Structure and dynamics of an Alpine potential‐vorticity banner

A detailed analysis is undertaken of the primary Alpine shear zone that occurred on 1 October 1999 during the Mesoscale Alpine Programme. This shear zone emanates from the south‐western Alpine tip in north‐westerly flow conditions, develops in response to the Alpine‐scale flow splitting, and separates the northerly mistral wind to the south‐west of the Alps from the quasi‐stagnant air within the Alpine wake. The study is based on in situ flight‐level and dropsonde data from two research aircraft. The data are used for the construction of an objective analysis in a cross‐section perpendicular to the shear zone, and for diagnostic computations of the potential‐vorticity (PV) flux between vertically stacked flight legs. The observed flow structure is compared with simulation results of the Swiss Model and the non‐hydrostatic Canadian Mesoscale Compressible Community Model (MC2), operating at horizontal resolutions of 14 and 3 km, respectively. Immediately downstream of the topography, the observed shear zone has a surprisingly quasi‐steady structure that becomes evident provided a slow westward migration is accounted for. Further downstream, the shear zone shows increased signs of transience. Near the surface, it has an overall width of ∼150 km, but narrows to about 25 km near the top of the mistral inversion at a height of ∼2 km. Detailed analysis shows that the shear zone has a complex substructure consisting of at least three positive and three negative PV filaments. These shear lines are characterized by a pronounced vertical coherence in the mixed layer and throughout most of the inversion layer. One of the banners is also evident in low‐level tracer constituents. Diagnostic computations of the PV flux along the banners using the generalized Bernoulli theorem reveal that most of the associated PV flux is due to vertically oriented vorticity within the inversion layer. Within the shear zone, there is a spanwise circulation of appreciable strength. It consists of a deformation flow with the axes of deformation oriented perpendicular to the shear zone. This deformation flow in effect squeezes the PV banners and thereby makes them resilient with respect to dissipation and barotropic instabilities. Detailed intercomparison with numerical model results shows that models with a grid spacing below ∼15 km are able to capture the overall structure of the shear zone. The MC2 model in addition credibly replicates some of the finer‐scale sub‐structure of the shear zone, in particular at low levels. The high predictability and steadiness of the PV banners supports the idea that individual banners are generated by flow‐separation and/or gravity‐wave breaking events upstream. It is argued that the overall width of the shear zone is thus in effect determined by the geometry of the upstream topography and the dynamical processes that govern the downstream advection of the PV banners.