Studies of Internal Gravity Waves in the Stably Stratified Troposphere.

Available from UMI in association with The British Library. Requires signed TDF. In 1986, the British Antarctic Survey installed the State Antarctic Boundary Layer Experiment at Halley Base. The purpose of the experiment was to investigate the structure of the nocturnal boundary layer. It was found that during periods of stable stratification, wavelike phenomena were always detected in the surface layer. The main objective of the work presented in this thesis is to determine the characteristics of these waves and to identify possible mechanisms for their generation. A fully nonlinear, two-dimensional finite difference Navier-Stokes model has been developed to model the lower troposphere at Halley Base and to study the behaviour of Kelvin-Helmholtz type instabilities. The model is used to investigate the radiation of shear-induced internal gravity waves away from the level at which they are generated. It is deduced that the short bursts of large amplitude waves may be caused by regions of shear instability above the surface layer. Statistical methods are used to analyse data from meteorological instruments mounted on a triangular array of masts in order to determine some of the properties of the waves. The dispersion relation of the waves is deduced and their direction of propagation is related to local topographic features. The results indicate that the smaller amplitude waves are generated by topographic forcing. Theoretical methods are used to derive the cross -spectral phases associated with three different types of internal gravity waves. The results obtained are used to identify which types of waves tend to be dominant in the surface layer at Halley Base. It is found that when the surface layer is strongly stably stratified that the waves present are trapped, neutral modes and when the surface layer is only weakly stably stratified, locally generated instabilities dominate. In order to study the effect of the waves on the mean flow, data from slow ascent radiosondes released in the vicinity of Halley Base are analysed. It is shown that the waves have a significant influence on the mean flow.