The wake south of the Alps: Dynamics and structure of the lee‐side flow and secondary potential vorticity banners

Abstract

AbstractThe dynamics and structure of the lee‐side flow over the Po valley during a northerly föhn event, which occurred in the framework of the Mesoscale Alpine Programme Special Observation Period (on 8 November 1999 during Intensive Observation Period 15), has been investigated using aircraft data and high‐resolution numerical simulations. Numerical simulations were performed with the mesoscale non‐hydrostatic model Meso‐NH, using three nested domains (with horizontal resolutions 32, 8 and 2 km), the 2 km resolution domain being centred on the Po valley. The basic data–model comparison, and back‐trajectory and tracer release analyses, provided evidence that the jet/wake structure of the flow above the Po valley could be reasonably identified with the mountain pass/peak distributions. Measurements from three aircraft flying below the Alps crestline (at 2700, 1500 and 600 m above sea level) along two 350 km east–west legs, designed to be approximately perpendicular to the northerly synoptic flow, were used to compute the potential vorticity (PV) experimentally assuming the lee‐side flow to be two‐dimensional. (The simplified form of the PV under these assumptions is hereafter referred to as SPV). Due to increasing lee‐side flow curvature with decreasing altitude (caused by flow splitting at the scale of the Alps), the experimentally derived SPV was compared to its simulated counterpart. In situ measurements showed that coherent secondary PV banners (PVB2s) do exist downstream of the complex Alpine terrain, as observations show oscillations between positive and negative values of SPV as expected from the simulations. The details of the structure of the SPV field simulated with Meso‐NH were found to be different from the observations (i.e. the location of observed maxima and minima of SPV did not match their simulated counterparts at particular points). This is because the correspondence between observed and modelled velocity and potential temperature fields was not good enough to expect good correspondence between differentiated quantities such as vorticity and potential temperature gradient (since less‐reliable shorter‐scale features are thereby accentuated). Parametrized processes such as surface drag and internal diffusion, and ‘numerical’ processes, such as model filters, to which PV production mechanisms are sensitive and whose role is difficult to assess, also contribute to the poor agreement between observed and modelled SPV fluctuations. Finally, simulations suggest that lowlevel gravity‐wave breaking was the main mechanism responsible for the formation of the elongated PVB2 modelled/observed over the Po valley.