35 Bora flow over the complex orography of the mid-Adriatic region

Monday, 20 August 2012
Priest Creek AB (The Steamboat Grand)
Kristian Horvath, Meteorological and Hydrological Service, Zagreb, Croatia; and Z. Vecenaj and B. Grisogono

While severe northern-Adriatic downslope windstorms are since long in the research focus, strong bora winds in the hinterland of mid-Adriatic coast are much less studied, yet frequent phenomenon in the region. The predictability of these events is considerably lower than for its northern counterpart due to the flow complexity induced by the chain of secondary orographic mountain sub-ranges and deep valleys.

A late winter strong anticyclonic bora event in the mid-Adriatic was analyzed with the use of measurements and numerical sensitivity experiments carried out with the WRF model. The analysis used wind tower measurements at 10 m, 22 m and 40 m levels above the ground at 5 Hz sampling frequency. The WRF model was configured with grid spacings ranging from 9 km in the outermost domain telescoping to 333 m in the innermost domain.

The three-dimensional bora flow in the mid-Adriatic had a pronounced directional vertical wind shear within the bora layer, while valley circulations interacted with the near-surface bora flow in deep valleys. The strongest observed wind speed pulsations had periods of 7 – 11 minutes and were found to originate near the primary mountain wave region. The pulsations propagated far away from the point of origin during the daytime convective boundary layer, but were quickly dissipated during the stable nighttime conditions. The secondary orography had a crucial role on the propagation of the bora flows away from the primary mountain range, and apparently promoted 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.

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