Observations of Gravity Waves at Atmospheric Fronts

Available from UMI in association with The British Library. An observational study of pressure perturbations associated with the passage of atmospheric fronts over the British Isles using a triangular array of sensitive microbarographs reveals the preponderance of gravity wave activities in the vicinity of the surface cold front (SCF). Examination of the time series of these pressure perturbations in the frequency domain shows an enhancement for frequencies less than the local buoyancy frequency N after the passage of the SCF. The spectral analysis also shows two predominant frequency peaks usually located near N and N/2 s ^{-1}. Isolating these frequencies shows that there is a systematic amplitude modulation with an amplification near the SCF and at a region 2-3 hours before and after the SCF passage. The cross-correlation analysis reveals that the gravity waves in the post SCF region propagate towards the SCF. As these waves approach the SCF, the across front component of the phase speed decreases and the direction of propagation of the wave rotates in an anticlockwise manner. It is found that a consistent description of the gravity waves can only be made if first the waves are assumed to be ducted, i.e. there is a reflecting layer aloft, and second that as these waves propagate through the frontal environment, due to the inhomogeneity, they are refracted. A number of conceptual models are then developed to account for the observed wave behaviour in a frontal region. In this investigation it is shown that the stable layer associated with the frontal zone can form a good upper reflector for non-hydrostatic gravity waves. It is also argued that the slope of this layer plays an important role in the refraction of the observed gravity waves. A model of wave propagation in a wedge is then used to account for this slope. This model however predicts a clockwise rotation of the direction of propagation as the wave propagates toward the SCF. This rotation is the opposite to that observed. In order to account for this, the meridional shear at the SCF must also be taken into consideration. In the non-uniform environment of the front there is not only wave refraction but there is also the interaction between the wave and the frontal environment. Based on the energy flux equation it is argued that in order to account for the observed increase in the rate of energy change at the SCF, the interaction terms cannot be neglected. It is suggested that the wave reflection and refraction would be able to confine the gravity wave energy to the SCF vicinity. The interaction between the wave and the non-uniform frontal environment could play an important role in the dynamic of the SCF itself and should be investigated.