Abstract. The Extrapolar SWIFT model is a fast ozone chemistry scheme for interactive
calculation of the extrapolar stratospheric ozone layer in coupled general
circulation models (GCMs). In contrast to the widely used prescribed ozone,
the SWIFT ozone layer interacts with the model dynamics and can respond to
atmospheric variability or climatological trends. The Extrapolar SWIFT model employs a repro-modelling approach, in which
algebraic functions are used to approximate the numerical output of a full
stratospheric chemistry and transport model (ATLAS). The full model solves a
coupled chemical differential equation system with 55 initial and boundary
conditions (mixing ratio of various chemical species and atmospheric
parameters). Hence the rate of change of ozone over 24 h is a function of 55
variables. Using covariances between these variables, we can find linear
combinations in order to reduce the parameter space to the following nine
basic variables: latitude, pressure altitude, temperature, overhead
ozone column and the mixing ratio of ozone and of the ozone-depleting families
(Cly, Bry, NOy and HOy). We will show that these nine variables are
sufficient to characterize the rate of change of ozone. An automated
procedure fits a polynomial function of fourth degree to the rate of change
of ozone obtained from several simulations with the ATLAS model. One
polynomial function is determined per month, which yields the rate of change
of ozone over 24 h. A key aspect for the robustness of the Extrapolar SWIFT
model is to include a wide range of stratospheric variability in the
numerical output of the ATLAS model, also covering atmospheric states that
will occur in a future climate (e.g. temperature and meridional circulation
changes or reduction of stratospheric chlorine loading). For validation purposes, the Extrapolar SWIFT model has been integrated into
the ATLAS model, replacing the full stratospheric chemistry scheme.
Simulations with SWIFT in ATLAS have proven that the systematic error is
small and does not accumulate during the course of a simulation. In the
context of a 10-year simulation, the ozone layer simulated by SWIFT shows a
stable annual cycle, with inter-annual variations comparable to the ATLAS
model. The application of Extrapolar SWIFT requires the evaluation of
polynomial functions with 30–100 terms. Computers can currently calculate
such polynomial functions at thousands of model grid points in seconds. SWIFT
provides the desired numerical efficiency and computes the ozone layer 104
times faster than the chemistry scheme in the ATLAS CTM.