Gravity wave ducting in the upper mesosphere and lower thermosphere duct system

We report on a numerical study of gravity wave propagation in a pair of ducts located in a region where dramatic changes in the airglow most likely associated with ducted wave trains are observed. We examine ducting in an upper mesosphere inversion (INV) and an always present lower thermosphere stable layer (LTD) for a range of phase speeds and horizontal wavelengths characteristic of ducting events. We analyze the propagation and modal structure of ducted waves for backgrounds with increasing realism, starting with a climatological temperature profile where only the LTD is present. In succession, we add the INV based on the work of Smith et al. (2003), climatological winds, and winds in the upper mesosphere based on the work of Smith et al. (2003). We examine ducting for phase speeds between 40 and 100 m s−1 and horizontal wavelengths between 20 and 60 km. We find that without winds, only the LTD supports ducting of waves forced from below. When observed winds and temperatures are included, strong ducting is evident in both regions. For waves forced from below, the strongest ducted modes are those with slower phase speeds, and of these the third gravest agree reasonably well with the observed phase speeds and wavelengths, indicating that the observations are consistent with linear ducted waves. For waves forced in the INV, we find an intense and strongly dominant fundamental mode. This is a fast mode having phase speeds ∼100 m s−1 for a horizontal wavelength of 30 km in the INV and much faster in the LTD. That the fundamental is not seen in Smith et al.'s (2003) observations indicates that the waves were forced from below and that the lowest mode was blocked by an evanescent barrier below the INV. Our results show that the two ducts communicate: the upward extensions of waves ducted in the INV are seen in the LTD. This is particularly significant in the case of in situ forcing, where the fundamentals combine to give amplification exceeding a factor of 10 in the LTD.