In this study the sensitivity to horizontal resolution of the total mountain drag in the Met Office Unified Model (UM) is assessed. The UM has therefore been multiply nested over different mountain ranges and run at horizontal resolutions ranging from 40km (the current global forecast resolution) down to 1.5km (the finest resolution now used operationally over the United Kingdom). Case studies which include significant mountain drag, e.g. associated with observed downslope windstorms, have been selected and run for each mountain range. The design of the experiments therefore represents a significant test of the UM representation of orography and, in particular, of the UM orographic drag parametrizations.
In this talk, the key results from these case studies will be described and illustrated. The sensitivity of the mountain drag to resolution will be evaluated, with consideration given both to the surface pressure drag and to the vertical profile of the momentum flux. Implications for the future development of orographic drag parametrizations will also be discussed.