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Model and experiments

The GCM used in this study is a global spectral model developed at our laboratory, with triangle truncation at wavenumber 21 in the horizontal direction and 37 vertical layers extending from the surface to about 83 km. The GCM includes realistic topography and has a full set of physical processes, such as the boundary layer, hydrology, dry and moist convection, and radiative processes. Reyleigh friction and gravity wave drag parameterization are introduced for the zonal momentum equation to represent the drag force due to unresolved motions. The ozone mixing ratio is calculated for the region up to about 55 km on the basis of a parameterized Chapman cycle proposed by Hartmann [1978], in which the catalytic destruction of ozone due to HOx and NOx is parameterized through the tuning of reaction coefficients, whereas the ratio above that level is prescribed by climatological values. The ozone destruction near the surface is expressed by introducing a suitable deposition velocity around 1 km altitude. Hence, the ozone field is coupled interactively with the radiative and dynamical fields in the GCM. For details, see Miyahara et al. [1995].

In order to simulate the ozone depletion, a parameterized loss term is added in the continuity equation for the ozone mixing ratio. The loss term is switched on for the region between 120 and 16 hPa, when three conditions are met, i.e., a noontime zenith angle less than 85o, a temperature lower than 198 K, and a latitude higher than 54o. We performed here an 'ozone depletion experiment' including the loss term over successive 40 years, along with a 'control experiment' without the loss term over successive 20 years.