An existing time-dependent 3-dimensional numerical chemical transport model (CTM) was improved and coupled with two different versions of a dynamical model. All physical processes believed to be important are simulated, including chemical interactions, photochemical dissociation, eddy and molecular diffusion, and advection. The most essential improvements concern the implementation of a new transport scheme marked by almost zero numerical diffusion and the derivation of the solar Lyman-α flux from the sunspot number as proxy and its consideration in the photolysis rate of water vapor. The CTM was coupled with the dynamical models calculating climatologic means (COMMA-IAP) and computing the dynamical state for real dates (LIMA). These coupled models were applied to study some particular phenomena in the mesosphere-lower thermosphere (MLT-region) such as the influence of the variable Lyman-α radiation on the aeronomy, the autocatalytic water vapor production as a source of large mixing ratios within the middle to upper mesosphere in high summery latitudes during, the so-called tertiary ozone maximum formation, the investigation of nonlinear effects of the chemistry, trends of mesospheric minor constituents due to the increase of methane, nitrous oxide and carbon dioxide since the pre-industrial area.
The autocatalytic water vapor production makes clear why large water vapor mixing ratios occur in the upper mesosphere in high-latitude summer under conditions of strongest solar insolation where photolysis should effectively reduce its value. The analysis of this finding by means of the 3-dimensional model supplies evidence that water vapor is autocatalytically formed below a crossover height of about 65 km, upward transported by an accelerated vertical wind in summer and destroyed by photolysis above this altitude. The global dynamics explain the existence of a double maximum of the water vapor mixing ratio in the mesosphere in summer.
The influence of the solar cycle on the chemical composition was studied. Likewise the spatio-temporal behavior of the ozone mixing ratio in the upper mesosphere/mesopause region under nearly polar night conditions was investigated and compared with measurements at Lindau (51.66° N, 10.13° E) and at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR, 69.29° N, 16.03° E). The most marked features of the modeling results are a pronounced nighttime ozone maximum around 72 km occurring close to the polar night terminator and a strong drop of the mixing ratio above approximately 80 km. The reason of these findings was analyzed. The so-called photochemical Doppler-effect has an essential influence particular on the nighttime ozone mixing ratio.
Nonlinear effects such as period-doubling were found in the model under the condition of small integration time steps. Their spatio-temporal patterns have been investigated and their impact on the chemical heating rate was analyzed.
Trend calculations revealed an increase of the middle atmospheric humidity but a decrease of ozone. The change of the composition depends on altitude, latitude, season and local time. Also the hydroxyl concentration (OH) grew since the pre-industrial time. A sub-model of relaxation of excited hydroxyl was developed that could be applied for future studies. Sudden stratospheric warming events reproduced by the model have a strong influence on the dynamical parameters temperature and wind components which impact considerably the distribution of all chemical constituents. In this connection the change of the chemical heating rate was calculated.