Brunt‐Doppler ducting of small‐period gravity waves

Abstract

The variation of the Brunt period with height lends itself to a natural ducting and filtering mechanism for low‐altitude short‐period gravity waves. We investigate this mechanism in combination with Doppler ducting produced by the variation in horizontal winds. Both the frequency dispersion at fixed propagation direction and the direction dispersion at fixed frequency have been examined in a COSPAR background atmosphere with zonal and meridional winds. Our results show that not only are the low‐altitude short‐period gravity waves ducted, but unlike the usual ducting mechanisms due to uneven structure and dissipation which produce only partially guided modes, this mechanism produces primarily guided modes in the absence of winds and a mixture of fully and partially guided modes with winds. The wind effects are very large on the higher modes and less significant on the few lowest modes, including the Lamb mode. Investigations of viscous dissipation, nonlinearity, and instability have shown that viscosity is unimportant for most altitudes of interest and that nonlinearity and instability can play a role for all but the lowest guided modes. We propose that simultaneous continuous observation of airglow at mesospheric and ionospheric altitudes be made to verify not only the low‐altitude Brunt‐Doppler ducting for short‐period gravity waves, but also the vertical energy distribution of the medium‐ and large‐scale TIDs.