A hydrostatic/non-hydrostatic grid-switching strategy for computing high-frequency, high wave number motions embedded in geophysical flows

Hydrostatic and non-hydrostatic models were used to simulate the generation of internal surges and associated soliton-like trailing waves from the non-linear steepening of low-frequency basin-scale waves. Results confirmed that the process cannot be modelled using the hydrostatic approximation. A grid-switching strategy was developed to reduce the simulation run-time of the non-hydrostatic model; a low-resolution grid using a hydrostatic computation of the flow field is dynamically switched to a high-resolution grid in the region of propagation of the leading internal surge, using a non-hydrostatic computation of the flow field. The strategy takes advantage of the small time scale required for non-hydrostatic effects to become important such that a high-resolution grid is invoked only when and where these effects become large. Run-time reduction, conservation of the interpolation scheme involved in the grid switching and strategies for field scale studies were addressed. In relation to the laboratory experiments, the grid-switching strategy predicted the phase speed and the amplitude of the leading internal surge similarly to the uniform-grid models, however, the trailing soliton-like waves lost some of their signature. All non-hydrostatic models predicted the features of the energy flux path between low- and high-frequency waves.