Computational modes: What are they and how bad are they for a weather/climate model dynamical core?
Seminar Room 2, Newton Institute Gatehouse
In this presentation we will take a broad definition of `computational mode', namely any gross mis-representation of linear wavelike behaviour of the continuous system by the discrete system. Computational modes are potentially problematic for a dynamical core because they are likely to be excited by nonlinear processes, physical parameterizations, or data assimilation. Some well-known and less well-known examples of computational modes will be reviewed, including isolated computational modes and entire branches of computational modes. Spatial computational modes will be related to grid staggering and to degrees of freedom of prognostic variables. The triangular and hexagonal C-grid cases will be examined in detail. It will be argued that the extra branch of Rossby modes that occurs for the hexagonal C-grid is, in fact, well behaved in the presence of a background flow provided an adequate advection scheme is used. The relation of computational modes to `parasitic' mo des and to trapped modes on non-uniform grids will be discussed.