Dynamic features of bora wind vortices

Abstract Wind speed in bora (downslope windstorm) events at the northern Adriatic Coast is often found to be pulsating in a quasi‐periodic manner with a period of a few minutes. The characteristic horizontal rotational motion of these pulsations at the town of Senj, Croatia was previously studied using tower measurements. In the present work, this analysis is extended to a larger domain using a hectometre‐scale numerical simulation (WRF‐ARW) of a summer bora event. The model successfully reproduced the rotational motion at the position of the tower. The near‐ground wind velocity vector within the band of periods between 3 and 11 minutes traces out a highly elongated ellipse in the counterclockwise direction, with its major axis aligned with the shear vector at the top of the leeside low‐level jet. The pulsations are associated with the Kelvin–Helmholtz instability between the low‐level jet and the stagnation zone. The most interesting finding is that the predominant rotation direction over the rest of the domain, especially over the sea, depends strongly on the directional shear within the low‐level jet, that is, which direction the wind turns with height. It is argued that the predominant rotation direction is caused by the deformation of the laterally unstable Kelvin–Helmholtz billows by the directional shear.