Mesospheric Momentum Flux Studies at Adelaide, Australia: Observations and a Gravity Wave–Tidal Interaction Model

We present here the results of an analysis of gravity wave momentum fluxes in the mesosphere and lower thermosphere, inferred using a dual-beam Doppler radar near Adelaide, Australia during June 1984. Our analysis reveals that over 70% of the momentum flux and of the inferred zonal drag was due to gravity waves with observed periods less than one hour. This suggests that it is the gravity waves with high intrinsic frequencies and small horizontal scales that are most effective at transporting momentum into the middle atmosphere. The temporal variations in the momentum flux and flux divergence due to high-frequency motions were also examined in detail. In addition to daily variability, a strong diurnal modulation was observed to occur. This was found to be correlated with the phase of large-amplitude diurnal tidal motions. As a result of these observations, a gravity wave–tidal interaction model was proposed which accounts for all of the major features of the observed data, including a reduction in the inferred diurnal tidal amplitude and an advance of its phase with time. We believe that this gravity wave–tidal interaction model may explain a majority of the tidal variability observed worldwide.