Simulating the submesoscale rotating structures in the bora wind

Wind speed within 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. In an earlier work, the characteristic horizontal rotational motion of these pulsations at the town of Senj, Croatia was studied using tower measurements. In the present work this analysis is extended to a larger domain using a hectometer-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 min traces out a highly elongated ellipse in the counterclockwise direction, its major axis aligned with the shear vector at the top of the leeside low-level jet. The pulsations are associated with 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 directional shear within the low-level jet, i.e., which direction the wind turns with height. It is argued that the cause of the predominant rotation direction is deformation of the laterally unstable Kelvin-Helmholtz billows by the directional shear.