Transilient Turbulence Theory. Part II: Turbulent Adjustment

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  • 1 Boundary Layer Research Team, Department of Meteorology, University of Wisconsin, Madison, WI 53706
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Abstract

Turbulent adjustment is a scheme where dynamic instabilities in the flow are eliminated by turbulence. It is a form of first-order turbulence closure that is applicable to numerical forecast models of the atmosphere. The responsive form of transilient turbulence theory (developed in the companion paper) is shown to be identical to turbulent adjustment for mixing between two points in the vertical. For mixing in a column of many contiguous grid points in a numerical model, turbulent adjustment only approximates the atmosphere's tendency to eliminate instabilities; whereas, transilient turbulence better describes typical atmospheric responses by allowing large-eddy mixing. The utility of turbulent adjustment is demonstrated with a case-study one-dimensional simulation of the Wangara Day 33–34 boundary layer, where nocturnal stable-layer characteristics develop in response to surface forcings. Turbulent adjustment, although inefficient in its ability to eliminate instabilities, is much simpler and quicker-executing than transilient mixing, and might be useful in fine-mesh boundary-layer, cloud, and small-mesoscale models having short timesteps.

Abstract

Turbulent adjustment is a scheme where dynamic instabilities in the flow are eliminated by turbulence. It is a form of first-order turbulence closure that is applicable to numerical forecast models of the atmosphere. The responsive form of transilient turbulence theory (developed in the companion paper) is shown to be identical to turbulent adjustment for mixing between two points in the vertical. For mixing in a column of many contiguous grid points in a numerical model, turbulent adjustment only approximates the atmosphere's tendency to eliminate instabilities; whereas, transilient turbulence better describes typical atmospheric responses by allowing large-eddy mixing. The utility of turbulent adjustment is demonstrated with a case-study one-dimensional simulation of the Wangara Day 33–34 boundary layer, where nocturnal stable-layer characteristics develop in response to surface forcings. Turbulent adjustment, although inefficient in its ability to eliminate instabilities, is much simpler and quicker-executing than transilient mixing, and might be useful in fine-mesh boundary-layer, cloud, and small-mesoscale models having short timesteps.

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