Towards a turbulence closure based on energy modes
A new approach to parameterising sub-grid scale processes is proposed: The impact of the unresolved dynamics on the resolved dynamics, i.e. the eddy forcing, is represented by a series expansion in dynamical spatial modes that stem from the energy budget of the resolved dynamics. It is demonstrated that the convergence in these so-called energy modes is faster by orders of magnitude than the convergence in Fourier-type modes. Moreover, a novel way to test parameterisations in models is explored. The resolved dynamics and the corresponding instantaneous eddy forcing are defined via spatial filtering that accounts for the representation error of the equations of motion on the low-resolution model grid. In this way, closures can be tested within the high-resolution model, and the effects of different parameterisations related to different energy pathways can be isolated. In this study, the focus is on parameterisations of the baroclinic energy pathway. The corresponding standard closure in ocean models, i.e. the Gent-McWilliams (GM) parameterisation, is also tested, and it is found that the GM field acts like a stabilising direction in phase space. The GM field does not project well on the eddy forcing and hence fails to excite the model’s intrinsic low-frequency variability but it is able to stabilise the model.