The impact of a new orographic gravity wave drag scheme in the Met Office Unified Model

At the Met Office a single global circulation model, known as the Unified Model (UM), is used for both operational numerical weather prediction and climate research. For a number of years now, sub-gridscale orographic effects have been incorporated 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 significant improvement in the UM 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 the low level drag associated with this response has been found to be beneficial, it would appear that this drag is, in the main, being attributed to the wrong mechanism.

Therefore, a new GWD scheme has been developed which addresses the above weakness (and other weaknesses) of the current scheme. Changes have been made to improve the accuracy of the diagnosis of both the low level flow characteristics and the low level Froude number. These changes affect which regime is triggered and how much GWD results from that regime. Coupled to this, a parametrization of low level flow blocking has been included. This helps offset the reduced triggering of the hydraulic jump response, as well as improving the overall realism of the GWD scheme.

In this paper the new scheme, and its behaviour in the UM, will be described and illustrated.