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2. Description of model and experiment performed

The Met. Office Unified Model (UM) (Cullen, 1993) is here used in extended form with 64 levels (upper boundary 0.01 hPa, Figure 1) and with a

Figure: Location of the model levels for the 49 and 64 level versions of the UM. For clarity, only every 3rd level is shown, starting at level 1.
\begin{figure} \vspace{8cm} \special{psfile=/home/austin/umfigs/umlevels.ps voffset=-260 hoffset=10 hscale=70 vscale=70} \end{figure}

latitude-longitude resolution of 2.5$^{\circ }$ by 3.75$^{\circ }$. The model contains a complete range of stratospheric chemical reactions allowing representation of all the main ozone formation and destruction processes. The chemistry is also coupled to the model radiation scheme. By resolving the whole of the stratosphere and most of the mesosphere the model is able to simulate possible dynamical feedbacks, such as changes in global mean meridional circulation, important for ozone transport. Running on our local massively parallel Cray T3E, the model takes about 40 hours per model year using 36 processors. The model has undergone many improvements (see Austin, 2000) since the earlier version of (Austin et al., 2000a,b).

Table 1. Long-lived tracer amounts used in the model simulations.
Year (1 Jan) 1980 1985 1990 1995 2000 Notes
Cl$_y$ (ppb) 1.63 2.21 2.72 3.26 3.53 1
Br$_y$ (ppt) 9.4 10.1 11.5 13.6 15.5 1
Effective halogen (ppb) 2.10 2.72 3.30 3.94 4.31 1,2
NO$_y$ (ppb) 18.4 18.6 18.8 19.1 19.4 3
CFC11 (ppt) 173 222 263 291 289 4
CFC12 (ppt) 295 382 477 532 545 4
N$_2$O (ppb) 302 306 310 314 319 4
CH$_4$ (ppb) 1603 1651 1700 1749 1810 4
CO$_2$ (ppm) 337 346 355 365 375 4
Notes: 1. Values are for the equator at the top model level. 2. The effective halogen is determined by Cl$_y$ + 50.0 $\times$ Br$_y$, and represents crudely the net impact of halogens on ozone depletion if all the Bromine were in the form of Chlorine. 3. The peak value is given. 4. The values for the greenhouse gases are for the troposphere. The model was initialised on 1 March 1979 as in Austin et al. (2000a) and a simulation of 20 years 10 months was completed with specified sea surface temperatures and sea ice from the Atmospheric Model Intercomparison Project (AMIP) (Gates et al., 1999). Concentrations of the key molecules and greenhouse gases (Houghton et al., 1996, Table 2.5 and Figure 2.3) are indicated in Table 1 for the period 1980-2000.

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