Flow past the Alpine topography is often associated with flow-splitting events and the formation of elongated potential vorticity (PV) banners downstream. Here a detailed analysis is undertaken of the structure of the primary PV banner that occurred at the south-western Alpine tip during MAP IOP4 (October 1, 1999). During this IOP, the weather situation was characterized by a north-westerly flow impinging upon the Alps, and the formation of an elongated PV banner over the Gulf of Genova. The study uses in-situ and drop-sonde data from research aircraft and includes a detailed comparison against high-resolution numerical model simulations.

To explore the structure of the potential vorticity field in a vertical section, a pressure-coordinate version of the generalized Bernoulli theorem is employed. This theorem allows to estimate the PV flux from in-situ aircraft data on two vertically stacked flight levels, without any need to restrict the analysis to the vertical component of vorticity. In this way, a quantitative estimate of the PV flux along the primary Alpine shear line is derived. Analysis of results confirms the presence of high values of PV and suggests that the high-resolution structure of the investigated banner is determined by the presence of several PV filaments (or narrow PV banners). The overall width of the banner is thus determined by the merging of several PV features, which in turn is controlled by the detailed structure of the upstream topography. Numerical simulations appear to confirm the feasibility of this mechanism.

The results are compared against intermediate-resolution (SM, grid-increment 14 km) and high-resolution (MC2, 3 km) model simulations. In general, we find a qualitatively satisfactory agreement between observations and simulations, suggesting that the models capture key-aspects of PV generation in flow past topography