Global three-dimensional modeling of the water vapor concentration of the mesosphere-mesopause region and implications with respect to the noctilucent cloud region

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 p roblem 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 th e specific feature of the water vapor distribution, particular attention wa s paid to the problem of the effective characteristic chemical time and the comparison of this time with the characteristic transport time. The result s confirm measurements which show highest concentrations during the summer months and lowest concentrations in the winter in the middle and high latit udes of the mesosphere because of the seasonal variation of the vertical wi nd system and change in the altitudes of levels of constant pressure as a r esult of a mean cooling or warming below the respective heights. The equato r region is marked by a semiannual variation with maxima around the equinox es. By way of contrast, the Halogen Occultation Experiment (HALOE) measurem ents show maxima around solstices, however, with some exceptions for which a maximum also occurs during the equinox. This points to a dynamically sens itive equatorial region. At high latitudes the extremely low temperatures w ithin the mesopause region in summer and the relatively high water vapor co ncentrations 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 t o model specific events on the basis of such a coarse model as the occurren ce of NLCs at middle latitudes. There is no direct hemispheric transport fr om the summer to the winter hemisphere within the mesosphere-lower thermosp here, but the meridional transport occurs in a more complicated manner.