engleski

Numerical simulation of pulsations in the bora wind

Numerical simulation of a long bora episode is presented and compared with measurements. The goal is to resolve the quasi-periodic oscillations of the bora gusts, i.e. the pulsations. The model reproduced well the approximately 7 min periodicity of pulsations and the upstream structure of the atmosphere, and can thus be used for the detailed dynamical considerations. The results of previous studies are confirmed, as well as the hypotheses on the mechanisms responsible for the disappearance of pulsations. Specifically, it is shown that the upper-tropospheric jet induces strong positive shear throughout the troposphere and consequently the local nonlinearity of the incoming flow weakens. Henceforth, the low-level wave breaking is diminished and so the pulsations cease. From the three previously proposed generating mechanisms of pulsations, the results obtained point to the Kelvin-Helmholtz instability as the primary mechanism in this case. Additionally, it seems that the situations with absence of pulsations may be related to the formation of the mountain-wave-induced rotor.

downslope windstorm; wave breaking; jet; trapped lee waves; rotor; Kelvin-Helmholtz instability

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