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Numerical simulation and observational analysis of the bora of Pag's ribs

The severe bora case that lasted from 13 to 15 November 2004 has been selected for the analysis of the bora of Pag’s ribs, which occurs in the northern part of the eastern Adriatic coast over the Pag island area (Croatia). According to the measurements from automatic stations, the MM5 numerical model is successful in the 10-min mean wind speed prediction at 10-m height. The vertical analysis of the wind speed and potential temperature also gave satisfactory results. At the commencement of the bora the modelled wind had a magnitude of 20ms−1 at 10-m height in the Pag island area, which sharply attenuated in the cross direction and to the open sea. In this way the model has proved successful in predicting the characteristics of the bora of Pag’s ribs. Potential vorticity (PV) at 600m has lower values within PV banners than during the developed bora. The consequence is a strong jet emanating from the nearest gap. The vertical cross-sections through the centre of the gap point out a permanent hydraulic-like flow. At the time of the bora of Pag’s ribs the highest modelled turbulent kinetic energy is found in the jump-like region above the inversion and within the boundary layer along the lower boundary, ranging from 6–8m2 s−2. It is concluded that the dissipation in the hydraulic jumps and the wave breaking regions are the reasons for PV generation.

bora; MM5 numerical model; Pag island area (Croatia); Severe bora episode

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