Evolution of the spectra of secondary acoustic-gravitational waves near critical levels in the atmosphere

      A high-resolution nonlinear numerical model is used to simulate the propagation of acoustic gravity waves (AGWs) from the troposphere to the upper atmosphere. This simulation takes into account background wind profiles containing critical levels at which the horizontal wind velocity becomes equal to the horizontal phase velocity of the AGW. According to traditional linear theories of atmospheric waves, near critical levels, the vertical wavelength approaches zero, which leads to a strong dissipation of AGWs propagating from the troposphere and does not allow them to reach the upper atmosphere. Our numerical simulation is carried out using wave sources in the form of vertical velocity perturbations propagating along the Earth's surface. Jet streams in the atmosphere are approximated by Gaussian profiles of the average zonal wind with maxima at altitudes of 110 km and 50 km.

     Calculations show that the AGW amplitudes decrease significantly above the altitude critical levels. For critical levels at altitudes from 30 to 70 km, part of the wave energy can penetrate through them and spread further into the upper atmosphere. In the nonlinear model, there is an increased formation of secondary wave modes near the critical level. The analysis of the spectra of wave fields near critical levels and at a distance from them is performed. It has been found that the instability of waves near critical levels intensifies the energy transfer from primary AGWs propagating from surface sources. This increases the height of spectral peaks at wavelengths of 1/2 and 1/3 of the horizontal length of the primary AGW. Therefore, at altitudes of more than 100 km, modes with shorter horizontal wavelengths than the wavelength of the primary AGW prevail, and the amplitudes of these secondary waves can exceed the amplitudes of the primary AGW in the absence of critical levels in the middle atmosphere.