On the Treatment of Subgrid Orography in a Baroclinic Forecasting Model

The primitive equations for baroclinic flow confined to a channel are integrated numerically in time using a grid size of 350 km and three levels in the vertical. At the axis of the channel there is a subgrid mountain covering an area of less than 4 grid squares. At each time step an additional very simplified integration of the model equations is performed in the mountain area to determine the influence of the subgrid orography on the flow. The results of 24-hr forecasts are compared with those obtained by an integration with a grid size small enough to treat the orography in the usual way. Runs have been made for several types of mountains and for two types of initial flow. The correspondence is found to be quite satisfactory in all cases.