Tunneling of acoustic-gravity waves through critical levels to the upper atmosphere

Abstract

Using a high-resolution nonlinear numerical model, simulations are performed to study the propagation of acoustic-gravity waves (AGWs) from the troposphere into the upper atmosphere. These simulations take into account background wind profiles containing critical levels, where the horizontal wind velocity becomes equal to the horizontal AGW phase speed. According to conventional linear theories of atmospheric waves, the vertical wavelength approaches zero near critical levels, resulting in strong dissipation of AGWs propagating from the troposphere and preventing them from reaching the upper atmosphere. Our numerical simulations are carried out using wave sources in the form of plain wave perturbations of vertical velocity, propagating along the Earth’s surface. Jet streams in the atmosphere are approximated by Gaussian profiles of the mean zonal wind with maxima located at altitudes of 110 km and 50 km. Calculations reveal that AGW amplitudes are significantly reduced above the high-altitude critical levels. For the critical levels at altitudes 30–70 km, part of wave energy can penetrate through them and propagate further into the upper atmosphere. In the nonlinear model, increased generation of secondary wave modes occur near the critical level. Therefore, modes with vertical wavelengths longer than that of the primary AGW dominate at altitudes exceeding 130 km, where amplitudes of these secondary waves may surpass the amplitudes of the primary AGW in the absence of middle atmosphere critical levels.