Stratospheric inertia–gravity waves generated in a numerical model of frontogenesis. I: Model solutions

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The generation of stratospheric inertia–gravity waves is investigated with the aid of a dry two‐dimensional anelastic numerical model. The model calculations follow the development of upper‐level and low‐level fronts together with their attendant jets. Long‐wavelength low‐frequency inertia–gravity waves are generated in the model runs. The key constituents of the model basic‐state are: (1) a time‐dependent shear flow normal to the front in thermal‐wind balance with the along‐front potential temperature; and (2) a time‐ and height‐independent confluent‐deformation wind‐field with its axis of dilation parallel to the front. When the cross‐front geostrophic wind shear A is negative, and consequently the upper‐level jet advects cold air along the upper front, prominent vertically propagating inertia–gravity waves are evident in the lower stratosphere. Like those observed in the atmosphere, the modelled waves have frequencies close to the inertial frequency, horizontal wavelengths of order 400 – 1200km, and vertical wavelengths in the range 2 – 10km. Numerical experiments in which Λ ⩾ 0 showed little gravity‐wave activity.