Numerical Modelling of Inertia-Gravity Wave Emission by Fronts and Jets

Abstract

Idealized numerical-model calculations following the development of upper-level and low-level fronts and their attendant jets are presented. As the fronts and jets evolve, long-wavelength low-frequency inertia-gravity waves are emitted. The key components of the model basic-state are: a time-dependent cross-front geostrophic shear flow in thermal wind balance with the along-front potential temperature, and a time-independent height-independent confluent deformation wind field with its axis of dilation parallel to the front. When the cross-front geostrophic wind shear, Λ, is negative and consequently the upper-level jet advects cold air along the upper front, vertically propagating inertia-gravity waves are a prominent part of the model solution, especially in the lower stratosphere. Like those observed in the stratosphere, the modelled waves have frequencies close to the inertial frequency, horizontal wavelengths of order 400 – 1200 km, and vertical wavelengths in the range 2–10 km. Numerical experiments with Λ ≥ 0 show little gravity wave activity. The time-space mean of the vertical flux of horizontal momentum in the stratosphere associated with the modelled waves is approximately 1.4 x 10^-4 N m ^-2, where the average is taken over a 24 h time interval, and over a 5000 km wide x 14 km high box in the lower stratosphere, centred over the jet/front system.