Inertio‐gravity wave induced accelerations of mean flow having an imposed periodic component: Implications for tidal observations in the meteor region

Abstract

The semidiurnal harmonic exhibits great day‐to‐day variability in amplitude and phase. In addition, the variability appears to be substantially local and random, suggesting a connection with gravity wave activity. We suggest that a significant contribution to the observed semidiurnal harmonic at meteor heights might result from inertio‐gravity wave induced accelerations of the mean flow. The rate of wave forcing of the mean wind is related to the Doppler‐shifted phase velocity, so that during alternate phases of an imposed mean wind oscillation interactions with waves that accelerate the mean wind in opposite senses may be favored. Thus the imposed mean wind may modulate the mean‐flow acceleration at the imposed frequency. In this view, the semidiurnal character of the acceleration is a manifestation of the modulation of the interaction process by the semidiurnal tide. The variability of the semidiurnal harmonic would then reflect the local variability of inertio‐gravity wave fluctuations and also nonlinear feedback on the waves. Calculations with a simple time‐dependent wave‐mean‐flow model indicate that a wave‐induced component of the semidiurnal harmonic with amplitudes comparable to the semidiurnal tide itself is possible.