For a number of years now, sub-gridscale orographic effects have been incorporated into the global version of the Unified Model(UM) via an orographic form drag scheme and a gravity wave drag (GWD) scheme. The GWD scheme includes a representation of linear hydrostatic waves and, for low Froude number flows, a hydraulic jump response. It also accounts for the anisotropy of the sub-grid orography and trapped lee waves. The introduction of these two schemes led to a very significant improvement in global numerical weather prediction (NWP) performance, especially in northern mid-latitudes in northern winter.

However, a number of weaknesses have been identified with the GWD scheme. The most striking of these is the excessive triggering of the hydraulic jump response; typically it is diagnosed at about 50% of grid points every timestep. Whilst this low level drag is beneficial, it would appear that it is being attributed to the wrong mechanism.

Therefore, a new sub-grid orography scheme has been developed which addresses the above weakness (and other weaknesses) of the current scheme. The new scheme is much simpler than the current scheme, using the "usual" expression for linear two dimensional flow in the absence of friction and rotation to calculate the total surface pressure drag. However, the amplitude of the parametrized gravity waves is set equal to the depth of air flowing over the mountains rather than being set equal to the full sub-grid mountain height. The remaining surface stress is assumed to be due to low level flow-blocking and is deposited uniformly below the sub-grid mountain tops.

The rationale behind this simple scheme and its benefits on global NWP performance will be described and illustrated. Results showing the impact of the scheme in the mesoscale version of the UM may also be presented.