DEVELOPMENT OF A CHEMISTRY MODULE FOR GCMS - FIRST RESULTS OF A MULTIANNUAL INTEGRATION

The comprehensive chemistry module CHEM has been developed for applica tion in general circulation models (GCMs) describing tropospheric and stratospheric chemistry, including photochemical reactions and heterog eneous reactions on sulphate aerosols and polar stratospheric clouds. It has been coupled to the spectral atmospheric GCM ECHAM3. The model configuration used in the current study has been run in an ''off-line' ' mode, i.e. the calculated chemical species do not affect the radiati ve forcing of the dynamic fields. First results of a 15-year model int egration indicate that the model ECHAM3/CHEM runs are numerically effi cient and stable, i.e. that no model drift can be detected in dynamic and chemical parameters. The model reproduces the main features regard ing ozone, in particular intra-and interannual variability. The ozone columns are somewhat higher than observed (approximately 10%), while t he amplitude of the annual cycle is in agreement with observations. A comparison with HALOE data reveals, however, a serious model deficienc y regarding lower-stratosphere dynamics at high latitudes. Contrary to what is concluded by observations, the lower stratosphere is characte rized by slight upward motions in the polar regions, so that some of t he mentioned good agreements must be considered as fortuitous. Neverth eless, ECHAM3/CHEM well describes the chemical processes leading to oz one reduction. It has been shown that the mean fraction of the norther n hemisphere, which is covered by polar stratospheric clouds (PSCs) as well as the temporal appearance of PSCs in the model, is in fair agre ement with observations. The model results show an activation of chlor ine inside the polar vortex which is stronger in the southern than in the northern winter hemisphere, yielding an ozone hole over the Antarc tic; this hole, however, is also caused to a substantial degree by the dynamics. Interhemispheric differences concerning reformation of chlo rine reservoir species HCl and ClONO2 in spring have also been well re produced by the model.