GMDGeoscientific Model DevelopmentGMDGeosci. Model Dev.1991-9603Copernicus PublicationsGöttingen, Germany10.5194/gmd-10-2671-2017Update of the Polar SWIFT model for polar stratospheric ozone loss (Polar SWIFT version 2)WohltmannIngoingo.wohltmann@awi.dehttps://orcid.org/0000-0003-4606-6788LehmannRalphRexMarkusAlfred Wegener Institute for Polar and Marine Research, Potsdam, GermanyIngo Wohltmann (ingo.wohltmann@awi.de)13July20171072671268925January20177March201722May20178June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://gmd.copernicus.org/articles/10/2671/2017/gmd-10-2671-2017.htmlThe full text article is available as a PDF file from https://gmd.copernicus.org/articles/10/2671/2017/gmd-10-2671-2017.pdf
The Polar SWIFT model is a fast scheme for calculating the
chemistry of stratospheric ozone depletion in polar winter. It is intended
for use in global climate models (GCMs) and Earth system models (ESMs) to
enable the simulation of mutual interactions between the ozone layer and
climate. To date, climate models often use prescribed ozone fields, since a
full stratospheric chemistry scheme is computationally very expensive. Polar
SWIFT is based on a set of coupled differential equations, which simulate the
polar vortex-averaged mixing ratios of the key species involved in polar
ozone depletion on a given vertical level. These species are O3,
chemically active chlorine (ClOx), HCl, ClONO2 and
HNO3. The only external input parameters that drive the model are the
fraction of the polar vortex in sunlight and the fraction of the polar vortex
below the temperatures necessary for the formation of polar stratospheric
clouds. Here, we present an update of the Polar SWIFT model introducing
several improvements over the original model formulation. In particular, the
model is now trained on vortex-averaged reaction rates of the ATLAS Chemistry
and Transport Model, which enables a detailed look at individual processes
and an independent validation of the different parameterizations contained in
the differential equations. The training of the original Polar SWIFT model
was based on fitting complete model runs to satellite observations and did
not allow for this. A revised formulation of the system of differential equations
is developed, which closely fits vortex-averaged reaction rates from ATLAS
that represent the main chemical processes influencing ozone. In addition, a
parameterization for the HNO3 change by denitrification is included.
The rates of change of the concentrations of the chemical species of the
Polar SWIFT model are purely chemical rates of change in the new version,
whereas in the original Polar SWIFT model, they included a transport effect
caused by the original training on satellite data. Hence, the new version
allows for an implementation into climate models in combination with an
existing stratospheric transport scheme. Finally, the model is now formulated
on several vertical levels encompassing the vertical range in which polar
ozone depletion is observed. The results of the Polar SWIFT model are
validated with independent Microwave Limb Sounder (MLS) satellite observations and output from the
original detailed chemistry model of ATLAS.
List of equations used in the original and new Polar SWIFT version.
Terms A to L are specified in Table . … is the vortex
mean, z is a free fit parameter. FAP and FAPs are fractions of
the polar vortex below different threshold temperatures for the formation of
PSCs (see Sect. ).
The importance of interactions between climate change and the ozone layer has
long been recognized e.g.,. Hence, it is
desirable to account for these interactions in climate models. Usually, this
is accomplished by coupling a full stratospheric chemistry module to a global
climate model (GCM) in models referred to as chemistry climate models (CCMs)
e.g.,. Since this approach is computationally expensive,
ozone is usually prescribed in the type of climate model runs that are used
in the Intergovernmental Panel on Climate Change (IPCC) reports ,
where long-term runs and multiple scenario runs are required. There is
however a growing number of models incorporating interactive ozone chemistry
e.g.,, either by using simplified fast schemes like the
Cariolle scheme or Linoz scheme
or by using CCMs. The fast stratospheric chemistry
scheme Polar SWIFT was developed to enable interactions between climate and
the polar ozone layer in time-critical applications of climate models and to
improve quality and performance compared to existing schemes. The original
version of the Polar SWIFT model was presented in . Here, we
present an update of the Polar SWIFT model. The Polar SWIFT model is
complemented by an independent model for calculating extrapolar stratospheric
ozone chemistry (Extrapolar SWIFT), which is presented in a separate
publication .
Polar SWIFT simulates the evolution of the polar vortex-averaged mixing
ratios of six key species that are involved in polar ozone depletion by
solving a set of coupled differential equations for these species on a given
vertical level . The model includes four prognostic variables
(ClONO2, HCl, total HNO3, and O3) and two diagnostic variables
(chemically active chlorine ClOx= ClO + 2Cl2O2 and HNO3 in the gas
phase). The differential equations contain several free fit parameters, which
were fitted to match satellite observations in the old model version
see and which are fitted to vortex-averaged reaction rates
from the ATLAS Chemistry and Transport Model in the new version.
Polar SWIFT is driven by time series of two external input parameters. The
first is the fraction of the polar vortex area that is cold enough to allow
for the formation of polar stratospheric clouds (fractional area of PSCs,
abbreviated FAP) and the second is the 24 h average of the fraction
of the polar vortex that is exposed to sunlight (fractional area of sunlight,
abbreviated FAS). A system of four differential equations is formulated that
describes the chemical rate of change of the prognostic variables as a
function of FAP, FAS and the mixing ratios of the species only (the term for
HNO3 also includes the rate of change caused by denitrification). The
equations comprise terms for the most important chemical processes involved
in polar ozone depletion, e.g., the effect of the catalytic ClO dimer cycle.
Since only a single value per vertical level and species is used in Polar
SWIFT, which is constant over the polar vortex, and since the model is able
to use a large time step of typically 1 day and a simple integration
scheme, it is possible to calculate the ozone evolution of a complete winter
in a few seconds.
The original system of equations is shown in Tables
and , together with the new model formulation, which is
presented in more detail in Sect. .
Table contains the fitted coefficients.
Other fast ozone schemes developed for climate models, such as the Cariolle
scheme or the Linoz scheme , were
originally designed to model only extrapolar ozone. In contrast to these
schemes, the SWIFT model is not based on a single linear differential
equation based on a Taylor series expansion, but on a set of coupled
nonlinear differential equations representing the main processes changing
polar ozone. This has the advantage that the model is not required to be
linear and can cope with the nonlinearities occurring in polar ozone
chemistry. Together with the fact that the model equations are closely based
on the real atmospheric processes, we expect our model to behave more
realistically than a Taylor series based approach, especially in conditions
far away from the current atmospheric mean state. The Polar SWIFT model is
therefore able to represent ozone–climate interactions during climate change
(in combination with the Extrapolar SWIFT model).
The latest version of the Cariolle scheme also includes a
parameterization for heterogeneous polar chemistry, but is based on a quite
different approach using a temperature tracer. The newest version of the
Linoz scheme uses a simple parameterization based on an earlier
version of the Cariolle scheme .
In Sect. , an overview of the new Polar SWIFT model is
given and the fitting procedure is described in detail. In Sect. ,
we present the new differential equations for the
four prognostic variables of the model (HCl, ClONO2, HNO3 and
O3), and the fits to the modeled reaction rates. In
Sect. , the Polar SWIFT model is validated by comparison
to Microwave Limb Sounder (MLS) satellite data and the original detailed chemistry model of ATLAS.
Section contains the conclusions.
List of the terms used in the differential equations in the original
and new Polar SWIFT version. … is the vortex mean. a to l and
y are free fit parameters. FAP and FAPs are fractions of the
polar vortex below different threshold temperatures for the formation of PSCs
(see Sect. ). FAS is the fraction of the vortex exposed to
sunlight (see Sect.).
Expression (original) Expression (new) Term A: heterogeneous reaction HCl + ClONO2a⋅[ClONO2]⋅[HNO3]⋅FAP[HCl]>27ppta⋅[ClONO2]⋅[HNO3]2/3⋅FAP[HCl]>1ppta⋅[HCl]⋅[ClONO2]⋅[HNO3]⋅FAP[HCl]<27ppta⋅[HCl]⋅[ClONO2]⋅[HNO3]2/3⋅FAP[HCl]<1pptTerm B (and G in the original model): net change by ClONO2 gas phase reactions b⋅[HNO3]g⋅FAS[ClOx]>135pptb⋅[ClOx]⋅[HNO3]g⋅FASB and Gb⋅[ClOx]⋅[HNO3]g⋅FAS[ClOx]<135pptreplaced by Bg⋅[ClONO2]⋅FASTerm C: reaction Cl + CH4c⋅[ClOx]/[O3]⋅FASc1⋅[ClOx]/[O3]⋅FAS2+c2⋅[ClONO2]/[O3]⋅FAS3Term D: ozone depletion by ClO dimer and ClO–BrO cycle d⋅[ClOx]⋅FASd⋅[ClOx]⋅FASunchangedTerm E: denitrification e⋅[HNO3]⋅max(FAP-y),0e⋅[HNO3]⋅FAPsTerm F: reaction ClO + OH f⋅[ClOx]⋅FAS2Arcticf⋅[ClOx]⋅FAS20.25f⋅[ClOx]⋅FAS2AntarcticTerm H: heterogeneous reaction ClONO2+ H2O h⋅[ClONO2]⋅max(FAP-y),0h⋅[ClONO2]⋅[HNO3]2/3⋅FAPTerm K: reaction HCl + OH k⋅[HCl]⋅FAS2Term L: heterogeneous reaction HOCl + HCl l⋅[HOCl]⋅[HCl]⋅[HNO3]2/3⋅FAP[HOCl]=[ClOx]⋅FASOverview of the changes in the new Polar SWIFT versionRevision of the system of differential equations based on ATLAS results
The original formulation of the system of differential equations is revised
based on results of the Lagrangian Chemistry and Transport Model ATLAS. A
detailed description of the model can be found in .
The model includes a gas phase stratospheric chemistry
module, heterogeneous chemistry on polar stratospheric clouds and a
particle based Lagrangian denitrification module. The chemistry module
comprises 47 active species and more than 180 reactions. Absorption cross
sections and rate coefficients are taken from recent JPL recommendations .
Vortex-averaged mixing ratios of all model species and vortex-averaged
reaction rates of all modeled reactions are used to identify the important
processes involved in polar ozone depletion, and to identify the relevant
reactions, their relative importance and their time evolution. Results are
based on two model runs for the southern hemispheric winters 2006 and 2011
(1 May to 30 November) and two model runs for the northern hemispheric
winters 2004/2005 (15 November to 31 March) and 2009/2010 (1 December to 31 March).
The identification of the most important processes and reactions is discussed
in a companion paper . The present paper concentrates on
the technical aspects, such as the fitting procedure and finding appropriate
parameterizations for the processes.
Details of the model setup are described in and we will
only repeat the most important facts here. Model runs are driven by
meteorological data from the ECMWF ERA Interim reanalysis .
Chemical species are mainly initialized by MLS satellite data .
The initial horizontal model resolution is 150 km. The runs use a
potential temperature coordinate and vertical motion is driven by total
diabatic heating rates from ERA Interim. In addition to the binary background
aerosol, the model simulates three types of polar stratospheric clouds, which
are supercooled ternary HNO3/H2SO4/H2O solutions (STS),
solid clouds composed of nitric acid trihydrate (NAT), and solid ice clouds.
The number density of NAT particles in the runs is set to
0.1 cm-3, the number density of ice particles is set to
0.01 cm-3 and the number density of the ternary solution droplets
to 10 cm-3. A supersaturation of HNO3 over NAT of 10
(corresponding to about 3 K supercooling) is assumed to be necessary
for the formation of the NAT particles. For ice particles, a supersaturation
of 0.35 is assumed. The settings for the polar stratospheric clouds largely
favor the formation of liquid clouds (binary liquids and STS clouds) over the
formation of NAT clouds, and activation of chlorine predominantly occurs in
liquid clouds in the model runs.
Vertical levels
Fitted parameters for the differential equations from
Tables and are obtained for
five pressure levels, which roughly encompass the vertical range in which ozone
depletion is observed. Here, the choice of the pressure levels is guided by
the pressure levels of the EMAC (ECHAM/MESSy Atmospheric Chemistry) model
(39 level version) in this altitude range , which is the
first model in which Polar SWIFT is implemented. The levels used are at
approximately 69.7, 54.0, 41.6, 31.8 and 24.1 hPa (see Table for exact
values). Results at intermediate levels can either be obtained by vertical
interpolation of the fitted parameters or by running the Polar SWIFT model at
the two enclosing levels and averaging the results. In the following, we will
only show results from the 54 hPa level in the figures for clarity.
Fit of the free parameters to ATLAS reaction rates
The fitting procedure for the original model version was based on fitting the
time series of species mixing ratios of a complete Polar SWIFT model run to
satellite observations at a given vertical level. This approach has several disadvantages:
The fit is nonlinear, since the solution of the differential equations
depends nonlinearly on the fit parameters. This requires a nonlinear
fitting algorithm, which may only find a local and not a global minimum for
the residuum of the fit.
In addition, the fitting procedure is iterative and is computationally more
expensive than a linear fit. Every iteration of the fitting procedure
requires a complete run of the Polar SWIFT model.
Transport effects are implicitly included. The rate of change of the
satellite data at a given level is the sum of the chemical rate of change and
the rate of change by transport. Hence, the fit parameters include a
transport effect. This effect is most pronounced for O3, where the rates
of chemical change and of change by subsidence in the polar vortex are comparable.
Satellite data of the species that are fitted have to be available. For
species like ClONO2, the availability of measurements is limited.
Here, we employ a new method that avoids these disadvantages. We take
advantage of the fact that all of the equations of the system of differential
equations from Tables and on a given
vertical level are of the form
dXntidt=cpn1fpn1X1ti,…,XNti,ti+…+cpnM(n)fpnM(n)X1ti,…,XNti,ti,
where … is the vortex average, Xn is the
vortex-averaged mixing ratio of species n and N is the number of species
(n= 1, …, N). The fp(…) are functions of the mixing ratios (and
of fixed parameters such as FAS and FAP), which represent the parameterizations
for the different processes p= 1, …, P introduced in . The
processes fp are the terms A, B, etc., in Tables
and . The cp are the associated fitted coefficients for
each parameterization (a, b, etc., in Table ). The pnm
assign a parameterization to a specific species n and the additive term m
of that species. M(n) is the number of additive terms for species n.
Different pnm are allowed to contain the same number (i.e., the same
parameterization can be used for different species). The net chemical rate of
change dXn(ti)/dt for every species and
all fp(…) terms can be obtained as fixed values from the ATLAS runs
for a number of model time steps ti (i= 1, …, T), since both the
vortex-averaged mixing ratios and the vortex-averaged reaction rates are
available from the ATLAS model. This gives a system of T⋅N equations
that can be solved for the cp. The system consists of simple linear
equations for the cp, which can be solved by a least-squares fit (since
the number of equations T⋅N is much larger than the number of
coefficients P, the system is overdetermined). Equations with different
time steps ti but the same species n are coupled since they contain the
same cp. Additionally, equations with different species may also contain
the same cp.
To simplify the fit further, we split the left-hand side into a sum of the
rates of change that are caused by single chemical reactions
dXndtti=dXndt1ti+…+dXndtK(n)ti,
where dXn/dt|k is the rate of change
caused by the kth reaction changing species n in ATLAS, with K(n) the
number of reactions changing species n (k= 1, …, K(n)). In many cases,
it is feasible to assign a single reaction (or a sum of a very few reactions)
to one of the parameterizations fp(…) on the right-hand side. This
way, the system of differential equations decouples into many independent
linear equations, which can simply be solved by fitting the cp as a factor
that scales the parameterization fp(…) to the rate of change of the
corresponding chemical reaction:
dXndtkti=cpfpX1ti,…,XNti,ti.
The time series of the northern hemispheric runs and of the southern
hemispheric runs are concatenated and fitted at the same time to obtain one
set of fit parameters valid for both hemispheres. This is done because the
physical and chemical foundations are the same in both hemispheres and the
same parameterizations can be used. Since the conditions in the Northern and
Southern hemispheres cover a wide range of temperatures, this approach ensures
that the model does respond correctly to changes in temperature, e.g., temperature
trends caused by climate change.
Vortex averages
The vortex-averaged mixing ratios of the species Xn in the
Northern Hemisphere are obtained by assuming that the vortex edge is situated
at the 36 PVU contour of modified potential vorticity (PV) at all
altitudes. Modified PV is calculated according to , with
θ0= 475 K. In the Southern Hemisphere, the vortex edge is
assumed to be situated at the -36 PVU contour. Note that the vortex
tracer criterion described in is not applied here.
The air parcels of ATLAS that are inside the vortex are vertically binned
into bins centered at the five pressure levels of Polar SWIFT to obtain the
mixing ratios Xn for these levels by averaging. The edges of
the bins are in the middle between the Polar SWIFT levels (in the logarithm
of pressure). ATLAS model output is available at 00:00 and
12:00 UTC. Vortex means from 00:00 and 12:00 UTC on
a given day are averaged to obtain daily means. Vortex-averaged reaction
rates are calculated as 24 h averages over the diurnal cycle by the
method described in .
Usually, it is easy to find a parameterization for the rate of a specified
reaction or the mixing ratios of a chemical equilibrium if only looking at a
given location inside the vortex (i.e., a reaction
A+B→C leads to the equation
d[C]/dt=k[A][B] with
[A] the mixing ratio of A and k the rate coefficient). However,
problems arise if vortex averages are used. If we assume that either the
mixing ratios of the species are sufficiently constant over the area of the
vortex, or that the differential equations do only contain terms linear in
the mixing ratios, we can use vortex averages. Care has to be taken if
products of mixing ratios appear in the equations. If X1 and X2 are the
mixing ratios of two species, the vortex average of the product is not
necessarily the same as the product of the vortex averages (if their
covariance is not zero)
X1X2=X1X2+covX1,X2.
There are several possibilities to cope with this problem. If at least one of
the species is long-lived and constant over the vortex, approximate equality
can be assumed. If both species are short-lived, the vortex can be divided
into a sunlit part and a dark part, and two separate constant mixing ratios
have to be assumed in the sunlit and dark part.
However, we will see in the following that it is not possible in all cases to
transform the original expression for the chemical reaction at a single
location to an equivalent expression that only uses vortex averages. We use
expressions that are empirically derived in these cases. Here, the quality of
the approximation is assessed by the goodness of fit for the wide range of
climate conditions observed in the training data set.
The external parameters FAP and FAS
The 24 h averaged fraction of the polar vortex in sunlight (FAS) and
the fraction of the polar vortex below the formation temperature of polar
stratospheric clouds (FAP) are calculated from the same ERA Interim data that
is used for running the ATLAS model for consistency.
Two different FAP parameters are used in the new version of the Polar SWIFT
model, which are called FAP and FAPs. Evidence from modeling
studies and observations suggests that a considerable part of chlorine
activation occurs on clouds composed of liquid binary and supercooled ternary
solutions (STS) and that nitric acid trihydrate (NAT) clouds only form when
large supersaturations of more than 10 are reached (for a detailed discussion
and references, see ). By
chance, the required supercooling of 3 K also corresponds roughly to
the temperature at which binary liquid aerosols begin to take up HNO3 in
significant quantities and are transformed into ternary solutions, which
increases the reaction rates on liquid aerosols significantly. Hence, we
calculate the area of the polar vortex above a supersaturation of HNO3
over NAT of 10 according to the equations of and divide the
values by the vortex area. This quantity is called FAPs in the
following. However, chlorine activation already sets in at higher
temperatures than the NAT threshold temperature minus 3 K on the
liquid aerosols, albeit with smaller rates. Hence, we also calculate a
quantity called FAP by assuming no supersaturation. The decision to use FAP
or FAPs is based empirically on the quality of the fit for the
single equations. A special case is the denitrification, which is based on
sedimenting NAT particles in the ATLAS model and is parameterized with FAPs.
For FAS, the area below a solar zenith angle of 90∘ inside the vortex
is calculated and divided by the vortex area. To obtain a 24 h
average, the polar vortex obtained from ERA Interim is assumed to be fixed
for a virtual 24 h period. Then, the solar zenith angles are
calculated for many intermediate time steps in this 24 h period.
The area below 90∘ solar zenith angle is calculated for each
intermediate step. Finally, the results are averaged over the intermediate time steps.
The parameterizations
In the next sections, we present the new differential equations for the four
prognostic variables of the model (HCl, ClONO2, HNO3 and O3),
and the fits to the modeled reaction rates. The terms fp are indicated by
upper case letters A, B, C, etc., in the following, to comply with the
notation in . Mixing ratios of species are denoted by putting the
name of the species into brackets, e.g., [HCl] for the mixing ratio
of HCl. … is the vortex mean again.
HClOverview
The equation for HCl is changed from
d[HCl]dt=C+F-A
in the original model to
d[HCl]dt=C1+C2+F-A-L-K
in the new model. Term C of the original model and terms C1 and
C2 of the new model represent the effect of the reaction
Cl+CH4→HCl+CH3,
which is responsible for deactivation of Cl into HCl under ozone hole
conditions in the Southern Hemisphere and is the main HCl production reaction
in both hemispheres. In the new parameterization, we split term C into two
terms C1 and C2 to account for two different Cl sources
(Cl2O2 photolysis and the ClO + NO reaction). The
less important reaction
Cl+CH2O+O2→HCl+CO+HO2,
which also depends on Cl, is subsumed into term C in the new model.
Term F represents the effect of the reaction of ClO with OH
ClO+OH→HCl+O2,
which helps HCl reformation in both hemispheres. Term A accounts for the
effect of the most important heterogeneous reaction activating chlorine
HCl+ClONO2→Cl2+HNO3.
We introduce a new term L for the heterogenous reaction
HOCl+HCl→Cl2+H2O,
which is responsible for a considerable part of the activation in the
Southern Hemisphere and for a non-negligible part in the Northern Hemisphere.
Another reaction that consumes HCl not included in the original model that
turned out to be significant in late winter and spring is
HCl+OH→H2O+Cl,
which is considered by a new term K.
Vortex-averaged mixing ratio of Cl2O2 for the Arctic
winter 2004/2005, the Antarctic winter 2006, the Arctic winter 2009/2010 and
the Antarctic winter 2011 at 54 hPa (from left to right). Vortex average
(solid blue) and parameterization for the mixing ratio by
[ClOx](1-FAS) (dashed blue) and average
over sunlit part of vortex (solid magenta) and parameterization for the
mixing ratio by [ClOx] (dashed magenta). Tick
marks on the horizontal axis show start of months.
Term C
Term C represents the effect of
Cl+CH4→HCl+CH3.(R1)
This reaction is responsible for chlorine deactivation under ozone hole
conditions and is the main production reaction of HCl in both hemispheres
. If we assume that CH4 is sufficiently constant,
the rate of production of HCl by this reaction is only proportional to Cl:
d[HCl]dtR1∼[Cl].
Term C of the original model is split into two additive terms
C=C1+C2 in the new model. These two terms account for two different
sources of Cl. Under sunlit conditions, Cl is mainly determined
by two source reactions that produce Cl
Cl2O2+hν→2Cl+O2ClO+NO→Cl+NO2
and a reaction that removes Cl
Cl+O3→ClO+O2.
Reaction () is coupled to the catalytic ClO dimer cycle. It can
be shown by using the equilibrium condition
d[Cl]/dt= 0 that the mixing ratio of Cl
under sunlit conditions is roughly approximated by
[Cl]day=2kR7Cl2O2kR9O3day+kR8[ClO][NO]kR9O3day,
where …day is the average over the sunlit
part of the vortex and the kR are the rate coefficients. The first term
on the right side corresponds to term C for the Cl2O2 photolysis
in the original model and to term C1 in the new model. The second term
corresponds to the new term C2 for the ClO + NO reaction.
Vortex-averaged mixing ratios can be obtained by an area-weighted average
[Cl]≈FAS⋅[Cl]day+(1-FAS)⋅[Cl]night≈FAS⋅[Cl]day
under the assumption that there is no Cl at night.
Term C1
The vortex average of the photolysis coefficient kR7 is
assumed to be proportional to FAS. Figure shows that
[Cl2O2]day is proportional to
[ClOx] in good approximation, since a relatively constant
fraction of ClOx is in the form of Cl2O2 during day in the
covered time period. We assume that ozone is sufficiently constant over the
vortex so that [O3]day=[O3] and
that the division and the vortex mean can be interchanged. Hence, term
C1 is parameterized similarly as the original term CC1=c1⋅ClOx/O3⋅FAS2.
This is the original term multiplied by FAS. The two different FAS factors in
the new model have their origin in the area-weighted average and in the
photolysis coefficient, respectively. The term
1/[O3] makes sure that if ozone concentrations
become low, Reaction () of the ClO dimer cycle becomes less
efficient and the ratio of Cl over ClO increases.
Fit of term C for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Sum of the vortex-averaged reaction rates modeled by ATLAS for the
reactions Cl + CH4 and Cl + CH2O (blue), the fitted terms
C1 (orange) and C2 (brown) and the sum C1+C2 (red).
Tick marks on the horizontal axis show start of months.
Term C2
Term C2 accounts for Reaction (), which produces Cl in
large quantities in spring and was not considered in the
original model. Looking at Eq. (), it is tempting to
model term C2 by assuming that
[ClO][NO]O3day
is equal to
[ClO]day[NO]dayO3day
and then finding parameterizations for
[ClO]day,
[NO]day and
[O3]day. Unfortunately, it turns out
that this approximation is not valid, since the spatial distributions of ClO
and NO are very different and
[ClO][NO]day≠[ClO]day[NO]day.
The reason is that there is an equilibrium between ClONO2 on the one
side and ClO and NO2 on the other side, which limits the
amount of ClO and NO2 (and in turn NO) that can exist
at the same location. ClONO2 is to a good approximation in an
equilibrium between
ClONO2+hν→Cl+NO3→ClO+NO2
and
ClO+NO2+M→ClONO2+M,
and the partitioning of NOx is governed, to a good approximation, by
the equilibrium between
O3+NO→NO2+O2ClO+NO→Cl+NO2(R8)NO2+hv→NO+O.
By using the equilibrium conditions d[NO]/dt= 0
and d[ClONO2]/dt= 0, the product of ClO and NO
can be expressed by
[ClO][NO]=kR10+kR11kR14ClONO2kR12kR13O3+kR8[ClO].
Unfortunately, the vortex mean of this function can again not be replaced by
this function formulated in terms of the vortex means
[ClONO2], [O3] and
[ClOx].
That is, a formulation of term C2, which is quantitatively correct and
only depends on the vortex means of the variables, is not possible. It is only
possible to find a parameterization that results in a good fit and takes into
account some important properties of the above equations.
A very good fit for term C2 can be achieved by the parameterization
C2=c2⋅ClONO2/O3⋅FAS3.
The three FAS factors take the involved photolysis Reactions ()/() and () and the area-weighted average into account. The dependence on ClONO2 in
Eq. () is considered by multiplying by
[ClONO2]. The shift of the equilibrium towards high
Cl values for low O3 values by the Cl + O3 and
NO + O3 reactions is parameterized by dividing by [O3].
The sum C1+C2 is fitted to the sum of the modeled rates of the two
Reactions () and (). Reaction () is a less
important reaction that also depends on Cl. Figure
shows the sum of the reaction rates modeled by ATLAS (blue), the fitted term
C=C1+C2 (red) and the two components C1 (orange) and C2
(brown) at the second fitted pressure level (54 hPa). The fitted
coefficients can be found in Table .
Vortex-averaged mixing ratio of ClO for the Arctic winter 2004/2005,
the Antarctic winter 2006, the Arctic winter 2009/2010 and the Antarctic
winter 011 at 54 hPa. Vortex average (solid blue) and parameterization for
the mixing ratio by [ClOx]⋅FAS
(dashed blue) and average over sunlit part of vortex (solid magenta) and
parameterization for the mixing ratio by [ClOx]
(dashed magenta). Tick marks on the horizontal axis show start of
months.
Vortex-averaged mixing ratio of OH for the Arctic winter 2004/2005,
the Antarctic winter 2006, the Arctic winter 2009/2010 and the Antarctic
winter 2011 at 54 hPa. Vortex average (solid blue) and parameterization for
the mixing ratio by FAS2 (dashed blue) and average over sunlit part of
vortex (solid magenta) and parameterization for the mixing ratio by FAS
(dashed magenta). Tick marks on the horizontal axis show start of
months.
Term F
Term F represents the effect of the reaction of ClO with OH
ClO+OH→HCl+O2,(R3)
which helps HCl formation in both hemispheres. The reaction starts to become
important only in late winter, when sunlight comes back and OH and
ClO are produced in photolytic reaction cycles. Since ClOx levels
decrease in spring, it is only important for a relatively short period
(February to March in the Northern Hemisphere, September to October in the
Southern Hemisphere; ). The
rate of change of HCl by this reaction is given by the area-weighted rate of
change under sunlit conditions, since there is no OH and very little
ClO at night
d[HCl]dtR3≈FAS⋅d[HCl]dtR3day.
The rate of change during day can be expressed by
d[HCl]dtR3day=kR3[ClO][OH]day.
Contrary to the situation in the last section, the vortex average and the
multiplication can be interchanged, to a good approximation, for ClO
and OH
[ClO][OH]day≈[ClO]day[OH]day.
Hence, term F is parameterized by
F=f⋅[ClO]day⋅[OH]day⋅FAS
with the FAS factor from the area weighting. The
mixing ratio [ClO]day is modeled by
assuming proportionality to ClOx[ClO]day∼ClOx
since a relatively constant fraction of ClOx is present as ClO
during day. Figure shows that this assumption works well.
The mixing ratio [OH]day is modeled by assuming
[OH]day∼FAS.
Figure shows that this is a sufficiently good assumption,
partly due to the fact that the mixing ratios of OH are relatively
similar in the Northern and Southern hemispheres. In conditions not disturbed
by heterogenous chemistry on PSCs, it can be shown that the stratospheric OH
abundance is in relatively good approximation a linear function of the solar
zenith angle, mostly independent from the concentrations of other species
. In addition, the average solar zenith angle in the vortex
is in good approximation a linear function of FAS. However, this is not true
anymore under conditions of heterogeneous chemistry, and significant
deviations from this behavior occur, especially in the Southern Hemisphere
(see Fig. 11 in ; the effect is also visible in Fig. ).
Fit of term F for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged reaction rate modeled by ATLAS for the reaction
ClO + OH (blue) and the fitted term F (red). Tick marks on the
horizontal axis show start of months.
Production and loss processes of HOx= OH + HO2 are
fairly complicated . In particular, it is not
possible to find a simple equation that relates the mixing ratios of the
relevant source gases CH4, HNO3 and H2O to the mixing
ratio of HOx. In addition, the partitioning inside HOx
depends in a complicated way on O3, ClOx and NOx and there
are considerable differences in the partitioning of HOx between
OH and HO2 in the Northern and Southern hemispheres
see. Hence, only an empirical parameterization that is
not a function of the source gases is given here. Note that this means that
the Polar SWIFT model implicitly uses the water vapor and methane levels of
the ATLAS model runs and that it is not possible to model responses to
changes in these source gases with Polar SWIFT.
Term F is fitted to the modeled rate of Reaction ().
Figure shows the modeled reaction rate and the fitted term F.
Term A
Term A accounts for the effect of the most important heterogeneous reaction
activating chlorine
HCl+ClONO2→Cl2+HNO3.(R4)
The parameterization of term A remains similar to the parameterization in
the original model. For high HCl, it is given by
A=a⋅ClONO2⋅HNO32/3⋅FAP
and for low HCl it is given by
A=a⋅[HCl]⋅ClONO2⋅HNO32/3⋅FAP.
The threshold for HCl is set to 1 ppt. Reaction rates for
heterogeneous reactions are proportional to the surface area density of the
liquid or solid particles in the ATLAS model. The surface area density is
modeled by [HNO3]2/3⋅ FAP. Here, we
assume that cloud particles are mainly composed of HNO3, that all
HNO3 is in liquid or solid form in the area below the threshold
temperature used for FAP and that all HNO3 outside this area is in the
gas phase. In the original model, the parameterization
[HNO3]⋅ FAP was used. In the new model,
[HNO3] is raised to the power of 2/3 to account
for the difference between particle volume density (proportional to the
mixing ratio of liquid or solid HNO3 per volume of air) and particle
surface area density (surface is proportional to volume raised to the power
of 2/3).
Normalized pseudo first-order rate coefficients as a function of HCl
mixing ratio for the heterogeneous reactions ClONO2+ HCl (blue) and
HOCl + HCl (cyan) on liquid STS surfaces. Reaction rates were calculated
for T= 190 K, p= 50 hPa, 10 ppb HNO3, 0.15 ppb
H2SO4, 4 ppm H2O and 1 ppb
ClONO2.
Fit of term A for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged reaction rate modeled by ATLAS for the reaction
HCl + ClONO2 (blue) and the fitted term A (red). Tick marks on the
horizontal axis show start of months.
For heterogeneous reactions on NAT, reaction rates are not proportional to
HCl; i.e., the change of HCl is given by
d[HCl]dtR4=-kR4ClONO2,
where kR4 is a pseudo first-order rate coefficient,
which is not a function of HCl. This is not the case for reactions on liquid
STS surfaces, where the rate depends on the concentration of HCl
(Fig. ). Activation mainly occurs on liquid surfaces in
the model runs. The sensitivity of the reaction rate on the HCl concentration
is nonlinear, with a rapid increase between 0 and 0.3 ppb. The
reaction rate becomes relatively independent of HCl above 0.3 ppb.
Hence, a parameterization that does not depend on HCl is a good approximation
and gives a better fit than a parameterization that depends linearly on HCl.
Term A is fitted to the modeled rate of the heterogeneous
Reaction (). Figure shows the modeled rate
of this reaction (blue) and the fitted term A (red).
Term L
The new term L accounts for the effect of the heterogeneous reaction
HOCl+HCl→Cl2+H2O,(R5)
which can activate a significant part of chlorine in the Southern Hemisphere
and a non-negligible part in the Northern Hemisphere. In the original model,
this contribution was implicitly subsumed into term A. However, since we
use term A both in the HCl equation and in the ClONO2 equation
with the same fit parameter a, we introduce an additional term here to
represent the HCl loss by HOCl + HCl. Similar to the approach for term A,
term L is parameterized by
L=l⋅[HOCl]⋅[HCl]⋅HNO32/3⋅FAP.
HOCl is parameterized by
[HOCl]∼[ClO]day⋅FAS.
HOCl is in a fast equilibrium between
ClO+HO2→HOCl+O2
and
HOCl+hν→Cl+OH
under sunlit conditions. Using the equilibrium condition, we obtain
[HOCl]day=kR15kR16[ClO]HO2day.
Now, we assume that the ratio of [HO2] and kR16 is nearly
constant, since both terms depend on the amount of sunlight, which gives
[HOCl]day∼[ClO]day.[ClO]day is parameterized by [ClOx]
(see discussion of term F and Fig. ). If we assume that there
is no HOCl during night, we obtain
[HOCl]=[HOCl]day⋅FAS.
This assumption is not straightforward. If the heterogenous reaction
HOCl + HCl did not take place, night-time mixing ratios of HOCl would
remain at mixing ratios similar to the daytime values, since the Reactions ()
and () do not proceed during night. However, the
parameterization for term L is only different from zero when heterogenous
reactions can proceed (due to the FAP term) and when enough chlorine is
activated (due to the [ClOx] term). Under these
conditions, HOCl is depleted by the HOCl + HCl reaction during night.
HOCl + HCl is a heterogeneous reaction, whose reaction rate will be
proportional to FAP. The rate of the HOCl + HCl reaction
shows a more linear dependency on HCl mixing ratios than the
ClONO2+ HCl reaction (Fig. ). Hence, we
include the HCl mixing ratio as a linear factor in term L, which improves
the fit compared to a parameterization that does not depend on HCl. Still,
term L shows one of the poorer fits compared to the other parameterizations.
Term L is fitted to the modeled reaction rate of the heterogeneous
Reaction (). Figure shows the modeled reaction
rate of this reaction and the fitted term L.
Fit of term L for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged reaction rate modeled by ATLAS for the reaction
HOCl +H Cl (blue) and the fitted term L (red). Tick marks on the
horizontal axis show start of months.
Fit of term K for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged reaction rate modeled by ATLAS for the reaction
HCl + OH (blue) and the fitted term K (red). Tick marks on the
horizontal axis show start of months.
Term K
A reaction not included in the original model that affects the redistribution
of HCl and ClONO2 in late winter and spring is
HCl+OH→H2O+Cl(R6)
as shown in . In spring, this reaction consumes much of
the HCl that is produced by Cl + CH4. The reaction is
represented by a new term K, which is parameterized by
K=k⋅[HCl]⋅[OH]day⋅FAS.
The equation is derived analogously to the equation for term F. We multiply
by FAS again to take the average over the sunlit area into account. Term K
is fitted to the modeled rate of Reaction ().
Figure shows the modeled reaction rate and the fitted term K.
Fit of term B for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Sum of the vortex-averaged reaction rates modeled by ATLAS for the
reactions ClONO2+hν (both channels), ClO + NO2,
ClONO2+ Cl, ClONO2+ OH and ClONO2+ O (blue) and
the fitted term B (red). Tick marks on the horizontal axis show start of
months.
ClONO2Overview
The equation for ClONO2 is changed from
dClONO2dt=B-A-G-H
in the original model to
dClONO2dt=B-A-H
in the new model. ClONO2 is in a near equilibrium between
ClONO2+hν→Cl+NO3(R10)→ClO+NO2(R11)
and
ClO+NO2+M→ClONO2+M.(R12)
In the new model version, the net effect of these reactions (and of some
additional ClONO2 loss reactions) is described by term B, while in the
original model, there were two different additive terms B and G. The net
change of ClONO2 by the above reactions is responsible for deactivation
of active chlorine in the Northern Hemisphere. Term A accounts again for
the effect of the heterogeneous reaction
HCl+ClONO2→Cl2+HNO3,(R4)
which both activates HCl and ClONO2, while term H accounts for the
effect of the less important heterogeneous reaction
ClONO2+H2O→HOCl+HNO3
which only activates ClONO2.
Term B
Term B represents the net effect of the Reactions ()/()
and (). ClONO2 is
in an equilibrium between Reactions ()/()
and () . Changes in ClONO2 by a shift in
this equilibrium are mainly induced by the production of NOx
(NOx= NO + NO2+ NO3+ 2N2O5). Since
NOx is mainly produced by the comparably slow reactions
HNO3+hν→NO2+OHHNO3+OH→H2O+NO3
these reactions determine the net production of ClONO2. The equilibrium
condition for ClONO2 can be written as
ClONO2∼[ClO]NO2.
Production of NOx will increase NO2. In turn, ClONO2
will increase almost instantly at the expense of NO2 to match the
equilibrium condition again. In this sense, ClONO2 can be considered a
part of NOx, which is mainly partitioned into NO, NO2
and ClONO2. Term B is parameterized by
B=b⋅ClOx⋅HNO3g⋅FAS,
where [HNO3]g denotes HNO3 in the gas phase. This
parameterization is obtained empirically, since it is again difficult to
derive an expression from the chemical equations, and qualitatively takes
into account the properties of the ClONO2 equilibrium. The change of
NOx is parameterized empirically as
[HNO3]g⋅ FAS taking into
account that NOx is produced from HNO3 under sunlit conditions.
Term B is fitted to the sum of the modeled rates of the following reactions
ClO+NO2+M→ClONO2+M(R12)ClONO2+hν→Cl+NO3(R10)ClONO2+hν→ClO+NO2(R11)ClONO2+Cl→Cl2+NO3ClONO2+OH→HOCl+NO3ClONO2+O→ClO+NO3
The main channel of the photolysis reaction is into Cl + NO3
(Reaction ), but the minor channel Reaction () into
ClO + NO2 is also included in the fit. In addition, we include
several reactions of the form ClONO2+X in the fit, where
X is one of Cl, OH or O. Figure
shows the sum of the modeled reaction rates for these reactions and the
fitted term B.
Term A
See explanation in Sect. .
Term H
Term H accounts for the effect of the heterogeneous reaction
ClONO2+H2O→HOCl+HNO3.(R17)
Term H is parameterized by
H=h⋅ClONO2⋅HNO32/3⋅FAP
in the new model. The parameterization for term H remains similar to the
original model. The term max(FAP -y, 0) of the original
model (with y a fitted parameter) is exchanged by
[HNO3]2/3⋅ FAP.
h is fitted to the modeled rate of Reaction (). The fit has
a rather large residuum both with the original and the new parameterization.
This is relativized by the fact that the ClONO2+ H2O reaction
is only of minor importance for chlorine activation and ClONO2 removal.
One of the reasons for the disagreement may be the complicated dependence of
the γ value of the reaction on H2O (see
Fig. ). Figure shows the modeled
reaction rate and the fitted term H.
HNO3
The change in the total amount of HNO3 (i.e., the sum of the gas phase
and the condensed phase) is given by
dHNO3dt=-E
both in the original and in the new model. Changes in HNO3 are dominated
by changes due to denitrification, i.e., the irreversible removal of
HNO3 by sedimenting cloud particles. Term E is parameterized by
E=e⋅HNO3⋅FAPs.
The term max(FAP -y, 0) of the original model is replaced
by FAPs in the new model. The rate of change by sedimenting
particles is assumed to be proportional to the volume of HNO3 condensed
in the cloud particles. For this, it is assumed that the amount of HNO3
that is in the cloud particles is proportional to the total amount of
HNO3 inside the area indicated by FAPs and that the
HNO3 mixing ratio is proportional to the particle volume. Additionally,
it is assumed that there are no cloud particles outside the area indicated by
FAPs. FAPs is chosen because denitrification is
modeled by sedimenting large NAT particles, which form above a given supersaturation.
e is fitted to the modeled change by sedimenting particles plus the modeled
sum of all reactions changing HNO3 (which is small).
Figure shows the modeled rate of change of HNO3 and the
fitted term E.
Normalized pseudo first-order rate coefficient of ClONO2 loss
as a function of H2O mixing ratio for the heterogeneous reaction
ClONO2+ H2O on liquid STS surfaces. Reaction rates were
calculated for T= 190 K, p= 50 hPa, 10 ppb HNO3,
0.15 ppb H2SO4, 2 ppb HCl and 1 ppb
ClONO2.
Fit of term H for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged reaction rate modeled by ATLAS for the reaction
ClONO2+ H2O (blue) and the fitted term H (red). Tick marks
on the horizontal axis show start of months.
The partitioning between HNO3 in the gas phase and in the liquid and
solid phase is calculated by
HNO3g=1-FAPs⋅HNO3+z⋅FAPs⋅HNO3
in the new version. In the original version, FAP was used for
FAPs. z is obtained by a simple linear fit from
Eq. () and the model results for FAPs,
[HNO3] and [HNO3]g.
Fit of term E for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged rate of change of HNO3 by denitrification
modeled by ATLAS plus the rate of change of HNO3 by chemical reactions
modeled by ATLAS (blue) and the fitted term E (red). Tick marks on the
horizontal axis show start of months.
O3
The rate of change of ozone is given by
dO3dt=-D,
where term D is parameterized by
D=d⋅ClOx⋅FAS.
Term D remains unchanged compared to the original model. The
parameterization is based on the fact that the combined effect of the most
important catalytic ozone destruction cycles (the ClO dimer cycle and
ClO–BrO cycle) is a nearly linear dependence of ozone
destruction on ClOx. In addition, the rate of change of
ozone depends on the amount of available sunlight due to the ClO dimer
photolysis reaction.
The ClO dimer cycle alone would lead to a quadratic dependence on ClO
in the sunlit part of the vortex, since the reaction
ClO+ClO+M→Cl2O2+M
is the rate-limiting step in the cycle. That would lead to the
parameterization [ClOx]2FAS, since
[ClO]day∼[ClOx]
(see Fig. ) and ClO is only present in the sunlit part of
the vortex. The nearly linear dependence of the rate of change of ozone on
ClOx for the sum of the effect of both cycles has several reasons: the
dependence of the rate of change of ozone on ClOx caused by the
ClO–BrO cycle, the dependence of the rate of change of ozone
on ClOx caused by the ClO dimer cycle, and the repartitioning of ClO and BrO.
The fact that most of the ClOx that is set free from the heterogeneous
reactions is in the form of Cl2 in early winter and needs to be
photolyzed into Cl can be ignored for the purpose of our model, since
this happens before substantial ozone depletion is observed.
The amount of Bry is not explicitly parameterized in the Polar
SWIFT model. The effect of Bry is implicitly included since the
magnitude of the rate of change of ozone depends on the
ClO–BrO cycle. The ATLAS runs that are used for the fits are
initialized with a maximum Bry of 19.9 ppt. Note that this
means that the Polar SWIFT model implicitly uses the bromine levels that are
given in the ATLAS model runs and that it is not possible to model responses
to changes in bromine with Polar SWIFT.
Term D is fitted to the sum of the modeled reaction rates of all reactions
changing ozone. Figure shows the modeled reaction rates and
the fitted term D. The figure shows that the parameterization works very
well for ozone.
Fit of term D for the Arctic winter 2004/2005, the Antarctic
winter 2006, the Arctic winter 2009/2010 and the Antarctic winter 2011 at
54 hPa. Vortex-averaged rate of change of O3 by all reactions modeled
by ATLAS (blue) and the fitted term D (red). Tick marks on the horizontal
axis show start of months.
Validation
The species mixing ratios simulated by the Polar SWIFT model are compared to
corresponding measurements of the MLS satellite
instrument and to simulations by the full stratospheric scheme of the ATLAS
model for validation. Polar SWIFT is implemented into the ATLAS model for the
validation runs and uses the transport and mixing scheme of the ATLAS model,
while the detailed stratospheric chemistry scheme of the ATLAS model is
replaced by the simplified Polar SWIFT model. Runs are driven by ECMWF ERA
Interim reanalysis data. This approach is needed to obtain results from Polar
SWIFT that can be compared to measured data.
Polar SWIFT is implemented in ATLAS by adding the rate of change of ozone
calculated by Polar SWIFT for a given layer to the ozone value of every air
parcel inside the vortex and inside this layer. Note that this means that the
ozone field does still vary across the vortex. The same is done for the other
species HCl, ClONO2 and HNO3. The vortex means of these species,
which are needed as input at the start of every time step, are obtained by
averaging over all air parcels inside the vortex in the layer. Outside of the
polar vortex, O3, Cly, HCl, HNO3 and ClONO2 are
re-initialized every day with seasonal climatologies. For O3 and
HNO3, a seasonal climatology based on all available MLS data is used
(i.e., which is a function of the month of year, with data from all years
averaged). Cly, ClONO2 and HCl are taken from a seasonal
climatology derived from ATLAS runs with the full chemistry model. While HCl
is available from MLS data, it is not used here so that the sum of HCl and
ClONO2 is consistent with Cly.
Simulations of the Arctic winters 1979/1980–2013/2014 and the Antarctic
winters 1980–2014 are conducted. The simulated interannual variability of
ozone is compared to the observed interannual variability derived from MLS
satellite data for the years 2005 to 2014.
For every winter and hemisphere, a new run is started, which is initialized
with species mixing ratios from the same MLS and ATLAS climatologies that are
used for the re-initialization described above (i.e., the same starting
conditions in every year). Runs start on 1 November and end on 31 March in
the Northern Hemisphere and start on 1 May and end on 30 November in the
Southern Hemisphere. The long-term change in the chlorine loading of the
stratosphere is considered by multiplying the Cly, HCl and
ClONO2 values by a number obtained by dividing the equivalent effective
stratospheric chlorine (EESC; ) of
the given year by the EESC of the year 2000.
Interannual variability of vortex-averaged ozone mixing ratios in
Arctic winter at 46 hPa for Polar SWIFT (blue) and MLS (red), on the last
day before vortex breakup. The date differs for different years due to
different dates of vortex breakup; see
Table .
Interannual variability of vortex-averaged ozone mixing ratios in
Antarctic spring at 46 hPa on 1 October for Polar SWIFT (blue) and MLS
(red).
Figure shows the vortex-averaged mixing ratios
at 46 hPa simulated by Polar SWIFT in the Northern Hemisphere at the
end of the winter compared to the mixing ratios obtained from MLS ozone data.
Note that the date used in the plot differs for every year, since the date of
the breakup of the polar vortex is different in every year. The dates are
given in Table . Figure
shows the same for the Southern Hemisphere and on 1 October. Both the
magnitude and the interannual variability of the MLS measurements are
reproduced well by the Polar SWIFT model runs in the Northern Hemisphere. The
interannual variability is larger and reproduced better in the Northern
Hemisphere than in the Southern Hemisphere.
Time evolution of vortex means of O3 and HCl in the northern
hemispheric winter 2004/2005 for Polar SWIFT driven by the ATLAS transport
model (a), for the full chemistry model of ATLAS driven by the ATLAS
transport model (b) and for MLS satellite measurements (c).
The black line marks the approximate breakup date of the
vortex.
Same as Fig. for the southern
hemispheric winter 2006.
Figure shows the time evolution of vortex
averages of O3 and HCl for the winter 2004/2005 in the Northern
Hemisphere. The first column shows the results of the Polar SWIFT model run
driven by ATLAS and ERA Interim, the second column the results of the full
chemistry model run of ATLAS and the third column the corresponding
measurements of MLS. Figure shows the same for
the year 2006 and the Southern Hemisphere. The time evolution of ozone is
reproduced well in both hemispheres. Since the long-term ozone climatology
used for the initialization of Polar SWIFT is different from the actual
measured values, some differences show up in early winter. The evolution of
HCl shows some differences, which are partly caused by the fact that the full
ATLAS model has a parameterization that partitions a significant part of HCl
into the liquid phase to overcome a discrepancy between modeled and measured
HCl values (for a detailed discussion, see ). Polar SWIFT is always fitted to the total HCl mixing
ratios of ATLAS and has no parameterization for HCl in the liquid phase. MLS
measures HCl in the gas phase, and consequently, the figures for the full
chemistry model and MLS show HCl in the gas phase. Hence, some differences
between the total HCl values of Polar SWIFT and the gas phase values of MLS
are observed. This is however of secondary importance, since the only
variable of Polar SWIFT that is used outside of Polar SWIFT in a GCM is ozone.
Conclusions
This study presents an update of the Polar SWIFT model for fast calculation
of stratospheric ozone depletion in polar winter. The update includes a
revised formulation of the system of differential equations, a new training
method based on model results of the ATLAS Chemistry and Transport Model and
an extension from a single level to the vertical range in which polar ozone
depletion is observed.
The model is validated by comparison to MLS satellite data and the full
stratospheric chemistry scheme of the ATLAS model. It is shown that Polar
SWIFT is able to successfully simulate the interannual variability and the
seasonal change of ozone mixing ratios in the Northern and Southern
hemispheres (Figs. to ).
Polar SWIFT was specifically developed to enable interactions between climate
and the ozone layer in climate models. So far, climate models often use
prescribed ozone fields, since a detailed calculation of ozone chemistry is
computationally very expensive. The computational effort needed is
significantly reduced when using the Polar SWIFT model. The computing time
for a complete winter simulated by Polar SWIFT is on the order of a fraction
of a second on a single processor core, while the computational effort for
the detailed chemistry model of ATLAS is on the order of several days per
winter on 50 cores on current machines.
Polar SWIFT models the response of ozone to temperature changes and changes
in the chlorine loading well, since care has been taken to represent the
underlying chemical and physical processes in the model equations. This is
also shown in Figs. to .
As far as possible, the equations are derived by
mathematical derivation, but note that some model equations are derived by
empirically finding parameterizations that closely fit the training data set,
since no closed equation can be derived for them. Bromine, methane, water
vapor and some effects of HNO3 are not variable in the model equations,
which limits the ability of the model to respond to changes in these species
and should be kept in mind.
The source code is available on the AWIForge repository
(https://swrepo1.awi.de/). Access to the repository is granted on
request under the given correspondence address. If required, the authors will
give support for the implementation of SWIFT.
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the BMBF under the FAST-O3 project in the MiKliP
framework programme (FKZ 01LP1137A) and in the MiKliP II programme
(FKZ 01LP1517E). This research has received funding from the European
Community's Seventh Framework Programme (FP7/2007–2013) under grant
agreement no. 603557 (StratoClim). We thank ECMWF for providing reanalysis
data.
The article processing charges for this open-access publication
were covered by a Research Centre of the Helmholtz
Association.
Edited by: Fiona O'Connor
Reviewed by: two anonymous referees
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