A three-dimensional mechanistic model of the middle atmosphere is used to model various classes of equatorial wave motions that are observed in the atmosphere. These waves are thought to be largely responsible for the forcing of the quasi-biennial oscillation (QBO) in the tropical lower stratosphere. By generating a combination of different classes of equatorial waves in the model, an oscillation which has many similarities to the observed QBO is produced in the model. The numerical model used is run in a variety of configurations, including running it at different vertical resolutions and with two different radiation parameterisation schemes. It is found that model used in the project must be modified to allow the accurate modelling of equatorial waves. Several modelling problems are encountered while applying the modifications necessary in the model; the steps necessary to rectify these problems are detailed in this thesis. Equatorial waves are then forced in this modified model under a range of conditions and their interaction with the mean flow is observed. Their dissipation mechanisms and the influence of changes in model conditions on these waves are investigated. The model is found to be generally very successful in modelling these equatorial waves. Modelling of the QBO is one of the principle aims of this project and a QBO is successfully generated in a variety of model configurations. The modelled QBO is found to be sensitive to changes in the temperature structure of the model (brought about by changes in the model's radiation scheme) and several experiments are performed in order to learn what processes affect this sensitivity. A QBO is then generated in series of model runs where the state of the model is varied from very idealised (where temperatures in the model are relaxed towards an isothermal state by the radiation scheme) to a state that is far more realistic (a perpetual January run with realistic boundary information). A fairly realistic QBO is generated throughout many of the experiments. The properties of this QBO are investigated and compared to the observed QBO. The model is then run with planetary waves forced in addition to the QBO. The interaction between the planetary waves and the QBO is investigated. It is found that the planetary waves have little effect on the QBO propagation. The QBO however has a fairly strong modulating effect on the planetary waves in certain regions.

The resonance of stationary waves in a mid-latitude (beta)-plane model is examined. In a series of linear experiments it is found that a large, quasi-resonant response occurs even in the presence of realistic damping. With forcing of reasonable amplitude the resonant growth rates are found to be such that a period of several weeks is required for the resonant mode to achieve large amplitude in the troposphere. When nonlinear effects are included the growing resonant wave is found to cause a stratospheric warming and zonal wind reversal and thus drives the system off resonance. We therefore examine flow configurations for which the nonlinear terms vanish. It is found that these flow configurations can resemble realistic blocking patterns. When diabatically forced, the resonant growth of these patterns does not give rise to any nonlinear interactions, although such interactions do occur if other models, which do not satisfy the non-interaction conditions, are present. We conclude that resonant growth can lead to the establishment of large-amplitude, stationary, long waves even when realistic damping and non-linear effects are operating.

No description available.

No description available.

No description available.

No description available.

No description available.