Parametrizing the effects of orography on the boundary layer: An alternative to effective roughness lengths

Abstract A new method of parametrizing the effects of form drag due to unresolved orography is proposed in which an explicit orographic‐stress profile is specified. This approach is evaluated using the results of numerical model simulations and the performance is found to be at least as good as that obtained with the traditional effective‐roughness‐length parametrization for cases of neutral flow normal to ridges. The new method is more readily extended to represent the changes in form drag as a function of wind direction for flow over anisotropic orography, and also simplifies the problem of parametrizing the scalar transfer. It is also suggested that this more explicit approach will provide a more convenient framework for parametrizing form drag in stable conditions and for understanding and improving the interactions with gravity‐wave‐drag parametrization schemes.