatic Data Center. In early 1997 1-second data began to be archived at 7 Caribbean stations (operated by the individual national governments but using equipment supplied by the US NWS).
The Gravity Wave Initiative co-chairs have made approaches to other national meteorological services to encourage the archiving of high-resolution data. The current situation is somewhat complicated with some countries now routinely archiving the data and making it widely available, while others store the data (at least for some period of time) but want considerable payment for making it available to researchers, while others are not saving this data. The Gravity Wave Initiative has focused recently on enlisting individual scientists who are able to obtain high resolution data from their national meteorological services, at least for some stations and for limited times. All these data will be used in a global climatology of the gravity wave variations as seen in radiosonde observations. About 10 principal investigators have joined from various countries at this point and more participants are being actively solicited. Each investigator will analyse at least a year of daily or twice-daily high-resolution radiosonde profiles from stations in his or her country. The analysis will aim at characterising the vertical wave number spectra of temperature and horizontal wind as a function of time of year. Wind and temperature data will also be combined to compute the dominant horizontal propagation directions for gravity waves. The aim is to standardise the methods of analysis among the various investigators to the greatest extent possible, so that geographical variations can be clearly determined. Figure 3.5 shows the locations of those stations for which data are now known to be available. The distribution of the number of these stations in different latitude bands is given in figure 3.6. The climatology that results should be very valuable for comparison with GPS/Met soundings that are now becoming available.
The development of comprehensive GCMs that include a large number of model levels in the stratosphere has proceeded rapidly over the last decade. There are now about 10 groups world-wide that have significant projects in this area (see Hamilton, 1996, for a general review). Early efforts to extend climate GCMs into the stratosphere typically produced simulations of the winter high-latitude flow that were much too close to radiative equilibrium (and hence were characterised by a much too cold pole and unrealistically strong westerly flow in the polar vortex). Even the current generation of such models have fairly basic problems
Figure 3.5. Meteorological stations for which the data from operational balloon-borne radiosondes are now available at high resolution.
Figure 3.6. Latitudinal distribution of the stations shown in figure 3.5.
remaining. In particular, while the "cold pole" problem in the northern hemisphere winter stratosphere has been alleviated to some degree in some current models (particularly those run at high horizontal resolution), the simulation of the southern hemisphere winter circulation is still generally very unrealistic. Similar problems occur in the upper mesosphere, with the simulated temperatures in the summer (winter) polar regions being warmer (colder) than observed. A consensus is growing that the remaining deficiencies in the extratropical middle atmospheric simulation must be due - at least in part - to the omission of the mean momentum transport from unresolved gravity waves, resulting in a strong effective drag on the mean flow in the mesosphere. There have already been some proposed parameterisations of the drag which can be tested in GCMs.
The question of how to parameterise tropospheric/lower stratospheric gravity wave drag in GCMs is in some respects even less settled than in the case of mesospheric drag. The waves most likely to have an effect in this region are those generated by flow over topography. While the excitation for such waves is in principle well known (in contrast to non-stationary waves), and while there has been considerable experience with parameterisations of mountain wave effects in GCMs following the pioneering works of Palmer et al. (1986) and McFarlane (1987), some basic questions remain. One concern is that at high resolution GCMs may tend towards a state with unrealistically large poleward eddy momentum flux in the resolved flow. Thus the inclusion of a "topographic gravity wave drag" in the momentum equation may simply cover up some more fundamental problem with the model simulation. What is needed here is a careful diagnosis of the mean zonal momentum budgets in models and in observations with estimates of uncertainty due to observation/assimilation errors (for the observations) and to uncertainties in the other physical parameterisations (notably the surface drag formulation). SPARC can play a role in fostering the kind of co-operation among climate modellers, numerical weather prediction specialists and atmospheric dynamicists needed to attack this important problem.
Some widely-varying approaches to the practical parameterisation problem for non-stationary gravity waves have been proposed thus far in the literature. The differences among various proposals relate to both the specification of sources of subgrid-scale waves and the treatment of their vertical propagation and dissipation. At one extreme Rind et al. (1988a,b) impose a source of waves that depends at each grid point and time step on the local tropospheric conditions (so that subgrid-scale gravity waves are "launched" at the tropopause at grid points where there is convective adjustment or where the resolved-scale Richardson number falls below some threshold). The treatment of the problem beyond that point in the Rind et al. scheme is rather simple, however, with an assumption that the subgrid-scale waves have a unique wave number and phase speed, and can be treated by steady-state WKB approximations, with a further assumption that non-linear processes lead to a saturation of wave amplitudes (following Lindzen, 1981).
The other set of approaches that have been advanced begin with the assumption that a statistically-steady spectrum of waves is emerging from the troposphere at all times (the spectrum can, of course, be specified as a function of location and time of year). The next step is to compute the momentum flux convergence that results from the propagation and dissipation of these waves. Here again there are some quite different proposals that have been published. One simple approach is to follow Lindzen (1981) and compute the effects of a number of discreet waves individually (i.e. ignoring any non-linear interaction among the waves). This has been employed in the simple middle atmosphere models of Holton (1982) and Garcia and Boville (1994) and is now being adopted in the NCAR CCM. Fritts and van Zandt (1993) and Fritts and Lu (1993) have formulated a more sophisticated treatment involving a continuous spectrum of waves, but again essentially assuming that each part of the spectrum attains a non-linear saturation independently of the rest of the spectrum. By contrast, Hines (1997a,b) presents a theory of the upward propagation of a spectrum of waves that considers the mutual interaction among the waves as the key to their propagation and dissipation. In particular, Hines has produced a formalism in which the dissipation of waves (and hence their momentum flux convergence) is controlled in large part by a Doppler spreading induced by the winds of the wave spectrum itself. It would seem that the Hines approach would be appropriate if the gravity wave field were very steady, while the approaches based on saturation of individual waves would be relevant to a case where the wave field was extremely transient, with nearly-monochromatic waves dominating each local region for significant times. The range of validity of each of these approaches needs to be understood more completely, and these idealised theories have to be adapted to produce results relevant to the complicated situations encountered in GCMs. One problem apparent in many of the current schemes is a tendency for the waves to break significantly only at high altitudes (near and above the mesopause). This leads to a predicted gravity wave drag that has a distribution that is centred at higher altitudes than that required by numerical simulation models in order to obtain realistic results for the global circulation. A possible solution to this dilemma might be obtained if the subgrid-scale gravity wave fluxes are assumed to be quite intermittent. A crucial question for observationalists is to characterise the degree of intermittence in the real atmosphere so that this aspect of the formulation of gravity wave parameterisations can be constrained.
A number of fundamental scientific problems remain in formulating realistic gravity wave parameterisations for the middle atmosphere. Notable is the question of the horizontal spectrum of waves near the tropopause that is needed as input to any parameterisation. There is currently a lack of detailed observations to characterise the spectrum and its relation to possible wave sources. Another important issue is the characterisation of the longest vertical wavelength components of the spectrum emerging into the middle atmosphere from below. This end of the spectrum is extremely hard to characterise from observations, but is very important high in the atmosphere, since the longest vertical wavelengths correspond to the highest phase speeds and vertical group velocities. Important contributions to our understanding of these issues can come from detailed modelling studies building on recent numerical simulations of the gravity wave breaking process (e.g., Fritts et al., 1994) and of the tropospheric excitation of gravity waves (e.g., Alexander et al., 1995). In April 1996 SPARC helped sponsor an international workshop on the gravity wave parameterisation problem in Santa Fe, USA, at which observational and limited-area modelling studies were reviewed (see Hamilton, 1997).
Another approach to the tropopause problem is one that focuses on the performance of various schemes in actual climate model integrations. One issue is the large number of options that currently exist for implementation of parameterisations. There are several general approaches that have been advanced and each proposed scheme has a great deal of flexibility in terms of the input parameters (notably in specifying the spectrum of waves assumed to be emerging from tropospheric levels into the stratosphere). The practical numerical implementation of the schemes (particularly schemes which involve a numerical iteration) also can be done in significantly different ways. In order to characterise the performance of various schemes and choices of parameters, SPARC is organising the development of a set of standard "test bed" atmospheric profiles that various modelling groups can use to compare their gravity wave parameterisation implementations in an efficient "off-line" approach. The profiles are being prepared to reflect a superposition of typical diurnal mean conditions and the diurnal tide. The off-line calculations will be performed by each of several climate modelling groups now working on the parameterisation problem in various countries. SPARC also will coordinate some more ambitious projects involving comparison of results of complete global model integrations employing gravity wave drag schemes. This is now in the initial planning stage and all efforts will be co-ordinated with the SPARC GRIPS initiative. Two types of comparisons are envisaged. One would involve running different climate models with exactly the same formulation of a particular gravity wave parameterisation. The second would be to compare results from different models, in each of which the parameters for the drag have been tuned to produce a realistic zonal mean circulation.
While much progress has been made in observational studies of gravity waves in the middle atmosphere, formulation of parameterisation schemes is still limited by the lack of detailed knowledge of the spectrum of waves entering the stratosphere and its relation to possible sources. SPARC will pursue the organisation of a large international field experiment to help address this deficiency. Following discussion at the Santa Fe workshop and the SPARC General Assembly in 1996, a decision has been taken to initially concentrate on the problem of stratospheric gravity waves forced by deep tropospheric convection. A proposed field campaign entitled "Convective Excitation of Gravity Waves Experiment" (CEGWE) is described in Annexe II.
The experiment is envisaged as a deployment of an extensive array of instruments as well as one or more aircraft in a limited region over a period of the order of two weeks. Given the very limited work that has been done on convective forcing of gravity waves, the experiment would ideally be located in a region of predictable and fairly simply-organised diurnal convection. To be most successful, it would be necessary to measure the complete wind and temperature fields as well as important minor constituents such as water vapour over as large a height range as possible. High-resolution observations within and near the convection cells and to altitudes in excess of 30 km are essential. It is envisioned that the field program would include a wide variety of ground-based and airborne remote sensing and in situ observations as well as observations from satellites. Balloon-borne sondes, drop-sondes, radars, lidars, imagers, radiometers, spectrometers, and satellites sensors are all expected to make important contributions.
The characteristics of the gravity wave fields generated by convection are related to many features of the source including geometry and lifetime, large scale motion, and strength of the updrafts. The convection must be characterised in sufficient detail to permit the strength and properties of the gravity response to be related to the strength and properties of the convective activity. An extensive network of relevant meteorological observations will be essential for monitoring key parameters of the convective activity, its physical extent, and temporal evolution.
The wave spectrum can be measured by using radiosonde balloon observations of the wind and temperature profiles, radar observations of wind profiles, lidar measurements of temperature profiles, and in-situ airborne measurements of the winds and temperatures. To be most useful these observations should extend well above the tropopause to altitudes in excess of 30 km if possible. Vertical and horizontal profiles as well as time series are desirable so that the full vertical wave number, horizontal wave number, and temporal frequency spectrum can be characterised. Wave propagation directions in the troposphere and lower stratosphere can be derived from radiosonde balloon profiles of temperature and winds (Vincent et al., 1997) and from airborne in-situ and remote sensing observations. To adequately characterise the anisotropy of the wave field, balloons must be launched from several sites surrounding the convection region and aircraft must make several transects at various azimuths over the convection cell.
As the wave field propagates to higher altitudes in the middle atmosphere, some components of the spectrum will grow in amplitude as the density decreases while other components will be damped by instability processes and critical layer interactions associated with the height varying background wind field. Measurements of the wind and temperature profiles throughout the middle atmosphere as well as measurements of the wave spectrum are required to characterise and quantify these effects. Ground based radars, and ground based and airborne lidars, airglow imagers, and airglow spectrometers can provide much of the requisite data.
Plans for this experiment were discussed at a SPARC-supported workshop held in Victoria, Canada in June 1997. The participants identified Darwin, Australia as the preferred venue and late 2000-early 2001 or late 2001-early 2002 as the preferred time for the experiment. These choices were influenced by the meteorological regime of the area, which includes a very predictable intense daily thunderstorm over Bathurst and Melville Islands just north of Darwin, along with weaker oceanic and continental convection (Keenan and Carbone, 1992). If the proposed gravity wave experiment spans the late pre-monsoon and early monsoon periods, then a wide variety of convective events could be sampled.
The Darwin venue is also attractive for practical reasons. Darwin was the site of the 1995 Maritime Continent Thunderstorm Experiment (MCTEX) which involved detailed study of the tropospheric convection in the late pre-monsoon period. The results of this experiment are now being evaluated, and are being studied in conjunction with numerical model simulations by a number of research groups. The experience gained in MCTEX will be extremely valuable in planning the proposed gravity wave experiment. Darwin is also a ground truth station for the Tropical Rainfall Measuring Mission satellite program, and as a consequence is well instrumented with radars, wind profilers, a rain gauge network, and other facilities.
Alexander, M.J., J.R. Holton and D.R. Durran, The gravity wave response above deep convection in a squall-line simulation, J. Atmos. Sci., 52, 2122- 2226, 1995.
Allen, S.J., and R.A. Vincent, Gravity wave activity in the lower atmosphere: seasonal and latitudinal variations, J. Geophys. Res., 100, 1327-1350, 1995.
Fritts, D.C., J.R. Isler and O. Andreassen, Gravity wave breaking in two and three dimensions, 2. Three-dimensional evolution and instability structure. J. Geophys. Res., 99, 8109-8123. 1994.
Fritts, D. C. and T.E. van Zandt, Spectral estimates of gravity wave energy and momentum fluxes. Part I: Energy dissipation, acceleration and constraints. J. Atmos. Sci., 50, 3685-3694, 1993.
Fritts, D. C. and W. Lu, Spectral estimates of gravity wave energy and momentum fluxes. Part II: Parameterization of wave forcing and variability. J. Atmos. Sci., 50, 3695-3713, 1993.
Garcia, R.R. and B.A. Boville, "Downward control" of the mean meridional circulation and temperature distribution of the polar winter stratosphere. J. Atmos. Sci., 51, 2238-2245, 1994.
Hamilton, K., Comprehensive meteorological modelling of the middle atmosphere: A tutorial review. J. Atmos. Terr. Phys., 58, 1591-1628, 1996.
Hamilton, K. (ed.), Gravity Wave Processes: Their Parameterization in Global Climate Models. Springer-Verlag, Heidelberg, 414 pp. 1997.
Hamilton, K., and R.A. Vincent, High-resolution radiosonde data offer new prospects for research. Eos, 74, 497-507, 1995.
Hines, C.O., Doppler-spread parameterization of gravity-wave momentum deposition in the middle atmosphere. Part 1: basic formulation. J. Atmos. Terr. Phys. 59, 371-386, 1997a.
Hines, C.O., Doppler-spread parameterization of gravity-wave momentum deposition in the middle atmosphere. Part 2: broad and quasi monochromatic spectra and implementation, J. Atmos. Terr. Phys. 59, 387-400, 1997b.
Holton, J.R., The influence of gravity wave breaking on the general circulation of the middle atmosphere. J. Atmos. Sci., 40, 2497-2507, 1983.
Keenan, T., and R. Carbone, 1992: A preliminary morphology of precipitation systems in tropical Northern Australia. Quart. J. Roy. Meteorol. Soc., 118, 283-336
Lindzen, R.S., Turbulence and stress due to gravity wave and tidal breakdown. J. Geophys. Res., 86, 9707-9714, 1981.
McFarlane, N.A., The effect of orographically excited gravity wave drag on the general circulation of the lower stratosphere and troposphere. J. Atmos. Sci., 44, 1775-1800, 1987.
Palmer, T.N., G.J. Shutts and R. Swinbank, Alleviation of a systematic westerly bias in general circulation and numerical weather prediction models through an orographic gravity wave parameterization. Quart. J. Roy. Meteorol. Soc., 112, 1001-1039, 1986.
Rind, D., R. Suozzo, N.K. Balachandran, A. Lacis, and G. Russell, The GISS global climate-middle atmosphere model. Part I: Model structure and climatology. J. Atmos. Sci., 45, 329-370, 1988a.
Rind, D., R. Suozzo, and N.K. Balachandran, The GISS global climate-middle atmosphere model. Part II: Model variability due to interactions between planetary waves, the mean circulation and gravity wave drag. J. Atmos. Sci., 45, 371-386, 1988b.
Vincent, R.A., S.J. Allen and S.D. Eckermann, Gravity wave parameters in the lower stratosphere. Gravity Wave Processes: Their Parameterization in Global Models (K. Hamilton, ed.). Springer-Verlag, 7-25, 1997.
Currently, much international research is focused on the issues of measuring and understanding the UT/LS region. SPARC activities in this area have focused on complementing the existing research by providing a means of communication/interaction between the numerous communities studying this region of the atmosphere. This has been carried out through focused workshops and tutorial review papers, and by providing synthesised input on coupled processes to the WMO/UNEP Assessment on Stratospheric Ozone Depletion 1998. This work will continue, and will evolve, and future co-operation with IGAC will aim to stimulate further research in this region.
The SPARC working group on gravity wave processes and their parameterisation has concentrated on establishing a global climatology of gravity wave activity. Their efforts will continue over the coming years and will be complemented by new field experiments designed to measure gravity wave processes under specific conditions.