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Model description and experiments

Model experiments have been performed with the KASIMA model, see Kouker et al. (1999). For the present studies, an updated model version was applied allowing for a vertical resolution that is variable with height. The model extends from 10 km to 120 km with 63 layers. From the lower boundary up to 25 km, the vertical resolution is 750 m. From 25 km up to the upper boundary the vertical spacing between the levels gradually reaches 3.8 km. The triangular truncation T21 corresponds to a horizontal resolution of about . At the lower boundary, the geopotential and temperature fields are given by the analyses of the European Centre of Medium range Weather Forecast (ECMWF) and the model is relaxed to the analyzed temperature, divergence and vorticity using a forcing term with a time scale of 4 h below 10 hPa. Above 10 mb, the primitive equations are integrated without additional forcing.

The trend of is described by the function of Volk et al. (1997) (their equation (15)) and their expression for the mean age is used. The mixing ratio is set according to the trend function within an equatorial wide belt extending up to 100 hPa, thus simulating the intrusion into the tropical pipe (Plumb, 1996). As a rough initialization a linear profile between 15 and 35 km ranging from 0 to 7 years is assumed.

At first, the model has been run for 5 years using repetitive ECMWF analyses of the year 1990 to establish stationary conditions. After that, the model was run using the ECMWF analyses of the years 1990 - 1997. Hence, the model results include the appropriate meteorological conditions for comparison with observations of different authors (Harnisch et al., 1996; Strunk et al., 2000; Patra et al., 1997; Moore, 1999). All eight scenarios for chemistry were run with the model and in addition a pure inert tracer was transported.

Chemistry of in the middle atmosphere