Assessment of which scales of orography can be credibly resolved in a numerical model

Idealized simulations are performed in order to examine which scales of oro graphy (relative to the grid scale) can credibly be represented in numerica l models. A wide range of Row regimes is considered. These include neutral flow over small hills embedded in the boundary layer and highly stable flow over and around large mountains. In none of these cases do simulations wit h only two grid points per hill show any appreciable skill. Four and six po ints per hill give qualitatively reasonable results, but still tend to unde restimate the drag relative to that obtained in the best resolved simulatio ns. Implications for the smoothing of the resolved orography fields (and fo r the inputs to the sub-grid parametrizations) in large-scale models are di scussed.