Frontal‐wave development by interaction between a front and a cyclone: Application to FASTEX IOP 17

The way new cyclones appear along a baroclinic zone is central to the problem of the prediction of mid‐latitude cyclones. A new barotropic mechanism for frontal‐wave development is proposed in the present paper. The ingredients are a frontal environment characterized by horizontal wind shear and a finite‐amplitude initial wind perturbation orthogonal to the main deformation axis associated with the front. It is found that the initial wind perturbation can be provided by a low‐level vortex maximum located north‐westward with respect to the position of the new cyclone. Interactions between these synoptic features are documented using local energetics. The theory is only valid at the very early stage of development. After a few hours the main growing phase, which is mostly baroclinic, will take place. A wide set of experiments is performed in order to find the preferred location of the vortex at the initial time in the frame of the barotropic vorticity equation. The result is found to be consistent with the Fronts and Atlantic Storm‐Track Experiment case studied in this paper. Frontogenetic forcing provided by a deformation field does not inhibit the perturbation amplification. It is also found that the impact of the vortex is significantly larger than potential instabilities developing along the front where the unstable feature is defined as the most unstable singular vector.