Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Appendix B STRATOSPHERIC PERTURBATIONS--THE ROLE OF DYNAMICS, TRANSPORT' AND CLIMATE CHANGE Robert E. Dickinson National Center for Atmospheric Research Boulder, Colorado INTRODUCTION The purpose of this paper is to review the role of transport, dynamics, and climate change in the question of stratospheric perturbations, with emphasis on progress over the last two years. Atmospheric dynamics and thermal structure are major factors in quantitative evaluations of the possible changes in the concentrations of strato- spheric ozone. The distribution of ozone itself below 25 km is controlled more by transport by atmospheric motions than by chemical sources and sinks. Furthermore, atmos- pheric transport between troposphere and stratosphere determines the concentrations of the various chemical families that determine the catalytic destruction of ozone. In particular, the transport of organic chlorine species from the troposphere to levels above 25 km provides the radical chlorine species whose effect is of special concern here. The concentration of total odd chlorine species derived from photodissociation of chlorocarbons depends on the balance between production and downward transport to the troposphere. The longer- lived chlorocarbons such as Fell and F-12 whose only loss is by photodissociation in the stratosphere have average lifetimes in the troposphere inversely proportional to their rate of transport into the stratosphere. Likewise, the concentrations of total stratospheric odd nitrogen as derived from N2O generated in the troposphere are also controlled by atmospheric transport. Finally, the concentrations of water, which provides the OH radicals so crucial to ozone chemistry in the lower stratosphere, are determined by exchanges with the troposphere. 159
160 Atmospheric thermal structure is important for deter- mining the rates of various photochemical processes. At lower temperatures, most chemical kinetic processes, including those responsible for the catalytic destruction of ozone, proceed at a slower rate. Consequently, lowering of temperatures in the upper stratosphere, for example, as a result of ozone loss or increase of carbon dioxide, tends to increase stratospheric ozone. The atmospheric trace species discussed in this report are of concern not only because of possible changes in ultraviolet fluxes due to this impact on ozone change but also because of possible climate change. Climate change is possible either because of the ozone change or because of the direct radiative effects of the species. There have been no significant modifications in the last two years of our understanding of possible climate change due to the direct radiative effects of the CFMs. However, currently projected ozone change profiles imply a much larger change in the energy balance of the tropospheric energy balance than was inferred from ozone profile change estimates of two years ago. PROGRESS IN QUANTITATIVE MODELS OF TRANSPORT One-Dimensional Models Current evaluations of possible ozone depletion are still primarily based on one-dimensional empirical diffusion transport models. Quantitative approaches for objectively obtaining optimum eddy diffusion coefficients K(z) for such models were discussed at length in NRC (1976) and NRC (1979a). The basic concept is to determine K(z) empirically to reproduce one or more of the long-lived stratospheric species, in particular, NoO, CHa, On (below 25 km) or the CFMs. Stratospheric H2O is poorly simulated by one-dimensional models; it is not expected that the global average profiles of all the above- mentioned tracer species would simultaneously be accurately modeled by any particular K(z). Eddy diffusion parameterizations are not inferred from known physical processes but rather are simply representations of the time scales for vertical transport as indicated by the profile of a given tracer. Insofar as all the tracers have somewhat different sources and sinks, they all are expected to have somewhat different vertical transfer rates.
161 Little progress has been made in the last two years in deriving improved K(z)'s, and it is believed that remaining uncertainties in transport inferred from one . . ~ Dimensional models should be due more to the physical unreality of the approach than inaccuracies in the derivation of K(z). It was previously estimated (NRC 1979a) that projections of global average ozone depletion were uncertain by a factor of two due to inaccuracies in transport calculations. This estimate was somewhat subjective, but there is no current basis for improving it. Current models provide reasonable agreement with the observed vertical distributions of both N2O and CH4, but they calculate concentrations of Fell and F-12 above 20 km that are somewhat too large in comparison with that observed. It was reported two-dimensional Two-Dimensional Models previously (NRC 1979a) that a number of ~ empirical transport models were on the verge of completion. About a dozen of these models are now operational, but at the time of the May 1981 NASA workshop only one such model had obtained a projection of steady state ozone depletion with currently recommended chemical rates. This projection did not depart signifi- cantly from those of one-dimensional models (Hudson et al. 1982). If such a model were to simulate latitudinally varying vertical profiles of O3, H2O, CH4, and the CFMs, it could be regarded as providing a major improve- ment in the parameterization of transport over that given by one-dimensional models. If it also gave a reasonable simulation of stratospheric H2O, it would be a remark- able success. Some current two-dimensional models appear to simulate the latitudinal-seasonal variation of total ozone quite well but not the latitudinal variations of stratospheric N2O and CH4 (Hudson et al. 1982). Besides possibly improving estimates of global average ozone depletion, two-dimensional models can provide the latitudinal and seasonal patterns of ozone change. As reported in NRC (1979a), Pyle and Derwent (1980), and Hudson et al. (1982), the two-dimensional models indicate ozone depletions to be greatest at high latitudes in winter where there is the least hazard of excess W. It is evident that multidimensional models are required for detailed studies of the impacts of ozone change even if
162 the estimates they provide of global average ozone change are no better than those of one-dimensional models. Three-Dimensional Models Three-dimensional model studies of transport to the troposphere from the stratosphere have been carried out recently by Mahlman and his collaborators at the Geophysical Fluid Dynamics Laboratory in Princeton. No attempts have been made to include realistic chlorine chemistry. They have largely been concerned with the transport of various tracer species as inferred from winds generated from a past general circulation model simulation. In particular, they have analyzed in detail two model simulations of a tracer whose source is similar to ozone (Mahlman et al. 1980); they have used the second of these simulations to study the sampling errors for total ozone measurements in a global network of stations. ADVANCES IN THEORETICAL UNDERSTANDING OF STRATOSPHERIC TRANSPORT Considerable advances have been made in our theoretical understanding of stratospheric transport (e.g., Matsuno 1980, Pyle and Rogers 1980). Transport in the latitude- altitude plane depends on the phase relationships between poleward and vertical eddy velocities, and the relative magnitude of the photochemical source terms compared to advective transport by motions. The phase difference between poleward (v) and vertical (w) velocities depends on fluctuations in wave amplitude and dissipative processes perturbing the motions. For a simple model of a stationary planetary wave, Pyle and Rogers show that the symmetric components of the diffusion coefficient tensor (i.e., Kyy, Kzz) for a particular species depend on the rate at which that species damps to photochemical equilibrium, and on the strength of its chemical coupling to other species. The latter term can so drastically change the inferred K's that only for quasi-conservative species or families of species does the assumption of a species-independent diffusion tensor seem approximately justified. Fortunately, it is the quasi-conservative constitutents whose distribution is determined by transport.
163 It is not currently known whether or not complexities of the motions not included in the simple planetary wave models are less important than the photochemical phase shifts considered by Matsuno and Pyle and Rogers. CONNECTIONS BETWEEN STRATOSPHERIC OZONE, STRATOSPHERIC TEMPERATURE STRUCTURE, AND CLIMATE CHANGE In discussing stratospheric ozone, it is important to recognize possible effects of changes in stratospheric temperature on ozone concentrations. Such changes will occur either due to changes in the ozone concentrations themselves, e.g., Penner and Luther (1981), or due to changes in the concentrations of the other species that are important for stratospheric radiative balance, i.e., CQ2 and H2O. The concentration of H2O in turn can be affected by changes in the temperature of the tropical tropopause. Our understanding of these feedbacks has changed since NRC (1979a) primarily because of the recent changes in the assumed chemical rate constants for the lower stratosphere and consequent ozone perturbations there. In particular, small increases of O3 in the lower stratosphere, as now inferred in steady state CFM scenarios, imply a warmer tropical tropopause (as does the direct radiative heating by the CFMs), hence likely increases in stratospheric H2O concentrations. This water vapor-temperature feedback has not recently been examined quantitatively, but it should amplify, somewhat, the ozone depletion. It has been argued in the past that atmospheric CO2 would double in 50 years due to burning of fossil fuel. The stratospheric cooling due to such a doubling (10°K at 50 km according to Fels et al. (1980)) would increase O3 by 2 to 4 percent (Hudson et al. 1982) compared to the ozone column without the cooling, given the odd- chlorine concentrations expected if current CFM releases were to continue indefinitely. This CO2 effect now appears to be much more impor- tant than the 2 percent effect suggested in NRC (1979a), because it is a much larger fraction of the anticipated ozone depletion (one-fourth to one-half of it). However, it should be noted that doubling of CO2 in 50 years is no longer regarded as a credible scenario. Current scenarios for CO2 growth (Rotty and Marland 1980)
164 suggest only a 30 to 40 percent increase of CO2 in 50 years. There has been considerable progress in developing an understanding of possible changes in stratospheric temperature and winds consequent to changes in strato- spheric radiative heating terms. In particular, Fels et al. (1980) studied the stratospheric response to either a 50 percent reduction of O3 or a doubling of CO2. They used both a three-dimensional general circulation model (GCM) and simpler radiative equilibrium models. They showed that a simple model that assumed pure radiative balance for the perturbation, an approximation also used by Ramanathan and Dickinson (1979), gave temperature changes closely resembling those predicted by the GCM. This conclusion is very important for the development of two-dimensional chemical models for it provides a simple means to include temperature feedback in photochemical sensitivity studies. The recommended procedure is to assume observed temperature structure plus whatever temperature changes are needed to balance changes in radiative heating due to changes in ozone. The effects of various radiative perturbations on tropospheric climate continue to be a major concern in climate studies. Anticipated increases of CO2 still give the largest effect. However, most other likely changes in atmospheric composition also lead to warming and therefore exacerbate the problem. In particular, an increase of CFM concentrations to 1 ppb Fell and 2 ppb F-12 would heat the troposphere by about 20 percent, as much as would a doubling of CO2 (NRC 1979b). It was inferred previously that the anticipated ozone decrease due to CFMS would provide a slight cooling due to a somewhat greater increase in thermal infrared cooling than the increase in solar heating. However, current projections of ozone change suggest ozone increases in the lower stratosphere, especially in the tropics where sensitivity to radiative changes is greatest (as shown by Ramanathan and Dickinson (1979) and Fels et al. (1980)). Hence the ozone change itself now also implies signifi- cant tropospheric warming; the change due to continuation of present emission would give about 5 to 10 percent as much warming as a doubling of CO2 in the atmosphere.
165 REFERENCES Fels, S.B., J.D. Mahlman, M.D. Schwarzkopf, and R.W. Sinclair (1980) Stratospheric sensitivity to perturbations in ozone and carbon dioxide: Radiative and dynamical response. Journal of Atmospheric Sciences 37:2265-2297. Hudson, R.D., et al., eds. (1982) The Stratosphere 1981: Theory and Measurements. WHO Global Research and Monitoring Project Report No. 11. Geneva: world Meteorological Organization. (Available from National Aeronautics and Space Administration, Code 963, Greenbelt, Md. 20771.) Mahlman, J.D., H. Levy II, and W.J. Moxim (1980) Three- dimensional tracer structure and behavior as simulated in two ozone precursor experiments. Journal of Atmospheric Sciences 37:655-685. Matsuno, T. (1980) Lagrangian motion of air parcels in the stratosphere in the presence of planetary waves. Pure and Applied Geophysics 118:189-216. National Research Council (1976) Halocarbons: Effects on Stratospheric Ozone. Panel on Atmospheric Chemistry, Committee on Impacts of Stratospheric Change, Assembly of Mathematical and Physical Sciences. Washington, D.C.: National Academy of Sciences. National Research Council (1979a) Stratospheric Ozone Depletion by Halocarbons: Chemistry and Transport. Panel on Chemistry and Transport, Committee on Impacts of Stratospheric Change, Assembly of Mathematical and Physical Sciences. Washington, D.C.: National Academy of Sciences. National Research Council (1979b) Protection Against Depletion of Stratospheric Ozone by Chlorofluoro- carbons. Committee on Impacts of Stratospheric Change, Assembly of Mathematical and Physical Sciences and Committee on Alternatives for the Reduction of Chlorofluorocarbon Emissions, Commission on Socio- technical Systems. Washington, D.C.: National Academy of Sciences. Penner, J.E. and F.M. Luther (1981) Effect of temperature feedback and hydrostatic adjustment in a stratospheric model. Journal of Atmospheric Sciences 38:446-453. Pyle, J.A. and R.G. Derwent (1980) Possible ozone reductions and W changes at the earth's surface. Nature 286:373-375. Pyle, J.A. and C.F. Rogers (1980) Stratospheric transport by stationary planetary waves--the importance of
166 chemical processes. Quarterly Journal of the Royal Meteorological Society 106:421-446. Ramanathan, V. and R.E. Dickinson (1979) The role of stratospheric ozone in the zonal and seasonal radiative energy balance of the earth-troposphere system. Journal of Atmospheric Sciences 36:1084-1104. Rotty, R.M. and G. Marland (1980) Constraints on fossil fuel use. In Interactions of Energy and Climate, edited by W. Back, J. Pankrath, and J. Williams. Boston, Mass.: D. Reidel.