The excitation of ducted modes by passing internal waves

Abstract

A local peak in the atmospheric wind or stability profile supports ducted (vertically trapped) modes. If the duct is rather high in the atmosphere it cannot be directly excited by the major atmospheric sources such as convection, but an internal gravity wave radiating out of such sources transfers energy to the duct via nonlinear processes related to the resonance triad interaction. The amplitude of such a transfer is computed. The modal excitations remain after the internal wave has left the region, and while each passing wave adds only a small amount of activity to the duct such contributions are cumulative and drive the modal activity up to a level where it is limited by local dissipation. This suggests the possibility of monitoring the mean flux of internal waves through the region by measuring activity within elevated ducts, a procedure that offers several advantages over monitoring the wave flux directly.