Observations of potential vorticity banners during the Mesoscale Alpine Programme

The generation of potential vorticity (PV) in flow past high mountains was first suggested on theoretical grounds a little over ten years ago. As the horizontal resolution of numerical models has increased over the past five years, PV banners have become a ubiquitous feature of numerical simulations of flow in complex terrain. The predicted distribution of PV downstream of a complex mountain barrier oriented perpendicular to the large-scale flow can resemble a series of smoke plumes extending downstream from several point sources. Thus, the term plume or "banner" is used to describe this feature. PV is conserved in the absence of diabatic and dissipative processes. Thus, mountain-generated PV has the potential of representing a physical link between mesoscale mountain disturbances and the larger-scale balanced flow. On the other hand, the widths of these banners seem to be mainly determined by the model resolution, raising some concern these features may be spurious features of the numerical simulations. Although observational evidence indicates elongated banners of modified flow can be created as air encounters isolated mountains or islands, no systematic observations have been made to confirm or refute their existence in the lee of major continental mountain ranges. The Alps, with their compact structure and irregular ridge line, provide an ideal testing ground for mountain-generated PV banners.

The Mesoscale Alpine Programme is a coordinated international effort to explore the mesoscale effects of complex terrain. The objectives of the PV banner project of MAP were to: (1) verify the existence of PV banners predicted by numerical models, (2) document the lateral, vertical, and downstream structure and persistence of PV banners, and (3) provide insight into the origin (generation mechanism) of PV banners. During the Special Observing Period (SOP) of MAP, which took place between 7 September and 15 November 1999, the PV banner project had four aircraft available for missions around the Alps (NOAA P-3, NCAR Electra, CNRM Merlin, and INSU Fokker-27 ARAT). Six missions involving one to four aircraft were dedicated to the PV banner project. These missions, which included a good mix of cases, used in situ measurements, downward-pointing lidar measurements of backscatter and water vapor, and dropsondes to document the characteristics of the flow downstream of complex terrain. The tail radars on the P-3 and Electra may also provide useful information about the downstream flow field for one, possibly two, of these missions. Jet-like structures in qualitative agreement with real-time mesoscale forecasts were observed during each of these missions. This presentation will provide an overview of the aircraft missions dedicated to the PV banner project of MAP.