Lamb waves originating in nongeostrophic disturbances: A case study

Abstract

On July 1, 1973, large amplitude atmospheric waves occurred simultaneously in the troposphere and ionosphere over the southern part of Africa. The waves were observed at eight stations on a ground level pressure record, on vertical incidence ionograms, and on Faraday rotation records. Both a Fourier and a band filter analysis show significant wave amplitudes in two narrow period bands around 85 and 65 min, respectively. The waves were excited at low heights near the south coast of Africa and propagated toward the equator with horizontal phase trace speeds of the order of the lower atmosphere sound speed. An analysis of surface weather maps indicates that there was a close spatial and temporal coincidence of the observed waves and nongeostrophic winds, confirming the hypothesis that atmospheric waves may be generated during geostrophic adjustment processes. Wave calculations in a realistic model atmosphere over a rigid surface show that the observed wave properties can be best explained in terms of the Lamb mode. An energy estimate confirms that the nongeostrophic disturbance could in fact provide the energy necessary to explain the amplitudes and the period of occurrence of the observed waves.