Misalignment between the propagation direction of the bora wind and its pulsations

The bora is a well-known downslope windstorm that blows at the eastern coast of the Adriatic Sea from inland towards the sea, mainly from the northeasterly direction. One of the notable bora characteristics is the quasi-periodic gust pulsations. We address their characteristics during strong to severe the bora events by applying rotational spectral analysis on 9 months of continuous high-frequency sonic anemometer measurements in the town of Senj, Croatia. The analysis shows that the orientation of the pulsations' plane of oscillation does not correspond to the measured near-ground wind direction. In particular, these two directions seem to be negatively correlated. Although the structure of the pulsations is mostly rectilinear, the positive rotational component was larger than the negative one in all except one the bora event. The coincidence of the wind shear direction within the lee-side jet with the orientation of the plane of oscillation leads to a tentative conclusion that its orientation was governed by the Kelvin-Helmholtz instability, at least in the cases presented. We use rotational spectral analysis to explore the characteristics of the quasi-periodic pulsations in the bora wind, based on 9 months of high-frequency measurements. Specifically, the structure of the motions in the horizontal plane and the relation of the structures' orientation to the mean wind direction are explored. Analysis shows that the structures are mostly rectilinear in nature but prefer rotation in the counter-clockwise direction. Possible reasons for the observations, such as large directional shear or potential vorticity banners, are discussed. image