18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Tuesday, 31 July 2001: 4:00 PM
Structure of wake north of the Alps: Study of PV banners during an episode of deep South Foehn
Vanda Grubišic, DRI, Reno, NV
Poster PDF (191.5 kB)
As the horizontal resolution of numerical models has increased during the past five years, orographic PV anomalies (banners), indicators of significant horizontal wind shear in the presence of stratification, have become a ubiquitous feature of numerical simulations of flow in complex terrain. One of the objectives of the Mesoscale Alpine Programme (MAP) experiment was to observationally document shearlines and PV banners generated by the Alps. Extensive MAP data sets provide an excellent data source for verification of high-resolution numerical simulations, and can provide clues, if not direct answers, to aid in solving dynamical puzzles such as orographic PV generation.

Six PV banner missions were realized during the Special Observation Period (SOP) of MAP in Sept-Nov 1999. During MAP IOP 8, on November 8, a single-aircraft PV banner mission was flown by NCAR Electra over the Bavarian plane, north of the east-west oriented portion of the main ridge of the Alps during an episode of deep South Föhn. The Electra carried out multiple transverses within a single Alps-parallel vertical plane, which was covered additionally by a curtain of dropsondes. The wind speed variation along the flown tracks, within and above the planetary-boundary layer, reveals a strong variation of the strength of the southerly wind along the Alps and reveals existence of multiple narrow shearlines and associated PV banners. The position of two major documented jets correlates well with two large gaps in the terrain representing the exit regions of the Inn and the Rhine Valleys.

High-resolution simulations of this case carried out with the Naval Research Laboratory's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) compare favorably with the MAP observations. Spatial structure, temporal evolution and budget of vorticity concentrated in shearlines and secondary PV banners in the lee of the Alps, generated by a deep South Foehn flow over the Alps, will be compared to the observed and simulated flow and PV banners in the lee of the Dinaric Alps driven by Bora (MAP IOP15, Nov 7 1999).

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