engleski

The complex bora flow in the lee of Southern Velebit

The complex bora flow structure in the lee of Southern Velebit, Croatia, is investigated by means of very high-resolution numerical simulations carried out with the NRL COAMPS model with the aim of identifying reasons for the strong spatial variability of the bora flow in the wider Zadar region and uncharacteristically weak winds in the city of Zadar during bora episodes. The primary focus is placed on the severe bora episode of 20 December 2004 during which a sodar system was operated at the Zadar airport. The numerical model results are verified against the available surface and upper-air observations. The bora flow is characterized by a synoptically- induced critical level and several temperature inversions defining the upstream flow layer. In the lee of the highest portion of the Southern Velebit terrain a hydraulic jump forms, downstream of which a pronounced highly unsteady wake gets established over the Zadar peninsula. The wake is influenced by both the changes in the upstream atmospheric structure and the diurnal boundary-layer evolution. During the most developed phase, in the late morning, the wake scales the length of the entire Zadar peninsula and extends up to the critical level. At the surface, a wake vortex develops as the flow reverses in the wake centerline. In the evening hours, large amplitude trapped waves form on top of the jump, inducing boundary layer separation and rotor formation. Sensitivity experiments, conducted to examine the effects of the Zadar peninsula topography and the height of Southern Velebit on the structure of bora, show the influence of Velebit being particularly strong, governing the onset and strength of the bora. The terrain of the Zadar peninsula is also shown to influence the characteristics of the developed bora flow, especially the location of the flow separation point and the strength of the rotor flow.

Atmospheric rotors; Boundary layer; Critical level; Hydraulic jump; Wake; Wake vortex

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