Sina Khani Research Scientist/Engineer 4 skhani@apl.washington.edu |

Education

B.S. Mechanical Engineering, Zanjan University, 2007

M.S. Mechanical Engineering, Iran University of Science and Technology, 2010

Ph.D. Applied Mathematics, University of Waterloo, 2015

Publications |
2000-present and while at APL-UW |

An anisotropic subgrid-scale parameterization for large-eddy simulations of stratified turbulence Khani, S., and M.L. Waite, "An anisotropic subgrid-scale parameterization for large-eddy simulations of stratified turbulence," Mon. Wea. Rev., 148, 4299-4311, doi:10.1175/MWR-D-19-0351.1, 2020. |
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1 Oct 2020 |
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Subgrid-scale (SGS) parameterizations in atmosphere and ocean models are often defined independently in the horizontal and vertical directions because the grid spacing is not the same in these directions (anisotropic grids). In this paper, we introduce a new anisotropic SGS model in large-eddy simulations (LES) of stratified turbulence based on horizontal filtering of the equations of motion. Unlike the common horizontal SGS parameterizations in atmosphere and ocean models, the vertical derivatives of the horizontal SGS fluxes are included in our anisotropic SGS scheme, and therefore the horizontal and vertical SGS dissipation mechanisms are not disconnected in the newly developed model. Our model is tested with two vertical grid spacings and various horizontal resolutions, where the horizontal grid spacing is comparatively larger than that in the vertical. Our anisotropic LES model can successfully reproduce the results of direct numerical simulations, while the computational cost is significantly reduced in the LES. We suggest the new anisotropic SGS model as an alternative to current SGS parameterizations in atmosphere and ocean models, in which the schemes for horizontal and vertical scales are often decoupled. The new SGS scheme may improve the dissipative performance of atmosphere and ocean models without adding any backscatter or other energizing terms at small horizontal scales. |

Diagnosing subgrid mesoscale eddy fluxes with and without topography Khani, S., M.F. Jansen, and A. Adcroft, "Diagnosing subgrid mesoscale eddy fluxes with and without topography," J. Adv. Model. Earth Syst., 11, 3995-4015, doi:10.1029/2019MS001721, 2019. |
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1 Dec 2019 |
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General circulation models use subgrid‐scale (SGS) parameterizations to represent the effects of unresolved mesoscale eddies on large‐scale motions. Most of the current SGS parameterizations are based on a theoretical understanding of transient eddies, where the mean flow is a temporal average. Here, we use a spatial filtering analysis to better understand the scale‐dependent characteristics of the SGS fluxes. Specifically, we apply the filtering approach to diagnose SGS eddy volume fluxes and eddy velocity scales in a hierarchy of model configurations from a flat‐bottom channel to an idealized Southern Hemisphere. Importantly, SGS volume fluxes include significant contributions from standing meanders; unlike for transient eddies, the vertically integrated SGS volume flux does not necessarily integrate to zero. To accommodate net vertically integrated eddy fluxes, we define a SGS eddy diffusivity based on planetary potential vorticity (PV) diffusion. We diagnose the transient and standing contributions to SGS fluxes and associated effective diffusivities. In the presence of bottom topography or continental barriers the standing component of the PV diffusivity becomes dominant at large filter scales in the westerly wind region, while the transient component remains dominant in the easterly wind region. Our results suggest that the diagnosed PV diffusivity can be parameterized using mixing length theory based on a priori estimates of SGS velocity and length scales. |