Stratified turbulence in the atmosphere and oceans: A new subgrid model
- NASA, Goddard Institute for Space Studies, NY (United States)
Turbulence in a stratified medium is studied with emphasis on stable stratification, as it occurs in the atmosphere and oceans, and on the construction of a subgrid model (SGS) for use in large eddy simulation (LES). The two basic assumptions of all SGS models are (1) that the unresolved scales are isotropic and (2) that they can be described by a Kolmogorov spectrum and are no longer valid in a stably stratified medium. Generation gravity waves invalidates the second assumption, while the damping vertical motion induces a degree of anisotropy considerably higher than in unstably stratified flows. Weinstock's model is used to find that the energy dissipation rate [epsilon][sub [theta]] of temperature variance increases with stability. The effect of shear on the subgrid scales is neglected. Because of the higher anisotropy of stably stratified flows, even the most complete SGS model presently in the use must be enlarged to include new higher-order terms. A new second-order closure model is proposed in which the three components of the flux u[sub i][theta] can be obtained by inverting a 3 x 3 matrix and u[sub i]u[sup j] can be obtained by inverting a 6 x 6 matrix. An approximate procedure is suggested to avoid the 6 x 6 matrix inversion and yet account for anisotropic production. The kinetic energy e is a solution of a differential equation. It is shown that in a deep LES, where the buoyancy scales are fully resolved, the standard models for [epsilon] and [epsilon][sub [theta]] are probably adequate, whereas in a shallow LES, where the buoyancy range may not be fully resolved, the above effect on [epsilon] and [epsilon][sub [theta]] must be accounted for. Preliminary results indicate that the (total) kinetic energy dissipation length scale increase with stability, in accordance with LES results but in disagreement with Deardorff's model that suggested a decrease of all dissipation scales in presence of stratification. 41 refs., 5 figs.
- OSTI ID:
- 6125506
- Journal Information:
- Journal of the Atmospheric Sciences; (United States), Vol. 50:13; ISSN 0022-4928
- Country of Publication:
- United States
- Language:
- English
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