The Turbulence Structure of Nocturnal Slope Flow

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  • 1 Pacific Northwest Laboratory, Richland, Washington
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Abstract

Measurements of the turbulence structure of nocturnal slope flow are used to test the hypothesis that slope flow turbulence in the region above the low-level wind maximum is decoupled from the surface and has a local structure similar to that found by Nieuwstadt for stably stratified flow over flat terrain. A turbulence covariance model is used to determine local scaling relations for slope flow. These relations predict that slope-flow turbulence variables, scaled with the local values of the momentum and heat fluxes, are nearly identical to those for stably stratified flow over flat terrain, the principal exception being the normalized eddy diffusivities.

Comparison of observations with local scaling predictions above the slope flow wind maximum indicates that local scaling is a promising approach for the description of slope flow turbulence. In particular, the slope flow turbulent kinetic energy (TKE) is found to be proportional to the local stress, with the constant of proportionality in close agreement with that observed by Nieuwstadt over flat terrain. This result, along with an empirical evaluation of the TKE budget, supports the hypothesis that the slope-flow TKE budget is principally a local balance between shear production and viscous dissipation, and supports the assumption of local scaling that turbulence flux divergence can be neglected in the turbulence second-moment equations.

The observations also support the prediction of local scaling that the normalized eddy diffusivities above the slope-flow wind maximum can be considerably larger than for stably stratified flow over flat terrain. The increased eddy diffusivities are caused by an additional buoyancy term in the TKE budget that depends on the slope-parallel heat flux; the flux Richardson number above the wind maximum can be considerably smaller than that for identical wind and temperature gradients over flat terrain, implying increased TKE production and increased turbulent transport. The gradient Richardson number, however, does not account for this additional term in the TKE budget and does not correctly indicate the stability of the flow.

Abstract

Measurements of the turbulence structure of nocturnal slope flow are used to test the hypothesis that slope flow turbulence in the region above the low-level wind maximum is decoupled from the surface and has a local structure similar to that found by Nieuwstadt for stably stratified flow over flat terrain. A turbulence covariance model is used to determine local scaling relations for slope flow. These relations predict that slope-flow turbulence variables, scaled with the local values of the momentum and heat fluxes, are nearly identical to those for stably stratified flow over flat terrain, the principal exception being the normalized eddy diffusivities.

Comparison of observations with local scaling predictions above the slope flow wind maximum indicates that local scaling is a promising approach for the description of slope flow turbulence. In particular, the slope flow turbulent kinetic energy (TKE) is found to be proportional to the local stress, with the constant of proportionality in close agreement with that observed by Nieuwstadt over flat terrain. This result, along with an empirical evaluation of the TKE budget, supports the hypothesis that the slope-flow TKE budget is principally a local balance between shear production and viscous dissipation, and supports the assumption of local scaling that turbulence flux divergence can be neglected in the turbulence second-moment equations.

The observations also support the prediction of local scaling that the normalized eddy diffusivities above the slope-flow wind maximum can be considerably larger than for stably stratified flow over flat terrain. The increased eddy diffusivities are caused by an additional buoyancy term in the TKE budget that depends on the slope-parallel heat flux; the flux Richardson number above the wind maximum can be considerably smaller than that for identical wind and temperature gradients over flat terrain, implying increased TKE production and increased turbulent transport. The gradient Richardson number, however, does not account for this additional term in the TKE budget and does not correctly indicate the stability of the flow.

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