Interactions between gravity waves and the diurnal tide in the mesosphere and lower thermosphere

Interactions between breaking internal gravity waves with phase speeds of 0, ±10, ±20, ±30 ms-1, and the diurnal tide in the mesosphere and lower thermosphere (70-120 km) are investigated using a time dependent numerical model of the tide. The gravity wave breaking and stress are calculated using a modified Lindzen's parameterization making allowance for interactions with a diurnally varying zonal wind superimposed on a zonal mean wind field. In order to avoid difficulties with the WKB approximation used in the parameterization, the interactions are only considered in the extratropics. However, the gravity wave stress is shown to suppress the amplitude of the diurnal tide at all latitudes in the upper mesosphere and lower thermosphere. It is also shown that gravity wave stresses modified by the diurnal tide induce significant semidiurnal and terdiurnal tides in the mesosphere and thermosphere.