Modelling of LargeScale Unstable Waves in the Middle Atmosphere.
Abstract
Available from UMI in association with The British Library. Requires signed TDF. Observations of the zonal mean state of the middle atmosphere suggest that it may be unstable, in certain regions and certain seasons, to largescale wavelike disturbances of appropriate zonal wavenumbers and phase speeds. Possible candidates for such waves have also been observed, including a 4day wave in the winter stratosphere and a 2day wave in the summer mesosphere. In this thesis we attempt to model the observed waves, extending the linear calculations of earlier authors to look at the nonlinear evolution. Some of the mechanisms which may limit the growth of unstable waves are elucidated. We also investigate the effect of the unstable waves on the zonal mean circulation, and the competition between this wave feedback and thermal and frictional forcing of the mean circulation. In order to address these questions some analytical models are developed, including a weakly nonlinear model, and also a numerical model which uses a fully threedimensional spectral representation of the stream function to integrate the quasigeostrophic potential vorticity equation in spherical geometry. A systematic derivation of the quasigeostrophic potential vorticity equation in spherical geometry is given, along with several results which are extensions of earlier theorems in the field of hydrodynamic stability or analogues of results in primitive equation theory or quasigeostrophic betaplane theory. The numerical model reproduces the wavenumbers and phase speeds of the 4day wave and the 2day wave, but not all of the details of the wave structures agree with the observations. In the case of the 4day wave, it is suggested that the observed warm pool may, in fact, have an origin other than instability of the type modelled here.
 Publication:

Ph.D. Thesis
 Pub Date:
 1988
 Bibcode:
 1988PhDT........94T
 Keywords:

 Physics: Atmospheric Science