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OCR for page 159
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
OCR for page 160
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.
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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
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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.
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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)
OCR for page 164
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.
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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
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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.
Representative terms from entire chapter:
stratospheric ozone
