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Entrainment and Transport in Idealized Three-Dimensional Gravity Current Simulation

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  • 1 Computational Research Division, Lawrence Berkeley National Laboratory, Berkeley, California
  • | 2 AcuSea, Inc., Albuquerque, New Mexico
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Abstract

A purely z-coordinate Dietrich/Center for Air Sea Technology (DieCAST) ocean model is applied to the Dynamics of Overflow Mixing and Entrainment (DOME) idealized bottom density current problem that is patterned after the Denmark Strait. The numerical results show that the background viscosity plays a more important role than the chosen coordinate system in the entrainment and mixing if the background viscosity is not small enough. Both higher horizontal viscosity and coarser resolution leads to slower along-slope propagation. Reducing vertical mixing parameterization also leads to slower along-slope propagation with thicker plume size vertically. The simulation gives consistent results for the moderate- and fine-resolution runs. At a very coarse grid the dense water descends more slowly and is mainly dominated by diffusion. Time-averaged downstream transport and entrainment are not very sensitive to viscosity after the flow reaches its quasi-steady status. However, more realistic eddies and flow structures are found in low-viscosity runs. The results show good convergence of the resolved flow as expected and clarify the effects of numerical dissipation/mixing on overflow modeling. Larger numerical dissipation is not required nor recommended in z-coordinate models.

* Current affiliation: Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

Corresponding author address: Yu-Heng Tseng, Department of Atmospheric Sciences, National Taiwan University, No. 1, Roosevelt Road, section4, Taipei, Taiwan 10673. Email: yhtseng@webmail.as.ntu.edu.tw

Abstract

A purely z-coordinate Dietrich/Center for Air Sea Technology (DieCAST) ocean model is applied to the Dynamics of Overflow Mixing and Entrainment (DOME) idealized bottom density current problem that is patterned after the Denmark Strait. The numerical results show that the background viscosity plays a more important role than the chosen coordinate system in the entrainment and mixing if the background viscosity is not small enough. Both higher horizontal viscosity and coarser resolution leads to slower along-slope propagation. Reducing vertical mixing parameterization also leads to slower along-slope propagation with thicker plume size vertically. The simulation gives consistent results for the moderate- and fine-resolution runs. At a very coarse grid the dense water descends more slowly and is mainly dominated by diffusion. Time-averaged downstream transport and entrainment are not very sensitive to viscosity after the flow reaches its quasi-steady status. However, more realistic eddies and flow structures are found in low-viscosity runs. The results show good convergence of the resolved flow as expected and clarify the effects of numerical dissipation/mixing on overflow modeling. Larger numerical dissipation is not required nor recommended in z-coordinate models.

* Current affiliation: Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

Corresponding author address: Yu-Heng Tseng, Department of Atmospheric Sciences, National Taiwan University, No. 1, Roosevelt Road, section4, Taipei, Taiwan 10673. Email: yhtseng@webmail.as.ntu.edu.tw

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