engleski

Downslope windstorms over very complex orography: formation and development of pulsations

While severe northern-Adriatic downslope windstorms are since long in the focus of interest, strong bora winds in the hinterland of mid-Adriatic coast are much less studied, yet frequent and eqaully severe phenomena. The predictability of these events is considerably lower than for its northern counterpart due to the inflow complexity induced by the upwind chain of secondary orographic steep mountain sub-ranges and deep valleys. A strong late-winter anticyclonic bora event at the mid-Adriatic was analyzed with the use of ultrasonic measurements, a SODAR and numerical sensitivity experiments carried out with the WRF model. The three-dimensional bora flow was characterized by a shallow bora layer, a pronounced directional vertical wind shear, and interaction with valley circulations in deep valleys. Two regimes of pulsations were found: i) Regime A – pulsations observed predominantly during the night and morning hours with periods of 5 – 8 minutes and ii) Regime B – pulsations observed predominantly during the afternoon with periods of 8-11 minutes. According to the model simulation, pulsations of regime A propagated far away from the point of origin, while pulsations of regime B quickly dispersed and dissipated during the stable nighttime conditions. The roles of gravity-wave breaking, Kelvin-Helmholtz instability and surface fluxes were analyzed to study the formation and development of pulsations. Furthermore, the secondary orography appeared as essential for the propagation of the bora flow away from the primary mountain range, by promoting the hydraulic-jump type of flow recovery. Finally, main differences in the bora subtle structure, there and over the northern areas, the latter pertaining to more known bora cases, are pointed out.

Bora; Adriatic coast; Pulsations

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