Abstract. A global three-dimensional model of the dynamics (0-150 km) and chemistry (30-150 km) of the middle atmosphere has been developed and applied to the problem of the water vapor distribution of the mesosphere-mesopause region. The mesopause region is one of the most intricate domains of the atmosphere and requires an extraordinarily careful modeling. In order to interpret the specific feature of the water vapor distribution, particular attention was paid to the problem of the effective characteristic chemical time and the comparison of this time with the characteristic transport time. The results confirm measurements which show highest concentrations during the summer months and lowest concentrations in the winter in the middle and high latitudes of the mesosphere because of the seasonal variation of the vertical wind system and change in the altitudes of levels of constant pressure as a result of a mean cooling or warming below the respective heights. The equator region is marked by a semiannual variation with maxima around the equinoxes. By way of contrast, the Halogen Occultation Experiment (HALOE) measurements show maxima around solstices, however, with some exceptions for which a maximum also occurs during the equinox. This points to a dynamically sensitive equatorial region. At high latitudes the extremely low temperatures within the mesopause region in summer and the relatively high water vapor concentrations cause a supersaturation of water vapor which is the condition for the formation of noctilucent clouds (NLCs). The calculated seasonal and latitudinal border of the domain of supersaturation corresponds quite well with the mean areas of the occurrence of NLCs; however, it is impossible to model specific events on the basis of such a coarse model as the occurrence of NLCs at middle latitudes. There is no direct hemispheric transport from the summer to the winter hemisphere within the mesosphere-lower thermosphere, but the meridional transport occurs in a more complicated manner. IntroductionMost of the middle atmosphere models include the mesosphere as an upper model domain and exclude the sensitive mesopause region as done, e.g., in the model of Rasch et al. [1995]. The aim of these models is the investigation of the stratosphere containing the ozone layer, but the insight that the mesopause region is also important for both dynamical and chemical processes of the stratosphere is receiving growing acceptance. A large number of models are two dimensional

[1] The strong turbulent mixing within the middle atmosphere should prevent a change of the value of the total hydrogen mixing ratio regardless of the transformation of the hydrogen species in one another within the homosphere. Within the heterosphere the corresponding mixing ratio should drastically increase due to the action of molecular diffusion. We show on the basis of a global three-dimensional model of the dynamics and chemistry of the middle atmosphere that the total mixing ratio decreases under certain conditions when an (escape) flux flows through the domain. The effect occurs particularly above and around the mesopause region, reaching a distinct minimum at about 108 km height. Hunten and Strobel [1974] also found a small reduction on the basis of simplified model calculations, but they stated that the total mixing ratio of hydrogen atoms remains nearly the same. The cause of this apparently paradoxical behavior lies in the fact that under the condition of a hydrogen flux just in the domain of transition of predominant turbulent diffusion to predominant molecular diffusion, a strong decomposition of the heavier H 2 O component into the light hydrogen constituents takes place, and the temperature increases strongly with height above the mesopause. The marked reduction of the total hydrogen mixing ratio within the extended mesopause region is extremely important for all questions concerning the physics and chemistry of this domain.

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