Role of Small Scale Gravity Waves in Generating Nonmigrating Tides in the Mesosphere

We demonstrated with the Numerical Spectral Model (NSM) that nonlinear interactions between planetary waves (PW) and migrating tides could generate in the upper mesosphere nonmigrating tides having amplitudes comparable to those observed. The NSM incorporates Hines' Doppler Spread Parameterization for small scale gravity waves (GW), which affect in numerous ways the dynamics of the mesosphere. The latitudinal (seasonal) reversals in the zonal circulation and temperature variations above 70 km, largely caused by GWs (Lindzen, 1981), contribute to the instabilities that generate the PWs as well as filter them. The PWs in turn are amplified by the momentum deposition of upward propagating GWs, as are the migrating tides. The GWs thus affect the migrating tides and PWs, the building blocks of non-migrating tides. We present here the results of two computer experiments, which indicate that the GWs also contribute significantly to the process of nonlinear coupling between PWs and tides. In one, we turn off the GW source to show the effect on the nonmigrating tides. In the second case, we demonstrate the effect on the stationary non-migrating diurnal tide by selectively suppressing the GW momentum source for zonal wavenumber m = 0.