A Comparison of Several Forms of Eddy Viscosity Parameterization in a Two-Dimensional Cloud Model

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  • a Naval Environmental Prediction Research Facility, Monterey, CA 93940
  • | b The Rand Corporation, Santa Monica, CA 90406
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Abstract

A two-dimensional model is used to test the effects of using several forms of eddy viscosity parameterization to simulate subgrid-scale turbulence. A well-documented observation of a quasi-steady cumulus cloud which formed over a refinery is used for simulation and comparison. The control parameterization of eddy viscosity is one based on both the deformation and buoyancy fields. When compared to observations, this control run overestimates somewhat the liquid water contents and slightly underpredicts the vertical velocities. Parameterizations based on deformation alone, two-dimensional turbulence theory, and several constant values of eddy viscosity result in cloud simulations that are deficient primarily in their significant overprediction of liquid water content. These experiments confirm that a buoyancy term in the prescription of eddy viscosity is necessary when thermal instability plays an active role in the subgrid forcing.

Abstract

A two-dimensional model is used to test the effects of using several forms of eddy viscosity parameterization to simulate subgrid-scale turbulence. A well-documented observation of a quasi-steady cumulus cloud which formed over a refinery is used for simulation and comparison. The control parameterization of eddy viscosity is one based on both the deformation and buoyancy fields. When compared to observations, this control run overestimates somewhat the liquid water contents and slightly underpredicts the vertical velocities. Parameterizations based on deformation alone, two-dimensional turbulence theory, and several constant values of eddy viscosity result in cloud simulations that are deficient primarily in their significant overprediction of liquid water content. These experiments confirm that a buoyancy term in the prescription of eddy viscosity is necessary when thermal instability plays an active role in the subgrid forcing.

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