Testing Lagrangian Theories of Internal Wave Spectra. Part I: Varying the Amplitude and Wavenumbers

Abstract

A growing body of literature has been built on the premise that kinematic advection produced by linear superpositions of sinusoidal Lagrangian gravity waves confined to lower vertical wavenumbers can provide an explanation for quasi-universal Eulerian spectral tails commonly found in the oceans and the atmosphere. Recently, Hines has established criteria delineating the circumstances in which Eulerian and Lagrangian spectra differ. For conditions in which Hines claims Lagrangian linearity and the production of quasi-universal Eulerian m?3 spectra, a kinematic advection model based on ensembles of seven nonstanding Lagrangian waves reveals the presence of gross violations of continuity and adiabaticity as well as severe departures from hydrostatic balance. Similar infractions are found for other seven-wave ensembles having a broad range of amplitudes and wavenumbers typical of saturated wave fields in the middle atmosphere. Furthermore, m?3 spectra are found only as the Lagrangian wave field approaches a singular state. The singularities in the Lagrangian to Eulerian transformation are induced by stretching deformation fields that form during the superposition of sinusoidal waves with nonparallel wave vectors. Such deformation fields are known to be unstable with respect to three-dimensional vortices. The results strongly suggest that saturated middle atmosphere wave fields are frequently accompanied by small-scale turbulent eddies.