Structure and dynamics of an Alpine potential‐vorticity banner

• DOI: 10.1256/qj.02.47
• Publication Date: 2003-03-31T15:20:00Z
• Authors: Christoph Schär; Michael Sprenger; Daniel Lüthi; Qingfang Jiang; Ronald B. Smith; Robert Benoit
• Keywords: Hydrostatic equilibrium

Abstract A detailed analysis is undertaken of the primary Alpine shear zone that occurred on 1 October 1999 during Mesoscale Programme. This emanates from south‐western tip in north‐westerly flow conditions, develops response to Alpine‐scale splitting, and separates northerly mistral wind south‐west Alps quasi‐stagnant air within wake. The study based situ flight‐level dropsonde data two research aircraft. are used for construction an objective a cross‐section perpendicular zone, diagnostic computations potential‐vorticity (PV) flux between vertically stacked flight legs. observed structure compared with simulation results Swiss Model non‐hydrostatic Canadian Compressible Community (MC2), operating at horizontal resolutions 14 3 km, respectively. Immediately downstream topography, has surprisingly quasi‐steady becomes evident provided slow westward migration accounted for. Further downstream, shows increased signs transience. Near surface, it overall width ∼150 but narrows about 25 km near top inversion height ∼2 km. Detailed complex substructure consisting least three positive negative PV filaments. These lines characterized by pronounced vertical coherence mixed layer throughout most layer. One banners also low‐level tracer constituents. Diagnostic along using generalized Bernoulli theorem reveal associated due oriented vorticity Within there spanwise circulation appreciable strength. It consists deformation axes zone. effect squeezes thereby makes them resilient respect dissipation barotropic instabilities. intercomparison numerical model models grid spacing below ∼15 able capture MC2 addition credibly replicates some finer‐scale sub‐structure particular low levels. high predictability steadiness supports idea individual generated flow‐separation and/or gravity‐wave breaking events upstream. argued thus determined geometry upstream topography dynamical processes govern advection banners. Copyright © 2003 Royal Meteorological Society.