Impact of a spectral gravity wave parameterization on the stratosphere in the met office unified model

The impact of a parameterized spectrum of gravity waves on the simulation of the stratosphere in the Met Office Unified Model (UM) is investigated. In the extratropical mesosphere, the gravity wave forcing acts against the mean zonal wind and it dominates over the resolved wave forcing. In the extratropical stratosphere, the gravity wave forcing gives a small acceleration in the direction of the mean zonal wind. Both summer and winter stratospheric jets have improved maximum strength and tilt with height when the parameterized gravity wave forcing is included, although the southern winter jet is still more vertically aligned than in observational analyses. The timing of the seasonal breakdown of the southern winter vortex is also improved by the addition of gravity wave forcing. In the Tropics, the most obvious impact is that the model reproduces the quasi-biennial oscillation (QBO) with a realistic mean and range of periods. It also reproduces most of the observed asymmetries between the easterly and westerly phases of the oscillation. The sensitivity of this modeled QBO to horizontal diffusion parameters is investigated and it is shown that diffusion set to damp out grid-length disturbances can also attenuate the QBO due to its long period, particularly in the narrower westerly phase.