Efficient form of the LANS-alpha turbulence model in a primitive-equation ocean model

Abstract

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.