The current parametrization of the effects of sub-gridscale orography (SSO) in the Unified Model (UM) has been used operationally at the Met Office in both mesoscale and global forecast configurations since August 2002. The scheme uses the simple analytical expression for linear hydrostatic flow over a two-dimensional ridge in the abscence of friction and rotation as its prediction of the surface pressure drag. This drag is then partitioned into blocked-flow and gravity-wave components depending on the low level Froude number. The scheme is essentially, therefore, an empirical fit to the idealised simulations of Olafsson and Bougeault (1997, JAS, 193-210, hereafter OB97) of flow over an elongated three-dimensional ridge in the presence of both friction and rotation.

In this study the range of parameter space explored by OB97 has been extended to assess whether the effect of rotation on the surface pressure drag could be important after all. To this end, a series of simulations with ridges significantly longer than those used by OB97 have been performed. The results from these simulations will be presented and their implications for the current UM parametrization of SSO will be discussed.