The Interaction between an Internal Gravity Wave and Turbulence in the Stably-Stratified Nocturnal Boundary Layer

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  • 1 CSIRO Division of Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia
  • | 2 School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, GA 30332
  • | 3 IFA/CNR c.p. 27, 00044 Frascati, Italy
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Abstract

Observations have been made of a stably-stratified nighttime boundary layer perturbed by Kelvin-Helmholtz internal waves with critical levels around 600 m. Significant turbulence intensities were measured although the time-mean gradient Richardson numbers were large and positive. It is shown by constructing energy budgets of wave and turbulent components separately that there is an essential flow of kinetic energy from wave to turbulence and that the mechanics of this exchange process depend upon the nonlinear character of the wave field.

Turbulent energy budgets were followed through a wave cycle and revealed that turbulence production occurred during only one quarter of a wave period, the rest of the time being taken up by redistribution of turbulent kinetic energy (tke) among the three orthogonal components, relaxation under the effects of density stratification and dissipation. The principal path of energy dissipation is through conversion of vertical component tke to density fluctuations, which are in turn dissipated by molecular conductivity. Direct viscous dissipation of tke is negligible in comparison. This behavior is consistent with the quasi-two-dimensional character imposed on the turbulence by the strong stability and is clearly apparent in the behavior of the velocity and temperature spectra.

Abstract

Observations have been made of a stably-stratified nighttime boundary layer perturbed by Kelvin-Helmholtz internal waves with critical levels around 600 m. Significant turbulence intensities were measured although the time-mean gradient Richardson numbers were large and positive. It is shown by constructing energy budgets of wave and turbulent components separately that there is an essential flow of kinetic energy from wave to turbulence and that the mechanics of this exchange process depend upon the nonlinear character of the wave field.

Turbulent energy budgets were followed through a wave cycle and revealed that turbulence production occurred during only one quarter of a wave period, the rest of the time being taken up by redistribution of turbulent kinetic energy (tke) among the three orthogonal components, relaxation under the effects of density stratification and dissipation. The principal path of energy dissipation is through conversion of vertical component tke to density fluctuations, which are in turn dissipated by molecular conductivity. Direct viscous dissipation of tke is negligible in comparison. This behavior is consistent with the quasi-two-dimensional character imposed on the turbulence by the strong stability and is clearly apparent in the behavior of the velocity and temperature spectra.

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