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LANS-alpha Turbulence Parameterization

The Lagrangian-averaged Navier-Stokes-alpha (LANS-alpha) model is a turbulence parameterization that increases eddy activity near the grid-scale. As a post-doctoral researcher at LANL, I implemented LANS-alpha in the POP ocean model. Simulations with LANS-alpha in an idealized channel model domain produce turbulence statistics like a doubling of resolution, including kinetic energy, eddy kinetic energy, and temperature proles. The cost of adding LANS-alpha is only 30%, versus a factor of ten for doubling the horizontal resolution.

LANS-alpha, which is derived using Hamilton's principle, models the momentum equation with an extra nonlinear term and a smoothed advecting velocity. The alpha parameter controls the strength of smoothing in a Helmholtz inversion operator, and thus the strength of the turbulence model. We show that ecient lters can replace the Helmholtz inversion, with similar results.

A linear dispersion analysis of LANS-alpha shows that it increases the eective Rossby Radius, thus allowing more eddy activity near the grid-scale in low resolution ocean simulations. Preliminary simulations in a North Atlantic domain produce deeper and more realistic penetration of eddy kinetic energy in the North Atlantic Current.

Summary of LANS-alpha results

Our LANS-alpha work is described in detail in the following papers:

Hecht, M.W., D.D. Holm, M.R. Petersen*, B.A. Wingate: 2008, Implementation of the LANS-alpha turbulence model in a primitive equation ocean model, J. Comp. Physics, 227 5691

Petersen, M.R., M.W. Hecht, B.A. Wingate: 2008, Efficient form of the LANS-alpha turbulence model in a primitive-equation ocean model, J. Comp. Physics, 227, 5717

Hecht, M.W., D.D. Holm, M.R. Petersen*, B.A. Wingate: 2008, The LANS-alpha and Leray turbulence parameterizations in primitive equation ocean modeling J. Physics A, 41 344009