The biharmonic filter is an extra term that is added in the horizontal turbulent viscous flux, J, that filters high frequency oscillations in a numerical model of advection-diffusion of a property (i.e. oscillations with a wave-length equal to one or two widths of the horizontal resolution):
where is Fick's law diffusivity. A ready made formula that would estimate the order of magnitude of the coefficient for momentum would be
where U is a typical flow velocity and is the numerical model horizontal resolution.
In mohid, you need to add in the Hydrodynamic_x.dat the following keywords:
BIHARMONIC : [0/1] <-- Activates de biharmonic viscous flux BIHARMONIC_COEF : [Real] <-- Sets the value of
Delhez, E., & Deleersnijder, E. (2007). Overshootings and spurious oscillations caused by biharmonic mixing. Ocean Modelling, 17(3), 183-198. doi: 10.1016/j.ocemod.2007.01.002.