The onset of a severe summer bora episode near Oštarijska Vrata Pass in the Northern Adriatic

Abstract

A severe bora episode which lasted from 22 to 25 July 2012 has been selected for the 3D numerical model analysis of the start of the bora near the Oštarijska Pass in the Northern Adriatic of Croatia. The 10-m wind analysis shows that the gap flow turns and rotates in the central part of the Vir Sea at the start of bora. The MM5 model validation shows that the modelled wind matches well the automatic stations measurements and the validation of the vertical profiles shows that the model adequately captured the basic thermodynamic and flow structure during the bora episode. The vertical cross sections show the turning of the horizontal wind in the first 500–800 m over a relatively small area over the Vir Sea. The analysis implies that the wind turning can be attributed to the impact of the Etesian windflow, the frontal passage and the positions of the outer islands and coastal area. The analysis of the horizontal vorticity of the surface wind shows vorticity banners downwind of the gap with high vorticity values, up to 4 × 10⁻⁵ s⁻¹, in the central part of the Vir Sea. A secondary northeastern vorticity maximum is developed due to the orography of the coastal area and the rotational windflow.