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MAP Potential Vorticity Banners |
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Idealized and real-case numerical experiments suggest that flow past three-dimensional topography is often accompanied by the formation of elongated filaments of vorticity and potential vorticity (so-called PV banners or shearlines), which trail downstream from topographic obstacles. The PV banners usually occur in pairs of positive and negative filaments, and their formation is associated with irreversible topographic processes such as flow-splitting, boundary layer separation and gravity-wave breaking. |
During MAP Special Observation Period (SOP) PV banners were observationaly documented in several locations downwind of the Alps. The above figure summarizes research flights carried out in support of the PV Banner project during MAP SOP. |
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As the spatial resolution of NWP models has increased over the recent years, PV banners have become routinely simulated. However, since the spatial resolution of NWP models has far outstripped resolution of the existing regular observational data networks, before MAP it was difficult to asses the value of high-resolution numerical model forecasts. MAP data sets offer an unprecedented opportunity for verification of high-resolution mesoscale numerical model predictions, and for answering scientific questions related to orographic PV banners. |
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| Two mesoscale model forecasts of PV and wind field at 850 hPa geopotential surface for 1 October 1999 during IOP4. Under northwesterly flow conditions, both the SM model (horizontal grid spacing 14 km; left panel) and the MC2 model (horizontal grid spacing 3 km; right panel) predict a "primary" (positive) PV banner that originates from the west edge of the Alps and extends over the Ligurian Sea, and a number of "secondary" PV banners over the Po Valley originating from the individual massifs along the Alps ridgeline. The PV banners in the MC2 forecast are, however, significantly more complex and display fine-scale structure and variability. |
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