A Chemical Tracer Model (CTM) that can use wind field data generated by the General Circulation Model (GCM) is developed to implement chemistry in the three dimensional GCM of the middle atmosphere. Initially, chemical tracers with simple first order losses such as N2O are used. Successive models are to incorporate more complex ozone chemistry

Garcia, Maria M.

Prather, Michael

English

NASA Technical Reports Server

N92-14536
Research Summaries FY 1988-89: Upper Atmosphere Theory & Data Analysis Program
A. Stratospheric Chemistry and Transport
B9 Dr. Michael Prather (P.I.), NASA/GISS
Dr. Maria M. Garcla (Co-I.), Columbia U.
C* Research Objectives of this project were to implement chemistry in the 3-D general
circulation model of the middle atmosphere developed by Dr. David Rind (GISS) under
another task. The objectives were to develop an operational chemical tracer model (CTM)
that used the wind fields from the GCM. The first work under this project was to use
chemical tracers with simple, first-order losses such as N20 and then to develop a
successively more complex ozone chemistry. A major direction of the CTM is the
examination of climate perturbations (predicted by the GCM for doubled CO 2) and their
impact on stratospheric circulation and ozone.
D. Progress and Results
A study of the Antarctic ozone hole has been made with a 3-D chemical transport model
(CTM) using a linearized photochemistry for ozone. The tracer model uses the winds and
convection from the GISS general circulation model (8 ° x 10° x 23 layers). The GCM
develops an Antarctic circumpolar vortex in early winter with strong westerlies that
reverse in Austral spring and the circulation compares favorably with the observed
climatologies. A 4-year control run of the CTM with annually repeating winds produces
ozone distributions that compare reasonably with the observed climatology. We examine
different linearizations of the ozone chemistry and show that the calculated column ozone
is sensitive to the chemical time constants in the lower stratosphere. In separate numerical
experiments, a hypothetical Antarctic ozone "hole,' is induced on September 1 and on
October 1; the CTM is integrated for 1 year with a linearized model that assumes standard
photochemistry, not including the heterogeneous reactions and unusual chemistry
associated with formation of the ozone hole. The initial depletion, assumed to be 90% of
the Os poleward of 70°S between 22 and 200 mbar, amounts to about 5% of the total Os
in the Southern Hemisphere.
As the Antarctic vortex breaks down and the ozone hole is dispersed, significant
depletions to column ozone, of order 10 Dobson units (_3 %) occur as far north as 40°S
during Austral summer. One year later, only 30 % of the original depletion remains,
mostly below 100 mbar and poleward of 30°S. The October 1 initialization is continued
for a second year, the ozone hole being reinduced one year later with the same
parameterization. The cumulative effects from the year before are noticeable, but add
only 20 % to the depletion. A budget analysis for the southern high-latitude stratosphere
(10-350 mbar x 31-90°S) indicates the ozone hole is replenished equally by photochemical
regeneration and by reduced transport of ozone into the troposphere, with a lesser
fraction being filled in by an increased flux from the tropical stratosphere.
A model for the chemical mixing of stratospheric air over spatial scales from tens of
kilometers to meters was developed (Prather and Jaffe, 1989). Photochemistry, molecular
diffusion, and strain (the stretching of air parcels due to wind shear) are combined into a
single one-dimensional model. The model is applied to the case in which chemically
perturbed air parcels from the Antarctic stratosphere are transported to midlatitudes and
strained into thin ribbon-like filaments until they are diffusively mixed with the ambient
stratosphere. We find that the parcels may be treated as evolving in chemical isolation
until the final mixing. When parcels reach a transverse thickness of 50-100 m in the
lower stratosphere, they are rapidly dispersed by the combination of molecular diffusion
and strain. The rapidity of the final mixing implies a lower limit to the vertical scales of
inhomogeneities observed in the lower stratosphere.
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For this sensitivity study, we consider four types of Antarctic air: a control case
representing unprocessed polar air; heterogeneous processing by PSCs that has
repartitioned the CIx and NO v families; processing that also includes denitrification and
dehydration; and alIprocessifig plus 90 % ozone depletion. Large abundances of C10,
resulting initially from heterogeneous processing of stratospheric air on PSCs, are
sustained by extensive denitrification. (One exception is the case of Antarctic air with
major ozone depletion in which CIO is converted rapidly to HCI upon release of small
amounts of NO_ as a result of the extremely non-linear CIx-NO,, chemical system.) CIO
• P.
concentrations In the midlatitude stratosphere should be enhanc6d by as much as a factor
of 5 due to the mixing of air processed around the Antarctic vortex and will remain
elevated for most of the following season.
Chemical propagation of the Antarctic ozone hole occurs in two phases: rapid loss of
ozone in the heterogeneously processed parcels as they evolve in isolation; and more
slowly, a relative recovery of ozone over the following months. Another important effect
is the transport of denitrified Antarctic air reducing NO x and hence the total catalytic
destruction of ozone throughout the southern midlatitudes. In Antarctic air that has
already been depleted of ozone within the vortex, little additional loss occurs during
transport, and the propagation of chemically perturbed air acts partially to offset the
deficit at midlatitudes caused by dynamical dilution of the ozone hole. In air which has
not experienced substantial ozone loss, chemical propagation can generate a net ozone
deficit of order 2-3 % at midlatitudes, which may be part of the recently detected trend
at northern midlatitudes in late winter.
E9 Publications:
Michael J. Prather and Jos6 M. Rodriguez. Antarctic Ozone: Meteoric Control of HNO 3,
Geophys. Res. Lett., 15, 1-4, (1988).
Michael J. Prather. European Sources of Halocarbons and Nitrous Oxide: Update 1986, J.
Atmos. Chem., 6, 375-406, (1988).
Michael J. Prather, ed. An Assessment Model for Atmospheric Composition, NASA
Conference Publication, CP-3203, 64 pp, (1989).
Michael Prather, Maria M. Garcia, Robert Suozzo, David Rind. Global Impact of the
Antarctic Ozone Hole: Dynamical Dilution with a 3-D Chemical Transport Model, J.
Geophys. Res., 94, xxxx-xxxx, (1989).
Michael J. Prather and Andrew H. Jaffe. Global Impact of the Antarctic Ozone Hole:
Chemical Propagation, J. Geophys. Res., 94, xxxx-xxxx, (1989).
[3-D model results in:] Jackman, C.H., R.K. Seals Jr. and M.J. Prather, eds. 2-D
Intercomparison of Stratospheric Models, NASA Conference Publication, CP-3042, 608
pp, (1989).
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General Circulation Modeling of
Stratospheric Dynamics and Transport
Byron A. Boville and Jeffrey T. Kiehl
National Center for Atmospheric Research
P.O. Box 3000
Boulder CO 80307
Research Objectives
The purpose of the research funded under this proposal is to develop and use three
dimensional models of stratospheric dynamics and transport together with simplified chem-
istry to further our understanding of the stratosphere. The dynamical model will be tested
at several resolutions with simplified forcing in order to determine the resolution required
for transport experiments. The generation of equatorial waves in the model will be stud-
ied for varying vertical resolution and tropospheric physical parameterizations in order to
determine the feasibility of simulating the quasi-biennial oscillation (QBO). The research
on transport will begin with the advection of small number of species (two or three) with
parameterized chemistry.
Summary of Progress
During the five months that this proposal has been funded, several of the proposed
projects have been initiated. The implementation of a diurnal cycle has been completed
and a study of tides in the troposphere and stratosphere is in progress. The radiation
parameterization has been further refined to include the effects of Voigt line shapes and
the (apparently significant) effects on the stratospheric circulation are being studied.
Short (150 day) simulations at three vertical resolutions (with level spacings of about
3kin, 1.5km and 700m) have been performed to examine the effect of vertical resolution on
the generation, propagation and absorption of the short vertical wave length, equatorially
trapped mixed Rossby gravity waves believed to be important in forcing the QBO. Early
analysis indicates that a doubling of the vertical resolution over that typically used may
be sufficient to represent these waves adequately, which may make an attempt to simulate
the QBO feasible in the near future.
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