The atmosphere is a continuous, layered medium with vertical gradients in density caused by gravity forces. When a volume of gas is displaced from its equilibrium position it starts oscillating around its normal position. This movement generates waves in the fluid known as internal, gravity or bucancy waves. Gravity waves can be generated by wind blowing over mountains, strong windshears and by vertical currents in the atmosphere.
The internal waves are influenced by the vertical velocity distribution of the background winds. The wind causes a Dopplershift in the frequency of the wave. Dependent of this frequency shift the wave will be deflected, reflected, absorbed or amplified. The level at wich the Dopplershift equals the wave frequency is called the critical level. At this level the wind-velocity is equal to the horizontal phasevelocity of the wave. The behaviour of the wave at this level depends strongly on the windgradient.
The motion of the air near the critical level is modelled by the Taylor-Goldstein enuation. Usually the influence of viscosity and thermal conductance on the air movements is small, at the critical level however these effects cause important changes in the behaviour of the waves.
The first chapter of this report gives a short survey of the properties of the atmosphere and the various kinds of waves in the atmosphere.
The second chapter gives a detailed analysis of the motion of a gravity-wave in an atmosphere with a vertical windgradient. This analysis is based upon the dispersionrelation of the waves.
In the third chapter the behaviour around the critical level is treated.
Approximate and exact solutions of the Taylor-Goldstein eq. are derived for various windprofiles.
In chapter four the influence of viscosity and thermal conductivity around a critical level is studied.
The last chapter gives some notes on the observation of the waves in the ionosphere by means of the radiosignals of a navigationsatellite.
E.R. Stolp. Voortplanting van zwaartegolven in de atmosfeer en rond een kritieke laag
KNMI number: WR-85-01, Year: 1985, Pages: 80