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Brief description of the Model and USSP for gravity waves

The troposphere-stratosphere configuration of the Met. Office Unified Model uses 55 quasi-horizontal levels to represent the atmosphere from the surface up to 0.01hPa on a 2.5$^{o}$ by 3.75$^{o}$ latitude-longitude grid. The physical parametrizations are documented by Pope et al. (1999) and used in tropospheric climate prediction experiments. Of these parametrizations, the orographic gravity wave parametrization of Gregory et al. (1998) is applied up to the lower stratosphere and is uncoupled to the USSP. Climatological, annually varying sea-surface temperatures and atmospheric trace gases are specified and no interannual variability is imposed in the model. The USSP (see Warner and McIntyre 1999 for full details) represents the forcing from a continuous spectrum of gravity waves on the mean flow. We assume an isotropic and homogeneous wave source at the surface with 4 wavevector directions (N,S,E,W). The total launch momentum flux is 0.0066 Kg/m/s2 distributed equally between the 4 wavevector directions. The launch spectrum is continuous in vertical wavenumber (m) and is proportional to m at small m and m^-3 at large m. Assuming Coriolis effects can be ignored, the hydrostatic dispersion relation for gravity waves is used to Doppler shift the wave spectrum by the change in horizontal wind in the Unified Model as the spectrum propagates up from one model level to the next. A semi-empirical saturation limit is applied to the gravity wave spectrum after Doppler shifting and excess pseudomomentum flux is removed from the spectrum and used to calculate the wave induced force on the modelled atmosphere.