Interpretation of subplanetary-scale diurnal variance in the middle atmosphere

Numerical studies indicate that diurnal waves of intermediate scale (zonal wavenumbers between 4 and 15) are significant momentum sources for the stratopause and mesopause semiannual oscillations. This paper presents observational support for such waves in the middle atmosphere. Day-night differences from the Limb Infrared Monitor of the Stratosphere (LIMS) temperatures are analyzed as proxies for the diurnal tide. The variance in zonal wavenumber 9-16 is weakest in the mesosphere when the underlying zonal mean winds are directed westward in the lower stratosphere, and eastward in the upper stratosphere and lower mesosphere. This behavior is consistent with critical level filtering of diurnal subplanetary scale waves by the zonal mean winds. However, this mechanism cannot be verified without information about the propagation directions of the diurnal waves. SABER sampling patterns enable such definitions. Preliminary analyses of SABER diurnal temperatures show further support for the existence of intermediate- scale waves, and filtering by the mean winds in the upper stratosphere. The westward-propagating waves dominate their eastward counterparts between zonal wavenumbers 7 and 11.