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S. Juricke, S. Danilov, A. Kutsenko, and M. Oliver
Ocean kinetic energy backscatter parametrizations on unstructured
grids: Impact on mesoscale turbulence in a channel,
Ocean Model. 138 (2019), 51-67, doi:10.1016/j.ocemod.2019.03.009.
Abstract:
We present a new energy backscatter parametrization for primitive equation ocean models at eddy-permitting resolution, specifically for unstructured grids. Traditional eddy parametrizations in terms of viscosity closures lead to excessive dissipation of kinetic energy when used with eddy-permitting meshes. Implemented into the FESOM2 ocean model, the backscatter parametrization leads to a more realistic total dissipation of kinetic energy. It maintains a reservoir of
dissipated energy and reinjects this subgrid energy at larger scales at a
controlled rate. The separation between
dissipation and backscatter scales is achieved by using
different-order differential operators and/or spatial
smoothing. This ensures numerical model stability.
We perform sensitivity studies with different choices of parameter
settings and viscosity schemes in a configuration with a
baroclinically unstable flow in a zonally reentrant channel with a
horizontally uniform mesh. The best backscatter setup substantially
improves eddy-permitting simulations at 1/4° and 1/6°
resolution, bringing them close to a 1/12° eddy-resolving
reference. Improvements are largest for levels of kinetic energy and
variability in temperature and vertical velocity. A selected optimal
default scheme is then tested in a mixed resolution setup - a channel
with narrow transitions between an eddy-permitting and an
eddy-resolving subdomain. The backscatter scheme is able to adapt
dynamically to the different resolutions and moves the diagnostics
closer to the high resolution reference throughout the domain.
Our study is a first step toward using backscatter in global
variable-mesh ocean models and suggests potential for substantial
improvements of ocean mean state and variability at reduced
computational cost.
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