Excitation of ducted gravity waves in the lower thermosphere by tropospheric sources

Short-period, small-scale gravity waves are frequently observed in nighttime airglow imaging experiments. These waves are often found to be ducted and may be confined to a thin region of altitude in the mesosphere or lower thermosphere. An apparent paradox of high-altitude ducted waves is the nature of the source; it is necessary that a ducted wave be excited in situ or have been able to tunnel into the duct from another atmospheric region. In this paper, analytical and numerical solutions are presented for simple thermally ducted gravity waves that are Doppler-shifted by constant background winds. Using a continuous analytical model, duct dispersion properties are calculated for three case studies. Using a fully nonlinear numerical model, several scenarios are explored by which a tropospheric source can excite these thermally ducted wave modes. First, we validate the analytical and numerical models for the classical case of linear wave tunneling. Second, we examine the nonlinear excitation of ducted waves due to resonant wave self-interactions associated with realistic propagation and small-scale wavebreaking, for propagation in the same direction as the wind flow. Third, we consider the case of ducted wave excitation and propagation opposite to the direction to wind flow. Specifically, where horizontal group and phase velocities exhibit opposite sign in the ground-relative frame. The results suggest that ducted waves of very short period can be excited in the lower thermosphere by tropospheric sources, via simple linear and nonlinear processes. These excitation mechanisms are likely to be robust for a range of realistic thermal and thermal-Doppler ducts.