Critical level influence on spectra of secondary gravity waves in the middle and upper atmosphere

A high-resolution nonlinear numerical model is used to simulate propagation of internal gravity waves (IGWs) 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 speed of IGW. According to traditional linear theories of atmospheric waves, near critical levels the vertical wavelength approaches zero, which should lead to a strong dissipation of IGWs propagating from the troposphere and may significantly decrease their ampli-tudes in the upper atmosphere. The wave sources in the model are defined as vertical velocity perturbations propagating along the Earth surface. The mean horizontal wind in the atmosphere is approximated by the Gaussian profile with a maximum at an altitude of 50 km. We analyze the spectra of wave fields near criti-cal levels and at a distance from them. It has been found that the instability of waves near critical levels intensifies the energy transition from primary IGWs propagating from surface sources to secondary wave modes. This causes an increase in spectral peaks at wavelengths shorter than the horizontal length of primary IGW. Therefore, above critical levels, spectral modes with shorter horizontal wavelengths begin to prevail with increasing altitude, and the amplitudes of these secondary waves at the same altitudes can exceed the amplitudes of analogous primary IGW propagating in the absence of critical levels in the middle atmosphere.