The Lagrangian-Averaged Navier-Stokes alpha (LANS-alpha) model is a turbulence parameterization that has been shown to capture some of the most important features of high resolution ocean modeling at lower resolution. Simulations using LANS-alpha in the POP primitive-equation ocean model resemble doubled-resolution simulations of standard POP in statistics like kinetic energy, eddy kinetic energy, and potential temperature fields. The computational cost of adding LANS-alpha is only 27% for our most efficient implementation, as compared to a factor of 8-10 for a doubling of resolution.
The LANS-alpha model improves turbulence statistics with an additional nonlinear term and a smoothed advecting velocity. In this work we investigate different kinds of smoothing techniques and their effect on the LANS-alpha model's results and efficiency. We show that we can substitute convolution filters for full Hemlholtz inversions and produce similar results at a significantly lower expense.
When constructing filters for LANS-alpha in a primitive-equation ocean model the filter weights must be chosen carefully, otherwise a pressure-velocity instability will be excited. We show analytically that certain ranges of filter weights are unstable, and confirm this with numerical experiments. Our stability criterion also guarantees that the kinetic energy is well defined, and that the filtered velocity is smoother that the original velocity.
We consider the problem of coarse-graining in the context of finite-volume fluid models. If a variable is defined on a high-resolution grid it may be coarse-grained so that it is defined on a grid of lower resolution. In general this will cause some information about the variable to be lost. In particular, horizontal divergences, gradients or other operators calculated on the coarse grid after projecting may differ from those calculated on the fine grid. In some cases we are able to choose averaging weights for coarse-graining such that the coarse-grid operators will give a result approximatin…
The global stratification and circulation of the ocean and their sensitivities to changes in forcing depend crucially on the representation of the mesoscale eddy field. Here, a geometrically informed and energetically constrained parameterization framework for mesoscale eddies --- termed GEOMETRIC --- is proposed and implemented in three-dimensional primitive equation channel and sector models. The GEOMETRIC framework closes mesoscale eddy fluxes according to the standard Gent--McWilliams scheme, but with the eddy transfer coefficient constrained by the depth-integrated eddy energy field, prov…
This study compares the skills of two numerical models having the same horizontal resolution but based on different principles in representing meso- and submesoscale features of ocean dynamics in the Lakshadweep Sea (North Indian Ocean). The first model, titled LD20-NEMO, is based on solving primitive equations using the NEMO (Nucleus for European Modelling of the Ocean) modelling engine. The second one, titled LD20-SDD, uses a newer Stochastic-Deterministic Downscaling method. Both models have 1/20o resolution and use the outputs from a Global Ocean Physics Analysis and Forecast model at 1/12…
