Numerical simulation of pulsating bora wind gusts

Quasi-periodic pulsating behavior of wind gusts is a phenomenon regarded as inherent to downslope windstorms. These pulsations have periods of the order of 1 - 10 min and have so far been observed in the Boulder downslope windstorm, downslope windstorms on Iceland and in the bora wind. The bora wind blows off the east Adriatic coast and has been shown to greatly influence the dynamics of the Adriatic Sea. As the most prominent feature of the bora wind is its gustiness, which is responsible for the appearance of even hurricane-strength wind speeds, it is expected that the effect of gusts on the sea is also significant.

This study deals with the pulsating behavior of the bora wind gusts. High-resolution numerical simulation of a long bora episode is presented and compared with measurements, with the goal to resolve the quasi-periodic pulsations. The model reproduced well the approximately 7 min periodicity of pulsations and the upstream structure of the atmosphere, and is thus used for detailed dynamical considerations. The results of previous studies that reported on the possibility of disappearance of pulsations within a bora episode are confirmed. Specifically, it is shown that the positive tropospheric wind shear related to the upper-level cross-mountain jet diminishes the low-level wave breaking and so the pulsations cease. Additionally, it seems that the situations with absence of pulsations may be related to the formation of the mountain-wave-induced rotor. The origin of the pulsations seems to be related to the Kelvin-Helmholtz instability appearing in the region between the low-level bora jet and wave breaking aloft.