Spectra of Internal Waves and Turbulence in Stratified Fluids. Part I. General Discussion and Indications from Measurements in Stably Stratified Atmosphere and Ocean

摘要

A unified spectral description of internal waves and turbulence in stably stratified atmosphere and ocean is proposed. We envisage that the fluctuating motions in a free atmosphere with scales smaller than the synoptic scale consist of internal waves and turbulence. At low wavenumbers, internal waves predominate; this range of wavenumbers may be called the internal wave subrange. The internal waves can be identified from the characteristics of velocity-scalar co-spectra and quadrature-spectra, as shown recently by Pao in a laboratory experiment. When the internal waves are sufficiently strong and distinct, the different harmonics of internal waves may present themselves in the auto-spectra of velocity and temperature, where the peaks and valleys are clearly identifiable. This, we believe, explains the presence of peaks and valleys in some velocity auto-spectra of clear air turbulence measured in strongly stratified regions of the atmosphere. At intermediate wavenumbers (buoyant subrange), the buoyancy effect is still strong but the turbulent scrambling process becomes important; the higher-order harmonics of internal waves are no longer distinct and cannnot be detected in the auto-spectra. (Author)