Bora-driven potential vorticity banners over the Adriatic

A case study is presented of the secondary potential vorticity (PV)banners over the northern Adriatic that occurred in an early stage of a bora on 7 November 1999 during the Mesoscale Alpine Programme (MAP) Special Observation Period. The dynamics and structure of the lee-side and cross-mountain flow past the Dinaric Alps were investigated using data collected in a dual-aircraft (NCAR Electra and NOAA P-3) MAP Intensive Observing Period 15 mission over the Adriatic and high-resolution numerical simulations. The observational study employs flight-level, dropsonde, and Scanning Aerosol Backscatter Lidar data. The observed flow structure is compared with simulations results of the COAMPS model run at a horizontal resolution of 3 km. The Dinaric Alps, the north-west/south-east oriented coastal mountain range of Croatia, has an irregular ridge line with a number of peaks in the range of 1.5-2 km with several prominent mountain passes. The identified jet and wake structure within the east-north-easterly bora over the Adriatic was found to be well correlated with the upwind distribution of mountain passes and peaks. The wake flow structure was found also to be in excellent agreement with the climatological profile of the bora strength along the Croatian coast. The attendant secondary PV banners separating individual jets and wakes, diagnosed by computing PV from the flight-level data, were found to have a characteristic horizontal scale of 10-25 km, and a maximum amplitude of up to -6 pvu within the boundary layer. Over the open sea, the thickness of the boundary layer, within which the east-north-easterly bora also achieved its maximum strength, was approximately 1 km. Detailed comparison with the numerical model results shows that, at the horizontal resolution of 3 km, the COAMPS model reproduces well the overall flow structure. The COAMPS-simulated PV field was found to be in good agreement with the PV derived from observations. The differences in substructure between simulated and experimentally derived PV profiles derive from minor differences between modelled and observed velocity and potential temperature profiles, which are subsequently accentuated by computing differentiated quantities such as vorticity and potential temperature gradients. The high predictability and steadiness of the PV banners, and a good correlation with the geometry of the upwind topography, support the orographic generation mechanism of PV with dissipation in hydraulic jumps and gravity-wave breaking regions as the likely main source of PV.